Top vs Bottom Balancing and

Dzl

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Below is an excerpt from marinehowto.com the source Will 'highly suggests' here for more information on lifepo4 and DIY lifepo4 building. I highly suggest reading the full article, its really really in depth and pretty well written. I learned a ton from it.

The excerpt I am posting relates to balancing. The author advocates top balancing for fractional C, 'off-grid' marine use (but would be just as applicable to on land vehicles and maybe stationary off grid situations). I won't summarize his reasoning here as it is all outlined below. Will and it seems most people here opt to bottom balance their cells. I have two reasons for this post:

  1. Its a request for comment on this method of balancing from those who prefer to bottom balance (or those who top balance)
  2. To share this method of balancing 'Parallel step-method top balancing'
What are you thoughts?


Beginning of excerpt:

A LiFePo4 Build Needs to Begin With Well Matched Cells
Cell Matching
10-LiFePO4-On-Boats.jpg


Any discussion about balancing the cells would be incomplete without discussing matching the cells for internal resistance and Ah capacity. When I got these cells there was less than a 0.32 Ah difference between the worst cell and the best cell. These cells were made in 2009 and were matched at the factory before being shipped to Balqon. Finding well matched cells today can actually be rather difficult. Over the last 8 years things seem to have gotten worse with LFP cell matching, not better. I really don’t know why, but it’s what I am seeing.

If you don’t build your pack from well matched cells there’s not a lot that can be done to keep them in balance when working into the knees, so do yourself a favor and stay out of the knees. If you don’t have a way to confirm the Ah capacity of each cell, you’re essentially shooting darts blind trying to build your own pack.

Cell Balancing
Cell balancing is an extremely important aspect of LFP banks. When you have lead acid batteries in series they can be purposely over charged/equalized to a 15.5V pack voltage and they will, in a sense, self balance. With LFP banks this will not happen due to the knee ranges. As a cell becomes full the voltage all of a sudden skyrockets and the cells need to be in balance in order to charge and discharge at matched voltages.

TOP BALANCE vs. BOTTOM BALANCE:
There is much controversy over top vs. bottom balance mostly due to confusion over differing uses.

BOTTOM BALANCE:
A bottom balance simply means the cells are balanced at the lowest “safe” voltage and all cells will converge and match exactly at say 2.75 VPC. In the EV world bottom balancing is almost always the preferred method, and makes the most sense. With high loads, and frequent opportunities to completely drain the bank, a bottom balance is critical with an EV pack. In an EV the car is then brought back to the garage and charged with ONE charge source.

Bottom Balance:
1-
Discharge cell using a 20-30A load to 2.50V
2-Let the cell rest at room temp for 24 hours and allow voltage to rebound
3-The cell will now be resting somewhere between 2.75V and 2.85V
4-Apply the load and stop discharging at exactly 2.65V
5-Allow voltage to recover for about 6 hours
6-Repeat load discharge to 2.65V until the resting stable voltage of each and every cell is 2.75V
7-As you get closer and closer to resting voltage of 2.750V a small resistor can be used as opposed to the large load.
Once all cells rest at 2.750V and stay there the cells are bottom balanced.

NOTE: A guy recently dropped off 4 cells he was having trouble “balancing”. He was attempting a bottom balance and intending on using these for fractional “C” use stopping at 70% DOD.. He had spent countless hours trying to bottom balance these cells, and he did.

So what’s the problem? The problem is that at a 14.0V pack voltage he had one cell at 3.65V and one cell still at 3.380V!!!! His cells tested at varying capacities and thus the cell with the lowest capacity was firing into the upper knee sooner than the rest, even at a pack charge voltage of 14.0V. These were cells with an absolute MAX cell voltage of 3.600V. With a bottom balance and used cells (I don’t suggest buying used cells) he was sending one cell into the dangerous upper knee even at just a 14.0V charge rate. I conducted a top balance for him and the cells now all remain well balanced at the upper charging voltage range. On the low end one cell will still fall off the cliff early, but at 70% DOD that does not happen.

