Looking for advice, doing a top balance after 2 years in storage (clean bus bars?)

[seprintz]

Hi guys, first time posting on this forum.

I built 2 x 24v LiFePO4 batteries a few years ago, but sold that camper and disassembled. I'm now onto a new project and I'd like to start with a good top balance and had a few questions. I am pleased to report, that all cells appear to have maintained charge, they're all between 3.288v and 3.350.

1.) Should I clean my CALB bus bars? They appear to have some tarnish/patina on them, what would y'all recommend?
2.) Should I clean the terminals on the cells? They all appear to be in good order.

I never did a top balance back in the day, I just didn't have the patience or time with my build, and I'd like to do it this time around to get more storage capacity at the top when charging.

I've seen a lot of discussion on this forum about this being needed/not needed, and dangers of going to too high of a voltage for too long. So I thought I'd post my plan below and see if y'all had any reasons not to.

Step 1: Wire my batteries in series, I have 16 cells, so that'd give me 2 x 24v batteries. Wired with my two overkill 8s BMS in place.
Step 2: Wire them all to my inverter/charger, set on the Classic LFP charge profile, which will take them up to 27.2 volts, or 3.4v per cell.
Step 3: Disassemble batteries.
Step 4: Assemble all cells in parallel.
Step 5: Set bench top power supply to 3.5 volts and top balance until current goes to near 0.
Step 6: Disassembly and re-assemble into my battery packs.

My logic for 3.5v for the top balance would be to get them all topped off and avoid the potential damage of going all the way to 3.65v.

TIA for any input.

 

[sunshine_eggo]

1) Yes.
2) Yes. Aluminum terminals look great, but they have a thin oxide layer that inhibits conductivity.

Scotchbrite, light sandpaper, brass/steel bristle brush.

Given the voltage range, top balance is essentially mandatory. Your cells are likely at wildly different SoCs. Assuming you're not ready to start charging today, given the voltage range, while it won't do much, I recommend you parallel the lowest volts with the highest volts (or all of them together) to try and at least normalize where they are now. Again, if you're ready to start today, don't bother. If you don't see yourself starting in a few days, use those days of down time to try and equalize their voltages.

Steps 1-6: Follow the top balancing guide in resources. You're mostly there, but you should read through it cover to cover.

3.5V is likely sufficient if you plan to charge to those levels. If you plan to charge to 3.55V, balance to that level.

FWIW, zero chance of damage at 3.65V. Damage doesn't being to occur until after 4.20V where the electrolyte breaks down. LFP used to be charged to 4.20V, but the gains between 3.65 and 4.20V are meager, and limiting to 3.65V dramatically increases cycle life.

 

[Browneye]

And no-lox or oxguard on terminal mating surfaces. The busbars are tinned?
I didn't like the balance lead ring terminals under the battery terminal nuts, so I drilled and tapped the bars.
A proper torque wrench is recommended.

I would charge to 3.65Vpc, fine tune again after reassembly. Then set boost to 3.5 or 3.55Vpc on the whole battery. They should be fine.

 

[seprintz]

I am ready to start today.

My inverter/charger is not programmable, and the LiFePO4 setting will take a 24v battery to 28v, which is 3.5v per cell, so I figure top balance to there and call it a day. I will be adding solar, but don't have that hardware yet.

Browneye is recommending top balancing to 3.65v, it didn't seem to me like there's much of an advantage to going that high if I'm not charging to that voltage. I am open minded, and willing to change my mind with good advice.

Thanks for your replies.

 

[sunshine_eggo]

I am ready to start today.

My inverter/charger is not programmable, and the LiFePO4 setting will take a 24v battery to 28v, which is 3.5v per cell, so I figure top balance to there and call it a day. I will be adding solar, but don't have that hardware yet.

I would balance to 3.55V, and it shouldn't take much longer to go from 3.5 to 3.55.

Browneye is recommending top balancing to 3.65v, it didn't seem to me like there's much of an advantage to going that high if I'm not charging to that voltage. I am open minded, and willing to change my mind with good advice.

