Daly BMS discharge protection failure

[MTH07]

I built a battery for my BBSHD powered bike in a 14s3p configuration using Samsung 25s 18650 cells and a Daly BMS (not smart BMS, just basic balancing unit).

After a short ride a few weeks ago depleting the battery to about 52 volts, I got home and forgot to unplug the battery from the bike. Apparently there is a significant phantom draw, and it drained the battery all the way to zero volts. The over-discharge protection did not appear to do its job at all. I initially thought it kicked in and opened the circuit, but it measures zero volts across the cells, and is confirmed dead.

I am going to build a replacement, and definitely will not be using a Daly BMS (likely either JK or JBD). But I am a little surprised about such a basic function on the Daly unit failing like that. I thought I would post on here and see if possible it was something I did incorrectly, or if other have had this experience with Daly BMS units. The battery worked great for a dozen or so rides/recharges, coming up to the right voltage and discharging consistently at the anticipated rate.

 

[chuyskywalker]

Zero volts measured how, exactly?

Even when a battery gets drained well past a safe place, it's almost never to a measurable, actual zero. Usually you get a pack of 1.3v cells, something like that.

 

[fechter]

Possibly the BMS drained the cells, but usually only one or two groups gets drained with a bad BMS.

 

[E-HP]

I built a battery for my BBSHD powered bike in a 14s3p configuration using Samsung 25s 18650 cells and a Daly BMS (not smart BMS, just basic balancing unit).

After a short ride a few weeks ago depleting the battery to about 52 volts, I got home and forgot to unplug the battery from the bike. Apparently there is a significant phantom draw, and it drained the battery all the way to zero volts. The over-discharge protection did not appear to do its job at all. I initially thought it kicked in and opened the circuit, but it measures zero volts across the cells, and is confirmed dead.

I am going to build a replacement, and definitely will not be using a Daly BMS (likely either JK or JBD). But I am a little surprised about such a basic function on the Daly unit failing like that. I thought I would post on here and see if possible it was something I did incorrectly, or if other have had this experience with Daly BMS units. The battery worked great for a dozen or so rides/recharges, coming up to the right voltage and discharging consistently at the anticipated rate.

So is your normal procedure for powering down your bike to unplug the battery? I wasn't aware that the BBSHD drew that much when switched off, but if that's the case, then perhaps after rebuilding the pack, adding a disconnect switch would make things simpler.

 

[MTH07]

Thanks all for the input.

Zero volts measured how, exactly?

Even when a battery gets drained well past a safe place, it's almost never to a measurable, actual zero. Usually you get a pack of 1.3v cells, something like that.

I discovered it when I was going to go for a ride, took the battery off the bike, and connected it to my charger. The charger just showed a green light - i.e. not charging. I measured the voltage across the charging plug on the battery, then across the power plug on the battery (it is a BMS that has those seperated). Both measured effectivly zero (a couple of millivolts), which did not seem right to me. I assumed that the BBSHD was draining the battery, and that the BMS shut down when it hit the discharge limit. In order to confirm that, I measured the voltage at the two ends of the pack itself - between the soldered connection for the main cables to the cells themselves, thus taking the BMS out of the equation. Same result. I tried it with a second meter (one a fluke, and the other one an off brand but it has been reliable for years). Same thing. So I stripped the shrink wrap off and pulled away enough tape to check periodically along the pack. I was getting about ~0.2 mV per parallel set, adding up to to about 1.5 mV for the entire 14s pack. I then started pulling the pack apart and checking individual cells. Same result.

I am a bit perplexed, I agree with comments about a bad BMS draining a couple of cells, or causing an isolated issue. I also did not realize that the BBSHD would have that level of draw powered off. It could be an issue with the motor too, but that doesn't explain why the BMS didn't protect the battery like it is supposed to and allowed the cells to be completely depleted.

Those cells will get recycled, and I have new ones on the way as well as a JBD smart BMS to build a new pack. I am fine with chalking this up to learning experience, but would like to know if there is something other than "don't count on Daly BMS to protect your cells" that I could learn.

In terms of my shutdown, normally I power the BBSHD down at the display then unplug the battery and set it aside to put on the charger when I have a chance. I have two of these batteries with the same cells and BMS units, which gives me about 30 miles of range on my Santa Cruz VP-Free. I play it pretty safe and swap them when they get down around 47-48 volts. This one sat plugged in for maybe two weeks, at most. I got back from a test and tune ride around the neighborhood at 52 volts, so it still had juice left and I planned to ride it a bit around town before I charged it but got busy. I have left them plugged in overnight when I was going to ride the next day and had no drawdown, etc. I have not had a chance to charge up my other battery and connect it to see if I can replicate the draw when the motor is powered down.

