My Battery Load Test Results - Advice, Please ?

[Poozer]

I finished building my load tester and ran my first test today. The load tester places a 21 amp load on the battery and shuts itself off when the terminal voltage drops below 10.6v. It records the maximum amps, minimum volts and total Ah consumed during the test.

I expected the batteries had degraded, but the the results are quite surprising, so I was hoping to get some help on interpreting what's going on here.

Test battery is a Toyama Hybrid Gel 250Ah capacity.

I fully charged the house bank (4 x 250Ah batteries) at 13.7v for a few days. I then disconnected one of the batteries from the bank and charger and ran the load test.

Here are the readings:

Terminal voltage after battery was disconnected for 30 minutes with no load applied = 13.72v

The 21.3 Amp load was turned on. After 15 minutes I measured the terminal voltage under load of 11.92v

The test ended when the battery voltage reached 10.58v.

*Amp hours consumed during test = 13.8Ah*

This was so low that I'm wondering if I did something wrong. Any thoughts? Or is this just a clear case of a faulty battery?

Thanks all.

[goboatingnow]

Looks to me to be a faulty battery. Have you tried the others.

Dave

[Poozer]

Not yet, I'll test the next battery tomorrow.

[motion30]

13.7 seems like a low charge voltage

[Poozer]

Originally Posted by motion30

13.7 seems like a low charge voltage

It charges at 14.4, then floats around 13.7. You think I might have a charger problem?

[djmarchand]

I see two problems with your test. Deep cycle amphours are typically rated at a discharge rate of 1/20 of capacity. You put almost double that load on it.

Secondly your cut off voltage is too high. I'll bet if you let the battery rest for 24 hours the voltage will go back up to 12 or better. With a load of 21 amps you will draw down the voltage significantly.

Batteries are dead when the RESTING (for 24 hours) voltage is down to 10.5.

All of this doesn't mean that your battery isn't bad but your test won't tell you that.

David

[btrayfors]

NO. Those charger voltages seem fine.

Battery load testing is usually done at the 20-hour rate, i.e., the load is selected so that it approximates the AH rating of the battery divided by 20. For your 250AH battery, that would be 250/20 = 12.5 amps.

So, the 21 amp load is more like a 12-hour load for your battery.

Still, the 13.8AH total delivered to the load is unacceptably low. It would appear that your battery is just about at the end of its useful life.

Note once again, as I've said many times, battery voltage is an indication of state of charge (SOC). It says practically nothing about the capacity of the battery to store and deliver energy.

Sounds like after charging you had a fully-charged 15 to 20AH battery (at the 20-hour rate) :-)

For comparison, here's a chart showing three banks of batteries under a 20-hour load test (1 flooded bank and 2 gell banks).



Bill

[btrayfors]

David posted while I was composing. I mostly agree with what he said. Still, the voltage draw down by the 21amp load notwithstanding, I would think your battery is greatly diminished in capacity.

In the graph above, look particularly at the precipitous voltage dropoff after the battery reaches about 11 volts under load. Not much more useful energy to deliver after that.

Bill

[Poozer]

Thanks for the data Bill, I'll redo the test at ~12 amps load and see how it goes.