charging new lifepo4

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.

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...

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.

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.

 

whats that all about ?

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...

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.

This one's beyond me I'm afraid. Be patient... someone here will definately assist.

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 BMS can cut the supply if current draw is too high.

it worked fine with sla batteries.

 

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?

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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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.

Edited October 22, 201015 yr by theskip1

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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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.

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.

 

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?

Give it a little time. Someone may well have some more positive advice.

 

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 ?

Edited October 22, 201015 yr by theskip1

I agree with NRG on this one. The BMS over amp protection is probably cutting power. This would likely need disconnecting before resetting.

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.

 

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:) hn

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.

 

Can 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.

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:) hn

 

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).

 

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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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

Can 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.

 

yes i can monitor the voltage thats what i aim to do ,without the bms conected, in the hope that it does not drop below 33v and thus pointing to the bms cutting the power which would be a lot cheaper to repair/replace.

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 found the listing (not the actual listing but the same type and seller ) for my battery.it does not say but it is a v power battery

36V 20AH LiFePO4 Battery Set

Edited October 25, 201015 yr by theskip1

That should be able to handle your motor without any of the problems you are experiencing, pointing even more strongly to this particular battery being faulty.

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That should be able to handle your motor without any of the problems you are experiencing, pointing even more strongly to this particular battery being faulty.

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UPDATE

you are quite right it is indeed the battery. put it under test and found it lost volts at an alarming rate, it was soon down below 35v and dropping to below 32v when opening the throttle. this means buying a new cell. i am not going to send it back to china as it will probably cost almost as much as repairing the battery with no garantees of return. so will have to save what i can from my pension, read up on replacing a bad cell, and place myself in the hands of providence.

thanks for all the advice. tony

Replacing lithiun cells is not normally done as it's very important that the correct cell is used, these constantly changing with development. It would be best to try and get a cell from the supplier of your battery to ensure compatibility and minimise risks of accidents.

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