How do I bypass the Battery Management System in GM's 48V 10Ah LiFEpo4?

[veloguy]

New here...My bms shuts my one year old, 48 volt, GM lifepo battery down after using just 3.5 amps out of a rated 10 amps. There appears to be plenty of juice left in the battery. After an hour or less recharge, it is fully charged again, so I don't believe it is a charger issue. Gary said to unhook the BMS and that instructions for doing this are in the forum. I haven't been able to find that post or any other instructions...so help finding it would be appreciated. Or...is this a bad idea? I checked each individual cell for voltage and found one to be a bit lower than the others. all but that one had the exact same reading. Also, is there a trick to removing the red wire harness? I couldn't get it off the pins by pulling. I didn't want to force it. Also, if I run without a bms, what should I set the low voltage cutoff reading on my Cycle Analyst?
Thanks

[Bikemad]

Hi andto the forum Veloguy.

It sounds as if you have a faulty cell, causing the voltage to drop much quicker than the others under load, resulting in the Low Voltage Cut-off being activated in order to prevent further damage to the cell.

Ideally the faulty cell should be replaced with a new one to enable the BMS to work as intended.

If you really want to bypass the BMS you should take a look at this post.

If you decide to bypass the BMS, it might be better to just bypass the discharge side of the BMS (connect the power output lead direct to the battery lead so that the BMS will still work as a cell balancer and should also prevent overcharging of the remaining good cells.

It might also help to put a 30 Amp diode across the faulty cell to allow an alternative path for the current from the other good cells in case the faulty cell gives up completely.

Alan
 

[veloguy]

Thanks Alan!
I think I will go with getting a new cell and see if that is the problem. Is there a trick to breaking out a cell from the pack? Best tools to use?

[Bikemad]

Check out Gary's LiFePO₄ battery thread for more details.

Alan
 

[GM Canada]

Hi Veloguy,

I don't think I would have said "remove the bms to fix it". This was the result of some email and telephone conversation first. I think what I suggested was since it was past the warranty period and you were concerned about he cost of cells or a bms you could unhook the BMS and still get more life from the battery without it costing you anything.

This obviously, is not the best was to get past your issue, but it is the least expensive.

Gary