That is a long explination, sorry, just trying to provide context. Thanks again for the thoughts/suggestions!

 

[molybdenum]

I was going to say to be sure to open up the battery and measure between the parallel groups as the BMS could very well shut down the pack. Seems like you did this, so I'm certainly perplexed... maybe a headlight or something that drew down the pack. Very unfortunate, but good to know the BBSHD could potentially draw a pack to zero. please update if additional info comes available.

 

[MTH07]

I was going to say to be sure to open up the battery and measure between the parallel groups as the BMS could very well shut down the pack. Seems like you did this, so I'm certainly perplexed... maybe a headlight or something that drew down the pack. Very unfortunate, but good to know the BBSHD could potentially draw a pack to zero. please update if additional info comes available.

Will do. I don't have a headlight on it, the only thing connected is the motor/controller/display. I am going to get my other battery charged up and connect it without changing anything, then measure to see if there is any draw. If there is an issue with the motor, I would like to find it before it messes up any more batteries (or leaves me stranded with a 15 mile uphill ride back to the truck...).

It seems to me that that has to be it, I don't know where else that energy could have gone. It still wouldn't solve the failure of the BMS to do its job, but would at least explain the condition that put it in the position of needing to shut down in the first place.

 

[E-HP]

Maybe you also forgot to turn it off. That could kill the battery in a few weeks.

 

[fechter]

The BBSHD will turn itself off if you forget. I leave mine connected to the battery all the time and have never had any noticeable drain even after months of non-use.

 

[MTH07]

The BBSHD will turn itself off if you forget. I leave mine connected to the battery all the time and have never had any noticeable drain even after months of non-use.

I will check to make sure that setting didn't get changed, because you can turn that off, but it has always turned itself off automatically before. Good to know that you have left a battery connected with no ill effect.

I will keep investigating and let you all know what (if anything) I figure out. Thanks again for the input, super helpful!

 

[chuyskywalker]

I was getting about ~0.2 mV per parallel set, adding up to to about 1.5 mV for the entire 14s pack. I then started pulling the pack apart and checking individual cells. Same result.

Very thorough check -- sounds like it was a bad bms in a super critical way. Like, PCB/component type failure that caused it to run the resistors on the cell groups forever instead of shutting off. (Essentially, allowing the balancing component of the board to instead be a drain to every parallel group.)

Unlikely the motor/controller. And, as you're aware, even if those DID draw small amounts of power overtime, the BMS should have stepped in a cut 'em off anyway.

Bummer about the pack. As for "Never again Daly!" I will say this -- I've had just about every major brand of BMS fail on me at some point or another. Daly's have typically held up pretty well for me, but there are certainly plenty of reports of them being...sub-par, shall we say.

 

[docw009]

Must have been a fake Daly, Popular brands in CHina get copied. I bought a Daly unit on aliexpress that didn't balance, Probably a fake. I now get them from sriko, which is a importer, but they're based in Illinois,

I've seen batteries with a cell group that discharged too low during winter storage, but it was always only one cell group, not every group. Further examination indicated a bad cell in the group, not unexpected since these batteries were made with cheap generic chinese cells,

 

[MTH07]

Very thorough check -- sounds like it was a bad bms in a super critical way. Like, PCB/component type failure that caused it to run the resistors on the cell groups forever instead of shutting off. (Essentially, allowing the balancing component of the board to instead be a drain to every parallel group.)

Unlikely the motor/controller. And, as you're aware, even if those DID draw small amounts of power overtime, the BMS should have stepped in a cut 'em off anyway.

Bummer about the pack. As for "Never again Daly!" I will say this -- I've had just about every major brand of BMS fail on me at some point or another. Daly's have typically held up pretty well for me, but there are certainly plenty of reports of them being...sub-par, shall we say.

Thanks for the comment on your experience with the various brands of BMS units. That is good to know. I had read some of the negative comments on the Daly chips, but in general the experience seemed to be positive.

I bought them through well-rated Ebay sellers, so I would think it was legit. But, it can be hard to tell for sure. I will take a closer look at it to see if I can see any signs of a counterfeit unit. I will probably open it up when I have some time to see if it looks legit inside, and or if I can see some damage internally that could explain the critical failure.

 

[fechter]

Once you remove the BMS try measuring resistance between adjacent cell wires. If the shunt transistors are shorted you will see the resistance of the shunt (typically around 100 ohms).
It’s also possible that something in the control circuit is turning them on.
I have not seen that happen before but designs vary quite a bit.

You may also have had a bad batch of cells. If the cells are good you should still be able to charge them but use a very low current until they reach around 2 volts per cell. After that you can charge like normal.
Cells that have gone below 2v may be unsafe so charge in a fire safe location.