TOP BALANCING:
On boats we have multiple charge sources, shore charger, alternator, solar, wind, hydro or even hydrogen fuel cells. Our risk of cell imbalance is more pronounced at the top end rather than the bottom end. We run a much higher risk of over charging imbalanced cells than we do by over discharging, like the electric vehicle (EV) guys do, but it can still be a risk.. For off-grid / marine use top balancing is quite often the preferred method so the cells converge or are in excellent balance at the top, when fully charged, rather than when dead or fully discharged…

In theory the BMS would always protect the cells at either the bottom or the top end but keeping the cells well balanced ensures an extra level of protection, just as keeping charging voltages out of the upper knee range does. Don’t discharge below 80% DOD and have a max charge voltage of 3.5VPC / 14.0V for a 12V bank, and your cells will be very happy.

Whether you choose to top or bottom balance is a personal choice. I chose a top balance for this bank and even after 700 cycles the cells have tended to converge in cell voltage rather than diverge.

PHOTO: In the photo the four Winston cells have been individually & very carefully charged to 3.75VPC with the bench top power supply shown. The cells were then wired in parallel and allowed to sit for multiple days but weeks or months is even better, if you have the time…
TIP: When ordering cells ALWAYS order extra cell jumpers so that you can wire the cells in parallel and top balance if you choose to do so.

Continued in next post
 
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Dzl

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Continuation of the above excerpt

Balancing – Wire The Cells In Parallel

11-LiFePO4-On-Boats.jpg

2010: As mentioned, I first charged these cells, INDIVIDUALLY, to 3.75VPC and X current taper. The bench-top power supply allows you to set the voltage to 3.XX and let the cell become “full” at 3.XX VPC. For these cells, based on the data available at the time, late 2010, I held the voltage at 3.75V and allowed the current to tail off to 20A then stopped charging and moved onto the next cell.

Within seconds of wiring these in parallel only 0.59A was moving between cells which means the balance to 3.75VPC was pretty close.. Leaving them in parallel will get them in closer balance but this can take lots & lots of time.

Updated Cell Balance Process: Parallel Step-Method Top Balance

My goal when balancing cells is always the following:

Keep the cells in the upper-knee for the shortest amount of time and still net a perfect balance

Trough testing and experimenting with numerous balancing processes I’ve found the “parallel step-method top balance” (PSMTB) has proven to be the absolute fastest method that also keeps the cells in the upper-knee the shortest. This means less upper-knee time for the cells. You will need a variable power supply capable of low voltage (3.6V) to do this. You will also want a model with the highest amperage you can source. Keep in mind that when we wire the cells in parallel the bank capacity grows tremendously. Four 400Ah cells become a 1600Ah 3.2V pack! Getting to 3.40V will take quite some time! The key with the PSMTB come from the fact that the cells are essentially full when you get to 3.40V and 0A. This 3.40V threshold is a perfectly safe voltage for the cells so no matter how long it takes to get there will not be causing damage to the cells. Once at 3.40V this means our steps to get to 3.5oV and then 3.60V are much, much shorter than the first step getting to 3.40V. The step to 3.50V is longer than the final step to 3.60V, which happens pretty quickly.

Parallel Step-Method Top Balance
:
1- Wire the cells in parallel
2- Set the power supply to 3.400V and 80% or less of the rated amperage (80% to not burn it out)
3- Turn on power supply and charge cells to 3.400V
4- When current has dropped to 0.0A at 3.400V turn off the power supply & set it to 3.500V
5- Turn on power supply and charge cells to 3.500V
6- When current has dropped to 0.0A at 3.500V turn off the power supply & set to 3.600V
7- Allow current to drop to 0.0A (or very close) at 3.60V
8- Done, pack is balanced.
WARNING: Top each cell up, to a similar SoC level, prior to wiring them in parallel.

Balancing Via Parallel Resting Voltages???
Many often assume that by simply wiring the cells in parallel they will magically get themselves in balance. This is not entirely true, if you expect it to happen in a timely manner. When cells are wired in parallel, the the cell voltages attain a parity voltage rather quickly. Once a parity voltage is attained the transfer or movement of current between cells, in order to balance SoC, slows to a crawl. Ohms law is in control here and we are talking 0.0001A level movements of current. Attaining a true balancing, by letting cells sit in parallel, at a resting non-charging voltage, takes a very long time. You can let them sit for a week or more, but again, this may not be enough time. Balancing ideally requires a voltage differential to move current between or into the cells. When cells are at the same voltage this transfer of current = slow.

