Bypass BMS with LifePo4 - good idea or not ?
I have an e-bike with 500w geared motor powered by 24v 20ah Lifepo4, I'm new to this and only recently got the kit together. I have a problem with the battery cutting out under small load, works fine when on a stand but as soon as I put any load onto it, it cuts out and I have to disconnect and reconnect to get power but then the same things happen again.
It seems it could be the BMS that's at fault, If I bypassed the BMS to see if that cures the problem I would then be able to confirm this, is this safe to do ? If it works can I run the bike like this ?
I'm thinking leave the BMS on for charging but for discharge go straight to motor, maybe with a 30amp fuse between.
what you plan to do is exactly what i did with two 12v 20Ah Yesa batteries (now 17'500km later).
the only consequence of bypassing the BMS on discharge will be not knowing when to stop discharge if a single cell runs out of charge significantly before the rest of the cells.
since you are still balancing on every charge this shouldnt be a problem (unless that one cell has failed, in which case even a working BMS couldnt have helped it).
the other thing a BMS does is prevent to rapid a discharge. so add a fuse that is rated slightly above your maximum current draw.
Do this, and you shouldnt have any problems.
Thanks for that i'll give it a go and post how I get on.
i might also add, you need a decent AH counter to let you know how charged the battery is.
you can still ruin a perfectly balanced battery by over-discharge, when theres no BMS to stop you.
fortunately everytime ive overdischarged a cell, it has charged up fine. but one of these days, someone is going to kill one.
I know of mixed results from doing this. In most cases, it works fine because the controller is also limiting output and watching for low voltage situation, and users are just trying to get a few more amps out the battery, and not disconnecting it to run some 100 amp monster drain motor. I think they put the BMS limits too tight on some of the batteries, and they cut out for no real reason in situations where no damage was going to happen. I do know a case where someone burnt out the BMS by disconnecting the output side and not the charger side, and the charger went nuts and overcharged. be careful the first time you charge it to make sure your charging BMS is a full separate system from the output one.
A better approach may be to find out why you are having the problem. A 500 watt motor shouldn't be drawing that much amps that it would cause a problem. I think you either have a minor problem, that can be cured by getting a new bms. Or a major problem, that killing your battery won't fix. Bypassing the bms won't make your motor stop drawing too much amps if that is what is happening. Bypassing the bms won't fix the problem if the problem is that there are bad cells in the battery either. Feel free to bypass for a test, and to help diagnose the problem, but if the bms is the problem, you can get another cheap.
Dogman is right. It could be the BMS or your motor seems to be drawing to many amps for a 500w motor. However keep in mind that electric motors usually draw more amps at low RPM's. At a full stop and at full throtle it could draw twice the amps that it normally would, or more, depending on a number of factors. The faster it spins the more back EMF (elctromotive force) it produces so less amps it draws but also, consequently, has less torque. So, if you have a heavy load on the motor it will stay in low RPM's and will draw more current. (That's why you can burn up an electric egg beater in cookie dough. It draws more amps than what it can handle.) My question is how much of a load are you putting on the bike and if the gear ratio is not too high? (This may be caused from wheel diameter as well.) Sometimes if you can lower the gear ratio you get more torque at the wheel which causes the motor to accelerate faster and pulls it out of the amp eating RPM's before your BMS detects it. As a test you could try starting out at half throtle then increase to full throtle gradually and see if it cuts out at high RPM's with full throtle with load. If so, then the gearing, and/or wheel diameter aren't right for that motor. If it still cuts out at high RPM's it maybe the BMS or the motor draws too many amps all the time. You could also try it with out the BMS just once to see if it lacks acceleration. Lack of acceleration would also indicate improper gear ratio. But still. I think it's probably what Dogman said.
What BMS is being used?
I would like to learn about its features and capabilities.
I recently acquired a xtreme xb300li. The lipo4 battery says it has a full charge on the LEDs on top of battery but like this post the battery shuts down at about half throttle or under the slightest work load. Have run a lead acid pack to the bike.. It''s fine.. The lipo pack seems to shut down but reset when I take out the pack and reseat it on the bike. I would like to try replaceing the BMS. Where is a good place to get another BMS? Where would I look for that?