Bypass BMS with LifePo4 - good idea or not ?

8 replies [Last post]
smarmstrong
Offline
Joined: 04/09/2009
Points: 5

Hi,

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.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
antiscab
Offline
Joined: 07/07/2007
Points: 1678
Re: Bypass BMS with LifePo4 - good idea or not ?

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.

Matt

__________________

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion
Spent so far: $5800 + $7000 + $2040 + $1200 + $425 + $800 + $140 + $3000
Cost to do it again: $1000 + $3800 + $2040 + $1200 + $425 + $800 + $140 + $1500 (lasts longer)
Cost for a Petrol bike:$6000 + + $1440 + $7560 + $1000 + $1600 + $4000 servicing
Total spent: $20405
Total to do again: $10905
Total to have used a petrol bike: $21600
Total distance travelled so far: 102'120km

Other vehicles:
2008 Mazda 2 conversion
2009 Blade Electron
1997 Prius - plugin hybrid mod

smarmstrong
Offline
Joined: 04/09/2009
Points: 5
Re: Bypass BMS with LifePo4 - good idea or not ?

Thanks for that i'll give it a go and post how I get on.

antiscab
Offline
Joined: 07/07/2007
Points: 1678
Re: Bypass BMS with LifePo4 - good idea or not ?

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.

Matt

__________________

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion
Spent so far: $5800 + $7000 + $2040 + $1200 + $425 + $800 + $140 + $3000
Cost to do it again: $1000 + $3800 + $2040 + $1200 + $425 + $800 + $140 + $1500 (lasts longer)
Cost for a Petrol bike:$6000 + + $1440 + $7560 + $1000 + $1600 + $4000 servicing
Total spent: $20405
Total to do again: $10905
Total to have used a petrol bike: $21600
Total distance travelled so far: 102'120km

Other vehicles:
2008 Mazda 2 conversion
2009 Blade Electron
1997 Prius - plugin hybrid mod

andys
andys's picture
Offline
Joined: 11/10/2007
Points: 262
Re: Bypass BMS with LifePo4 - good idea or not ?

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.

dogman
dogman's picture
Offline
Joined: 04/29/2008
Points: 830
Re: Bypass BMS with LifePo4 - good idea or not ?

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.

__________________

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

juanito
juanito's picture
Offline
Joined: 04/02/2009
Points: 37
Re: Bypass BMS with LifePo4 - good idea or not ?

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.

Example:
500w 24v.jpg

garygid
Offline
Joined: 12/20/2008
Points: 441
Re: Bypass BMS with LifePo4 - good idea or not ?

What BMS is being used?
I would like to learn about its features and capabilities.
Thanks

__________________

Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.

the flickster
Offline
Joined: 05/13/2014
Points: 21
Re: Bypass BMS with LifePo4 - good idea or not ?

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?
Tbanks!!

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

style="display:block"
data-ad-client="ca-pub-2401070691062219"
data-ad-slot="2079401426"
data-ad-format="auto">



Who's new

  • MJSfoto1956
  • Honda CB
  • sdf2000
  • Kile
  • Buffalo Chip

Customize This