Just how good are those used, sealed lead-acid batteries? [RE-wrenches]

[Mick Abraham]

Hi, Bob~

For real world battery testing, I suggest actual amp-hour measurement 
during discharge. The results from this approach are neither instant nor 
stress free to the battery, but they are reliable.

I built a gizmo that I affectionately call the "Battery Hammer". My rig 
uses a Tri-Star 60 controller with digital meter option, but a Trace 
C-series controller with DVM would also work. The integrated meter 
avoids the need for a separate amp-hour meter with shunt. 

If you configure the controller as a "low voltage disconnect", it will 
automatically terminate the discharge for you. The meter will tell you 
how many amp-hours the battery string delivered before reaching the low 
voltage cutoff point. Add a couple of breakers, cable, and a suitable 
load. The "suitable load" is the tricky part...I could post further on 
that if list members are interested.

With a rig like this, you can "set it & forget it", then come back later 
to see the results. However, I often spot check the voltage for the 
weakest segment in the string. If one segment is low by .5 volts when 
the string is at rest, that "delta V" typically becomes much wider when 
you impose a load. If the voltage drops precipitously on one segment of 
the string, the LVD switch stops the discharge while there is still 
usable energy in the other segments.  

(If you observe this problem, contact me off list about a battery 
balancer which can prop up the weak segments of the string by robbing 
energy from the stronger ones, thereby enabling higher amp-hour delivery 
before the LVD point is reached. You could replicate my own experiments 
with the balancer and post your findings to the List.)

The "Battery Hammer" has proven useful to me and to my clients. With all 
the used C-40 controllers that are in circulation, a similiar test rig 
could be built for relatively low cost. Considering all the times that 
I've looked at a battery bank and wondered about its capacity, it's 
great to now have a tool that can answer that question in genuine 
amp-hours.

Mick Abraham
Abraham Solar Equipment
Pagosa Springs, CO
www.abrahamsolar.com

Bob Clark wrote:

how do I know how "good" a used, sealed battery is?...how do I determine 
its condition (and capacity to hold a charge)? 


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