Load testing lead acid batteries

[TKC100]

I am involved in Solar installations and maintenance. A typical residential installations will have 16, 6 volt deep cycle lead acid batteries. Normally when preforming service to the battery bank and when evaluating it current condition I break the bank down into the individual batteries.
I then have a number of preliminary step I go through that finally ends in actually testing the battery's capacity.
I am aware of two methods of doing this. Load testing the individual batteries ( 3x Ah rating for 15 seconds)or performing a capacity test based on the reserve capacity (390 Minutes @ 25 amps @ 80º F).
Is one method of testing better or worse? Does one method provide better more accurate results? Is one or the other method less harmful to the battery?
From my experience load testing is quick and seems to work but if the other method has clear advantages I could easily build a tester that would provide the required load and record the time until critical voltage was reached.
An input on this subject will be greatly appreciated.
Thanks!

 

[Zogzog]

I think impedance testing would be easier, safer, and just as accurate.

 

[TKC100]

Impedance testing is an interesting idea but I do not have the resources to purchase such a tester. From what I know they start in the four digit range and that's before the decimal point.
What I am really needing assistance with is the actual nature of a lead acid battery.
I first encountered load testing a battery in a previous life in the automotive field. There it was routine to load test a battery using a carbon pile to 2 times its AH rating and monitor the voltage.
A good battery would not drop below 9.6 volts. (12 volt battery)
I am now being lead to believe that this is not a real test of capacity and not a good test for deep cycle batteries. It does give a fair idea of the CCA and it is also a good way to find a shorted cell but does not really relate to the RC.
In other words a battery could pass a load test and sill have a diminished capacity.
The only way I have of really testing the battery's capacity is to place a 25 amp load on it a monitor how long it takes for the voltage to drop to 10.5 volts. (12 volt battery)
I can compare this figure with the manufactures published data or hopefully with a similar test when the battery was put in service.
This is a time consuming and labor intensive task but for now the only method I have.
My question to any of you with a back ground in the engineering aspect of a lead acid battery are the statements made correct?
Thanks!

 

[oldfieldguy]

Absent the impedance tester, you can use resistive load banks to load the battery up to a specific level and monitor for the time it takes the string voltage to decay to an unacceptable amount. the best way to do this is to monitor each cell and stop the test when ONE drops to a minimum value. 1.75 or 1.85 volts is a commonly used value. This isn't too hard to do with a low voltage bank with a few cells, but for a 125-volt bank and sixty cells, it's a bit of a stretch. Companies that specialize in this work have means to continuously monitor the multitude of cells.

I and my technicians BUILT a load bank using commonly available resistive heating elements (our neighbor in the industrial park was an HVAC contractor. we got used elements for free)and manual switching. A shunt and a DC ammeter provided means to monitor current. It worked quite well for rough tests required by some clients, but if you want accurate results, you really need a good means of monitoring cell voltages.

After you've got a time and current figure, you can compare them to the battery manufacturer's data. If yours is less than 80% of nominal, you can consider your batteries at end of life.

old field guy

 

[Zogzog]

Think of the time you would save doing impedance testing, if this is something you do alot of it will pay for itself in no time.

 

[DTR2011]

You can rent an impedance test set for a few days for a few hundred dollars.