Still struggling to fully understand top balancing with bench top power supply

snoobler

Solar Honey Badger
Moderator
Charging to just 3.4V is going to get you well under 97.5%. You should likely go to 3.5 or 3.55. With the cells parallel balanced, you should see no wonkiness.

The BMS exists to save the day. .01V between the two means you're more influenced by meter accuracy. You should use your equipment to establish your SoC operating range.

BMS LVD not above 2.9V.
 

Solarfun4jim

Solar seduced :-)
Charging to just 3.4V is going to get you well under 97.5%. You should likely go to 3.5 or 3.55. With the cells parallel balanced, you should see no wonkiness.

The BMS exists to save the day. .01V between the two means you're more influenced by meter accuracy. You should use your equipment to establish your SoC operating range.

BMS LVD not above 2.9V.
Snoobler, that was what i expected after the first parallel charge, but unfortunately i saw wonkiness right after 3.42V
This was taken from averaging the total voltage shown on the chargery and dividing by 8
I watched the Wh rise all the way up, calculating as a percentage SOC against an expected 3.25v x8cells x 280Ah(7280Wh). So for each 10% threshold(initially) on SOC i calculated the average voltage. At 3.3638v = 70%, 3.3688v = 80%, 3.3725v = 90% of expected. At 3.38v exact, that equated to 95% SOC.
3.3862 = 96%, 3.3925 = 97%, 3.4175=98% and it terminated me off(one cell hit 3.65v) at 7190wh or 98.76% of expected. I have similar figures at the bottom end as well.
When the wonkiness started, this was the resulting figures....3.4238v = 7142wh, 3.4262v = 7144wh, 3.4275v = 7146wh, 3.4300v = 7148wh, and 3.4362= 7150wh (but these were averages during very expanding mV differences.)
Hope my ramblings make sense.

Edit....now here's the rub, i worked out the WH as per the chargery spec sheet suggestions, but this rated the cells at 3.25 as matter of fact. Looking at the eve spec sheet, these cells are 3.2v nominal not 3.25v
So, if we apply this info the total Wh to be expected would be 3.2v x 8cells x 280Ah = 7168Wh
In other words, at 3.42v average, i would have achieved 7190wh/7168wh = 100.30% (y)
 
Last edited:

FilterGuy

What, me worry?
Late to the conversation.

1) When top balancing with a small power supply (less than 20A) voltage drop across the leads does not matter by the time you are done. Why: As the cells charge and the current drops, the voltage drop across the leads will also drop. At the end of the top balance, the current will be zero or near zero and the voltage drop accros the leads will also be zero or near zero (Ohms LAW: V=IR). The cells that see the higher voltage will charge faster and get to the target voltage faster, but then they will just stop taking current..... As the current drops, the cells that see the lower voltage initially will eventually get to the higher voltage and charge till they stop taking current.

Note: This only applies to top balancing. In normal use, the voltage drop is bad and must be minimized.

2. I document my process for top balancing here:
https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/
 
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