Budget BMS on Ebay ~10

Batteries, Chargers, and Battery Management Systems.
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Pipi123   1 µW

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Budget BMS on Ebay ~10

Post by Pipi123 » May 28 2019 8:26am

Hi,
Looking to build my first 36v 10s12p e bike battery from recycled laptop cells to drive a 250w brushed motor.
I need to buy a BMS and was wondering if the BMS on Ebay for about £10 are sufficient to balance the charge and presumably the discharge Voltage?
Would this do?

https://rover.ebay.com/rover/0/0/0?mpre ... 3816168848

Any idea what is the difference between the £10 and the £30 ones?

https://rover.ebay.com/rover/0/0/0?mpre ... 3566456135

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amberwolf   100 GW

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Re: Budget BMS on Ebay ~10

Post by amberwolf » May 28 2019 10:43pm

bms must be rated for less current than the battery itself so it cn protect the battery.

pack incl batt and bms must be rated for more current than the controller will ever draw, peak or continuous.

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Re: Budget BMS on Ebay ~10

Post by jonyjoe303 » May 28 2019 11:37pm

laptop 18650 low discharge batteries won't be good enough for an ebike, you need powertool high drain 18650. Of the 2 bms, it looks like the cheap one can handle more watts then the expensive one. You multiply volts x amps, 1260 watts versus 720 watts. They both can handle more amps then the laptop batteries are willing to put out.

If the batteries are severely out of balance neither bms will balance them, they both balance at about 50ma which is nothing. A good balance charger for example balances at about 1000ma (or 1 amp).

Laptop batteries will heat up too much and the voltage will sag if you try to use the bike at full power. But I have seen people on youtube use them on bikes. And your motor is only 250 watts, so it might work on level ground. Going up hills might present problems.

Pipi123   1 µW

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Re: Budget BMS on Ebay ~10

Post by Pipi123 » May 29 2019 4:18am

Do you have the model # of the good balance charger that can do 1 amp?

How do I work out how many amps my battery pack can put out? Would 2amps per cell be over optimistic?
12 in parallel could do 24 amps?

Would this BMS be safer?

https://rover.ebay.com/rover/0/0/0?mpre ... 2990812863

I'm planning on using a brushed 36v 250w My1016 motor with a 350w controller.

Can the motor draw more than 250w?
250w/42V= 5.9Amps?
Or is it
350w/42V=8.3Amps

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amberwolf   100 GW

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Re: Budget BMS on Ebay ~10

Post by amberwolf » May 29 2019 4:33pm

Cotnrolelr current limit determines current pulled from bateries. Limit printed on controller is probably it's continous limit, may peak higher.

some brushed contorllers don't actually limit the current, and just have a power rating on them for what they can handle before you blow them up, so you then have to be careful not to exceed that by manually monitoring and limitng your throttle usage. you'd ahve to check the controller specs to find out.

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Re: Budget BMS on Ebay ~10

Post by john61ct » Jun 25 2019 8:27am

amberwolf wrote:
May 28 2019 10:43pm
bms must be rated for less current than the battery itself so it cn protect the battery.

pack incl batt and bms must be rated for more current than the controller will ever draw, peak or continuous.
Don't many setups bypass the BMS on power cables, IOW not rely on them for overcurrent protection?

That non-BMS OCP can then act as a failsafe limiter on the controller, right?

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amberwolf   100 GW

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Re: Budget BMS on Ebay ~10

Post by amberwolf » Jun 25 2019 10:13pm

john61ct wrote:
Jun 25 2019 8:27am
Don't many setups bypass the BMS on power cables, IOW not rely on them for overcurrent protection?
No, a few people with really high current stuff, hundreds of amps, and/or hundreds of volts, do this, because either no BMS exists that does it, or costs too much for their budget.

Most normal low to medium power systems use the BMS to shut off it's own output or input if any of the conditions it's designed for occur.


Some people don't run any BMS at all (like me) because they're just that certain their cells are all well-matched (thus will stay balanced), and use some other system for HVC and LVC. That could be the Cycle Analyst or similar system watchdog, controller, charger, or completely manual control.

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