Help me diagnose issue -- BMS potentially?

J

jaywood

100 µW
Joined
Jun 13, 2016
Messages
8
Hey endless,

Long time lurker here. Finally built my first ebike. Mongoose dolomite, with cyclone 3000w kit (all stock)

BMS: allow 50A continuous, peak 80A.
Battery nominal voltage 59.2V, full 67.2V. Capacity 23.2ah, max peak discharge current 80A. Using Panasonic PF cells

Everything ran great for the first 30 minutes. Full throttle, up steep hills, no problem. Continuous full throttle for minutes. And then right before I got home, I got immediate voltage cutoff. Everything read 0V. My voltage meter display at twist throttle died, checked voltage at charger port and discharge port, all read 0V.
Then, I plugged battery discharge connector back into the motor controller, and everything started working again (thinking this woke up the BMS?)
So I rode again, and this time the bike lasted 5 minutes or so before voltage cutoff. So I disconnected and reconnected battery to motor controller connection, and everything fired back up. No problem.
So I went to ride again, and this time it only lasted 2 minutes.
Now, I can't even get 20ft before voltage cutoff.
I have adjusted the motor controller as well, down to 50% torque. So max current I'm pulling (according to my bluetooth meter) is 18A, and I am still getting voltage cutoff.

However, with no load the battery will spin the motor all day. Ran it constantly full throttle (on a bike stand in the air) and it has no problem. I applied some disc brake, and the second it saw 'load' it shut off.

Is this a sign of a bad BMS?

Here are pics:

Bike by Jeff States, on Flickr

Bike by Jeff States, on Flickr

Bike by Jeff States, on Flickr

Keep in mind, the wires coming from the BMS are 14gauge I believe, they are not 12gauge.

Testing:
Untitled by Jeff States, on Flickr



One other problem, I can't seem to get the battery to charge. I know the charger works, have verified it, is 60-75V and 4.52A.

I am leaning towards a bad BMS. What do you think? Appreciate any and all help!
 
Does anyone have any insight on why the battery won't charge? Have verified charger works, pulls 4.52A at 60-75V. Battery is currently at 52.5V. Have verified connections are correct on BMS.
 
I agree with iceman, "pull off the BMS's balance plug and measure every single cell string".
Is it a 16s8p battery ?
 
Most likely:

If it works with no load but doesnt' under load, you have cell group(s) that are discharged enough to be near LVC, so when load is applied they sag below that.

You'll need to check every cell group's voltages to see which ones are low. Since it only happens under load you may need to also measure under load, if they appear to be the same voltage otherwise.

The cells that are lower voltage are lower SOC and need to be charged up (balanced) to the rest of the pack.

It's also possible that some cells either are not the same as others (internal problems), or they have a high-resistance connection to them, and so they are not contributing the same to the pack as the rest are. Then the groups they are in will sag more than others, causing what you see.
 
Appreciate the responses guys.

I will pull apart the pack, disconnect BMS and test V on each string of cells.

Why am I getting no charge from the charger? When I plug in charger to battery pack, nothing happens. I have verified that the charger works so I know the charger isn't at fault. Could it be that the BMS is a reading a string of cells that is already fully charged? In turn, not activating the charger to start charging?
 
Could your first ride have flattened the battery?

A 23 ah battery can supply 23 amps for one hour, 43 amps for 30 mins, etc. If your BMS allows peaks of 80amps, it would have flattened your pack in about 20 minutes. (Or is my math wrong?)
Or, 3000 watts at 67v == 44 amps draw.

Did you measure how much you took out of the battery? I'd be looking for cells that have been drawn too low to charge, or some bad cells. (Bad cells like to show they are full until a load is put on them.)

Hope this helps?

Colin
 
izeman said:
no. it's the opposite. it sees one or more string TOO LOW, so it doesn't start.
charging a dead battery can cause big problems like fires etc.
so the BMS is your firewall :)
Click to expand...

Understand. Will check on V of strings later tonight.

ColinB said:
Could your first ride have flattened the battery?

A 23 ah battery can supply 23 amps for one hour, 43 amps for 30 mins, etc. If your BMS allows peaks of 80amps, it would have flattened your pack in about 20 minutes. (Or is my math wrong?)
Or, 3000 watts at 67v == 44 amps draw.

Did you measure how much you took out of the battery? I'd be looking for cells that have been drawn too low to charge, or some bad cells. (Bad cells like to show they are full until a load is put on them.)

Hope this helps?

Colin
Click to expand...

Colin,

Interesting thought. I don't think that is the case however. I was not supplying that many amps constantly. I was on/off throttle. I also limited current allowed to motor controller by 30% decrease. I was pulling 18-32A, over 30 minutes of not continuous full throttle.

Also, when I got battery it read 59V. I tried charging then, before I even rode the bike and it still wouldn't charge.
 
Most batteries ship partially charged, don't they? I know my lifepo4 batteries did, and the seller recommended I give them a nice charge and let all the cells balance before using. I'd call your battery seller for advice.
Colin
 
This battery was partially charged when I got it.. 59.2V

Just pulled off BMS plug, and measured V on each string. Here are results:

3.308, 3.310, 3.300, 3.308, 3.308, 3.307, 3.308, 3.307

Is the 3.300V too much of a sag? Is this causing me problems?
 
