Buying a used battery. How to test?

[Just Give'r]

Hi All,

If I wanted to purchase a used ebike that someone has put together, what are my options for checking the battery age/condition?

If the seller allows a 15 minute test ride, for example. Would that be enough time to measure a voltage drop? I am eyeing a cheap 1000w hub 48v bike now.

Thanks in advance.

 

[e-beach]

15 minutes is really not long enough. Riding the bike until it drains the battery to low voltage cutoff (LVC) is really the only way to know how good the battery is.

 

[Eastwood]

If you think the sellers honest I guess you could at least ask him how many charge cycles are on the battery and or how old is the battery. Has it been charged to 100% or has it been charged to 80% throughout the lifecycle of the battery

 

[HK12K]

If you have a way to access the individual group voltages you can verify all of the groups are well balanced before a ride. Then hammer on it and watch their behaviour live, at best if it has a smart bms,, or at worst verify they're all still within tolerances after really getting on it for a while. That will at least let you know if the pack has any glaring issues. Won't guarantee capacity per-se, but better than nothing.

 

[E-HP]

If the seller allows a 15 minute test ride, for example. Would that be enough time to measure a voltage drop? I am eyeing a cheap 1000w hub 48v bike now.

Have the seller fully charge the battery before you get there, and check then check the voltage before the test ride. It should be close to 54.6V, but if it's a it's noticeably lower (e.g. 53V), that could indicate some bad cells in the groups pulling the voltage down.

 

[john61ct]

Best is a dummy load, start fully charged, and see how many Ah are drawn in a 1-hr CC load test.

Note that will be much lower than the rated capacity even when new

but if it comes up say 30-40% shy that's scrap.

Lots of batteries are crap to start with

might only get 50 good cycles

so that should be your default assumption

 

[dogman dan]

There is another method. Used batteries are valued at zero dollars. He can have 3 cycles on it, and its already ruined.

How could this be? He let it sit unused too long, till the bms drained one cell group till its dead, or nearly so. Lots of people let this happen over a winter.

But in 15 min, you will know if the battery is in that bad a condition. its time enough if the battery is truly junk. It will die fast if its dead meat. You can test the voltage before and after the 15 min ride. In 15 min, the battery should be in the middle voltage, between full and empty, not very near the empty voltage. But chances are, its at most worth half what he paid for it. So you only pay half that, meaning at most 1/4 what he paid new.

Even better would be to somehow watch the voltage as you ride. If your voltage after the initial 5 min ride is say, 52v, and then it drops to 42v when you apply half throttle, the battery is either ruined, or never was suitable to run that bike.

 

[MadRhino]

It is impossible to measure the required parameters without discharging and charging the battery one full cycle, and even then you need to know the cells specs to compare.

First thing you want to measure is the capacity, because it is the best indication of wear. A 20 Amp/h battery that has only 16 Amp/h capacity, had been used (or abused to) all of its rated life cycles.

 

[john61ct]

Problem is that 20Ah rating even if 100% honest, was derived in conditions that have nothing to do with your test parameters.

Doing the SAME test yourself when brand new get X Ah, then 100 cycles later get Y Ah

only that gives you accurate SoH% wear factor.

That X when new, may have been 19Ah or maybe 17Ah on a perfectly fine battery (or a crappy one) labeled 20Ah fresh off the production line.

 

[4πr^2]

Hi All,

If I wanted to purchase a used ebike that someone has put together, what are my options for checking the battery age/condition?

If the seller allows a 15 minute test ride, for example. Would that be enough time to measure a voltage drop? I am eyeing a cheap 1000w hub 48v bike now.

Thanks in advance.

The "...that someone has put together..." part would scare the crap out of me.

I guess if you could tear the battery down and examine it, and know what you're looking for, that might help. But a battery put together improperly by some rando seems like it would be just waiting to burn down your house. There are nearly infinite ways to put one together wrong...and one way to put everything together right...are connections tight, proper gauge wire used, solder joints good, spot welds good/solid, no wires crossed/pinched/chafing on each other, battery cells properly separated, proper separators used on end caps, no sharp edges, no points of chafing between case/battery, everything heat shrinked/insulated correctly, etc.

If you're OK with that aspect, as far as life, I guess a 15 minute test would do. You could note the battery state of charge, give 'er hell for 15 minutes, and note the state of charge again. If that doesn't seem like 'enough charge' for the money, walk away or negotiate a lower price. The battery may have been abused in the past, but what ever state it is currently in, it will most likely not get drastically worse with good care from here on out.

I guess you may also generate an opinion of the seller... does he seem on the level, bike clean and well maintained, battery doesn't seem to be abused, etc - then that would give a bit of confidence. But if the seller seems shady, bike looks like it was assembled from random parts a few days ago and the battery has hacksaw marks on it from where a lock might have been cut away... again, time to walk!

anyway, my .02

 

[999zip999]

Did this battery come with the bike that you're looking at what brand is the bike how old is the bike does he use it regularly ?