How do I measure capacity used?

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yodasnaps
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How do I measure capacity used?

I have purchased a new 12v, 86ah deep-cycle for my RV coach battery. I understand that the battery works best if it is "cycled", or discharged to about 20% capacity and then charged again. I have a volt meter, but I think voltage can show normal when battery capacity is running out. Do I have to load the battery to test capacity?

I think I may have "cooked" the last battery, so I am trying to figure out how to extend the life of this new battery.

Any help is appreciated.

reikiman
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Re: How do I measure capacity used?

Lead-acid will show lower and lower voltages as they're discharged. The voltage under load will read lower than the unloaded voltage, and also after you remove the load the voltage will recover some. However the fully charged voltage starts out at 13.2 and falls over the discharge of the battery.

Just because a lead-acid battery is marked "deep cycle" doesn't mean it's a good idea to deeply discharge the battery. A deep discharge isn't terribly good for the health of lead-acid batteries, it's just that deep cycle batteries are more able to tolerate the deep discharge.

On the other hand it does help to load the battery to test it. I've got a couple ways of doing this:-

One is a gizmo like this: http://www.7gen.com/product/battery-load-tester/23639-actron-cp7612-battery-load-tester It's a big honkin heating element, a pair of heavy duty battery clamps, and a volt/ammeter .. hook it up, flip the switch, the heating element starts heating, and you can compare what the battery does unloaded versus loaded. These gizmos are generally for 12v lead-acid batteries.

I also have a big honkin rheostat which I use when the battery pack is not 12v. I connect it using a Watts Up meter that shows voltage and amps and lets me see behavior at different rheostat settings.

yodasnaps
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Re: How do I measure capacity used?

I guess because the battery I just replaced would show about 13 volts and then quickly drain, I confused capacity with ampere hours. I hear you loud and clear that the voltage will drop as capacity (charge) drops. I think I should recharge at 50% capacity and cycle it regularly to get the best life out of the battery.

I may look into one of the load testers, or just turn on some lights with the volt meter connected.

Thanks for your input.

Love my Zappy
and eGo2 SE scooters
Cheers!
Yodasnaps

dogman
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Re: How do I measure capacity used?

If I understand you correctly, er, no.

Your battery will be happiest if kept charged but not overcharged, and never really used. Well that's not too usefull, so the best compromize is to keep it charged, and when using it, never get below 80% discharged, or if you really want it to last, some say 50%. If you ever ran the old battery till a 12 volt bulb wouldn't light at all, that was very bad. Also just as bad, is letting it sit any length of time while discharged. And lastly, some chargers kill lead acid batteries by overcooking them. If the RV recharges the battery off the alternator, it may be roasting the battery every mile you drive. Have a mechanic you trust check that. If you just charge with a charger, get a good one, not just a super cheap one from the discount store.

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yodasnaps
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Re: How do I measure capacity used?

Very good info too Dogman. I'm getting the hang of it. I will check the voltage regularly and charge as needed (80% but not lower than 50%). I have a 2amp/6amp bench charger that I need to monitor or it will cook the battery. The meter on it never reads 100% charged. Maybe a need a better charger.

I'll research the alternator output and read up on the RV inverter. I imagine that the inverter is a charger too. If so is it charging the battery all night long when we are plugged in? Hmmm...

I have some more learning to do, but I love this forum!

Love my Zappy
and eGo2 SE scooters
Cheers!
Yodasnaps

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