Testing old batteries with a Victron BMV600

[jmseas]

Anyone have a procedure for testing old(er) batteries to determine what what their effective 20 hour rating would be?
I have a pair of Costco GC2 6 volt batteries that are now 4 years old. Water has gone low on them at some point so I expect their 20 amp hour rating is significantly less than new. I have a Victron BMV602 to measure current/consumed AH/State of Charge/Voltage.

I'd like to adjust the the Battery Capacity (Cb) (& Peukert exponent?) to approximate what it is now. I see Maine Sail drops a few percent off each year for capacity, but I'd like something a little more accurate.

I ran lights for a few hours and pulled out 3 Amp Hours and my voltage is still at 12.81 which is still a full charge.
I'm guessing I need to pull out 50 AH or so, let the batteries rest overnight unloaded, measure the voltage and figure out the state of charge from a state of charge graph and then use some algebra to figure out the battery capacity?

 

[Stu Jackson]

John,

Find the Link 2000 manual here:

Link products:

http://www.xantrex.com/power-produc...oducts/discontinued-products-accessories.aspx

The Victron uses the same algorithm as the Link 2000 chip/software and the Link manual has a detailed discussion of how to do exactly what you're looking for.

Personally, I find it a little overkill. Have you considered just knocking a few percentage off your house bank size? If, as you said, things look good, why bother? Please remember, these aren't precision instruments, they're there to give you a much better picture of what you have, but they're not "good to the very last amp."

 

[mitiempo]

I agree with Stu, it's overkill.

To be meaningful you will have to do a 20 hour test at 5% of bank capacity (when new) but the difficult part is that the load should be constant during the test.

 

[Maine Sail]

The only accurate test is a full 20 hour capacity test at battery capacity divided by 20. As said above the current needs to be held as steady as possible.

Charge battery fully
Ensure battery is between 75F & 80F
Reset Ah counter to zero/100%
Turn on 20 hour load
Hold load current as steady as possible as voltage drops
Stop test the second the battery terminal voltage hits 10.5V
Note Ah capacity delivered from BMV screen
Recharge battery as soon as possible, at the 20 hour rate
Equalize battery if capable

If you really want to get fancy the BMV can drive a relay to stop the load at 10.5V..