You can drastically speed the process by presenting the parallel wired cells with a charging voltage.. The PSMTB method is the fasted way we know of to attain a perfect balance. Once all cells are at the same voltage and no more current can flow into the cells they are all at the identical SoC.

TIP: Never trust the volt meter on the bench top power supply as there will be voltage drop or inaccuracies between the supply & actual battery terminals. Always try to measure the actual battery terminal voltage, using an accurate DVM, when top balancing. You also don’t want to adjust your power supply voltage while it is under load. Again, this is because because there will be voltage drop between the batteries and power supply. Unlike the much more expensive BK Precison supplies we use here in our shop, the Mastech power supplies do not have a voltage sense circuit. While inexpensive they are certainly a bit less than full featured. Always set your power supply voltage into a zero amp load and do so based on your DVM’s accuracy not the power supply screen. Once voltage has been set you can then connect the load.

IMPORTANT: Please note that Winston recommended the 3.8V top balance voltage back in 2010. Today we don’t go anywhere close to this, no need to.
Current Moving Very Slow
12-LiFePO4-On-Boats.jpg

The current moving between cells dropped from 0.59A to 0.18A in a matter of seconds. It kept dropping very, very rapidly. This image and the previous one illustrates how parity voltages mean very little current movement. Within about 20 seconds the current moving between cells was below the resolution for this expensive clamp meter to read accurately. Was “balancing done” at this point, heck no…

Balancing Parallel Cells To 3.800 VPC
13-LiFePO4-On-Boats.jpg


IMPORTANT: This was 2010 and today we do not recommend top balancing to 3.800VPC as it is simply not necessary to push the cells to this level.
Top balancing, even at 3.600VPC needs to be closely monitored. Like equalizing flooded batteries you simply do not want to leave them unattended. Once the cells hit 3.600VPC (3.800 VPC in image) you may need to adjust your power supply, very carefully, so it does not overshoot target top-balance voltage. Watch your DVM not the power supply display.

When you balance in parallel you can hold voltage steady and allow the current to taper until flickering between 0.0A and 0.1A. Now the parallel pack is balanced and can be disconnected. After you top balance to 0.00A disconnect the charge source and you’re done.

EXPERIMENT: I recently conducted an experiment on some test bench CALB cells that paired a “balancing BMS” against a 3.65V parallel top balance to 0.00A on the power supply. A parallel top balancing to 3.65V & 0.00A actually re-balanced the cells in just under 3 hours.
Using the “balancing BMS“, after more than 7 hours at shunting / balancing voltages, the cells were still not “balanced“. The point of this was to see what it would actually take, in voltage held at high levels, to actually re-balance a pack. Breaking the pack down and performing a parallel top balance is significantly faster and meant considerably less time for the cells at a high voltage.

Continued in next post
 

Dzl

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Continuation of the above excerpt

Bench Top Power Supply

14-LiFePO4-On-Boats.jpg

As I mentioned earlier I am a believer that if venturing into DIY LiFePO4 it should be done as a system. Part of that system should include funds for a bench top power supply and other equipment to test for capacity etc.. In my opinion a bench top power supply with variable voltage and current should be a pre-requisite for DIY LFP. Can you make do without? Sure, and I am certain Bode Miller could ski with only one leg, but why..? In the whole scheme of things they are inexpensive and they have multiple uses not just for charging or top balancing LFP.

The bench top power supplies I sometimes use are made by Mastech, specifically the Mastech EX series. We own a 3030EX and a 3050EX. These are not the fanciest or the most expensive power supplies but they work and they work pretty well, especially for the price. Years ago these devices would have run four figures each but today they are very reasonably priced. A Mastech 3020EX (30V X 20A) will run you just $219.95. It will save you $400.00 in your time fiddling with top balancing alone. You will be looking for a 0-30V and 0-10A or larger model. This is my 3050EX. The EX in the Mastech line signifies these units are specifically designed for charging batteries, usually Li batteries. The dial second from the left is EX knob or the over voltage protection dial. Set this dial and the power supply will protect itself.

