Pocketbike Forum banner

Battery issue?

1K views 30 replies 4 participants last post by  TheRadDad 
#1 ·
So out of nowhere my 48v 20ah lithium ion battery pack started tripping the bms board at anything over half throttle. I swapped the motor for a different motor, still does it. I swapped speed controllers for another speed controller... still does it. Tried both new parts together, still does it! Did my battery pack **** the bed on me or something? Really confused and frustrated. Halp!
 
#2 ·
Every time a electronic BMS trips it gets a little bit weaker...

I always recommend putting a fuse or breaker that is rated for less than what the BMS trips at.

For instance if you had a 50 amp BMS I would put in a 45 amp breaker.

With that said let's get your pack up and running...

Try to locate where the BMS is so you don't have to cut off all of the heat shrink.

Use a plastic razor blade ( you can find them at the hardware stores in the paint section they are yellow plastic)

Carefully cut around the BMS to gain access.

Take pictures and notes exactly how the cables are plugging into the BMS if you plug it in backwards it will instantly fry the new one that you're going to get off of eBay or Amazon don't worry they are fairly inexpensive around $15 to $25 make sure you replace it with the exact one that was in the battery pack...

Use gorilla tape or flex seal tape to seal the pack back up
 
#4 ·
You have to match the BMS to your batteries cells discharge rate you absolutely cannot just throw a bigger higher amperage BMS on it!!!!

If you try to put a 60 amp BMS on a 30 amp pack and something happens to the speed control or motor it 100% definitely will catch on fire and you will be responsible for paying the fire department to come put it out with that special foam they use for lithium ion batteries...
 
#6 ·
I figured I'd try and describe what's going on a little better to see what you guys think wether it's the board or if my pack is crapping out. It's almost acting like it's not charging despite the battery meter reading full. You know that squelch noise an overvolted system makes on a full charge? Mine doesn't do that anymore and I found that I can do brief 1 second full throttle launches if I'm already moving and attempt even a slight hill without increasing throttle it will trip the bms... kinda like if the battery was dead and entering safety mode although the meter reads half life left.
 
#8 ·
Unfortunately I have to plug the pack back into the charger to get it to reset as I have no external reset button, and my battery gauge is push button style that lights up a series of LED lights to indicate charge level so sadly I don't know what my voltage read out is at the moment. I'm gonna see if my neighbor will help me out with his ohm meter thing and see if we can run some tests today.
 
#12 ·
Ok so it appears my battery has acquired the phantom voltage. I plugged it in for 4 hours, charger said it was full but my battery indicator says it's only 75% charged. Ran a voltage test and without load it reads 51v, under load it dipped down to 44v. Is this actually a battery problem or is there a chance my charger is being dumb?
 
#13 ·
In chinese batteries i do see a lot of voltage sag. It should come back up though even if you keep the load on. So for ex, battery is sitting at 50v, apply hard throttle, it drops to 45v, keep hard throttle on and battery should climb back up to 47 or 48v. Doesnt explain why you have to plug your charger in after it shuts off to get it working again though.
 
#14 ·
My guess is under load it's dipping below voltage cut off for some reason, which kinda makes sense if it's not properly charging from either a bad charger or some battery related issue
 
#19 ·
That sucks. If you decide to buy another battery, I would then find out what happened to that pack. And then repair. Try the BMS first, if that isnt it, gently pull apart the pack if possible, test each individual cell and then you can probably replace the few dead ones and get one of those Builder packs and rebuild your pack.
 
#24 ·
I'm starting to wonder, could it actually be a charger issue? According to the battery indicator the battery goes to 100% with the battery charger hooked up but when I unplug it the indicator reads 75%. I can swing a new charger for 25 bucks right now... but a $350 battery replacement is gonna take a month or so to factor into expenditures and I'd like to avoid shelling out money where I don't have to
 
#25 ·
The BMS is responsible for managing and charging the individual cells the plug in the wall box just gives the BMS power.

When the BMS starts having problems it leaves the pack un balanced or it does something really really stupid like give the cells a bunch of voltage then thinks the batteries are full and cuts off charging and never fills up the individual cells with the milliamp hours they need.

Or worse it starts over charging some cells leaving other ones completely empty and in this case it's going to catch fire eventually if you keep trying to use it while it has a problem.

The first thing that I would do if it was my pack would be to carve out the BMS figure out exactly which one it is and get a replacement.

The second thing that I would do would be to get out the RC car charger and start charging each cell individually then try it again with the new BMS.

If that didn't work then I would break the pack completely down and charge each cell individually with one of those fancy e-cigarette 18650 MOD chargers ( these style of chargers are surprisingly good at figuring out if the battery cell is good or bad)

Then I would replace any of the bad cells and rebuild the pack with one of those screw together kits.


This is why I always suggest just buying individual cells and the kit and the BMS and putting the battery together yourself then if anything goes wrong you can easily service it ( with plenty of how to guides all over the internet )
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top