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Old 06-26-2019, 06:24 AM   #1
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Proper way to test batteries

I purchased a battery analyzer (for the record a Foxwell BT705). Load test, starting,charging. I have 8D's, smart charger, combiner, inverter, and hooked up to shore power. Having an offline discussion with another TF'r about the correct way to test batteries. Do I need to rest the batteries prior to testing? Turn off all power, dead boat so to speak? Simply pull the boat off shore power ...disconnect the batteries totally? Thanks for any comments!
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Old 06-26-2019, 06:49 AM   #2
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I assume you are talking about house batteries. In that case the packaged battery testers won't tell you what you need to know. You need to know their amp hour delivery capacity and campare that against the battery's rating.

The only real way to do that is to remove all charging sources and hook up a load (lights, etc.) equal to 1/20 the rated capacity. Do this by hooking up a clamp on DC ammeter and adding lights until you hit the 1/20 value. Then wait and check the voltage at 10,12,14,16,18 and 20 hours and see when it drops it about 10V which is considered dead.

The real amphour capacity is the number of hours times the amps and maybe reduced as the amperage dies off at the end.

Group 8Ds are really not true deep cycle batteries and often don't do well on this test. Also you often don't even have an amphour rating to test against. Most 8Ds are rated at about 200 amp hour capacity though.

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Old 06-26-2019, 09:00 AM   #3
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Bingo. This is how you test the batteries deep cycle capacity. The off the shelf tester is for testing cranking amps. You would need to run a test like this when the batteries were new to set a baseline. Large constant resistors are the best, but they are not cheap and generate a lot of heat and so need to be safely mounted outside. i have a set for a much smaller battery 34 ah, and the amount of heat to deal with is impressive.
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Old 06-26-2019, 09:21 AM   #4
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My method - fully charge them then disconnect them for 24 hours then use a hygrometer and compare all the cells to the manufacturer’s chart.
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Old 06-26-2019, 09:40 AM   #5
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I’ve always charged the batteries to 100% - can’t get there, they are bad.

Turn off charger during the tests.

Charge to 100% After a couple of days with no significant load are they still above 90%. Then OK.

Charge to 100%. Run normal 12 volt appliances for a couple of days as if you were at anchor. Do the batteries still retain 75%, can you live with that? If yes your good for your purposes.

I’m no electrician and no expert on anything except paying off ex-wives so don’t be critical of a layman’s method.
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Old 06-26-2019, 10:31 AM   #6
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My method - fully charge them then disconnect them for 24 hours then use a hygrometer and compare all the cells to the manufacturer’s chart.

Or reference voltage after a 24 hour rest...% of charge will give you a good reference as to how much life has been used up and how much remains before they won't hold a relable charge for any period of time..

For instance, if your wet cells register 12.2 (50%) you can only count on 50% of the original amp/hour rating.

Those of us who live on 12V with no generator, watch this closely (Victron amp/hour gauge etc). Don't want the beer getting warm after a day on the hook..

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Old 06-26-2019, 04:58 PM   #7
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I hate electrical anything. Every time I go to learn something I find out I have to learn 3 other things before I can learn what I went to learn. Did I mention I hate electrical anything?

So, the battery tester I bought to test my batteries, my cranking, and my charging, will only test my starting battery. It won't even tell me the slightest thing good bad or indifferent on my house batteries. I do have a hydrometer and will use that on the house. The issue bringing all of this up was a dead starting battery a week or so ago which I don't understand. I think I mistakenly left the charger off when I hooked up shore power the last time coming in. Then I didn't use the boat for 4-6 weeks. Tried to start up and nothing, dead. Even without the charger being on the starting battery should have been fine. I have a marine electrician coming to look and to do a survey/audit for me because I don't understand the system. But the system is another story. Thanks for everyone's comments. Just surprising to learn that a battery tester won't test a battery. At least not house batteries.
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Old 06-26-2019, 05:25 PM   #8
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Greetings,
So there is no simple way to assess general battery condition without hiring an electrical engineer? What the heck are these things for then?


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Old 06-26-2019, 06:17 PM   #9
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That thing is great for testing the condition of the starting battery. Nearly useless for testing the capacity of the house battery. I say nearly useless because diminished AH capacity will also typically diminish cranking capacity.
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Old 06-26-2019, 06:42 PM   #10
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Old 06-26-2019, 07:02 PM   #11
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Back to my original question, yes, I do want to test my starting battery. It's not an 8D. Somewhat smaller. I will have to look at it but think possibly a 4D.

So, I should charge it up to full and then disconnect/turn off shore power and let the boat sit for 24 hours. Is that correct? Then do the battery, cranking and charging tests? Will also use the hydrometer on all of them.
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Old 06-26-2019, 07:45 PM   #12
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A starting battery left uncharged for 4-6 weeks should not be close to dead.
Either the battery is on its last life (internal short) or you have a drain.
Another reason to purchase a DC clamp-on ammeter.
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Old 06-26-2019, 08:12 PM   #13
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FB

I would start there, make sure your battery will take a full charge and then keep it for a day without discharging.
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Old 06-26-2019, 08:16 PM   #14
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Greetings,
So there is no simple way to assess general battery condition without hiring an electrical engineer? What the heck are these things for then?


I have one of those,cheap and ? nasty, bought on Ebay. But it does a reasonable job, used it only yesterday on the car battery(a 760CCA AGM) which at 7yrs old needed a new one. The load test is complete when you smell smoke being emitted, but you can use Lucas Replacement Smoke to recharge it.
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Old 06-26-2019, 08:33 PM   #15
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Firstbase,

For a start battery there is no need to let it sit for awhile before testing. It does need to be disconnected from shorepower though. The purpose of letting it sit is to remove any surface charge on the battery which is low current. In your case, you are trying to get a lot of current out of the battery in a short period of time (engine start) and still have sufficient voltage. The load tester will do that.

If it passes the load test but goes dead after several weeks then either the battery has an internal short (i.e. bad battery) or you have mystery load on the battery that is discharging it. The quickest way to isolate that issue is either ammeter in series (or clamp around) looking for any current flow when not trying to start the engine, or disconnect the battery and see if it still goes dead. If not then you have a mystery load that you will need to troubleshoot.

All the other test techniques are more intended for house batteries where you are not looking for a large "dump" of current but instead for a much smaller current draw over time.
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Old 06-26-2019, 08:40 PM   #16
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A starting battery left uncharged for 4-6 weeks should not be close to dead.
Either the battery is on its last life (internal short) or you have a drain.
Another reason to purchase a DC clamp-on ammeter.
Yep, have a clamp meter, DC, onboard as well.
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Old 06-26-2019, 09:08 PM   #17
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I have one of those,cheap and ? nasty, bought on Ebay. But it does a reasonable job, used it only yesterday on the car battery(a 760CCA AGM) which at 7yrs old needed a new one. The load test is complete when you smell smoke being emitted, but you can use Lucas Replacement Smoke to recharge it.


That’s complete nonsense. Lucas always outfitted with auto smoke recharge capability.
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Old 06-26-2019, 09:12 PM   #18
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Old 06-26-2019, 09:55 PM   #19
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Old 06-26-2019, 11:12 PM   #20
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To be fair,he seems to be brandishing a hydrometer, not a hygrometer as recommended in post 4. Provided it`s an open cell battery he`s in with a chance of getting a useful reading.
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