While the Mastech line represents a great value, our main work-horses in the shop are the BK Precision Model 1900’s. The BK Precision 1900 is a 1-16V, 60A variable power supply with dedicated voltage sensing leads. The voltage sense leads, to me, are really the driving factor as you get far more accurate voltage at the terminals without worrying about voltage drop through the cables & terminals. It is a very nice piece of gear but they run close to $600.00 each.

Knobs and Displays:
Left Digital Display = Current Output
Right Digital Display = Voltage
Red Light = Constant Voltage Mode (power supply is limiting voltage to 13.8V)
Left Knob = Current Control Dial
Second From left Knob = Over Voltage Limit
Third From Left Knob = Constant Voltage Fine Tune Adjustment
Right Knob = Constant Voltage Coarse Tune Adjustment

As you can see in this picture with 15A of current flowing the Mastech and the Fluke are in close agreement but I still trust my Fluke a lot more than the voltage display on the power supply.

TIP: When charging LFP cells or banks with a bench top power supply please dial the current back by about 20%. This will allow the power supply to run almost indefinitely and not cause undue wear and tear on the unit. I run my 30A model at 24A and my 50A model at 40A… I sometimes parallel them and charge at 64A when doing cycle testing.

Nothing makes top balancing easier than a bench top power supply:
#1 Charge individual cells to .05V below max top balance voltage and allow current to taper
#2 Wire cells in parallel and let sit, the longer the better.
#3 Charge cells to max top balance voltage Winston = 3.65V
#4 Allow current to go to 0.00A
#5 Turn off power supply and you’re done.

End of Excerpt
 

FilterGuy

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The key to top vs bottom balance is contained in this statement from the article:
"On boats we have multiple charge sources, shore charger, alternator, solar, wind, hydro or even hydrogen fuel cells. Our risk of cell imbalance is more pronounced at the top end rather than the bottom end. "

What he is saying is that the risk of one of the cells getting damaged due to over-voltage is more pronounced at the top end so he does top-balance.

If you top balance, then when you charge and discharge, your cell voltages will typically be closest to the same at full charge and furthest apart at fully discharged. If you are more likely to run batteries at max capacity, you want your cells to be matched at that point so top-balancing is a good idea.

However, if you top balance and then run your battery to completely empty, you are more likely to hit the minimum voltage on one cell sooner than the rest. Consequently, if you are worried about what happens at low charge you want to bottom balance. Put another way: top Ballencing allowes you to operate around the max voltages of the cells and bottom balancing allows to to play around the minimum voltage of the cells.

So what is worse? Having the cells most unequal at the top or at the bottom. That is almost a religious question and I am not sure I have a strong argument either way. However, as the article says, if you "stay away from the shoulders", it does not matter as much because it is less likely for one of the cells to ever hit min or max. Also, if you have a good BMS, it will cut things off if any one of the cells hits min or max to prevent damage to the cell.

In a different thread, one of the contributors said most of the balancing BMS' top-balance. Assuming that is true, you have to wonder if that is easier to design a circuit for or if they think top-ballence is better.

Here is one thing I have wondered about: I have read several places that LiFePo can "tolerate over voltage" better than other chemistries, but I have never heard LiFePO can tolerate under voltage. In fact, I have heard statements like "under-voltage will instantly kill LiFePo"
Given those two statements, I would think that bottom balancing is best for battery protection..... but that is a guess on my part.

Note: I avoid doing anything to that would come close to under or over voltage so I hope I am never able to give you personal-experience advice on what happens. :)
 

FilterGuy

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If you don’t build your pack from well matched cells there’s not a lot that can be done to keep them in balance when working into the knees, so do yourself a favor and stay out of the knees. If you don’t have a way to confirm the Ah capacity of each cell, you’re essentially shooting darts blind trying to build your own pack.

This is an important point. I have seen posts that seem to confuse matching voltages with matched cells. If you receive a bunch of cells and they are all at the same voltage, it is a good sign but it does not mean they are well matched. It only means they were charged to the same voltage. You can't tell if they are matched unless you do a capacity test on them.

