Cheap Reliable Battery Load Testing

I have an AGM 8D 250AH battery that I had to take out of the RV because it would run perfectly for a while doing low amp RV work (furnace etc) but then would collapse under higher amp inverter loads.

Apparently I must have damaged that battery doing high amp tests to find out whether my new inverter's low voltage alarm worked properly. The inverter worked properly, but after that, the AGM started to act up. Based on my reports here on just how it acted, Mex thought maybe it had warped plates.

I could not tell anything was wrong with that battery until it started to collapse one time while camping. I did a capacity check by running a load at its 20 hr rate for 10 hours to try for 50% SOC, and it only did 6 hours and voltage indicated it was at 50% SOC already.

So what I had was a damaged battery that would no doubt fail the "load test" like in this thread, but it was still a usable 150AH battery for low amp loads (60% of 250). So I still have it, but not in the RV.

This may be an odd case where the capacity loss is not from sulphation, but is from damage.

It now sits in the stick-house garage as part of a UPS set-up to keep the internet going if we have a power failure like when a tree falls on line in a storm. Had a power failure like that a few weeks ago that lasted for several hours till Hydro got it fixed, and the UPS worked great. So that broken AGM is still useful for something!

So I do think there is a difference between a broken battery that can be revealed by a "load test" and a good battery that just does not have much capacity anymore. The load test might well show that the good battery has lower capacity than it ought to have, but perhaps not show just how much usable it still has. That is what the capacity test does. But you must decide when the capacity remaining is not enough anymore for the job it was doing.

OTOH, if it also has low resting voltage or shows a bad cell by hydrometer, that would mean it is totally broken and you can't use it anymore.

MEXICOWANDERER wrote:
I also have 5 digit volt meters and 3 separate ammeters. Two inductive including a Fluke and one shunt type which uses a 1000 ampere shunt 100 mv Manganin type traceable to NIST. The HF is no more inaccurate than an Auto Meter, Sun VAT 40 or Snap On AVR I have used. My unit reads just under .2 volts high at 9.6 VDC. My bench DMM needs decertification again. Last time it has drifted seven ten thousandths of a volt low.
For hand grenade testing it is satisfactory to me. It was a $5,500 purchase in lieu of cash payment.
The validity of my AGM Load testing was approved by Concorde. I do not care to acquire the technique to test lithium batteries.

You are extremely smart when it comes to electronics. What bothers me about you is your "cliff" take on your posts. How many RV'ers have or even know about Concorde AIRCRAFT Batteries. Not me, I had to look them up. Then I find Concorde also distributes LifeLine. Here is what Lifeline states about testing THEIR batteries. Doug


"How do I know when it is time to replace my battery?
Perform a capacity test. Short duration load tests and impedance/conductance measurements are not reliable to determine the actual capacity of a battery".

I also have 5 digit volt meters and 3 separate ammeters. Two inductive including a Fluke and one shunt type which uses a 1000 ampere shunt 100 mv Manganin type traceable to NIST. The HF is no more inaccurate than an Auto Meter, Sun VAT 40 or Snap On AVR I have used. My unit reads just under .2 volts high at 9.6 VDC. My bench DMM needs decertification again. Last time it has drifted seven ten thousandths of a volt low.
For hand grenade testing it is satisfactory to me. It was a $5,500 purchase in lieu of cash payment.
The validity of my AGM Load testing was approved by Concorde. I do not care to acquire the technique to test lithium batteries.

I know you load tested your AGM's. BUT, I am very suspicious when you have a ultra cheap HF product and the Manual states NOTHING about AGM batteries. BTW, I have that tester and have never had occasion to take it out of the box. Maybe some day. Doug

I had a friend who loved to spout E=MC2 theory about uncomplicated problems in his anchored 65' motor yacht. After enduring yet another rave about why his widely spaced parallel connected battery bank was misperforming, I connected my AUTOMETER 1000 amp carbon pile load tester to the end bussbar and cranked it up to 400 amps.

"Yahhhhhh! SMOKE!" he yelled.
As he raced past me, fire extinguisher in hand I yelled after him. "And there, is your problem"

For the next year or so before I moved away, I heard not one more peep about ground loops, and charger peak inverse voltages.

So, it's Take Your Pick
Discover a weak battery today and then allow time to deal with it comfortably

Or do Breakdown Maintenance at 200% the cost.

Traveler .. Where did you get the idea of 3x the AHR capacity,
I did not see that mentioned
A 1000 cold cranking amp start battery, would be tested at 500 amp load, the biggest load my batteries see in use is the MW, with a few lights and the TV, Around 96 amps,
For testing I turn the load knob all the way up, what I want to see is the voltage stay in range then recover when load is removed, since you have to turn the knob , it is Not instant 500amp load, and you can adjust to to a preferred amp load if you want, the tester can handle 500 amps for 15 seconds, that doesn't mean my batteries or yours will supply 500amps into a carbon pile resistive load, that's a load , not a dead short like welding, or a surge like cranking over the engine, starter wiring has very little resistance so it's a high current spike operation , I think the highest amps I have observed while using the tester on the house batteries is 300+, which is over 3x my usual max draw
As stated this is a stress test ,
Watch the voltage and the amps, and the recovery or lack of recovery

Imo whatever it's worth,
for a start battery it has to maintain enough voltage to keep the starter cranking , I don't know what that is
And for house batteries
Not drop below minimum voltage for inverter use
Inverter runtime testing has been done by many members ,
Seems like BLF does a lot of this, and reports on it frequently, the rest of us don't have to do it, we just read his posts,
This tester gives us a way test batteries before install, and a way to test monitor degradation as they age, and trouble shoot problems when they arise

Thanks. Fingers agonizing today

Good to know - I found this 12/10/12 post.

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/26578587/gotomsg/26579192.cfm#26579192

I have posted how to load test GC220 batteries at least 10 times over 15 years on this forum.

I could do it another 20 times and it will still be ignored or requested "for funzies" It's discouraging.