Going above your planned charge voltage is sensible and top balancing to a higher voltage helps insure retained balance at lower voltage.

Don't be surprised if you cut off early on charging. There is likely a 40% SoC difference between the high and low voltage cells. Recommend you build one battery with the lowest voltage cells and the other with the highest voltage cells.

 

[seprintz]

OK, I hooked them all up to the inverter/charger last night and got the packs charged up that way as much as possible.

QQ before I top balance, can I do all 16 cells at once? Or since I'm building 2 24v batteries, should I top balance a set of 8, to be assembled into a battery, and then the 2nd set of 8 separately?

Thanks again!

 

[sunshine_eggo]

OK, I hooked them all up to the inverter/charger last night and got the packs charged up that way as much as possible.

QQ before I top balance, can I do all 16 cells at once?

Yes.

Have you fabricated new leads for your power supply?

 

[seprintz]

Yes.

Have you fabricated new leads for your power supply?

Yes, 10 gauge with ring terminals.

Also, I followed your advice yesterday and when I built my battery packs I put my lowest voltage cells in one pack and higher voltage cells in the other.

All the cells (except 1, that's at a higher voltage than the rest) are currently wired in parallel and connected to the bench top power supply. We'll see how many days it takes to top balance. Most cells are right around 3.34 volts, so this could take a while :-( But that's OK, I've allowed lots of time to do this correctly. I didn't allow for this the 1st time I built the batteries 2 years ago, and performance suffered because of it.

 

[sunshine_eggo]

Yes, 10 gauge with ring terminals.

Also, I followed your advice yesterday and when I built my battery packs I put my lowest voltage cells in one pack and higher voltage cells in the other.

All the cells (except 1, that's at a higher voltage than the rest) are currently wired in parallel and connected to the bench top power supply. We'll see how many days it takes to top balance. Most cells are right around 3.34 volts, so this could take a while :-( But that's OK, I've allowed lots of time to do this correctly. I didn't allow for this the 1st time I built the batteries 2 years ago, and performance suffered because of it.

I wouldn't hesitate to get that high voltage cell in the mix. If it's at elevated charge, it will only help feed the others.

 

[seprintz]

I wouldn't hesitate to get that high voltage cell in the mix. If it's at elevated charge, it will only help feed the others.

lol, that's what I would have done, but I was following step 1 in the tutorial that everybody referrences in this forum:

 

[sunshine_eggo]

ATTABOY!!! Quoting the guide back to me!!!!

IMHO, the 0.15V is overly-conservative. Andy with off-grid garage did a 0% SoC 280Ah cell in parallel with a 100% SoC cell (0.7V difference). Yes, initial current was very high, but it rapidly stabilized to below 0.5C. Given what you've done so far, I'm pretty sure you can assume that your low cells are much higher than 0% AND any current flow from the high cell would be shared among the others.

Lastly, whatever arrangement you made for the two 8S batteries, you should likely stick with moving forward when you go to build the 2X 8S batteries again. The differences in their voltages is proportional to their self-discharge rate, so matching them on that basis should mean that cells will drift similarly, and you should have less trouble maintaining the top balance. If you've lost track of which is which, oh well.

 

[seprintz]

I had the foresight to label the cells and track which cells were where via spreadsheet.

As an update, I'm excited, voltage at the terminals is up to around 3.41 from 3.35. Amps have dropped from 10.3 to 10.1 and voltage on the power supply has jumped from 3.5 to 3.6. From what I understand, these are all good signs that things are progressing. Since the cells are configured this way and charging I'll leave them like this till top balanced to 3.5, then add the higher voltage cell back into the mix and continue up to 3.6. I can't do 3.55 or 3.65 because my cheap power supply doesn't have a hundreds digit, lol.

Night y'all!

 

[Browneye]

I balanced all the way to zero current at 3,65. Still had to do some individual cell bleed off with the assembled battery to get them more closely matched. Not perfect cells, but they work great as a power source for the RV.
Ten charge cycles in about a year. Mostly they are idle, coach is parked.