However, if they are different voltages when you receive them it is a bit of a red flag. Different voltages probably means they did not do matching tests before they shipped them to you.
 

Dzl

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Thanks for the reply FilterGuy, your comment definitely adds some clarity to the topic.

If you top balance, then when you charge and discharge, your cell voltages will typically be closest to the same at full charge and furthest apart at fully discharged. If you are more likely to run batteries at max capacity, you want your cells to be matched at that point so top-balancing is a good idea.

This seems like a very good rule of thumb and a good way to look at it.

Note: I avoid doing anything to that would come close to under or over voltage so I hope I am never able to give you personal-experience advice on what happens.

This would be my intention as well. One of the reasons I am considering a top balance (apart from being swayed by that post), is that my system plans for 2-3 'days of autonomy,' so I wouldn't expect to dip down to lower voltages more than every once in a while. In those cases I could rely on my BMS to cut things off before it got to dangerous levels and/or disconnect loads before it became an issue. On top of this, because the bottom end of the voltage range would essentially just be backup capacity that I would rarely dip into, it would be comparatively less of a big deal to be conservative with lower voltage cutoff.

Here is one thing I have wondered about: I have read several places that LiFePo can "tolerate over voltage" better than other chemistries, but I have never heard LiFePO can tolerate under voltage. In fact, I have heard statements like "under-voltage will instantly kill LiFePo"
Given those two statements, I would think that bottom balancing is best for battery protection..... but that is a guess on my part.

Yeah, I've heard the same about the greater potential for quickly damaging your batteries at low voltage, that is definitely one logical approach and a prudent one. But I am looking at it a bit differently.

Would it make any sense to set conservative pack level and cell level limits at the low end and then top balance, so your cells are the most balanced near the top where they will spend most of their time, and where you are slightly more comfortable getting close to the upper limit. And than for the cases where you do get down towards the lower limit and your cells are more out of balance rely on a conservative pack level cutoff as well as cell level protection from the BMS. Does this make any sense? I'm still learning the concepts so I'm not quite confident in my own logic yet, but it seems like a possible logical approach and has multiple layers of protection at the low end.

However, as the article says, if you "stay away from the shoulders", it does not matter as much because it is less likely for one of the cells to ever hit min or max. Also, if you have a good BMS, it will cut things off if any one of the cells hits min or max to prevent damage to the cell.

Good points, and ideally I would like to design my system this way. Based on the article, it sounds like with low discharage and charge rates and good cells, its not hard to stay away from the shoulders and not sacrifice much capacity if you design it right.
 

FilterGuy

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Would it make any sense to set conservative pack level and cell level limits at the low end and then top balance, so your cells are the most balanced near the top where they will spend most of their time, and where you are slightly more comfortable getting close to the upper limit. And than for the cases where you do get down towards the lower limit and your cells are more out of balance rely on a conservative pack level cutoff as well as cell level protection from the BMS. Does this make any sense? I'm still learning the concepts so I'm not quite confident in my own logic yet, but it seems like a possible logical approach and has multiple layers of protection at the low end.
That sounds perfectly reasonable to me. I tend to be conservative with the settings on both ends. However, by top balancing you have a bit more control in the area your batteries will live in the most.

Balancing and matching cells is most important when you are trying to get every possible electron out of the battery. I am not doing that and it doesn't sound like you are either. In our case it is more about being gentle on our batteries and finding the best way to avoid damaging them.

I'm still learning the concepts so I'm not quite confident in my own logic yet,

You are doing the exact right thing: Read/watch multiple sources and ask a lot of questions.
 

FilterGuy

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If you don’t build your pack from well matched cells there’s not a lot that can be done to keep them in balance when working into the knees, so do yourself a favor and stay out of the knees. If you don’t have a way to confirm the Ah capacity of each cell, you’re essentially shooting darts blind trying to build your own pack.

I don't completely disagree, but I think it is a bit over stated. If your cells arn't well matched but you have a decent BMS, you can still build a good pack.... but the 'weakest' cell will define the capacity of the pack. (The capacity is not an average of all of the cells)

Also, if you have a BMS that can report on the cells, you will be able to see which ones are charging or discharging faster than the others. If one is way out of wack from the others you can swap it out and see how the replacement cell behaves.
 

