Premature BMS cutoff?

[joeworrall]

My battery is a 48v 10ah LiFePO4 with maximum continuos current output of 20a. When the battery is fully charged and still connected to the charger, it reads around 61.7v and when disconnected rapidly stabilizes at about 56v-57v. as im using it obviously there is some voltage sag but then im finding after around 19-24 miles the BMS cuts off the power at around 48.4v, just wondering if this is a typical voltage cutoff setting for a bms on a battery of my size or if something a little funny is going on beneath the duct-tape.

Thanks for all your expertise in advanced!

 

[Russell]

You didn't say if that 48.4V is the loaded or unloaded pack voltage (it makes a big difference). You also need to be sure it's the BMS which is tripping (requires a power reset) or if the controller LVC is being hit. If that 48.4V is the resting voltage then your LiFePO4 pack is indeed fully drained and either the voltage is hitting the LVC of the controller or the BMS is tripping due to at least one cell dropping to the individual cell LVC(~2.0V). In comparison the lowest resting voltage I ever recorded with my 48V/10Ah LiFePO4 pack was 51.45V after a ride which consumed 9.3Ah. During that ride the lowest voltage my wattmeter recorded was 42.90V and the BMS never tripped and the controller LVC was never reached. If the 48.4V you are talking about is under load then the BMS should not be tripping provided you are not exceeding the maximum current limit unless once again a cell is hitting the lower voltage cut-off level. In the first instance you are simply draining the pack completely, something you should not do on a regular basis if you want optimum life from the battery. In the second scenario you likely have one or more cells that are weak and they are hitting the individual cell LVC (way) before the others.

-R

 

[joeworrall]

The 48.4 was the resting voltage after i had unplugged the battery, left it a couple of minutes then re-plugged. During that 2 minutes or so the battery gauge on my thumb throttle doesn't illuminate even when connected to the battery, when it does finally decide to power back up i pedal up to speed then lightly press the throttle and within a few seconds of it being under load the thing cuts out again and there is no battery gauge/power. Maybe i have just drained the battery, seems a bit odd as when it turns its self back on the gauge reads full battery until under load then in goes to half immediately then cuts out. maybe the battery gauge just isn't very accurate when not under load

 

[Russell]

48.4V resting is dead for a 48V LiFePO4 and may even mean you have an unusually low cell or two. As I said the lowest resting voltage I ever have seen on my 48V LiFePO4 is 51.45V and that was with the pack nearly drained. Of course there's no way to be sure unless you check the voltage on each cell, it just may be that you really, really drained the pack. If you are getting the full theorectical 480Wh from the pack (you need a wattmeter to know) then that means you're using 20-24Wh/mile, which isn't unusual if you have a 500W+ motor and average 20+mph. Still it's not a good idea to drain the pack to nothing on a routine basis so either slow down or get a bigger battery. As for the LED indicators they can only give you a very rough voltage indication. Also a LiFePO4 battery will continue to exhibit a high resting voltage even when it's fully drained however as soon as a load is applied that voltage will sag way down which is exactly what you are seeing.

-R

 

[joeworrall]

Wouldn't be surprised if i had a cell or two thats slightly below standard, it was only a cheap 1st ebike project start-out duct-tape battery.. i plan to get batter batteries in future years.

Ive got Turnigy Watt meter, measures: minimum voltage, amp-hours, max current, wattage and so on but at the moment im struggling trying to wire it inline with my motor connector to the controller so until then i wont know how many amp-hours total im using.

Think i might have got it a bit wrong, the minimum voltage might have been 49.3v and the resting 50.2v (on the last ride where it cut out), im getting very confused as i often disconnect the battery from the meter after each ride so i get confused between what ive read.

I'll post another reply once i get my watt meter wired in-line with my motor and battery so i can give you a better idea of how many watt-hours total im using and stuff.

Thanks again. Joe.

 

[biohazardman]

19-24 miles on a 10AH 48V battery is good I managed 18-20 average with not much pedaling. A measurement of 48V unloaded after a 20-mile ride with 16 cells is 3v per cell so it's very near LVC at that stage. Once much of a load is applied, the pack will likely sag enough to cause LVC to kick in. Looks like things are just fine to me although your full charge voltage takes you into the 3.8V per cell range so is slightly high. Still voltmeters can be off a bit I have noticed so not to worry.

 

[joeworrall]

19-24 miles on a 10AH 48V battery is good I managed 18-20 average with not much pedaling. A measurement of 48V unloaded after a 20-mile ride with 16 cells is 3v per cell so it's very near LVC at that stage. Once much of a load is applied, the pack will likely sag enough to cause LVC to kick in. Looks like things are just fine to me although your full charge voltage takes you into the 3.8V per cell range so is slightly high. Still voltmeters can be off a bit I have noticed so not to worry.

Just got my pack on charge right now and its sitting at full and reading 60.47v, the 24 miles was really a big surprise to me as well but i was putting reasonable pedaling for about 5 miles, average speed 22mph for that ride. very relieved everything seems to be normal and i'll just ride a bit easier + pedal a bit more briskly when i need that little extra range

 

[dogman dan]

I agree, that's really a good distance for 10 ah, and the bms is not tripping premature. We do of course, all have to go through the same process of adjusting to the reality of how fast and how far our ebikes will go with the battery we use. It's pretty rare that we go as far as we expected from reading the sellers ads.

Slow is the key to range, since wind resistance get so much less, and the percentage of the wattage providied by the rider increases. My typical range with moderate pedaling with my 36v 20 ah battery is.
20 miles at 25 mph
30 miles at 15 mph
38 miles at 12 mph

I suspect my pedaling like hell is comprable to your moderate pedaling. No way I ever went that far on 10 ah, but if you did any pedaling without the motor, it doesn't count as battery range.