Pack/BMS Problem?

[dnmun]

you can charge the two parallel cells with a single cell charger while it is still wired in series with the other cells and attached to the BMS through the sense wires. be sure to get the polarity correct.!!

 

[MidniteTweeker]

Cells 15/16 have been on the charger for ~5 hours and ~12 a-h delivered. Now showing 3.32 volts, 57% charge, compared to 3.22 and 10% charge when they began.

Here's the current status of the pack:

[pre]Cell # Balance meter volts % of charge per balance meter 9/30/2013
1, 2 3.43 91
3, 4 3.4 90
5, 6 3.33 66 20.2 bank total, 86%
7, 8 3.37 86
9, 10 3.4 90
11, 12 3.3 35
13, 14 3.34 68
15, 16 3.32 57
17, 18 3.43 91 20.16 bank total, 84%
19, 20 3.32 65
21, 22 3.42 91
23, 24 3.34 68[/pre]

Cells 11/12 are the low ones now, standing at 35% charge. They're on the RC charger now. But their voltage at 3.3 isn't much different from the rest, .13 volts between high and low. How close should all of them be for the BMS to be able to properly do its thing?

Thanks,

MT

 

[dnmun]

when you measure the cells off the charger they show a resting voltage different from what the voltage is during charging. no telling what the voltage will be when you measure them while charging so until that happens it will be impossible to estimate how long it will take the BMS to balance it.

by measuring on the charger you will know which is the high cells and need to be drained with the power resistor. until then they will all read 3.3V.

 

[MidniteTweeker]

I guess I'm not clear on the function of the BMS. Or else I'm not clear on the function of my RC balance charger. Or else I'm not clear on the differences between LiPo and LiFe.

I thought that a balance charger would automatically bleed voltage from a high cell while still bringing a low cell up to the desired voltage. Between bleeding high cells and charging low cells, sometimes my LiPo chargers will time out trying to get the cells all balanced if the balance limits are set too narrow.

But the BMS works differently? It stops the entire charging process for all cells if one cell reaches a certain voltage? I thought that it would continue to charge the lower cells while bypassing the higher ones.

 

[dnmun]

the BMS charging mosfet is turned off when the HVC level is reached on any of the channels. if you do not drain the charge off that cell then the BMS will reamin turned of until the voltage on that cell reaches the reset voltage.

the shunt current is low so it takes a long time. but if those voltage measurements are made while the pack is charging then it is not close to being charged. i am under the impression you continue to post up voltages of the cells in the resting state, not while charging.

 

[MidniteTweeker]

Yeah, the recent readings were not taken while charging.

So I put the 6S2P banks on the RC charger one at a time with it set to charge, not balance. Plugged the balance meter into the JST plug and recorded the cell voltages.

[pre]Cell # Balance meter volts % of charge per balance meter 9/30/2013
1, 2 3.42 Bank 1 v
3, 4 3.4 v
5, 6 3.33 RC charger set for 6S charge only, not balance
7, 8 3.37 Charger terminated after only 2-3 minutes
9, 10 3.39
11, 12 3.35
13, 14 3.43 Bank 2 v
15, 16 3.41 v
17, 18 3.9 RC charger set for 6S charge only, not balance
19, 20 3.44 Charger terminated after only 2-3 minutes
21, 22 3.82
23, 24 3.43[/pre]

On bank 2, the balance meter captured 17/18 at 3.9 volts before the charger terminated. This is what you're looking for?

Oddly, on bank 1 the charger terminated also but I didn't get a reading on any cells near 3.9.

I wondered how charging with the bulk charger instead of the RC charger would compare:

[pre]Cell # Balance meter volts % of charge per balance meter 9/30/2013
1, 2 3.64 Connected to bulk charger blinking
3, 4 3.58 one long/3 short reds
5, 6 3.35
7, 8 3.45
9, 10 3.52
11, 12 3.35
13, 14 3.35
15, 16 3.33
17, 18 3.67
19, 20 3.35
21, 22 3.7
23, 24 3.35[/pre]

This shows nothing near 3.9 volts, but I have no idea what the charger is doing. BTW I compared the voltages on the balance meter with those from a DVM and they're essentially identical.

 

[dnmun]

yes, when the BMS detects the 3.9V on any cell, it will shut of the charging mosfet. to keep charging you have to drain the charge off that high cell while it is charging. i use a big cement power resistor, actually several, about 5 ohms, 5 watts, to drain the charge by attaching alligator clips on the ends of jumper wires, soldered to the resistors, onto the ends of the high cells, top and bottom and drain the charge off that way while it is charging and then the charger turns back on and continues charging until it hit HVC again, so you keep doing that until all the cells are high enuff in voltage that the high cells cannot get all the way up to the 3.9V level and then the pack voltage will be high enuff that the charger will go into the CV phase and the pack will balance then on the balancing current. that takes time but far less time than waiting for the charger to cycle on and off for the HVC cut offs.

 

[MidniteTweeker]

OK, I didn't realize that the shunt resistor was left in place during charge. Thought that it was used to drain down a cell after being charged.

So conceivably you could have a shunt resistor on 11 of the 12 cells in the pack while you're waiting for the 12th cell to catch up?

 

[dnmun]

yes, you have to be careful not to short stuff. i just ruined a BMS by shorting the pins of the sense wire plug with my voltmeter probe while measuring voltages. shorted the shunt transistor into junk.

i don't know what you have to work with. i use the small alligator clips and find some secure place to clip them so they cannot fall off. my big single cell charger has big clamps on each end like a auto battery charger so it is impossible to use and i have to use smaller alligator clip jumpers from it to the cells.

but you don't have to take the wiring apart to charge one cell, just be sure you connect only to the one cell and have the correct polarity.

we still don't know if your BMS is functional either.

 

[MidniteTweeker]

In the interest of trying to keep things neat and hopefully less likely to short, I'll use resistors instead of bulbs. Figure I can attach them to a piece of wood or something & alligator clip from there to the cell(s). Resistors & clips on order now. If only I hadn't gotten rid of all my dad's ham equipment. In the meantime I'm individually topping up cells that the charge meter says are low so hopefully they'll all be closer when the resistors arrive.

 

[dnmun]

you can buy the jumper wires at harbor freight with the clips already on them. i find the cement power resistors in old tvs and other junk. but you can buy them on ebay too.

you can use bulbs to drian the power but if you are discharging several cells in series you have to use a bulb that can handle the voltage or it will burn out. so don't use 3V bulbs to drain a lifepo4 cell that is sitting at 3.9V.