Measuring a battery's health

Hi,

I have a 48V 50A LiFePO4 battery in the shed. I've topped it up every couple of months, and would like to knows it health. I'm thinking of testing its capacity using a 180W load.

Battery testing is very new to me so I do apologise for my silly questions.

Since it's only a 180W load, I imagine that I can only discharge the battery at 3.75A. That is is very different to the 40A that the battery would need to supply to a 2000W motor.

1) If the battery were to achieve 45AH on the 180W load, can I expect the same capacity when using it on a 2000W ebike?
2) Would the 2000W ebike cut off sooner because of voltage sag for example?
3) Is the battery capacity the only measure of health that matters, or should I test something else as well?

Thank you,
Fidel

If you wanted to know how far you could sprint, would you see how far you can walk and attempt to extrapolate from that? You are taking a best possible case scenario for testing.

Your load is inadequate to provide the information you are seeking. Series about 5-6 of those and you will get a decent ballpark idea. Some will depend on age, number of cycles, quality of cells and construction of this particular pack. Also factor in throttle usage, coasting, stops, regen, hills, cargo, winds, etc.

You should get close to the batteries stated spec ah when you discharge it at 180w. Even a pretty toasted battery can do amazing good, at that low a c rate. If you get way less, like 80% of stated at 180w, its safe to say you will see real poor performance at 2000w.

You will need to come up with at least a 1000w load, like a space heater or something like that.


Best test of battery health is to measure internal resistance. This is done by a calculation, based on voltage sag under load. This you can do once you have a decent load. 180w won't sag enough to measure... Unless it does, then you do have a bad battery. Like if it sags 3v under 180w,, that is pretty high resistance inside those cells.

Battery **health** is defined by capacity loss.

SoH% compares Ah when new just broken in, to remaining Ah.

A standard rate CC load is important, from vendor spec Full down to a standard stop-voltage, coupled with precise timing for accuracy, rather than relying on a coulomb counter.

_____
IR measurement relates to high discharge rate performance, relevant for the use cases common here.

A relative benchmark showing resistance increasing over time / cycling usage, is however another useful indicator to judge how the battery is wearing.

But more of a challenge to get precisely comparable results.

A first-time measure of IR after the battery has **already** declined in SoH will yield limited information, both wrt remaining lifespan and real-life performance.

An actual high-amp discharge test looking at temperature rise and voltage sag would be more useful for purpose perhaps.

Thanks all, appreciate your responses.

The main message seems to be that a low current test will not reveal the performance at high currents.

What if I do three tests, at 60W, 120W and 180W. Could I extrapolate what happens at 2000W?

On the topic of internal resistance, should it be calculated at the individual cell level, or the whole pack level?

Thanks again,
Fidel

search youtube lipo ir testing

internal resistance dont change much

using high or low amps

as mandog explain

180w fine for basic capacity testing




important thing good balancing

following common bms balancing

many chargers wont reinitiate charge

therefore load small amount

recharge

rinse repeat several times

until close to full soc pack voltage

You can still get a fairly good idea if the cells are good with your 180W load. If you see a significant imbalance between cells toward the end of discharge, it may indicate problems. Any way to put a higher load on the cells will give you information about the internal resistance as the others have pointed out.

My experience with LiFePO4 is when the cells go bad, they will develop a higher self-discharge and the cells will go out of balance if just sitting around. If all the cell voltages are close, it's a good sign.