WARNING / REMINDER for users of BMS for discharge

Probably you know the deadly problem as I do but as a reminder:

NEVER GO DOWNHILL WITH FULL BATTERY - NO BRAKES!!!

I experienced a cut-off of the BMS due to overcharge protection twice. It’s no fun realizing that the brakes don’t work. The first time I could bail and barely run without crashing but this time it was different.

I fully charged the board at work to have enough juice to visit a friend and reach home afterwards. Well my work is on the top of a steep hill and I didn’t think about my BMS.

I was riding on the street because there were no cars and it got quite steep. Unfortunately I was already over 30km/h (19mph) when I realized my brakes don’t work and it got steeper. Man, that feeling when you know you are fu****.

Then I had to do something at least trying to footbrake which is tricky with the flexy Trampa deck and it didn’t work. I can’t remember the fall but I was sliding down the street and then watching my board rolling downhill. On the left and right side were parking cars. After some contacts with curbs the board crashed into a tire of a van.

Luckily the board and the van are OK. And most important I’m OK thanks to hard shell elbow pads and wrist guard gloves, just a bit road rash that’s it. Without protectors I definitely would have serious injuries after sliding with 35-40km/h (22-25mph) on concrete and cobblestones.



I live in a flat area so usually I don’t think about over-charge cut off, that’s why it is so dangerous when going downhill with a full battery once in a while. Hail Skatan, I pray it won’t happen again.

Stay safe!

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Close call man that definitely could have been worse

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Makes me wonder - if we had a BMS with extreme balancers

So like 84mA of balance per P-group, like normal, but also a 10A balance for the entire pack if it goes overvoltage…so like

S = series count
Bg = group balance
Bp = pack balance

Bp = S * Bg * 1.01

So, for an example 12S pack,
each P group balances above 4.20V at 84mA
the entire battery balances above 50.9V at 10A

Then the BMS wouldn’t need to cut power/charge on overvoltage until it hit like 52.0V… and you’d need to brake down a hill a few kilometers (miles) long at high speed to charge it that much, and if you just let off the brake slightly, it’d come back online in braking mode again… as long as you were pushing no more than 10A battery you could continue to do so.

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Oh man, Hope you are ok.

That is the reason why I have my battery pack at a overcharge protection of 4.25v for each cell and a balance to 4.2v.
So I don’t have such a problem on a steep hill.

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So true, lucky me.

@b264 I had to read your post several times until I understood what you mean, my knowledge about electronics is limited :joy:.

I can’t remember the name but was reading about an external device which transfers the power of regenerative braking into heat if the battery is full. I guess this is what you mean with balance for the entire pack but within the BMS.

Exactly like my BMS :laughing:

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Gotta dump all that heat somewhere and I’d say it’s probably best done seperately from the battery pack.

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Ha, this device I meant, thanks for posting.

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That looks designed to protect the ESC, not to protect braking ability.

It is very similar.

My fear with that would be if you weren’t bypassing your BMS, then you would still have brakes even if you went overvoltage, but as soon as you let off the brake, your ESCs might reboot. It depends if their filter caps kept them running long enough for your BMS to reset/recover.

Also I find it odd they chose 57V as a set point and don’t understand why it says “10S and below” for battery.

Overall, not enough details, don’t buy

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Glad you are ok. However not all discharge BMSs do this.

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I think it covers and fixes the full battery breaking situation and also read that they are working on a 12s model, I believe I got this from @hyperion1’s posts in the other place but subject to correction on it

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As you say this I realize my Bestech BMS on the MTB doesn’t cut off with full battery when braking on the same steep hill whereas the other BMS on the Carver does. But no idea if the Bestech overcharge the battery or dissipate the heat somehow.

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Maybe the specs from your other bms are wrong? Where you running a metr module? Maybe we can see the voltage when you was going down hill

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shit dude, thank god you’re okay. usually if my boards full I’ll pelt around the block once to get my voltage down a bit and then i go down the hill just to avoid this. it’s scary i feel ya

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The 223 bestech BMS uses different connection points for charge and discharge.
This coupled with the facts that

  1. I was able to increase my regen current way above the rated charge current.
  2. It has never cut out on me although I live on a hill and many times start fully charged
  3. @esk8manbabes had wired it the other way around and was discharging via the regen path.

Leads me to believe that the regen current does not go through the charge circuitry but directly to the battery.
This means that it will slightly overcharge your battery when starting fully charged on a hill but I will take that any day of the week vs no brakes.

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I’ve never had a bestech 223.

:man_shrugging:t4:

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No but you had a discharge bms that uses different charge and discharge connection points right?

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Yes and it would cut off while braking on a full battery. Like it should.

Ok, so yours probably has different circuitry than the bestech.
As far as what it should do, it is debatable. There are advantages and disadvantages in both approaches.

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Well the specs listed it would cut off above 4.25v which is what happens.

No just HM-10 plus the app (Ackmaniac) was off.

Even then the BMS measures cell voltages and should protect the battery above 4.25V/cell, just a guess.

All my BMS have charge and discharge on the same port and the Bestech didn’t cut off so I don’t think it’s about same or different port.

For comparison here the specs of the Bestech on my MTB which doesn’t cut off (overcharge protection 4.28V/cell)

I’m just thinking loud, maybe the proper Bestech ignores charging voltage and read cell voltages only. Maybe the cheaper BMS cuts off because of too high charging voltage and not because of cell voltages above 4.25V.

I can’t remember the value but once I watched the voltmeter during braking and was surprised how much the voltage increased.

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