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Lithium battery build questions

BeerBrewer

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May 14, 2020
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I own an Travel Trailer and need to upgrade the battery in it. The cheapo lead acid battery is no longer cutting it. So I've been researching a lot about lithium batteries and I am very interested in building my own using LiFePO4 3.2v cells. Can batteries be put together without using a power supply to ballance the cells before putting them in series? I've watched a few videos where they literally take the cells out of the box, measure the voltage across each, if close they just start putting the them together. I've seen others where they say balancing the cells by connecting them in parallel and applying voltage to them essential.

If ballancing the cells is essential (which I'm thinking) what size is the minum power supply required? I own a 12V DC power supply and I could easily use a adapt it into a variable DC power supply, but it would at most deliver 5 amps.

Does anyone knoow where I can find a site that has written instructions on how to correctly put together lithium battery? I just don't trust the all the videos that are out there, except those from Will Prowse. I'd like to build a 12V battery using 280AH or sightly larger cells.

Your hel[p is appreciated!
 
The solution I recommend is to add an active balancer or use a BMS with an active balancer. Skip the parallel top balance.
 
For you new battery you need a BMS anyway.
The fasted way to do a top balance, is to put the battery together in series, with the BMS attached. Now you can charge the battery as you woud do normally.
After the BMS cuts the charge because of overvoltage of one cell, you can disassemble the battery, put everyting in parrallel and finish the topbalance. For the last part, you need a powersupply that provides 3.65 or 3.6 volts.


There are also a lot of people that don't do the topbalance and trust that the BMS will take care of this. If you have a BMS with a good active balancer, you can do that. However, still I can take quite some time before the pack is balanced if you go that route. The JK BMS has a pretty good build-in active balancer, so if you don't do the top balance upfront, go for the JK.
 
Look under 'Resources ' on this site, and the Overkill site under downloads,

One technique that I have used with sucess is to build the battery in a containment structure, add the BMS, idealy with Bluetooth connection, and charge as a 12v battery with a charge voltage of 14.00 volts. As the battery reaches near full charge it's probable one cell will reach the protection level and cause BMS shutdown. Make up a DC load, a 12v car incandescent headlight bulb is suitable, and apply across the 'high' cell for several seconds whilst monitoring cell volts. With a little experience the duration needed for a given voltage fall will become easier. This is carried out with the assembled battery under charge. Once you have the cells with a resionable delta at 14 volts at the battery, say within 100mV , a BMS with passive balance will bring things in balance over time.
I would avoid if possible using an active ballancer unless you have really crap cells, in some situations they cause problems and if they fail may damage your battery.
It's important that the first full cell charge cycles are under taken with the cells under restraint.

Mike
 
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I was deffinetly planning to use a BMS, I haven't 100% decided on which, but I was leaning heavy towards the Overkill unit that Will Prowse recommends (Overkill Solar 12v 4S 120A BMS w/ Low temp Charging Protection). Is there a better one out there? I'm only planning on installing at max a 500Watt pure sine inverter, just to run a TV or small AC appliance, so the 120 amp BMS is big enough.
 
I was deffinetly planning to use a BMS, I haven't 100% decided on which, but I was leaning heavy towards the Overkill unit that Will Prowse recommends (Overkill Solar 12v 4S 120A BMS w/ Low temp Charging Protection). Is there a better one out there? I'm only planning on installing at max a 500Watt pure sine inverter, just to run a TV or small AC appliance, so the 120 amp BMS is big enough.
Many people are very happy with the Overkill. Some friends of mine have it, and it does the job. With regards to balancing, it has not a strong balancer.

If you choose this BMS, i would not skip the top balance.
 
I like the Overkill bms. The new app is great. My son has one in his Bass boat for his trolling motor for several years. The cells are flexible urethane potted into a battery box for vibration and ruggedness(I know, not serviceable). Even the capacitors were Shoe Goop-ed for vibration reduction. He has the Heltec balancer that can be turned on with a switch that just recently updated with an automatic controller. If you have this balancer and a way of reducing your charge current when you get at around 80% (when the cell voltage difference starts to spread) you won’t need to top balance the first charge. This balancer will keep your cell in check if you don’t frequently get to full charge or spend very little time at full charge. If you use a smart balancer be sure to disable the balance function in the app for the bms, they can confuse or fight each other and slow the balancing, especially near finish. The control is on eBay and the balancer from Amazon. Yeah it’s more wires but worth it.
IMG_0692.jpegIMG_0693.jpegIMG_0727.jpeg
 
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For you new battery you need a BMS anyway.
The fasted way to do a top balance, is to put the battery together in series, with the BMS attached. Now you can charge the battery as you woud do normally.
After the BMS cuts the charge because of overvoltage of one cell, you can disassemble the battery, put everyting in parrallel and finish the topbalance. For the last part, you need a powersupply that provides 3.65 or 3.6 volts.


