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Topping off or balancing a set of LiFeP04 batteries?

sidpost

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I have read a few things that say I would need to top balance a set of LiFeP04 batteries at 3.65V. What sort of constant current and voltage 'chargers' does everyone use? Anything specific recommended or that should be avoided?

As I understand it, I would take my 16 batteries and connect them all in parallel. Then I would top balance by charging everything to 3.65V. 16x302Ah is a huge power sink so, what sort of amperage do I need to top balance in a reasonable timeframe? I'm thinking days, not weeks here!

Thanks!
Sid
 
Most are using a 10 amp adjustable power supply. Yes with 16 large cells this will take an extended period. Can spend more for 40+ amps but seems like a bit much for most that will do this process just once.

Assuming your cells are quality and semi matched I will advocate for direct assembly in series, add the BMS and charge in series limiting the voltage to about 3.40 vpc and see what goes. Add an active balancer if needed.
 
How much of an imbalance could something like the JBD BMS handle? How close does the voltage need to be before I really need to buy a power supply to balance them?
 
Mine arrived with voltages withing a couple of millivolts. Still top-balanced them cuz that's what Overkill says to do - you can't tell their state of charge - SOC - very well by just voltage. The center of the charge-discharge graph is really flat.

Put a DMM on each cell, if they're all the same or within a millivolt or two, assemble with your bms and charge them. You can always bleed power off one or two cells if they get too far ahead of the others.

If you have a cheapo 10A DC CC CV adjustable power supply you can parallel charge them to full, all together. It just takes a long time - lots of amp hours. They're about $60-$70. I'm trying to figure out what else I can do with the one I bought to do mine. LOL
 
How much of an imbalance could something like the JBD BMS handle? How close does the voltage need to be before I really need to buy a power supply to balance them?
IIRC JBD will balance at 200 milliamps and that is very small and slow. 2 a/h would take 10 hours. And generally just balances above 3.40vpc only while charging.
 
Ya, the jbd is very low amp. I would put a 12V halogen headlight bulb on my high cell to bleed it off, along with the bms balance, until all the cells balanced. It took a few hours. But they all balance at high-knee now.
 
Mine arrived with voltages withing a couple of millivolts. Still top-balanced them cuz that's what Overkill says to do - you can't tell their state of charge - SOC - very well by just voltage. The center of the charge-discharge graph is really flat.

Put a DMM on each cell, if they're all the same or within a millivolt or two, assemble with your bms and charge them. You can always bleed power off one or two cells if they get too far ahead of the others.

If you have a cheapo 10A DC CC CV adjustable power supply you can parallel charge them to full, all together. It just takes a long time - lots of amp hours. They're about $60-$70. I'm trying to figure out what else I can do with the one I bought to do mine. LOL
Thanks! Yes, a very flat voltage curve could be deceptive with Multimeter. A 5A digital power supply on Aliexpress is ~$80 delivered so, probably a good investment for $2600 worth of LiFeP04 cells in my case. My friend might be getting some as well so, even a stronger case to invest in a good power supply.
 
I just connected mine in series and then connected up the Overkill BMS to the 16 cells. I purchased a light bulb with some alligator clips to bleed off the high cell if one cell starts climbing towards 3.65 Volts. To me it just was a lot of work and expense to hook them up and balance them. Here is a section from the Overkill manual:

Cheapest way: Connect a load to the high cell in your pack to quickly bleed off the excess energy. I tried this method using a random car light bulb with some alligator clips on the leads. (see Figure C.3 below) You need to watch the cell voltages closely because it’s easy to go too far.

It does work very well but when the cells get close to full charge then you need to be there and do the manual balance. I believe the cells drift over time unless you have an active balancer and will need to be balanced.

I have 32 cells rated at 280 ah. I set the two BMS to 300 ah and when the cells start to approach 300ah then I start to watch them. I have found that so far they have not approached the 3.65 v.
 
I found a 45' #14 wire will bleed about 30 amps. 25' of #16 will bleed about 20 amps. String the wire out as it will get warm. Did this on my first battery. Second and third battery received a Heltec balancer and put immediately in service.
 
I found a 45' #14 wire will bleed about 30 amps. 25' of #16 will bleed about 20 amps. String the wire out as it will get warm. Did this on my first battery. Second and third battery received a Heltec balancer and put immediately in service.
One wire connect to both pos and neg terminals? A direct short?
 
I found a 45' #14 wire will bleed about 30 amps. 25' of #16 will bleed about 20 amps. String the wire out as it will get warm. Did this on my first battery. Second and third battery received a Heltec balancer and put immediately in service.
That sounds like an awesome idea. Myself I just couldn't see spending a week balancing batteries when they could be placed in use. On a good day I generate 30kw + which totally charges the one set of batteries. The surplus would just be wasted without the second pack. I found that the batteries I purchased from Docan Tech were balanced good enough just to put in service and then if I need to then I will drain the voltage from the highest battery to equalize them. So far I have not had to balance the pack from Docan it was already balanced. They have some excellent batteries in my opinion. It saves a lot of time when you can rely on a good vendor and company. So many things that you buy from China end up in the scrap pile.
 
How much of an imbalance could something like the JBD BMS handle? How close does the voltage need to be before I really need to buy a power supply to balance them?
I just couldn't justify spending the money on a power supply that was only going to be used once. Then all of the time involved is another thing. I wanted to put the cells in service ASAP. I did not want to wait a week since I invested $2500 to save money.
 
Mine arrived with voltages withing a couple of millivolts. Still top-balanced them cuz that's what Overkill says to do - you can't tell their state of charge - SOC - very well by just voltage. The center of the charge-discharge graph is really flat.

Put a DMM on each cell, if they're all the same or within a millivolt or two, assemble with your bms and charge them. You can always bleed power off one or two cells if they get too far ahead of the others.

If you have a cheapo 10A DC CC CV adjustable power supply you can parallel charge them to full, all together. It just takes a long time - lots of amp hours. They're about $60-$70. I'm trying to figure out what else I can do with the one I bought to do mine. LOL
That is the problem with purchasing a power supply for $60. If it does not have another use then it just becomes another one of those things that you have lying around. I have so many of those now that I just have got to the point where I have to decided before I purchase something whether or not it is something that I am going to use. Even then somethings you think are going to be useful end put being paper weights.
 
That is the problem with purchasing a power supply for $60. If it does not have another use then it just becomes another one of those things that you have lying around. I have so many of those now that I just have got to the point where I have to decided before I purchase something whether or not it is something that I am going to use. Even then somethings you think are going to be useful end put being paper weights.
You can use it to electroplate you buss bars. Silver plating is the best.
 
That is the problem with purchasing a power supply for $60. If it does not have another use then it just becomes another one of those things that you have lying around. I have so many of those now that I just have got to the point where I have to decided before I purchase something whether or not it is something that I am going to use. Even then somethings you think are going to be useful end put being paper weights.
Tell me about it - 30 year collection of stuff in my garage!
And we’re planning to move in a year when I retire. Oh dear. :rolleyes::ROFLMAO:
 
Personally, I would think attaching a resistor in series to the wire would be a lot better than running a really long run of thin copper wire to provide this resistance to unacceptably high current levels.
 
20A for balancing does seem like a lot. I think the bms is like half an amp or something. I don't know how to calc my lightbulb thing, but my guess is about and amp or two. It takes quite a time to bleed off 50mah, for example. The bulb glows dimly, 3V I guess.
 
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