Help with BMS

Joined
Dec 11, 2018
Messages
12
Hey everybody my name is Colin this is my first post. I'm pretty new new to E-Bike building but i recently built an e-bike with a 500 watt 36 volt hub motor. I got a used 36 volt lithium battery pack from an electric scooter. My initial problem was the Bms would cut off power at 300 watts (the scooter had a 250 watt motor) so i replaced the Bms using this tutorial http://wiki.myelectricscooter.org/fr/xi ... _generique
and this bms from amazon https://www.amazon.com/gp/product/B073V ... UTF8&psc=1 .
for about three months everything worked perfectly and then all of a sudden this morning the battery was reading 15.6 Volt. im very confused that bms shouldn't allow it to discharge below 32 volts. also when the battery is plugged in it reads 42 volts. The only thing i did differently than the tutorial is instead of soldering a socket for the charger i soldered the charger directly to the bms. Could that be the cause of this problem or is it more likely to be a short or loose connection?
 
Hi Colin

I'd be happy to help, but with the information you gave it's very hard to give a solid diagnose.
The links you posted are corrupted and can't be used. Please correct that, so we can see what you bought.

a) Where did you measure those 15V?
b) Where did you measure those 42V with the battery plugged in (to what? The charger? The bike?)

I would open the battery and BMS and measure each cell pack directly. Best to be done at the big white connector (about 10pins wide) going into the BMS.
 
I tore the whole pack down this morning and found a loose connection thank you for the replies though the problem seems to be fixed. I did also want to ask about soldering the charger to the bms. Is this a bad design? My concern would be the battery would backfeed the charger, the circuit is broken when the charger isn't plugged in but i know most ac transformers still generate a magnetic field when one side of the circuit is broken (the reason cellphone chargers draw amps when no cellphone is plugged in) I don't know how ac to dc transformers work though. I haven't noticed any voltage drop when the battery isn't in use but i guess it could be a very slow discharge. Does anyone have any experience with this?
 
I'd say it's not the prefered way to do it, and most probably depends on the charger.
My big chargers have a relais at the output which is only activated when the charger is connected to a wall outlet and it's in charging mode. This would never discharge any battery.
On the other hand: If the discharge rate really is there, and it's so low that you don't notice it, or can't measure it: Why care? It will most probably not be an issue.
 
As long as the charger has an internal relay/etc that disconnects it when it is not powered by AC input, then it cannot drain the battery.

Similarly, if it has an output diode (sometimes used for polarity protection), no battery drain.


Many chargers have neither of those, and so they can drain the battery slowly, depending on the output stage design. If it has an LED on the output that lights up when the battery is connected (even if not plugged into the wall), then it will drain the battery via that LED, if nothing else. Very slow, but constant, could kill a pack in anything from days (if it's already mostly empty after a ride and just left sitting) to weeks to months, depending on pack capacity and state of charge and amount of drain.


If the BMS has only one port for both charge and discharge, then it is not likely to be a problem, because it will shut off it's output port, disconnecting it from both charger and bike, once it drops down to it's LVC. When the charger is plugged into the wall, it's output will still flow into the battery,

If it has a separate charge port, then there is no protection against draining the pack thru it into the charger.
 
Hi there.

I am having a really hard time finding a 150A+ BMS with 14s lipo input.

I really want to find one that I can connect to bluetooth and or PC to configure the BMS excatly to my tastes.

I don't want to drop the cells to low or too hight so they live longer.

if anyone has found anything suitable for this I would love to hear your advice.

N.
 
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