BMS problems

Batteries, Chargers, and Battery Management Systems.
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Callbrin   100 mW

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BMS problems

Post by Callbrin » Oct 08 2019 3:11am

ok so my bms has been acting up i just "installed" it on my battery and everything was fine and it balanced but the 2nd time charging it wouldn't power my ebike (voltage would drop a lot not sure on exact). and i tore the battery back open.

1st i reconnected the balance leads, no luck
2nd reconnected bms and it sort of worked but still wouldn't power my bike so i connected a watt meter and bam now it works so i will ride my bike till its close to 10% and charge again to see if its fine

any reason this happened? did i do something wrong? this is my 3rd BMS and if its bad I must have really bad luck at finding bms's

battery is 14s8p at 20ah samsung cells and bms is 14s rated at 45A.

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 08 2019 9:18am

It's hard to tell from the description. It seems like the BMS tripped for some reason and you had a hard time to reset it.
The BMS would normally trip if any cell measured too low (including a bad connection on a balance wire) or if the current was too high.

Once tripped, the output voltage will measure way low and not power the bike. Most require you to disconnect the load to reset, like unplugging the controller, wait a few seconds, then plug it in again.

It will be important to know exactly why it is tripping. Bad wiring connections or bad cells are most common causes.
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Callbrin   100 mW

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Re: BMS problems

Post by Callbrin » Oct 08 2019 11:41am

well now it just wont reset at all, i know its not the cells since its fully charged and the packs not even that old and the old bms was fine just didn't balance properly. so yeah I guess im unlucky and dont know what caused it. hopefully my bad bms streak ends soon. I think the wiring was fine as i followed the diagram

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 08 2019 2:07pm

One thing that can blow some BMS designs is connecting the wires in the wrong order.
I recommend:

First connect the main B- to the BMS
Then connect the balance wire plug and try to angle it so the B -end of the plug makes contact first
Last, connect the B+ wire.

Some designs are more tolerant of connection order than others, but I've seen quite a few fail from incorrect order.

If you have one that won't reset and all the voltages are correct, you can try briefly charging through the discharge port (assuming it is not a single port style). If this works, it means the BMS design sucks but you could wire a reset button that goes from B- to P- through a 100ohm resistor to manually reset. I have one that is like this and I'm still using it.
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Callbrin   100 mW

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Re: BMS problems

Post by Callbrin » Oct 09 2019 11:19pm

yeah that fixed the problem right up. bad design

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 10 2019 8:05am

The manual reset button is not so bad. It's better than having to unplug the controller. I bet your BMS is the same model as mine. You need a resistor in series with the push button to limit the current as it charges the controller caps. I think I used 47 ohms on mine.
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Callbrin   100 mW

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Re: BMS problems

Post by Callbrin » Oct 10 2019 2:58pm

im not to keen on wiring and many electrical things just the basics. if you could maybe provide a diagram or describe how I would wire that it would be of great help.

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 13 2019 11:55am

Sorry for the delay, busy around here.

You need to open the pack enough to get access to the BMS board. The board will have markings where the wires attach. You want the button setup to go between the B- and P- connections. These are big, heavy wires so the spots on the board will be pretty easy to solder onto. The resistor can be 47 to 100 ohms and is not critical. The push button can have long wires and be located remotely, but be sure the wires are routed so they can't get pinched and short out.

Here is a picture diagram:
BMS reset button.jpg
BMS reset button.jpg (102.01 KiB) Viewed 758 times
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Callbrin   100 mW

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Re: BMS problems

Post by Callbrin » Oct 15 2019 3:27am

thanks that helps. first, is it fine if its a switch? like on/off switch.

2nd, out of curiosity why is there a resistor? to keep the connection from shorting out through small power getting through of something?

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 15 2019 7:59am

A regular switch would work, but a push button is best. If you forgot to turn the switch off, the resistor could burn up.

You need the resistor to protect the switch when the controller is sitting at zero volts. Without the resistor, the switch would arc weld it's contacts as the pack tries to charge the controller capacitors. You want to limit the switch current to a level that's below the contact rating.

I suspect a fairly wide range of resistances would work. I'd pick 47ohm on the low end and around 1k on the high end.

Another option might be to just use a permanent resistor around 10k and no switch. If the BMS trips, you would likely need to unplug the controller to get it to reset like the original design was intended. If you're lucky, it would just automatically reset after the controller cuts off without messing with anything. I have not tried this.
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MJSfoto1956   100 kW

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Re: BMS problems

Post by MJSfoto1956 » Oct 15 2019 8:09am

fechter wrote:
Oct 13 2019 11:55am
...The resistor can be 47 to 100 ohms and is not critical ..
But the direction of the resistor is important -- correct?

M
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Callbrin   100 mW

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Re: BMS problems

Post by Callbrin » Oct 15 2019 11:28am

I almost forgot to ask but my bms has a B- and a C- port so would the wiring still be the same?

also what rating resistor should i get (watts) or does only the resistance amount matter?

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 15 2019 9:43pm

Don't use the C- port. Mine has that too.

Under normal conditions, the resistor can be pretty small like1/4W. If you have a shorted controller, the resistor will just smoke and be like a fuse. A larger wattage one won't hurt as long as it fits in the space.

It doesn't matter much which way the resistor goes but I'd stick it right to the B- so any shorts will be protected by the resistor. On mine, I have a pair of wires that goes from the BMS to a spot on the battery case where it was handy to reach the button, about 12" away. You could extend this any length but you don't want the wires to get damaged.
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Callbrin   100 mW

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Re: BMS problems

Post by Callbrin » Oct 16 2019 3:18am

mine actually only has C- and B-. no P- like other ones. So what would the connections be or should i just get a different one?

Heres the link. dont worry it is my battery is 14s and the bms is 14s as well just Chinese cant spell

https://www.aliexpress.com/item/3282375 ... 4c4dFDiWYl

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fechter   100 GW

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Re: BMS problems

Post by fechter » Oct 16 2019 7:54am

On one like that it would go from C- to B-. It's where the fat wires attach.
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