GEM / Chevy Volt Lithium - NO BMS UPDATE after 2 years

I just wanted to say that after 2 years of charging and discharging my 24s Chevy volt batteries I finally got around to checking the individual cell voltages after using NO BMS.

Cells are perfectly balanced to the 10th…

That is all…

But it do conservatively charge to only 4 volts per cell, and do not bring them to empty…


How are you controlling your charge level? I use the BMS only for charger control. I built an Arduino circuit to control the charge but haven’t gotten around to wiring it in yet. My BMS actually bleeds one of the cells down over time so its actually doing more harm than good except that I need it to shut the charger off at 4V/cell (96V total).

I use the Chargery 1500 charger and set it to charge to 4 volts.

These batts stay balanced. I was thinking of getting an active balancer for each 12 cell module on ebay. I’ve been doing some research on them and watching some youtube videos. They seem great, and balance at any charge level. But after testing voltages I don’t think it’s necessary. I bought one, to patch into the chevy volt harness and I’ll try plug that into each module once in a while and it will balance if needed. Just a science experiment…

BMS can do more harm then good and if it goes ‘wonky’ it can kill a battery. Jehu Garcia doesnt use them on his e-vehicles… I didnt know what I would find after 2 years and they are perfect.

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Trying this for science…

I agree 100%! My BMS has done more damage than good. I plug it in just long enough to control the max charge and then unplug it. It bleeds the one cell down enough while plugged in that I have to use my RC car charger to bring it back up to balanced every 5 or so charges. I need to wire in my charge controller I built but haven’t had time.

The Chargery BMS 24 can control your Delta Q or other charger. David makes and sells a plug in that connects to the Delta Q’s (white charge control wire)

Example. The BMS controls when the Charger shuts off, so if a random cell were to go crazy your factory charger would NOT see it and you could possibly have a Fire or Overcharge situation either 2 are a serious situation. I’m not trying to say if you don’t use the charge controller wire (the white on delta Q) your at risk, what I will say is I sleep with zero worry of Lithium Fire or overcharge because I use the BMS to monitor my pack. With out a BMS keeping a eye on your pack your counting on your pack to NEVER have a random failure. Some people are cool with that and some are not. Do yourself a favor if you don’t use a BMS with the charge control feature, don’t ever google or watch you tube videos of Lithium Fires.

Basically two schools of thought.

Some rely on the bms to protect the cells and not fail. Others feel the bms is another link in the chain that could possibly fail and be the cause of cell failure. (Ie above where the bms is bleeding one cell).

My chargery charger stops charging at whatever cell level you input.

Common debate. BMS or no BMS.

To BMS or not to BMS; that tis the question. The debate has raged since the inception of BMSs and we won’t solve it here. I will share my insights and experiences as anecdotes. Chevy spent millions of dollars developing their batteries and their BMS for the Volt (and others) knowing very well that they had to nail 2 criteria; battery life, and safety (fire avoidance). The BMS solution they use is highly robust, expensive and is redundant for safety. Moving to our application in the GEM. We are using highly refined and stable batteries in a very low demand application ( we can’t drain or charge at near the rate they are designed to handle). The BMSs we have available to us are cheap Chinese with the potential for a single point of failure to cause damage or worse. In my personal experience, this is what is happening. My batteries charge balanced and when my BMS is disconnected, it stays perfectly balanced. When my BT BMS is left connected, 2 of my cells leak down over several days. It is most likely leakage in the MOSFET as a single point of failure on each of the cells. In my opinion, the risk of failure in the BMS we have access to is significantly greater than the risk of a cell just failing in the Volt batteries. I don’t have statistical data to support it, but I am convinced that the BMS increases the odds of a catastrophic failure. Now the plus side of the BMS; it allows me to easily check the voltage of each cell at a glance. In my current application, I use it to control my max charge and to check my cell balance in the process. I must unplug it when charge is complete, and I must balance my cells every 5th or so charges using an external charger on the affected cell(s) because there is bleed down on one or more cells while the BMS is attached. A side note; I compared the BMS cell readings against a highly accurate lab grade voltmeter and they are not as accurate as one would hope. In conclusion, once I finalize my charge controller build, I will cease using my BMS altogether. I have the batteries to do my other GEM and when I do, I will not be using a BMS and will sleep better at night because of it. YMMV

what BMS do you use that discharges 2 cells ?

I’m using the BT BMS24 like the one in this picture

Check all the balance pins for leakage with nothing connected.

I ohmed them once with a handheld and all junctions showed open. I haven’t tried it with a better meter. I’m at the point where even if I can swap the MOSFETs and fix it, I don’t trust it. It used to work perfectly and then it failed. Had I not been monitoring it and watching the voltages, it would have destroyed my battery pack. I’m of the mindset that even a new unit in perfect working order is far more likely to cause a catastrophic failure than the batteries themselves causing it.

As you say, they are great cell monitors. You could clip all the transistor leads. (I think)

End leakage issues. I’m using the first one I bought several years ago. Never an issue.

If you want simple and reliable use shunt balance boards. I use them on life.

They are dormant, drawing near zero current until
high voltage turns one on.

No other monitoring needed, as led comes on when cell is full. Stays on until cell drains to a safe value.

Super simple to connect. No setup needed.