Top Balancing "How to"

Gazoo

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Some people recommend leaving the cells in parallel for at least 24 hours before seperating them to let them equalize. I know of one person who said the deltas were lower after letting the cells rest in parallel. Since the cells are way above the knee it's not going to take long. I didn't but would have had I known it could make a difference.

In this situation since the cells have been seperated, I would top balance that one cell again and see if the self discharge rate is slowed down, as has been suggested.
 

Draconius

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Some people recommend leaving the cells in parallel for at least 24 hours before seperating them to let them equalize. I know of one person who said the deltas were lower after letting the cells rest in parallel. Since the cells are way above the knee it's not going to take long. I didn't but would have had I known it could make a difference.

In this situation since the cells have been seperated, I would top balance that one cell again and see if the self discharge rate is slowed down, as has been suggested.
So charge Cell 1 to 3.65, disconnect and leave it be? Cells 2, 3, and 4 just leave them as is in the 3.5 range?
 

smoothJoey

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So charge Cell 1 to 3.65, disconnect and leave it be? Cells 2, 3, and 4 just leave them as is in the 3.5 range?
Charge each cell individually to 3.65 volts and 1 amp tail current.
Assemble into a battery and lets see how they do over as many cycles as you can give them before d-day.
 

Gazoo

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So charge Cell 1 to 3.65, disconnect and leave it be? Cells 2, 3, and 4 just leave them as is in the 3.5 range?
Yes. It won't take long so there is no need to top up the other cells. Also go to .100 milliamps tail current as you did initially to make sure that cell is fully saturated like the other ones were.
 

Just John

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Just hooked up cell 1 and it went from 10amp current to done in about 2 minutes and disconnected and the battery was at 3.64 after disconnecting the PS
Yes, there is only a couple of amp hours total up there, I mean REALLY a small amount.

Less than 2AH above 3.45v
 

Draconius

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Gonna see how it settles in an hour. Realized I don't have the right lugs for my BMS and can't use the regular 10awg connectors like I do with my fridge/solar. So now I get to wait til tomorrow for my lugs to arrive from Amazon :(
 

0rion

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Hi, I just finished top balancing my 4 202ah cells to 3.65v last night, took them off the charger but left them connected in parallel. Now I'm wondering if allowing them to rest together has thrown off the top balance. Is it better to disconnect them from parallel promptly after reaching 3.65v so they can each fall to their own specific resting voltage (at each's own pace) but remain in balance at the higher voltage?

I've read many tutorials that indicate that once they are balanced at 3.65v they can then be disconnected from parallel and put back in series for use, but none really indicate if they should be disconnected (promptly) or if it's a consideration.
 

Hedges

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Interesting thought. We don't think much charge gets transferred between paralleled cells of similar voltage, and they probably don't settle that differently.
I wouldn't worry. Separate, allow to settle further. If not widely divergent, connect is series and let balancer handle it.

What we sometimes see (or at least I hear) is one cell well below the rest after parallel balancing. Presumably poor contact. Separately topping it off fixed the issue.
 
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Just John

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Hi, I just finished top balancing my 4 202ah cells to 3.65v last night, took them off the charger but left them connected in parallel. Now I'm wondering if allowing them to rest together has thrown off the top balance. Is it better to disconnect them from parallel promptly after reaching 3.65v so they can each fall to their own specific resting voltage (at each's own pace) but remain in balance at the higher voltage?

I've read many tutorials that indicate that once they are balanced at 3.65v they can then be disconnected from parallel and put back in series for use, but none really indicate if they should be disconnected (promptly) or if it's a consideration.
I think disconnecting them from parallel and letting them settle gives a chance to find the very rare cell that has a substantial self discharge rate. But really, it doesn't matter either way. Or it shouldn't.
 

Hedges

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I know a forum member had better results after letting the cells sit in parallel for a couple days. I don't think it's important but might be worth mentioning.

Maybe that continued to balance by transferring charge. Or maybe, separately they diverged while up above knee of curve, but once down on the flat, voltages were the same. i.e. nothing accomplished by leaving paralleled. It was the 2 days which made the difference?

