Is my BMS doing its job?

[ironmonkey]

I have a 48V lifepo4 battery (16 3.2V lifepo4 cells purchased from alibaba that I assembled myself). After releasing off the charger from 57.5V, I get:

negative terminal: 3.3V,3.4V..........3.7V,3.6V,3.7Vositive terminal
negative terminal: 3.3V,3.4V..........3.6V,3.5V,3.6Vositive terminal several minutes after
negative terminal: 3.3V,3.3V..........3.4V,3.4V,3.4Vositive terminal at about 30min after, pack voltage at 54.5V
havent made more measurements after, will do tonight. Due to the battery cover I cannot measure the cells in the ........'s without major work.

I thought BMSs do their balancing during charge, but it appears that the cells balance after charge? Or is it just the cells balancing themselves and the BMS is actually faulty?

Note that I modified the shunt on the BMS so that it accepts alot more than the 10A current rating. I did accidentally short the BMS while moving the metal cover (while it was live) and it touched the shunt (a zap ensued). No damage I can see other than the zap mark on the shunt.

 

[dogman dan]

A "typical bms" such as signalab will be at work any time it's powered up. Lvc should function, max amps cutoff as well.

Balancing is done by both the charger and the bms. At a certain max voltage the charger will stop charging. Meanwhile the bms is discharging any cells that got too high. The discharging takes awhile. Any cells that are still low voltage won't get higher unless the charger is left plugged in. If still on the charger, eventually the battery voltage drops to a point where the charger will turn back on, repeating the process till all the low cells are fully charged.

If taken off the charger too soon, the bms will lower the voltage of the high cells, but the low ones will remain low, resulting in a partail balance.

Be nice if there was a simple and cheap bms tester we could buy, to go with our motor tester.

 

[ironmonkey]

alright thanks mate, then it seems my bms is working.

I've modified the shunt on the BMS, since the old shunt was only good for 10A. Would this have an adverse effect on the discharge FETs?

 

[dnmun]

the falloff in voltage after you remove it from the charger is normal. everybody calls that 'surface charge' and the cells will usually drop to a voltage just under the full charge, but not because it is being balanced, just each cell goes a little flat when the charging potential is removed.

if the cells were above the 3.65V level when it was off the charger then they would continue balancing, but then after that would drop down to the 'idle' level.

 

[Russell]

Depending on what voltage your BMS shunts are set for you may want to consider raising your charging voltage. For example if the BMS shunts are set to activate at 3.60V then ideally your charger should be set for at least 3.65-3.70V/cell or between 58.4 and 59.2V for a 16 cell LiFePO4 pack. Then what will happen is the cell voltages will all be in excess of 3.60V when the charging cyle is completed. When the charger is removed the shunts will bleed each cell down to 3.60V and the pack will be top-balanced. Again this is an example and your charging voltage should be based on the shunts for your BMS.

-R

 

[dogman dan]

Yeah, charging to 59v might help bring up the lowest cells, if the charger can be adjusted up a volt or two.

Good cells shouldn't drop to much more than 3.5v from losing surface charge, and new ones should hold surface charge better, at least for awhile. No way the bms should be causing them to drop to 3.3v. Hopefully you are getting them charged to at least 3.65v when you remove the charger. If not, leave it on the charger 24-7 for up to a week and see if things improve, or bring up the low cells with a single cell charger, or manualy discharge the highest cells to get it closer to balanced as the thing nears fully charged.

Oh, it just occured to me, If this pack is brand spanking new at this time, you'd expect some wierd readings and low capacities for the first 2-3 cycles. Verly short discharges, like a ride round the block, followed by long intervals on the charger is the routine for breaking in cells. By cycle 5-6 they should be charging more fully and balancing better and faster.

 

[ironmonkey]

Yeah, charging to 59v might help bring up the lowest cells, if the charger can be adjusted up a volt or two.

Good cells shouldn't drop to much more than 3.5v from losing surface charge, and new ones should hold surface charge better, at least for awhile. No way the bms should be causing them to drop to 3.3v. Hopefully you are getting them charged to at least 3.65v when you remove the charger.

I think I've probably left out one important detail, that is these batteries are not your ordinary lifepo4 battery, they are 'supercapacitor' batteries. The main feature of 'super capacitor' batteries is that you can charge them at high currents if you had a powerful enough charger - 30 minutes. Im not sure on what other differences supercapacitor batteries have. http://cyly.en.alibaba.com/product/495383046-212935359/48V_20Ah_Li_ion_Battery_Pack.html

Yes these are brand enw batteries. The cells are only rated up to 3.6V (their usable range is 2.7-3.6V) and were at 3.0v when I received them. All the cells I can measure are now 3.3V, so I'm guessing it is indeed the BMS discharging the high cells.

I'm not sure about leaving on the charger, I though the charger stops charging (green light) after the pack voltage reaches 57.5V? It'll only keep charging intermittently if I leave it connected with the controller, which I though could damage the charger from the numerous start stopping?

 

[dnmun]

nope, that is what the BMS does. the start -stopping is the BMS turning on and off the charging mosfet.

when the cells are outa balance, the highest cell will climb to 3.9V during charge and the BMS then shuts off the charging to allow the shunt resistor to drain off that excess charge on that channel.

when the voltage drops down to 3.8V then the charging mosfet turns back on and the cycle is repeated until the battery is charged.