Not if that's under a load.

If it's without a load on there it doesn't show you any voltage sag--you have to check with a load equivalent to what it sees on your bike for that.


Still, that's only 8 of your cell groups--maybe half your battery. What about the rest of them?

There must be at least 6 more (for a 14s pack), probably 8 (for a 16s pack)

But at only 3.3v / cell, that's getting close to fully discharged.


BTW, most meters aren't accurate to that last digit anyway, so I'd just count the first two digits and the third at best. :)
 
as amberwolf said: there are some values missing. your pack is 16s. so you need to check another 8 strings.
the voltages posted are well within normal range. 10mV delta from highest to lowest is maybe a little issue, but nothing that would stop the bms from working.
sag is something different. sag describes the amount of voltage a battery goes down when under load. so at a stop your battery shows 60V, while riding up a hill it shows 57V -> your voltage sag is 3V. what you see with your measurement is just some voltage difference of unloaded cells.
so please post the missing 8 cell's values as well.
 
Have measured the rest of the values. All +/- the same as before. All measured 3.3V
 
Hi

I have a question that I noticed about your pack.

Are there only 1 strip connecting the P groups? What is the thickness of those?
I have used 5 strips 0.2x1.0mm dimensioned for 50A current.

The battery wires seem also very thin to support high currents as for 50-80A.
 
i guess it's two stripes if i saw it correctly. and yes 14 awg is VERY thin for the load expected. i go 10 awg for 50A battery current.
so now that we know that all batteries seem ok, i guess it's safe to bypass the bms and try again. a new bms is not really expensive, and i would order one. just in case.
directly attach the battery +/- wires to the charger and monitor the battery while charging. no need to fully charge it. i'd say stop at 4,0V per cell and then go for a ride. take a voltmeter with you to monitor the cells from time to time and stop using the battery when one cell reaches 3,0V.
 
Starting to think the battery and charger are not compatible

Charger pulls 67V continuos, and 4.52A
I hear charger operate and fan turns on inside charger when I plug into multimeter to test for A.
When I plug charger directly into battery + / - I don't get any fan turning on in charger, and no LED lights turning on.

BMS sense wires disconnected, BMS - still connected however.
 
jaywood said:
Starting to think the battery and charger are not compatible

Charger pulls 67V continuos, and 4.52A
I hear charger operate and fan turns on inside charger when I plug into multimeter to test for A.
When I plug charger directly into battery + / - I don't get any fan turning on in charger, and no LED lights turning on.

BMS sense wires disconnected, BMS - still connected however.
Click to expand...

what you write makes no sense and leaves too much room for interpretation. you need to be more precise.
when does the charger pull any power? if so it would pull from mains. and it pushes into battery. if you connect a multimeter to test current: where do you connect it to? you need some load. so we can suspect you connect the battery to the charger and the multimeter in between. IF this shows 4.51A current, then the charger is charging.
but then you write that the charger is NOT charging. no wonder with balance wires disconnected. the bms won't do a thing w/o the wires connected. as already said: if the bms senses voltages below a certain threshold (maybe around 2.7V for your bms - just guessing) it will turn OFF the CHARGING and DISCHARCHING FETs. and if you disconnect the balance wires, exactly this happens. it sees ZERO volts. so you need to connect those again. this was only for testing.
if you say "when i plug charger directly into battery" you say you're bypassing the BMS completely? so you connect the charger to battery PLUS and GND directly? what happens then?
it seems very unlikely that both your BMS and your charger are both broken. if your charger output 67V that's fine, as this is almost exactly 16 x 4.2V. and if this is the case then your battery WILL charge if you directly connect the charger to the battery.
please go a bit more into detail. thanks.
 
Izeman,

Appreciate the response. Sorry I wasn't very clear.

I have the BMS sense wires disconnected. The only thing still connected from BMS to battery, is the ground wire from the BMS. I didn't disconnect because it was soldered and didn't have a desoldering iron here at home with me.

I pull 4.52A from the charger when I plug my multimeter leads directly into the xt60 charger connector. This is when it's standalone, and not plugged into the battery.

I then took extended leads, plugged them into the xt60 charger connector, and hooked it up directly to the battery + and - .... I got nothing. Battery would not charge.
 
It is a "load" in that the shunt in the meter is a dead short across the charger leads, whihc means that whatever current protection the charger has is is kicking in to prevent it's destruction when it is completely dead shorted like this.

That means that the measurement you get doesn't really have anything to do with whether the charger "works" in normal operation.

You woul dneed to put the charger's + lead on the meter's + lead when set on A, then the meter's - lead would go to the battery's + lead. Then the battery's - lead would go to the charger's - lead. That is how you would measure current output from charger.

You can susbstitute a different battery for the load on the charger, but it still needs to be hooked up the same way. Sometimes a resistivie load (heater, toaster, etc) can be used, as longn as the charger is not looking for a voltage on it's load before it turns on (some do, to be sure the battery is "ok").
 
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