"When I immediately disassembled at 3.61v their voltages drifted apart as they settled. Keeping them in parallel to settle for 2 days they remained identical voltage on disassembly."

I think what'll matter is that charge voltage is low enough that tail current tapers off before a runner gets too far away.
And each charge cycle gets high enough for balancing to take place.
If one cell has excessive self-discharge, that will be a problem, to be replaced or dealt with somehow.
 

Draconius

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Thank you Gazoo, Just John, smoothjoey and HRTKD! Was finally able to hook up my BMS and got 13.89v for the pack. The cells were a wee bit out, but after 5 minutes the pack balanced to only 0.020v differential.
 

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Draconius

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to run a load and charge, I was reading the tutorial and it talks about shorting stuff... but guessing that isn't the case hereSince I'm majorly top balanced, can I simply attach my PS at 14.4v and connect to positive of battery and the negative of the battery (not the C- on the BMS) and call that charge and connect my load to positive of battery and C-?

(My load is my power supply to my home office running a laptop and monitor. :p)
 

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smoothJoey

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to run a load and charge, I was reading the tutorial and it talks about shorting stuff...
What tutorial?
Please quote your source so we know what you are talking about.
but guessing that isn't the case hereSince I'm majorly top balanced, can I simply attach my PS at 14.4v and connect to positive of battery and the negative of the battery (not the C- on the BMS) and call that charge and connect my load to positive of battery and C-?
If the battery is full as in just finished the top balance and just let the cells rest then they are ready for the capacity test.
Letting the cells rest before the test is fine unless its like > 30 days or so.
(My load is my power supply to my home office running a laptop and monitor. :p)
That is not a deterministic load for a load test.
Are you doing a capacity test or are you just putting the cells in service or some third option?
 

Draconius

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What tutorial?
Please quote your source so we know what you are talking about.

If the battery is full as in just finished the top balance and just let the cells rest then they are ready for the capacity test.
Letting the cells rest before the test is fine unless its like > 30 days or so.

That is not a deterministic load for a load test.
Are you doing a capacity test or are you just putting the cells in service or some third option?
Source is the "Top Balance Tutorial" on page 1 of this thread. Section 3 (https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/)

Using "home office" as I am just testing that things are "working right" and the app is reading accurate. My cells have now evened out with a 0.014v difference, and they show as "yellow" at 3.24-3.31. These were top balanced, and topped off prior to setting up in series 3 days ago. So for them to already be at that range (60-70% based on this page: https://diysolarforum.com/threads/lifepo4-voltage-chart.3156/) seems a little "off" for these cells having been JUST balanced...so not sure if batteries or app...

I'm going to try and maybe run these cells down with about 200w of lights running.
 

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Just John

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to run a load and charge, I was reading the tutorial and it talks about shorting stuff... but guessing that isn't the case hereSince I'm majorly top balanced, can I simply attach my PS at 14.4v and connect to positive of battery and the negative of the battery (not the C- on the BMS) and call that charge and connect my load to positive of battery and C-?

(My load is my power supply to my home office running a laptop and monitor. :p)
Bypassing your BMS will sooner or later mean you need a new battery. Don't bypass the BMS, let it do its job.
 

Just John

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Source is the "Top Balance Tutorial" on page 1 of this thread. Section 3 (https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/)

Using "home office" as I am just testing that things are "working right" and the app is reading accurate. My cells have now evened out with a 0.014v difference, and they show as "yellow" at 3.24-3.31. These were top balanced, and topped off prior to setting up in series 3 days ago. So for them to already be at that range (60-70% based on this page: https://diysolarforum.com/threads/lifepo4-voltage-chart.3156/) seems a little "off" for these cells having been JUST balanced...so not sure if batteries or app...

I'm going to try and maybe run these cells down with about 200w of lights running.
The BMS capacity reading will not be accurate until you have done a discharge/charge cycle.
 

Draconius

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Bypassing your BMS will sooner or later mean you need a new battery. Don't bypass the BMS, let it do its job.
When you say bypass bms do you mean charging directly to the negative terminal? IE... put the PS to the C- when charging green arrow and not red
 

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