An odd battery fault - can any battery guru’s explain ?

[KirstinS]

My setup is 250w bafang hub, 36v bottle battery, 11.1v booster pack(homemade 18650 pack with 6 cells in each series), ku65 with 810 model LED

I was about 12 miles into my commute when the power failed. My LED went from 4 lights to 1 flashing light. Now this would indicate the LVC had tripped

But the kits is a 36v one and so is overvolted at 48v thus rendering the LVC useless

I had to cycle last few miles unassisted

At home I tested my batteries. My main was reading 39.5v but my booster only 6.5v. It turns out one of the series has packed up – no biggie I have loads of 18650 cells taken from laptops and they do tend to die on my every 6 months. Just not designed for the abuse I give them !

But what confuses me is that the overall pack voltage would still be 39.5 +6.5 = 46v so that’s still way over the 42v hot off charger the system expects from 36v input

So why did it read so low on the LED and why wouldn’t it go ? I used my spare 11.1v booster as soon I got home and the system worked perfectly again. As it did on my commute again this morning

I’m confused by this ? Can anyone explain this to me ?

 

[flecc]

The overall voltage of both would still read quite high when off load, but under load the booster pack could have been effectively flat and the main pack a bit lower too, so the actual working voltage then very much lower.

Maybe also the main pack was trying to reverse charge the failing booster which if it happened would certainly terminate things immediately.
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[KirstinS]

Thanks flecc

Pretty sure the Main pack couldn't be charging booster as connected in series.i use xt60 which can't be connected the wrong way round

The main pack is a brand me 12at pack which was at about 80% charge

The booster was near dead and low c rate cells so I bet you are correct there. The voltage was effectively zero under load

But that still leaves the main pack giving a solid, say, 38v under load

Thanks for help though

 

[mfj197]

The overall voltage of both would still read quite high when off load, but under load the booster pack could have been effectively flat and the main pack a bit lower too, so the actual working voltage then very much lower.

Maybe also the main pack was trying to reverse charge the failing booster which if it happened would certainly terminate things immediately.
.

I would imagine it's quite likely to be this latter point. That's also a reason why I'd be quite fearful of a booster battery. At some point in the ride either the booster or the main battery will effectively run down but the other battery will still be trying to drive current through it. Continuing to do this risks permanently damaging the empty cells, or even igniting them, by undervolting them.

You could minimise this by ensuring both the bottle battery and booster are of the same capacity (in mAh, not watt hours) - i.e. both 4p packs, but then you don't know what condition the cells are in, what capacity they have remaining etc. Also you could set the LVC in the controller to protect the entire 48V combination pack.

Michael

 

[KirstinS]

I usually have a lipo alarm on the booster but I trod on. my old one and New has not arrived. I also match the mah as close as as possible ( I capacity test each cell and match them )

It's not perfect though of course

 

[awol]

Not related but just out of interest, how many amps is your ku65 set to? or is it as standard.

 

[KirstinS]

Not related but just out of interest, how many amps is your ku65 set to? or is it as standard.

It's good question- they are usually 15a

But mine had 17a on the label. Maybe just misprint but d8veh had never heard of anyone running a ku65 at 48v long term. I've been doing so for over a year without issue on my commute. Controller barley gets warm

So it's possible I've an odd one with higher voltage caps and a couple.more amps

I did buy a spare ku65 recently and will compare soon as my new motor isbeing built into a wheel by my lbs

When I strip the bike down I'll open both ku65 and compare

 

[trex]

the flashing LED indicates that the voltage collapses then comes back. It oscillates between 0V (when the current through the bad cells collapses) and 50V. The LED panel is powered by a 5V buffered supply which is relatively unaffected, but the motor did not receive sufficient power to keep going.