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Old 12-02-2009, 13:19   #1
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How to Test a Gel Cell?

The dink battery was reading 9 volts, so off it came to be introduced to my favorite charger (Iota). After it spent the night getting pumped up, I disconnected the charger and let it sit for 2 days.

After the 2-day sit period, the battery reads 12.35 volts.

How do I tell if I've brought it back to life? It's not as if I can stick a hygrometer in there or anything....

Would like to know if it's good - without waiting to be left stranded of course....
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Old 12-02-2009, 13:30   #2
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Originally Posted by markpj23 View Post
The dink battery was reading 9 volts, so off it came to be introduced to my favorite charger (Iota). After it spent the night getting pumped up, I disconnected the charger and let it sit for 2 days.

After the 2-day sit period, the battery reads 12.35 volts.

How do I tell if I've brought it back to life? It's not as if I can stick a hygrometer in there or anything....

Would like to know if it's good - without waiting to be left stranded of course....
You will need to do a "Load Test" on it.

Do a Google for "Battery load test". Basically, you fully charge the battery then put a load on it for a determined amount of time, let it rest, then check the voltage. It's not rocket science. Just simple math.

If you have an inverter on board, it's pretty easy to put a good load on it.

I don't know what size battery or charger you have but it may take a few days to fully charge a good sized gel-cell with a 15A charger.
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Old 12-02-2009, 14:31   #3
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most car repair shops will have a battery load tester. Simply bring it over to them and ask them to test it.
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Old 12-02-2009, 14:51   #4
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Auto repair shops are used to checking a battery with a 500A load tester. They are interested in checking the battery for "Cranking amps".

If it were me, I would put a max 100 amp load on a deep cycle gel-cell and just keep the load on for a longer period of time. Again, it is important to know the capacity of your battery but I don't think that many gel-cells will hold up on a 500A load test.

Gel-cells don't break down like wet cell batteries do. If you let a wet battery get to 9V the chances of it recovering are slim. Gel-cells will recover easily in most cases.
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Old 12-02-2009, 15:47   #5
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Only practical way to test its capacity is to fully charge it (should read 12.7 volts or more after resting overnight without load), then put a known load on it and measure voltage decay over time.

Most marine batteries are rated for 20-hours, so it's best to put a load on it which, theoretically, would pull the voltage down to 10.5 or so (fully depleted) in 20 hours. Example: if you have a gelled battery rated at 20AH, put a 1A load on it (about 12 watts) and let it go for awhile. You don't have to go all the way to 10.5V to get a good idea of its capacity.

Another way, not terrific for deep cycle batteries but not bad either, is to borrow one of those expensive testers which measure internal resistance and give you a readout in CA, CCA.

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Old 20-02-2009, 22:44   #6
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As a follow up to btrayfors: the test device he is referring to is a Midtronics conductance tester. These have been out for several years and are superior to the waffle iron type load tester. Using high frequency, these testers pulse the battery and then compute the actual available amp-hour capacity. This is compared to the listed capacity and a recommendation is provided. Slick tool with about 50 patents.
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Old 21-02-2009, 07:55   #7
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The Practical Test....

After another 2 days on the Iota the at-rest voltage was 12.8 so I decided to check it out under load. It held up very well cranking the dink outboard as I tried to sort out its issues.

So I guess I've gotten lucky on this one.... until it leaves me stranded ashore of course. Then those oars will come in handy

Thanks to all who replied.

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Old 21-02-2009, 12:46   #8
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Practical tests are always the best.
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