Buying an eBay LiFePO Battery via Craigslist. Can I test it?

[mastershake916]

Okay, so I'm going to look at a LiFePO4 48v 20ah battery back that's for sale on craigslist for $200.
The sell says that he bought it on ebay a month ago for $600, but lost interest, which I believe.
If I test the voltage, what should I be looking for?
I was thinking about buying a power analyzer, and chekcing the voltage drop, etc., but that seems like it'll be more hassle than it's worth.
Worst case I'll have a pack that's not quite up to spec, and maybe need to replace a couple of cells, right?

Thanks for any help, this forum is great!

 

[dnmun]

put it on the charger and measure the voltage of each cell while charging. if the pack has dead cells or has been left uncharged then those rows will be much lower. examine the leads carefully to see if they have been torn away from the pack because that implies the tabs on the end cells may be damaged.

 

[mastershake916]

put it on the charger and measure the voltage of each cell while charging. if the pack has dead cells or has been left uncharged then those rows will be much lower. examine the leads carefully to see if they have been torn away from the pack because that implies the tabs on the end cells may be damaged.

I've very new at non-SLA battery packs, do you have any visual aid to illustrate the process?

Thanks.

 

[999zip999]

Some of those ebay batteries are pure shit. So be careful.

 

[dnmun]

do you have a voltmeter? use a digital voltmeter capable of easily reading to .01V on the 20V scale.

if you can reach the BMS of the battery, then measure the voltage on the BMS where the sense wire cable plugs into the pcb.

the battery has a series of cells that make up the total voltage. these individual cells are monitored and balanced by the BMS through those sense wires.

you want to measure the voltages on each cell by putting your voltmeter across the two adjacent pins for each cell from the bottom #1 to the top #16 of the pack. the first cell is measured between the black wire to the battery from the BMS, the ground, and the first pin on the sense wire plug, which is the top of #1.measure all of the cells and let us know what they read.

they should all measure close to the same voltage, between about 2.7-3.4V when removed from the charger, and up to 3.8V when on the charger.

if there is one row of cells that measure zero, that makes it a dead battery. it can be rebuilt but if you don't have much experience or skills then it may be too much work and having others work on it in the the US is very expensive because they pretend it is very difficult to understand how the BMS functions.

of course if it is dead, the guy knows this and will resist you examining the pack closely. if it is a Vpower pack, it may have come from the seller in china as a dud. which might be why he decided to sell it. that is the problem with CL, people lie and sell you junk knowing you will be gone in minutes.

by putting it on the charger you will be able to tell if the battery will take charge, which is the most critical function. the charger itself may be dead, but if the pack is severely damaged, the BMS may actually prevent it from charging too.

you can log on to this thread when you go look at the pack and let us know what the cells read and we can give advice about how bad or good it is, and how hard it will be for you to get it to useful levels of performance. maybe even log on while you measure it at the guy's house.

 

[dogman dan]

Here's a pic of getting a single cells voltage. In this case, I could unplug the bms, and then probe for voltage on one cell by tapping the contacts on the battery side of the plug.

Then for the next one, move both probes down the row just one space. Repeat and record the voltages.

An article on lifepo4 bike battery basic care and troubleshooting should appear pretty soon here. http://www.electricbike.com/

 

[dnmun]

if the guy gives you a hard time when you ask to measure the voltages on the BMS while it is charging, then you may wanna think twice about the purchase.

by measuring the voltages while it is on the charger you can learn a lot about how the BMS is working and if it is not working. the charger and the BMS represent about 20-40% of the residual value of the pack so you wanna know that they both work.

if the charger is plugged in but the light is green and it is not charging, then you wanna know why it is not charging. that is why you measure the voltages while it is charging.

if you find one cell is stuck at 3.9V and the charger is shut off, then that cell is causing the BMS to turn off the charging mosfet to prevent overcharging, called High Voltage Cutoff HVC. you wanna inspect that row of cells to determine why.

if there is little variation in the voltages when you measure using the 20V scale(so you can see the voltage in .01 increments) and the charger is charging and the cells all appear to be moving up in voltage together, then you can assume it is in fairly good shape.

