Load Test of Deep Cycle storage battery

[deandec]

Is the load test done at an Auto Parts Store valid on a deep cycle storage battery such as a Golf Cart battery?

[ARFFMAN]

If test is done with a load tester designed for car batteries, Then, YES is the answer

[wa8yxm]

If anything it is superior when done on DEEP CYCLE, since a DEEP CYCLE is not designed for the high load of a starting battery... IF it passes a full blown (3x C/20) Load test... It is very very good.

[Gary RVRoamer]

It depends somewhat on the tester (person and device). A load tester is calibrated for a certain battery capacity, so its important to use the right parameters to get a valid test. Simple load testers aren't adjustable (that's ok for the general run of passenger car cranking batteries, but the better units require that the load be set to match the size of the battery being tested. A big Group 31 deep cycle might be able to pass the test for a typical Group 24 size starting battery and still be on its last legs. However, in the vast majority of cases the load test will give valid results.

Think of it this way: a "fail" is always a "fail" but an "OK" still leaves a tiny amount of doubt. Not enough to negate the value of the test, though.

[TQ60]

Any and all tests to batteries that can be completed in less than 1 minute simply cannot indicate a good battery.

They can indicate a bad battery.

If the battery does not fail it is not really known to be good...it is only indicating it is not bad.

Confused?

A storage battery of say 80 amp hours is rated for 10 amps over 8 hours and can output hundreds of amps for seconds.

If it is age degradaded and is only at 50% capacity then it may only support 10 amps for 3 or 4 hours but still may output hundreds of amps for the few seconds of a load test.

Do not get me started with impeadance testing...

In short the short testing quickly finds bad batteries but to know one is good it needs a controlled load for measured time.

Like running the inverter with good load for certin amount of hours as benchmark then repeating same test later.

Search battery testing for much good reading.

[YC1]

Hi Dean, TQ60 is dead on. I have load tested many batteries and still had to replace them because they would die in a few hours with a relatively light load on them. Yes, even some that passed the hydrometer test. You just can't get physically into a battery to see broken plates, sediment, or weak connections.


One does not buy batteries. We only rent them for a time. Most batteries in RV's are assonated and are not given a chance to die of natural causes. Severe discharging, lack of maintenance, all combine to destroy good batteries.

[BFlinn181]

A load test would be only one factor in measuring the health of a battery. Voltage after charging and time for absorption of surface charge and specific gravity readings in each cell, all should be used to evaluate the battery.

[deandec]

Thanks to all.

So the auto store test can tell me if the deep cycle battery is "toast" but not that it has a "future" in its assigned application.

To determine its ability to perform, one could put a measured load on the deep cycle battery and see how many hours the battery lasts with that consistent load?

That would require a lot of patience on my part!

[TQ60]

Yes that is how it is done...

A battery is like a glass to contain water.

Draw a line about 1 inch from the top and fill to there.

That is a full NEW battery.

Pour that into a cup to see the volume and now you have capacity.

Drill a hole in the bottom and return the water and the time it takes until empty is the hourly rate.

Good so far?

Now pour out the water and 1/2 fill it with gravel and plug the hole and notice you can still fill it to the full mark but now it takes less time to drain.

The gravel represents degradadion of the battery.

The water level to full mark can be either end point voltage or sg as both indicate state of charge but the old worn out battery just cannot hold as much.

And if you tip the glass fast you still get a splash of water either way snd that is the seconds long load test.

[TQ60]

Benchmarking.

If you know the capacity or have the rate sheet and you can apply a controlled load you can perform shorter testing than hours and get fair results.

If you do not have an amp meter you still can do well.

Locate something that you can run on your inverter that will stay running for hours that has a resistive load.

Some floodlights make good loads.

Multiply the watts by say 1.2 to add in some overhead for the inverter and it does NOT need to be exact or close but do bocument it as repeat testing needs to use same setup.

Fully charge the batteries for a few days on float.

From rate sheet look at the capacity of your batteries at the load you are going to use.

Amp hours is time times load so determine how long yiu wish to test then estimate your use.

The voltage will drop in close to a linear curve so guess what the voltage should be based on the amount of volts between charged 12.5 and dead of 10.5 and turn them on.

Monitor the voltage and if it stays high good and drops fast not so much.

Done with fresh charged batteries and timed to say 11.5 volts then that is benchmark for later testing.

Repeat test yearly and you will see battery performance decrease every year a bit.

These tests are only falid for trending observation and if and when a battery fails it will be obvious.

To measure full capacity requires full discharge.

[YC1]

How old are the batteries in question Dean. If they are more than a couple of years old just replace them. I have seen 1 month old batteries fail. If your batteries are not giving you the service they once did and your charging system is working then toss the batteries. I know how meticulous you are so do not assume you have any dirty grounds or battery posts.

[deandec]

Myron, thank you for the info and concern.

My golf cart batteries are about 1/2 year old and no problem thus far.

The problem was, I think I just posted a recommendation on a forum somewhere to have the deep cycle batteries load tested without thinking about the concept.

Then I had second thoughts about using a starting battery load tester like the auto parts store has to test deep cycle batteries. So I posted this topic.

You folks have educated me with your responses as usual.

I seem to need a lot of that help......

[TQ60]

Dean you are a smart man who asks questions before acting.

Stupid questions are a lot easier than stupid actions...

[TQ60]

Harbor freight has a 500 amp capable load that does a real good job of stressing batteries.

One should open it up and make sure the ground wire is not touching the case...one of my guys welded one to a platform while testing a positive ground plant...hf swapped it no problem even with the large black hole...

They have an adustable load so they work well on small to large batteries and if a stopwatch and digital voltmeter is used you can use them to quickly benchmark your batteries.

When you have more than one identical units all should test same as usually only one fails first.