Determining maximum discharge (continuous and peak) of batt?

[potatonet]

ok so I have a 48V 25aH LiFePO4 battery that I got from a guy. Its composed for a TON of smaller battery cells.

Im trying to fixgure out what the current delivery capabilites are, is there any way to determine this?

I know I should be able to get at least a 1C discharge from the battery, continuous. but Im looking to run an X5 controller (soon), only a 20A crystalyte right now.

hoping that its a 2C discharge continuous?

 

[dogman dan]

Lots of voltage sag will indicate you are exceeding the packs capacity to discharge without damaging it. Theoretically the bms does this for you, so If you don't get cut outs, you should be on the right track. Higher rates will tend to make the pack act as though it is smaller than it is, the familiar effect from sla batteries..

One way to go about it would be to use a meter, like a watts up or cycleanalyst, and some kind of device to discharge the battery, like car headlights or stove elements. Measuring the amp rate, 25 amps would be 1 c, and then seeing how many ah it will provide can tell you what it can do at a rate it is supposed to be able to do. Then repeat the test, with a higher amp rate and see what happens. If capacity remains the same, the higher rate should still be in the range the battery can handle. When capacity drops, you clearly are in the rate the battery doesn't like.

If I were you. I'd assume these are 1 c cells. you should be ok with a 20 amp controller, especially if it has a cycleanalyst to limit spikes to 25 amps. If no cycleanalyst, just try to ride so as to avoid big spikes. Pedal hard on hills, and pedal some on starts. Easing back the throttle on hills and starts also helps. The 20 amp should keep you in a safe enough discharge rate most of the time.

 

[Doctorbass]

Generally the maximum allowable current limit of any rechargeable cell is determined by the temperature increase.

EX: an A123 can hold 30C continuous and 60C during 10sec.

The 10 sec max is due to the temp that the cell reach at that limit.


The max allowable current of a cell should be describes as:

The current that generate enough heat in the cell that the temperature reach just under the critical damage threshold or safety limit of this cell.

Doc

 

[potatonet]

guess I will just run it in my cycle analyst...

full throttle I guess...


we'll see

my SLA 48V 7aH pack puts out 2200W when full, about 1800-2000 when not at 100%

thats like 6C? is that right for burst on a SLA?

 

[dogman dan]

Id say you have enough amp hours of battery to safely run a big motor on 1c batteries. But whether the cells you have can do more than 1c is the big question. This is a round cell pack? If so, treating it like 1 c is the max for average discharge over an entire ride may be best. Most of us getting good lifespan out of cheap lifepo4 are getting it by treating the pack like 1c is the max. Even treated nice, cells can still get unbalanced since at these prices they may not have such well matched cells. If the pack won't discharge at 25 amps pretty close to the stated capacity, you have a few stinker cells in there. With a big motor like that, you need the pack to be pretty good to use it for long rides.

 

[potatonet]

the battery is not round cells, it is a bunch of flat cells,

I know it can put out 1C continuous which is more than my controller right now

I may just get some A123M1 bateries for the 48A controller when it comes...