What is the factory spec shutoff voltage for GM 36V 12Ah Battery Management System?

[tex tiles]

Hi the BMS in my new GM 36 volt 12 amp hr battery is shutting down at 37.5 volts. Is this correct? I was under the impression that it would be shutting down much lower than that...   What is the factory spec for this?
Thanks, Pete

[Bikemad]

Hi Pete, unfortunately I don't know what the Low Voltage Cut-off is set at on the BMS in the GM batteries.

Are you able to monitor the voltage during use, or was the 37.5V measured on the disconnected battery after it had cut out and had time for the voltage to recover slightly?

You also need to appreciate that the battery voltage will drop quite significantly under load, and the higher the current draw, the greater the voltage drop will be.
My Turnigy watt meter has recorded up to 5V difference on my battery pack between the lowest recorded voltage during the ride and the resting voltage observed at the end of it. My 29.6V LiPo pack is regularly subjected to 45Amp loads, and I still get this considerable voltage drop even though the cells are rated for 100Amps continuous with short bursts of up to 150Amps!

The 48V GM LiFePO₄ packs have 16 cells, so it seems logical that the 36V pack will contain 12 cells.
Assuming your battery is a 12 cell LiFePO₄ pack the nominal voltage could be 39.6V (3.3V x 12) and if the cut-off is set at 3.0V per cell it would cut out at 36V or more, depending on the balance of the cells.
In which case, your measured 37.5V may well be perfectly normal.

I suggest you give your battery a good balance charge by leaving it on overnight or all day etc. to allow the cells to fully balance themselves out, as a single low cell (or weak cell) will cause the BMS to cut out at a higher overall battery voltage.
Eleven good cells @ 3.3V and one faulty cell @ 3.0V could cause the BMS to disconnect the battery power, even though the total battery voltage was still reading 39.3V

Alan
 

[tex tiles]

Thanks for the reply Alan, I'll do that with the overnight charge. I was measuring with a volt meter after it cut out and I turned it back on, not under load. I thought that it was supposed to cut out at like 20% charge.

[Bikemad]

If you look at the graph below you can see that when the cell reaches 3V it will have used around 90% of its capacity.
But you need to consider that this discharge curve is for a 5 Amp current draw, if the current draw is increased it would reach 3V at around 80% or perhaps even less:



If the battery was disconnected by the BMS when it was 80% discharged by a 20 Amp load, I would expect the voltage to increase by at least 0.1V per cell. Therefore 12 cells at 3.1V would give a total battery voltage of 37.2V, which is pretty close to your measured voltage.

If you used a wattmeter (or Cycle Analyst etc.) to measure the capacity consumed it would probably be around 8Ah (80% of the 10Ah pack) so the state of charge remaining in the pack would be approximately 20%.

Alan
 

[Mrkrsh]

Great article Alan on understanding how the battery discharges this hopefully teaches members that by lowering your discharging amps. You will increase your range. But knowing some riders they can not resist the temptation to go full throttle.

[tex tiles]

Interesting. It is not a 10 amp hr pack, but a 12 amp hr. Not having any training in electronics I was looking at it differently, I still don't quite understand but I can see that it is not just the volts that are at play.

But I do know that my big battery (not GM) on my bike at home (which is 20 amp hr) cuts off at a much lower voltage. This one I have with me is a GM and smaller for my folding bike which I travel with.

[Bikemad]

But I do know that my big battery (not GM) on my bike at home (which is 20 amp hr) cuts off at a much lower voltage.

Is your other battery LiFePO4 as well, and is the voltage lower when it is fully charged?

If the motor load (Amps being drawn) is the same for both batteries, the bigger battery will also be under less load in relation to its output ability and will therefore have less voltage sag (Drop in voltage) under load, so it should discharge more of its capacity before the Low Voltage Cut-off (LVC) is activated which would also lower the voltage further.

If you were to use two 10Ah packs, one at a time until each pack was fully discharged you would not go quite as far as using both packs simultaneously (connected in parallel). When both packs are supplying the required current, each pack
is only having to supply half of the required current instead of all of it. Reducing the load on the battery by 50% also reduces the voltage sag.

If you look at the following graph you can see quite clearly how much difference the load makes:



A 10Ah battery delivering 20Amps is discharging at 2C, but two 10Ah batteries in parallel delivering 20 Amps (10Amps each) are being discharged at a rate of 1C.

If we assume that the LVC is activated when the cells reach 3V the batteries discharging at 1C would output around 85% of their capacity, whereas the single battery discharging at 2C would drop to 3V/cell much sooner, only delivering around 75% of its total capacity.

If the LVC was set at 2.8V/cell, according to the above graph, the 10Ah pack would deliver over 90% of its rated capacity (10Ah) at 1C, but would only deliver around 50% of its capacity at 5C (50Amps load).

I hope this makes sense.

Alan
 

[tex tiles]

Thanks for the replies guys, very interesting and knowledgable. Alan, yes the other packs I have are lifepo4 as well. I appreciate the explanation and diagrams.

I would like to know what the actual spec is on this battery, is there any way to find out what the factory spec is on these batteries as far as shutoff goes? I would have thought it would be available somewhere...   

[Bikemad]

I'm guessing that the BMS boards used in GM batteries will be sourced from more than one supplier, in which case the specifications are likely to differ. Unfortunately, obtaining technical information from GM can be very difficult at times, but if you can identify the manufacturer of the BMS board it might be possible to get more detailed information direct from them.

Alan
 

[tex tiles]

I'll try through Gary who I bought it from again, but I haven't received a response from him yet.

Thanks again.