safe discharge rate for 36v 10ah lithium ion pack

[eHomer]

I've been absorbing the detail on forums about battery pack capacity and discharge rates.

I now know what the "C" rating is, (the multiplier of the ah capacity to give the safe sustained discharge rate).

I don't know what that figure is for my packs though, it wasn't in the original spec and it's not marked on them.

I have two identical Lithium Ion 36v 10ah packs that lock into my rack (one at a time), or fit in my other bike's pannier bags.

They're in an aluminium case, with rear light, lock, fuse, main switch and battery level leds.

I've been using them so far with 15 amp max controllers. Though they were briefly used with a 36v 750w direct hub motor and controller, and seemed to work ok, though not "thrashed" on it).

Just thinking ahead, and wanting them to last as long as possible, would they support say, 25 amp controllers and motors, or would that significantly shorten their life ?

 

[Deleted member 4366]

Unless they specifically mention otherwise, assume 1C, which means 10 amps continuous and 20 amps max, so the answer to your question is no unless you run them in parallel. You only have to make a simple 2 into one connector for that, and disconnect it for charging. There's some theoretical risks in doing that, but you'd be very unlucky to suffer them in practice. If you wanted to be absolutely sure, you'd need to install a couple of blocking diodes, but, personally, I wouldn't bother with the diodes.

Used singly at 25 amps, you'd get massive voltage sag and a strong possibility of tripping the BMS over-current.

 

[eHomer]

Ok Dave, thanks.

The idea of a 2 into 1 lead appeals to me, in case I ever did go that way, and I guess it would be cheap and easy to make.

I like the idea of making it bullet proof, in case for instance it was ever inadvertently used with one fully charged and one discharged battery, and I wouldn't want the current to surge from one to the other when initially connected.

What type and rating of diodes would you suggest for those batteries, and how would you connect them ?

 

[Deleted member 4366]

Assuming that one battery shorts out, the other one would provide the whole current (under stress), so you need a diode rated at 25 amps or more and able to withstand a reverse voltage of 42V. There's a few round about 45v and 15 amps for solar panels, but I think I'd like a it of a safety margin, so 50v and 30 amps minimum. You could use two pairs of 15 amp ones on each line to share the current.

To install them, just stick one in each positive battery line pointing outwards.

A quick search found these.
Buy Rectifier & Schottky Diodes VS-40HFR20, Rectifier Diode, Switching 40A 200V, 2-Pin, DO-5 Vishay VS-40HFR20 online from RS for next day delivery.

These might work too. They're rated at 40 amps, but that seems a lot for those thin legs.
4x Infineon BYP302 40A/1200V TO-218 Fast Diodes | eBay

 

[eHomer]

Thanks Dave.

Yes, I'll go for the RS ones in the stud mounts, much more meaty, as you say.

I think I'll make up a short "Y" lead adapter with inline plugs and sockets, so that I can easily plug it in whenever needed (in any bike).

Even with the 14 amp controllers, it'll save having to change batteries half way on my longer rides, and presumably benefit the batteries by halving the discharge rate, even on them.