have had my new lifepo4 for around 2 weeks now and have been topping it up each time i use it to 41v. problem is the low volt cut out cuts in after only around 3miles but the voltage still shows 38.8v. i think it may be that i have a 750w motor which is making the volts drop below the low volt cut off point when i open the throttle. its a 20amp battery so should do a lot more than 3 to 4 miles. do i need to fully charge and discharge a new battery? or have i got a problem of some other sort like a bad cell? or bms ? any help would be apreciated.
charging new lifepo4
- Thread starter theskip1
- Start date
If you are not putting a full load on the motor, asking it to pull you uphill on full throttle without pedalling for instance, then you have a problem.
Where did you get your battery? A LiPO shouldn't need any conditioning as far as I know. Its own internalBMS will protect from the voltage falling below around 29V, so it would appear the problem lies elsewere.
Quite likely the controller, which should be matched to the motor. Perhaps the drain of your large motor is too much for the controller.
If the controller is matched and good then the battery could be suspect.
I can feel Flecc coming to our rescue...
Where did you get your battery? A LiPO shouldn't need any conditioning as far as I know. Its own internalBMS will protect from the voltage falling below around 29V, so it would appear the problem lies elsewere.
Quite likely the controller, which should be matched to the motor. Perhaps the drain of your large motor is too much for the controller.
If the controller is matched and good then the battery could be suspect.
I can feel Flecc coming to our rescue...
My eleven yr old daughter sits on the back of my bike, difficult to know how the battery and motor should perform. I do make it to the top of the hills now, i do struggle on the push bike, lol.
the battery was a vpower of ebay. just a thought but when it goes of it does not come on again unless i unplug the (external) bms then plug it back in again. but as soon as i turn the throttle it shuts of again.if it was the low volt cut off would it not keep turning on and of by itself ? the controller came with the motor so i would assume its matched. it worked fine with sla batteries.
I had seen this post but I'm a bit stumped, in part through lack of information. It depends to some extent on the real motor power, whether it's a nominal 750 watts with a peak consumption & output very much higher. In that case it may be exceeding the capability of the battery, but somehow I think that unlikely.
The quiescent voltage whether 41 or 38 volts doesn't mean much, it's the extent of that drop that concerns me a bit. That said, your controller was probably designed for li-ion manganese or compound cathode batteries and these nominally run at higher voltages (3.2 volts per cell) than LiFePO4types (3 volts per cell). Therefore where the controller protection cutout is normally at 32 volts for a 36 volt Li-ion system, it's sometimes best set at 30 volts for an LiFePO4 system which can cut out 2 volts early with the Li-ion controller. However, I can't be definitive since manufacturers often use more cells in LiFePO4 batteries to compensate for the 0.2 volts loss per cell
All that said, cutting out after only 3 miles is so early that I suspect a faulty battery and I'd pursue this with the supplier first, since I'd expect at the very least half range before cutout in these circumstances.
One query though, how did the bike perform on it's previous presumably li-ion manganese battery?
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The quiescent voltage whether 41 or 38 volts doesn't mean much, it's the extent of that drop that concerns me a bit. That said, your controller was probably designed for li-ion manganese or compound cathode batteries and these nominally run at higher voltages (3.2 volts per cell) than LiFePO4types (3 volts per cell). Therefore where the controller protection cutout is normally at 32 volts for a 36 volt Li-ion system, it's sometimes best set at 30 volts for an LiFePO4 system which can cut out 2 volts early with the Li-ion controller. However, I can't be definitive since manufacturers often use more cells in LiFePO4 batteries to compensate for the 0.2 volts loss per cell
All that said, cutting out after only 3 miles is so early that I suspect a faulty battery and I'd pursue this with the supplier first, since I'd expect at the very least half range before cutout in these circumstances.
One query though, how did the bike perform on it's previous presumably li-ion manganese battery?
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So, you've changed from lead acid to Lithium...