HighTechLab

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I will say, When I put my bank of 16 used LiFePO4 cells online, I bottom balanced them - In an off grid application we have a backup generator that we want to run as little as possible. I do have one of the Deligreen active balancers on my system though, one of the ones able to move 6 amps between cells.

My opinion is that adding this active balancer, the top or bottom balance is out the window as soon as you connect it and you end up with a "living balance"

In this case it would depend on where the cells spend most of their time in terms of state of charge, wouldn't it?

And if you have the proper battery management system that shuts off the batteries if any cell gets out of range, you shouldn't really have a huge difference between either?

Don't take this message as fact, strictly just trying to apply some of the things I have experienced to add some thinking and reasoning to the post.
 

Dzl

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I don't completely disagree, but I think it is a bit over stated. If your cells arn't well matched but you have a decent BMS, you can still build a good pack.... but the 'weakest' cell will define the capacity of the pack. (The capacity is not an average of all of the cells)

Also, if you have a BMS that can report on the cells, you will be able to see which ones are charging or discharging faster than the others. If one is way out of wack from the others you can swap it out and see how the replacement cell behaves.

Yeah, I agree with you here. If you read the entire article, you will see the author takes a very cautious, conservative, approach. I'm glad he does, those are often the best people to learn from, even if you end up being a bit less conservative in your design. But considering that his advice is oriented towards the marine crowd (where failure is a bigger deal), is writing for a general audience, and is representing a business, prudence makes sense. IIRC (been a couple weeks since I read the article in full), he also doesn't personally use a balancing BMS so well matched cells probably matter more for his application.

Personally, I think I am more willing to take the risk of buying cells off aliexpress to save a few hundred bucks for my application, and my system size.
 

Dzl

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My opinion is that adding this active balancer, the top or bottom balance is out the window as soon as you connect it and you end up with a "living balance"

In this case it would depend on where the cells spend most of their time in terms of state of charge, wouldn't it?

And if you have the proper battery management system that shuts off the batteries if any cell gets out of range, you shouldn't really have a huge difference between either?

Don't take this message as fact, strictly just trying to apply some of the things I have experienced to add some thinking and reasoning to the post.

All good points. I hadn't really thought of the 'living balance' thing, but that makes sense. Although for all the people buying the cheap BMS's that balance at like 50-150ma, I wonder if the initial balance is significantly more important.
 

HighTechLab

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All good points. I hadn't really thought of the 'living balance' thing, but that makes sense. Although for all the people buying the cheap BMS's that balance at like 50-150ma, I wonder if the initial balance is significantly more important.
I would presume it absolutely is more important - If you are charging a 100ah bank at 50 amps, then there is no way if a cell is 100% full, that the balancer can move 100% of the power out of this cell, hence the need for the cells to still line up pretty well.

The nice part of this 6 amp balance current is that slowly these mismatched cells are starting to wear in to a point that they all are matching and living in harmony.
 
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FilterGuy

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All good points. I hadn't really thought of the 'living balance' thing, but that makes sense. Although for all the people buying the cheap BMS's that balance at like 50-150ma, I wonder if the initial balance is significantly more important.

The initial balance certainly can't hurt!

I assume the 'trickle' balance of the small BMS's will eventually balance the cells, but it will take days or weeks for a high amp-hour pack. If you are using your cells in the mean time, who knows how it will all add up and if the BMS can ever really catch up. All the more reason to be conservative on your settings.
 

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Total green horn here: I'm about to fire up my four 100ah 12v Battle Born batteries (2s2p )to the 24v Growatt tomorrow. After reading this I'm wondering: Do I need to wire them in parallel and let them sit for a while first!? I don't have a trustworthy bench charger
 

Dzl

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Total green horn here: I'm about to fire up my four 100ah 12v Battle Born batteries (2s2p )to the 24v Growatt tomorrow. After reading this I'm wondering: Do I need to wire them in parallel and let them sit for a while first!? I don't have a trustworthy bench charger
When we talk about top and bottom balancing we are talking about balancing 'raw cells' before making them into a battery. Since you are buying a pre-made, high quality, beginner friendly batteries you don't have to worry about this.