 

[TorC]

If you haven't done so, I'd check that your cheap power supply is accurate to what it claims with a decent multimeter.

That said, as long as you plan to put the cells in service soon, it won't hurt much even if it's +.2-.3V.

You'll probably be nearly top-balanced by the time you read this. Have fun getting your system going.

 

[seprintz]

Thanks for all of the advice.

I'm reading 3.61 volts at the cell terminals, which has me happy.

Currently only at 1.35 amps, so I'm thinking we're near the end here.

@Browneye , any tips for how to do this? And how to determine if it needs to be done?

 

[Browneye]

Sounds like you're good.
If your powersupply/charger is voltage adjustable, then you want to set your charge voltage with a DMM and then connect to your parallel cells.
Once current gets down under an amp you can figure you're good to go, or just let them charge to zero - all the same voltage. Do the other set, reassemble your batteries. Your power supply voltage display probably isn't as accurate as a decent DMM.

When everything is back together and you're powering loads and fully recharge, you can bleed any runners off with a resistive load on the cell terminals - without any disassembly. I made up a 12v headlight bulb with clips so I could help bms balancing. It only takes a few minutes and you do it at high knee while charging, while watching the bms cell voltage. You can bring all cells into perfect balance this way.

These were the old charger leads that got replaced with 10 gauge and ring terminals as recommended in the wiki.

 

[Browneye]

Posted this on another thread on this topic...I think the doc came from the site wiki here.

 

[seprintz]

Thanks for all the help, I'll see if I need to do that once I get the packs re-assembled.

Last night at 8:00pm I was charging at 0.96 amps, I'm now down to 0.36 amps.

Question: Is there any harm in leaving it all hooked up until it goes down to closer to 0? Even if it takes days?

I ask because I'm not in a hurry to hook these up, I have a lot of other work to do on my travel trailer before I can install these, so I have the time/patience, but I don't want to do it if there's a risk of causing any kind of damage/degradation to the cells.

Follow up question: Once I'm done with the top balance, if it's going to be a month or so before I install the batteries, should I leave them all disassembled? Or go ahead and assemble them into 24v packs?

 

[sunshine_eggo]

Thanks for all the help, I'll see if I need to do that once I get the packs re-assembled.

Last night at 8:00pm I was charging at 0.96 amps, I'm now down to 0.36 amps.

Question: Is there any harm in leaving it all hooked up until it goes down to closer to 0? Even if it takes days?

I ask because I'm not in a hurry to hook these up, I have a lot of other work to do on my travel trailer before I can install these, so I have the time/patience, but I don't want to do it if there's a risk of causing any kind of damage/degradation to the cells.

Follow up question: Once I'm done with the top balance, if it's going to be a month or so before I install the batteries, should I leave them all disassembled? Or go ahead and assemble them into 24v packs?

Better to cut it off now and resume 24 hours prior to your build.

Also worth disconnecting them from their parallel connections and letting the individual cell voltages drift.

 

[Capt Bill]

Posted this on another thread on this topic...I think the doc came from the site wiki here.

Nice PDF file Read. Liked the Details. I have three 24v 280Ah Banks that need maintenance at the terminals. I have limited my LV2424 charge voltages to between 27.7v and 28v to stop the peaker cells from getting over 200mv diff. (got too many other projects on my plate for another top balance) ... From noticing signs of corrosion on one battery bank, I definitely will be cleaning all my terminal connects soon, plus adding some Ox Guard and copper washers (confirmed a post for a cheap copper washer kit from my local Harbor Freight) ... while upgrading to try out three Heltec Active Balancers (which I will dial in to only work at about 3.4v and above by wiring their on/off terminal links to a voltage sensing relay). ... I will then see how far I can up my charge voltage without peaker cells triggering my BMS now and then (with 28.2 v being OK for me). I also have one cell that drops voltage early on the low end. ... Looking for Dependable Low Maintenance Cycling of my Home Solar System Battery Banks, without BMS triggering. Thanks for the thread refresh info. here, ... plus this PDF.