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Top Balancing "How to"

Of course it does to you.
One potential problem for newbies is mixing up the polarity.
Probably won't hurt the cell much but the power supply may not like it.

Fuse much?

1615940569214.png

And yes, as a matter of fact, I do get polarity mixed up. Quite often.

I'm thinking of putting suitable fuse holders on safety banana plugs to use with my high-voltage differential probe. It's only meant for low current devices like CRT tubes, not line voltage.

1615940765842.png
 
Fuse much?

View attachment 41267

And yes, as a matter of fact, I do get polarity mixed up. Quite often.

I'm thinking of putting suitable fuse holders on safety banana plugs to use with my high-voltage differential probe. It's only meant for low current devices like CRT tubes, not line voltage.

View attachment 41268
I thought...
fuses protect wires.
and buck converters protect fuses. ;)
 
I thought...
fuses protect wires.
and buck converters protect fuses. ;)

Other things too.
(At least these are the scare stories I've been told as part of training at several recent jobs.)
But as we know, one forum member found his class-T fuse didn't protect a busbar. Fortunately had a cover.

1615941988288.png
 
Some observations on building my first LiFePO4 battery 4S 280AH pack with Overkill Solar BMS + Bluetooth

I received my battery cells that all had a voltage around 3.29V (closely matched at that voltage). I hooked everything up in series and attached the BMS. I attached a 10A Lithium charger (not variable which is why I went with a serial arrangement). Everything charged up until one of the cells reached 3.65V before the BMS shut down charging. The delta from the lowest to the highest cell was about 0.25mV so there was a need to top balance to get everything on an even basis. I started by making up a resistance load by hooking up an automotive headlight to some wires with alligator clips. It should be noted that the BMS balancing function was turned on and working but apparently, this works best to adjust cells that have already been reasonably top balanced as the small resistors built into the BMS cannot compensate for a large voltage delta.

Because my multimeter only reads to two digits, I used the BMS+Bluetooth and its Android app to monitor the voltage as I brought each cell down to match the lowest cell voltage using my load resistor. As I did this, the lower cells did drift downward a bit so ultimate target was about 3.375 volts. I closely monitored the voltage as I was making this adjustment and each of the 3 cells took 5-10 minutes to get to the target. Next, I reattached the 10A charger and let it again bring the pack up to the cutoff voltage. Again there were unbalanced cells, this time with a delta of about 0.15mV but since they were getting closer to being close to the top, I decided to attach a voltage supply made up of an old laptop charger and a CV/CC Buck Converter set at 3.65V/2A to individual cells to bring them each up to the voltage of the top cell. With the cells drifting towards their resting voltage, this ended up being right around 3.60V. After about a day with no charging, the cells drifted equally to a resting voltage of 3.57V with a delta of 0.05mV which I am happy with. I'm thinking the BMS should be able to keep things balanced from this point forward.

One other idea I had for achieving a good top balance without the benefit of a variable bench power supply is to monitor the cell voltage as it is approaching the end of the charge cycle and "assist" it in balancing cells by manually attaching load resistors to the high cell(s) to bring down their voltage to that of the remaining cells. This would have a benefit over the method I used in that my 10A/12V charger could still be used rather than my little 2A/3.65V charger. One thing I still want to do to make this method a bit easer is to make some "chair" contacts that would sit atop a set of battery posts rather than having to connect via alligator clips. This would allow the assist load resistor to be easily moved from one battery to another when fine tuning the balance.
 
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Good discussion on top balancing
 
Hello,

I get back to you again because I still have issues with my Lifepo4 100ah 4s battery pack. I tried balancing from the top with a laptop power supply coupled to a DC buck all connected in parallel. When my power came to 3.65v my cells were actually only 3.50v. So I stopped and connected in series with my BMS Daly 4S 80A Lifepo4. Then I connected my specific charger which delivers 14.65v. The charger only worked for 2 minutes and then turned green. Yet here are the readings that I made with my multimeter:

- Battery (without BMS) = 14v
- Battery + BMS = 13.74v
- Cell 1 = 3.44v
- Cell 2 = 3.55v
- Cell 3 = 3.44v
- Cell 4 = 3.55v

What is my mistake? I do not understand...

Thanks everyone,
 
When my power came to 3.65v my cells were actually only 3.50v.

1) make sure your cables are thick enough and use e.g. a ring terminal and not a alligator clamp.
2) you have to let your cells take in the power, not just cut it off when they reach a certain voltage. You should ideally monitor the current; 2 cells at 3.5V can very well be at different states of charge.
 
Thanks.

1- this is the method I used. big cable + rings.

but I gave up the parallel for the load in series and there I still have problems !!! I do not know what to do...
 
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