As for whether you need to wire your batteries in parallel before assembling, I'm not sure, it wouldn't hurt, but since they are probably shipped at a storage SOC (where the voltage curve is very flat) I suspect it wouldn't help too much either, I may be wrong.

There are many BB owners here, and many with experience with other drop in batteries that may be able to answer your question with more certainty.

I suggest you review the manual for your Battleborn batteries (as well as the FAQ's on their website), the manual includes installation instructions and if you are still unsure reach out to tech support. The way I look at it, this is one of the selling points of buying from reputable and pricey US brands--good support and documentation, so might as well get you money's worth :)
 

FilterGuy

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Total green horn here: I'm about to fire up my four 100ah 12v Battle Born batteries (2s2p )to the 24v Growatt tomorrow. After reading this I'm wondering: Do I need to wire them in parallel and let them sit for a while first!? I don't have a trustworthy bench charger
It would probably be a good idea to get them to the same state of charge before you put them in series. Ideally you would put them in parallel *and* on a 12v Charger.
 

GLC

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Total green horn here: I'm about to fire up my four 100ah 12v Battle Born batteries (2s2p )to the 24v Growatt tomorrow. After reading this I'm wondering: Do I need to wire them in parallel and let them sit for a while first!? I don't have a trustworthy bench charger
So I have 8 of the same batteries. 4 on one solar set and 4 on another. They are installed 2S/2P. First I have learned that these batteries do not come in fully charged. Since I did not have a charger, I set them up as 2s on my solar controller to fully charge for a couple of days. I then hooked them up 2S/2P on one of my solar setups. I had issues right off of the batt of them syncing up. All seemed to be all over the place as far as different volts at different times. I then put 2 back at 2S. I ran them for about a week hooked up to a solar controller but I also had a load on them daily so they cycled up and down. I did this with both sets. I then charged them fully again and reinstalled them 2S/2P. All were now reading and behaving normal. Do not know why this happened but it did. Maybe because I bought them at different times? All were bought within three of months for a total of 8. I still have the last 2 not playing well with 2 others. A couple of nights ago I had 2 batteries in parallel go to 0 in a series where the other 2 were at 13 volts. They are the latest and newest that i got. Will do a re-sync of the 4 again this weekend. I am not bashing Battleborn, best batteries out there. Just the more setups that you put together seems the more finicky things are.
 

Nicauna

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So I have 8 of the same batteries. 4 on one solar set and 4 on another. They are installed 2S/2P. First I have learned that these batteries do not come in fully charged. Since I did not have a charger, I set them up as 2s on my solar controller to fully charge for a couple of days. I then hooked them up 2S/2P on one of my solar setups. I had issues right off of the batt of them syncing up. All seemed to be all over the place as far as different volts at different times. I then put 2 back at 2S. I ran them for about a week hooked up to a solar controller but I also had a load on them daily so they cycled up and down. I did this with both sets. I then charged them fully again and reinstalled them 2S/2P. All were now reading and behaving normal. Do not know why this happened but it did. Maybe because I bought them at different times? All were bought within three of months for a total of 8. I still have the last 2 not playing well with 2 others. A couple of nights ago I had 2 batteries in parallel go to 0 in a series where the other 2 were at 13 volts. They are the latest and newest that i got. Will do a re-sync of the 4 again this weekend. I am not bashing Battleborn, best batteries out there. Just the more setups that you put together seems the more finicky things are.
Yikes! That makes me a little timid! So, if I hear you correctly: I should hook 2 of them up in series and run them on the charge controller with a load then do the same with the other two before putting them 2s2p?
 

Dzl

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So, if I hear you correctly: I should hook 2 of them up in series and run them on the charge controller with a load then do the same with the other two before putting them 2s2p?
This sounds counterintuitive to me. I still suggest you follow the manufacturers recommendations if they are given in the manual and if not ask support for clarification. I can't see how putting them in series would help balance them, we put cells in parallel to equalize charge, I expect it would be the same with BB drop-ins as well.
 
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