There are also a lot of people that don't do the topbalance and trust that the BMS will take care of this. If you have a BMS with a good active balancer, you can do that. However, still I can take quite some time before the pack is balanced if you go that route. The JK BMS has a pretty good build-in active balancer, so if you don't do the top balance upfront, go for the JK.

Which JK BMS would you recommend? There are quite a few.
 
Which JK BMS would you recommend? There are quite a few.
You need to look at the number of cells you want to put in series. In your case that is 4.
In addition you can look at the capacity of the balancer. If you don't want to do the top balance, i would go for the most powerfull (2A). If that is too much, you can configure the app and set a lower amperage. With large cells 2A is perfectly fine.
Finally the max output the BMS can deliver (max amps). In my opinion, more is always better, so (200 Amps).
I just looked at the Hankzore store (on Aliexpress) and think this one would adress the above specs.
Personally good experiences with this shop on Ali. I ordered 3 BMS's from them and recentely 2 Neey active balancers. All delivered very fast.

1684850597209.png
 
Comming back to your question about a top balance, today I made a screenshot and this is the perfect example.
I received 16 cells for a new battery that I want to add to my system.
How I do the top balance is assembling the pack with BMS and charge until cutoff, then in parrallel.
The cells I received had a very different SOC. Two are very low (too much for my taste, but we will see) others pretty high.
The BMS cutted charging within 10 minutes. It is possible to have the balancer take care of this, but in this scenario, I would just take ages to have this fixed by the BMS. Not days, but realy ages. I charged one cell up, but decided that does not make any sense so the pack is back in parrallel and I am peforming a normal top balance now.
Slamming the pack together and have the balancer take care of it, is in this case with such enormous differences in SOC no option. 1684876062631.jpeg1684876130282.jpeg
 
The BMS cutted charging within 10 minutes. It is possible to have the balancer take care of this, but in this scenario, I would just take ages to have this fixed by the BMS. Not days, but realy ages.
Active balancer would be a few days to a week as it runs 24/7. Yes passive balancing could be effectively never as shown above.

However yes if the individual cells are 500 mV different a quick session to 3.450 might be in order. But still I would sooner buy the active balancer over a power supply that still may take a week+ with 16 cells. OP has a 5 amp charger.
 
Active balancer would be a few days to a week as it runs 24/7. Yes passive balancing could be effectively never as shown above.

However yes if the individual cells are 500 mV different a quick session to 3.450 might be in order. But still I would sooner buy the active balancer over a power supply that still may take a week+ with 16 cells. OP has a 5 amp charger.
Active balancer in my BMS is 2 amps. Looking at the differences (a couple very high and a couple very low), even a 2 amp balancer won't be able to fix that fast. Now the top balance is in progress. with 30 amps. I assume the pack is 50% full on average (9 cells are above 3.4). So that would take me 3 days from now.
 
You need to look at the number of cells you want to put in series. In your case that is 4.
In addition you can look at the capacity of the balancer. If you don't want to do the top balance, i would go for the most powerfull (2A). If that is too much, you can configure the app and set a lower amperage. With large cells 2A is perfectly fine.
Finally the max output the BMS can deliver (max amps). In my opinion, more is always better, so (200 Amps).
I just looked at the Hankzore store (on Aliexpress) and think this one would adress the above specs.
Personally good experiences with this shop on Ali. I ordered 3 BMS's from them and recentely 2 Neey active balancers. All delivered very fast.

View attachment 150250
I went over to jkbms.com and I can't find any reference to this BMS. I searched the page and come up with nothing. Is that an old BMS?

I also noticed that none of their BMSs work with less than 8 cells. Am I misreading it?
 
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