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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #1  
Old 11-08-2009
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Battery Load Tester

I was wondering if anyone uses a battery load tester when conducting routine battery maintenance checks or when troubleshooting electrical problems (to rule out a bad battery). I understand the importance of checking the specific gravity of a lead acid battery, and checking the open circuit voltage for all types. Yet, everything I read says that a load test is the best method for checking that a battery is good. Plus you can't check the specific gravity on gels or AGMs. Is a battery load tester a good purchase or is a multimeter and a manual load test (using the engine starter motor or some other device) the best way?

If a load tester is a good option, any recommendations on a quality one? Thanks.

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Old 11-08-2009
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Many auto parts stores will have a load tester available for rental or loan. A proper load tester is a far better idea than trying to load test using the starter or some other makeshift device, as the results are going to be far more consistent.
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The "industry standard" for testing battery capacity these days is a sophisticated -- and pricey -- computerized device which measures internal resistance. The Micronics series are very popular, but they cost about $600 for a medium-range one.

I have used these for extensive battery tests over a couple of years, and they're pretty good if you use them right. But, they're not 100% accurate.

The best way to load-test a battery is to put a constant load on it calculated to be sufficient to deplete it fully in 20 hours, based on it's 20-hour rating. For example, for a 225AH battery bank such as two Trojan T-105's in series, you would use a load of just over 11 amps.

As well, it's good to put a little energy-monitoring device like a Watts Up in-line, to actually measure energy delivered (in watt-hours). That way, you can see how closely the battery's actual capacity compares to it's rated capacity.

Everything else is a "next-best".

Bill
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K.I.S.S. in the real world of cruising, some of these concerns are valid but of really not that critical. It is obvious by simple age (years) the basic status of your batteries. And then doing a simple "resting battery voltage" test will reveal if the batteries have reached the end of their lives. You fully charge the battery overnight and then disconnect all loads from the battery - take off a battery terminal cable and measure the voltage with a digital meter. Repeat the reading every hour for 5 hours or more and graph the results. The resting voltage will decrease from the charger voltage when disconnected down to a sort of steady voltage. Look at the resting battery voltage chart and you can get a good idea of the "life" left in the battery.
- - It is not "totally accurate" but accurate enough to tell you when it is time to get new batteries. The other half of the simple test is your batteries go flat when used - in half the time or less than they used to last.
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Would that it were that simple. It isn't. Sorry, but resting voltage bears only incidental relationship to battery capacity.

Resting voltage indicates STATE OF CHARGE, not BATTERY CAPACITY. It's perfectly possible to have a battery "near the end of it's life" which shows a resting voltage of, say, 12.6-12.7 VDC, but which has had it's CAPACITY greatly reduced by....
  • sulfation of the plates
  • stratification
  • corrosion
  • contamination
  • mistreatment

Even a "new" off-the-shelf battery can have a greatly reduced capacity due to sulfation caused by leaving it too long under-charged. The voltages will look fine, but a true load test will reveal it's reduced capacity.

By the way, on a customer's boat just this afternoon I found a "new" 6V golf-cart battery which had a greatly reduced capacity. Age had nothing to do with it.

Bill

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I bought a load tester from harbor freight for under $20. Seems to work fine, gets HOT. Similar to one at an APstore for $100. I'm sure ths $100 one will last longer but this one is fine for occasional use.
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Yep, cheap load testers such as this one: - Harbor Freight Tools - Quality Tools at the Lowest Prices

...can give some indication of battery capacity, but require interpretation, since deep-cycle batteries aren't designed to put out large amperages over long periods.

This tester puts a 50A load on the batteries. A similar one for $25 puts a 100-amp load on the batteries.

But, as indicated above, the only way to properly test a deep-cycle battery's 20-hour rating is with a load calculated to draw the battery down to 10.5 volts (depleted) in a 20-hour period.

You can build an inexpensive load tester, though. I built one consisting of eight switchable light sockets in which I put 12VDC bulbs of varying wattages. I can switch in those needed to put the proper load on a battery (bank) under test. Then, it's just a matter of waiting :-) And, of course, counting....I put a WattsUp meter in line to do the measurements for me.

Bill

Last edited by btrayfors; 11-09-2009 at 10:04 PM.
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btrayfors - you are missing what I am saying. It is a fine and dandy to discuss the esoterics of sulfation of the plates; stratification; corrosion; contamination; mistreatment and all the other stuff while you are securely located in the USA or other major first world country. Good information - but for us out here anchored off little island countries with a battery salesman with a bone through his nose and a top-knot hairdo, all that stuff doesn't do much real-life good. The equipment does not exist to do esoteric tests on batteries.
- - So my post has to do with what is done in the "real world" outside of the techno-maniac first world. We have only our digital voltmeters and the knowledge that our batteries just are not up to what they used to be. So we use "simple" common sense ways of evaluating when it is time to replace the batteries. And it is just that simple - batteries don't hold a charge, resting voltages collapse too soon - we buy new T-105's or truck batteries from the local witch doctor. And if you happen to have a live chicken handy the price gets real low.
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Osirissail, you obviously haven't ever read btrayfor's posts before. If you had, you would know that he is probably about to tell you how to do that fancy load test using a gum wrapper, a bottle top, and the ass end of 50 honey ants. And it'll work too!
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FWIW...I use one all the time...its one of my favorite tools.
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