 

[999zip999]

If the battery is made of of 220 round cells spot welded togethere ? These are the batteries with the most problems as just a couple spot weld pop then you have to find or replace a cell of one in 220. Some of these cells are b grade.

 

[iamsofunny]

it should be over 38v after a few minutes of use. Right off the charger it should be 43-44v.
Really you could hook it up and see how long it runs.

 

[mastershake916]

@dnmun & Dogman:
Thanks for the advice, that's a lot more than I though that I'd be able to do with a voltmeter!
I will post my results, and if I have any more questions, I will let you know.

@999zip999:
How would I be able to know if it is a 220 cell pack?

@iamsofunny:
Are you thinking of a 36v battery pack? This pack is 48v, what would the proper voltage be for that?

 

[dnmun]

by 220 count he means the Vpower packs. the small cylindrical cells only hold about 1.5Ah each so it takes 13 wide to make the 20Ah width.

those cells in parallel are spot welded to a metal ribbon strap and the spot welds are usually weak or inadequate and they tear apart so the capacity of the row is reduced dramatically.

you will not be able to see inside because i suspect it has tape on it. dogman mentioned on the other thread how you can weigh it to see if it is a pouch pack or a cylindrical steel cell pack. the 48V20Ah pouch pack will weigh about 13lbs and the Vpower pack of the same capacity 48V20Ah will weigh about 22lbs.

usually the rows with bad welds show up as reduced capacity rows so they charge up to 3.9V first and shut off the charging. that was why i mentioned that you should look specifically at the row that charges up fastest to 3.9V. sometimes it is just really obvious what is wrong.

there is more you can do with a $3 voltmeter, and a big power resistor too. you can even estimate the internal resistance of each row.!!!

 

[999zip999]

Hopeful the battery should be 58.6 fresh off a charger. A blue ping maybe 60v. They will set and level and if left uncharge for 3mos. The bms could eat at 1-3 cells thay may power the bms or something maybe. The battery should be above 48v. and not below 47v if used. Cells up to 3.65v to 2.7v for my flavor. Are there little blinky lights when charged on the bms ( maybe a ping ect,) My lvc voltage was 47v. Give a second and this thread will light up on my opinon it is to my liking.

 

[mastershake916]

Okay, after a little bit of sleuthing, the ebay seller is chinarichroc2009 (http://myworld.ebay.com/chinarichroc2009) AKA Zita Huang of RICHROC (http://myworld.ebay.com/chinarichroc2009)
Also selling through DIYTrade (http://www.aliexpress.com/store/612373) and AliExpress (http://www.aliexpress.com/store/612373) and DHGAte (http://www.dhgate.com/wholesale/store/ff80808133908beb0133a65fbf573fec.html)

Anyways, here is the only instance of a 48v 20ah battery that I could find, without the charger: http://www.aliexpress.com/store/product/48V-20Ah-LiFePo4-Battery-Pack-with-High-Quality-BMS-FOR-E-BIKE-Long-LiFe/612373_583641357.html

So according to the stated weight, it would seem that it is a Vpower / cylindrical cell pack, right?
Also, the stated cell type is "26650 3,000mAh" what would this mean? And how likely is this to be true?

 

[dnmun]

all you can do is go look at it, it may be gone already. if you can check voltages while charging, you now have a basis for having an opinion about buying it. the pictures show 5 cells in parallel for 15Ah so that is 3Ah/cell where the Vpower cells are much smaller.

just be certain the charger works if nothing else.

 

[999zip999]

These packs get a lot of fix it questions. They are a low on output in ah sometimes and have had problems with cells poping the spotwelds. losing so cells and some say not all cells of a grade maybe. So yes it may be a gamble. Cheap is not cheap in batteries. It's the thing that has the most problems.

 

[dnmun]

the pictures in the links he showed had 3Ah cylindricals. the Vpower packs were only half that size so this is not Vpower, but not pouch cells. that is why it had the 9.2kg.

offer $145. then if he lets you prove it worth more by testing it then you can spend a little time on it and post up here for advice. if it works well that $200 would be a fair price.

 

[mastershake916]

Okay, going to look at it in an hour or two, fingers crossed.

Stay tuned!

 

[999zip999]

If it annt all that don't get it.