# DIY Electric Car Forums> EV Conversions and Builds> Batteries and Charging

 NuWiki Register Blogs FAQ Members List Social Groups Calendar Search Today's Posts Mark Forums Read

#1
07-18-2012, 12:48 PM
 lowcrawler Senior Member Join Date: Jun 2011 Posts: 371
How to test capacity...?

How do I check how much capacity my cells have?

I finally drove mine far enough in one burst to get down to the bottom, which using my miniBMS I was able to find out which is my 'weakest' cell.

Now I'd like to test HOW much weaker it is than the rest, and potentially replace if if it is, indeed, considerably weaker.

... but I have no idea how to do that.
#2
07-18-2012, 01:12 PM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 5,244
Re: How to test capacity...?

Quote:
 Originally Posted by lowcrawler How do I check how much capacity my cells have? I finally drove mine far enough in one burst to get down to the bottom, which using my miniBMS I was able to find out which is my 'weakest' cell. Now I'd like to test HOW much weaker it is than the rest, and potentially replace if if it is, indeed, considerably weaker. ... but I have no idea how to do that.
I would suggest that you get a CellLog8s and connect it to several cells in series including your cell of interest and "log" a discharge (and charge) cycle. You can then compare cells on graphs like these http://www.diyelectriccar.com/forums...ight=celllog8s You won't know the actual current from a test like that so you cannot calculate the Ah.

To find the actual cell capacity (AH), you need to know the discharge current. For the tests I do, I hold the current constant by using a regulated load in the lab (a big resistor and SS regulator).

Here is another way to do it. http://www.westmountainradio.com/pro...avcode=/UPSCBA

Last edited by major; 07-18-2012 at 01:15 PM. Reason: addition
#3
07-18-2012, 01:21 PM
 ricklearned Senior Member Join Date: Mar 2012 Location: Southern California Posts: 301
Re: How to test capacity...?

If you want graphs that show you volts and Amp hours, there is the West Mountain Radio CBA for about \$150 or the Revolectric will do an individual cell up to an 8 cell pack for about \$250.

For less money I think you can get some data out of certain cellogs and some other RC Watt meters but using the logview software is not real time but a lot cheaper.
__________________
My 1973 VW Super Beetle Thread
http://www.diyelectriccar.com/forums...ild-74500.html
#4
07-18-2012, 01:37 PM
 lowcrawler Senior Member Join Date: Jun 2011 Posts: 371
Re: How to test capacity...?

I appreciate the link, but I'm not really looking for a \$250 dollar solution to test if a \$100 cell is in need of replacement. :-/

Isn't there an easy and cheap way to see how much capacity a cell has compared to another? That's really all I'm looking for... I don't need automated logging, graphs, etc. I'll probably do the test once, replace if needed, and never do it again for the life of the pack.
#5
07-18-2012, 01:59 PM
 Ziggythewiz Senior Member Join Date: May 2010 Location: Houston Posts: 3,747
Re: How to test capacity...?

Depending on your setup, if you're currently drained you could remove the weak cell, and keep discharging through an AH counter till the next triggers the cutoff, etc. Or if the charger is adjustable, you can do similar charging to the top.
__________________
1 EV - 8,530 e-miles in 2 years (2/11/13) - http://www.evalbum.com/4000

Last edited by Ziggythewiz; 07-18-2012 at 02:25 PM.
#6
07-18-2012, 03:57 PM
 dougingraham Senior Member Join Date: Jul 2011 Location: Rapid City, SD USA Posts: 579
Re: How to test capacity...?

Quote:
 Originally Posted by lowcrawler I appreciate the link, but I'm not really looking for a \$250 dollar solution to test if a \$100 cell is in need of replacement. :-/ Isn't there an easy and cheap way to see how much capacity a cell has compared to another? That's really all I'm looking for... I don't need automated logging, graphs, etc. I'll probably do the test once, replace if needed, and never do it again for the life of the pack.
The cheap way requires one or two DVM's so you can measure the voltage and current. And you need some sort of load that will support quite a lot of power. If testing a single 100AH cell you want approximately a 30 amp load. You can use a bank of automotive tail light bulbs as the load. You may need to experiment to see how many bulbs it takes to get 30 amps. And you need a clock.

Average the current reading voltages (or just use the current at the halfway point as it will be close enough) and then calculate the current based on this average. Voltage / shunt resistance will give the average current. Compute the time in hours. It will be minutes until the voltage dropped to 2.5V divided by 60. If you were using 30 amps the expected time for a 100AH cell would be around 200 minutes. 200/60=3.33 hours. Multiply the time in hours by the average current and you will have the AH capacity you measured. In this case 3.33 hours times 30 amps = 99.9AH.

You might need a bank of 20-30 bulbs to get 30 amps which would make that the most expensive part of the deal. It doesnt have to be exactly 30 amps, but if too low you could get bored and forget the battery and have it go flat making the whole thing a moot point because then you would have to replace it.

A watch, a volt meter, a shunt, and a load is all you need. You can enter the data into a google docs spreadsheet and make a nice discharge graph if you took down the numbers.

Pick another cell you think is a good one and test that too so you have something to compare against.

An alternative to all of this is just replace the cell you think is weak and keep it as a spare for when you have a real failure. Charge it so the resting voltage is a little over 3.30 volts for long term storage.
__________________
Doug Ingraham
Rapid City, SD
1985 Mazda RX-7 GSL (1400+ EV miles)
Now on the road but still under construction.
#7
07-19-2012, 03:29 AM
 eva-michael Junior Member Join Date: Apr 2010 Location: China Posts: 22
Re: How to test capacity...?

If it's not more than 60V nominal 140A rated. You can directly use a GT power capacity meter.
__________________
www.evassemble.com All for EVs. If you are finding any ev parts, this will be a website deserves your visit.
#8
07-19-2012, 04:58 AM
 GerhardRP Senior Member Join Date: Nov 2009 Posts: 449
Re: How to test capacity...?

Quote:
 Originally Posted by lowcrawler I appreciate the link, but I'm not really looking for a \$250 dollar solution to test if a \$100 cell is in need of replacement. :-/ Isn't there an easy and cheap way to see how much capacity a cell has compared to another? That's really all I'm looking for... I don't need automated logging, graphs, etc. I'll probably do the test once, replace if needed, and never do it again for the life of the pack.
First, find a data sheet for your cells and find the resting voltage vs. state of charge. Then, after you have run your weakest cell to its low limit, accurately measure the resting voltages of each cell. Finally look up the SOC of each cell from the data sheet. Cost = good DVM.
Gerhard

 Share or Bookmark this

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home General Forum     EV News     Chit Chat EV Conversions and Builds     All EV Conversions and Builds     Technical Discussion     Batteries and Charging     Controllers     Electric Motors     EV Performance     Electric Bikes     Non Road Going Vehicles DIY EV Wiki     EV Information     DIY Wiki Discussion Production EVs and Hybrids     Production EVs     Production Hybrids Marketplace     Classifieds     eBay EV-Related Auctions     Parts Vendors     Professional Conversion Services     EV Dealers EV Clubs     Dallas EV Club     TexomaEV.com     EVAoSD     NEAA     NZEVA     B-EVA     UtahEVInterest External Information     EVDL List

All times are GMT -6. The time now is 02:43 PM.