Do you have a link to the ebay ad? Or a picture of the new battery?
previous battery was 3 sla's at 15amph which worked ok got around 10mls per charge.is there any way of telling if its the battery. when newly charged it reads 41. 5 approx but after around 2 to 3 hours it has dropped to 40. 5 or so. is this usual ?
i might try running without the bms and see how fast the voltage drops.
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That voltage drop is normal, warm off the charger, cells read a bit higher but drop somewhat as they cool.
If 3 SLAs of 15 Ah managed ok with reasonable performance and range and no controller cutout, I'd expect this lithium iron phosphate one to do about the same. This makes it look more like the battery under performing but no easy way to check without knowing more about the motor, it's consumption under load and the exact usage circumstance. However, if the cutout at 3 miles is when riding along the flat, the battery is almost certainly to blame.
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If 3 SLAs of 15 Ah managed ok with reasonable performance and range and no controller cutout, I'd expect this lithium iron phosphate one to do about the same. This makes it look more like the battery under performing but no easy way to check without knowing more about the motor, it's consumption under load and the exact usage circumstance. However, if the cutout at 3 miles is when riding along the flat, the battery is almost certainly to blame.
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The drop in voltage of Lead Acid batteries is linear, whilst Lithium is fairly flat then a sudden drop-off.
SLA batteries keep going during the range, but you will notice the speed keep dropping, 'til fairly embarrasingly slow. Lithium will maintain the high speed, then suddenly drop-off.
If you were getting 10 miles on 15AH SLA I think it right to assume only 7 of those were anywhere near top speed (30+ volts)
Now you're using Lithium at 20AHs then you should expect 20/15 X 7 miles
i.e 9.2 miles. I think it's a dudd battery.
SLA batteries keep going during the range, but you will notice the speed keep dropping, 'til fairly embarrasingly slow. Lithium will maintain the high speed, then suddenly drop-off.
If you were getting 10 miles on 15AH SLA I think it right to assume only 7 of those were anywhere near top speed (30+ volts)
Now you're using Lithium at 20AHs then you should expect 20/15 X 7 miles
i.e 9.2 miles. I think it's a dudd battery.
i think you might be right but will try without bms first. if it is the battery i will try and find the bad cell myself as the seller is expecting me to pay the post back to china and i have no garantee that ill ever see it again.
Give it a little time. Someone may well have some more positive advice.
Do you have any details / pictures of the motor?
Do you have any details / pictures of the motor?
no pics but its big, black and heavy, and also from china. from someone called sanny kung ?
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I agree with NRG on this one. The BMS over amp protection is probably cutting power. This would likely need disconnecting before resetting.
C
Cyclezee
Guest
Slightly off thread here, but when you say LiPO, do you mean LiFePO4? If so is that correct, they do not need to be conditioned in the same way that other Lithium batteries do?
J
hnCan you monitor the voltage between battery and Bms at the time you open throttle causing power to cut? If this drops to below the expected low volt cut off there is probably a problem with the battery. This could be several things, bad cell/s, poor connections between cells/wiring or inadequate wiring thats not up to the job.
Have you any specs for battery/bms? I see vpower batteries are rated at 2c which isn't particularly high.
It is confusing, in the model world lipo is often carelessly used for LiFePO4. However, note Bob's correct use of capitals for the PO in LiPO, hinting at phosphate as in Lithium iron phosphate (chemical), rather than polymer (construction).Slightly off thread here, but when you say LiPO, do you mean LiFePO4? If so is that correct, they do not need to be conditioned in the same way that other Lithium batteries do?
J
I can't answer on the conditioning need, but if an integral meter is fitted in the battery, running the battery near empty to zero the meter can still be necessary, regardless of the chemistry. Some maintain that is the only reason for conditioning other lithium types.
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C
Cyclezee
Guest
Thanks for clearing that up Tony.
In other words, condition all Lithium batteries as it can do no harm even if it might be unnecessary in a few cases
J hn
In other words, condition all Lithium batteries as it can do no harm even if it might be unnecessary in a few cases
J
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