how long will a cranking (auto) battery last : LUSENET : TimeBomb 2000 (Y2000) Preparation Forum : One Thread

Everybody says "use marine or deep cycle batteries" and I agree that's good advice. But in an emergency, many people won't have them or be able to find them and so they will be using auto batteries. Any ideas how long before an auto battery lasts before it dies when used to power lights?

-- robert waldrop (, August 02, 1999


It depends partly on how old the batteries are, and how deeply you discharge them. They could last a year or two if they are only partly discharged (like 20% of their capacity used) but it would be hard to tell how much you are discharging them without some instrumentation or testing. Almost 30 years ago we used a 12 volt car battery for lighting on weekends when we built our first house, before we got our big batteries and our wind generator, and it held up find all summer, but that's my only experience using an automotive battery that long. Seems like we ended up putting it back in a car at the end of the summer, but it was only used after dark on a couple of nights a week. I do know a few people who have used a spare battery in their car to run lights in the house, etc. The battery is set up so whenever the car runs, it charges both batteries, but the second one is isolated from the auto's electical system with a diode so the car can't use it. A cord connects this battery to the house electrical system and is unplugged when the car is driven. A battery might last a year in this kind of use. Hope that this helps.

-- Jim (, August 03, 1999.

Thanks for this info. Could I tell how much a battery had been discharged with one of those electrical testers sold at auto parts stores? (The package says, "checks volts, ohms, amps, decibels, and batteries.) Or is something else called for?

-- robert waldrop (, August 03, 1999.


Jim is correct in saying the Bat will last, but remember you have to Charge it after using 20% of the charge. The following table is from Electric Sun Engineering:

Battery Open Circut Volts Charge Level

12.78 100%

12.66 90%

12.53 80%

12.81 50%

11.91 30%

11.52 0%

Conpensation factor -30mV/degree C ref 25C

Use a volt meter, Radio Shack or what ever (not battery powered) and measure the voltage.

You may want to look at the info at some of the Solar Power web sites: use Yahoo and type in Solar+Battery or other combinations.

Good Luck

-- helium (, August 03, 1999.

The proper tool for checking how much a battery is charged is a hydrometer. It looks like a big eyedropper or a baster and has balls in it that float or sink depending on the density of the electrolyte. You can get one at any auto parts store. Sometimes you can get a smaller pocketsized one at a motorcycle shop. Store it in a ziplock bag to keep it clean.

To some extent you can judge the condition of a battery by the voltage while there is a load on it, but with no load the voltage can be ok even though the battery is not fully charged.

-- biker (, August 03, 1999.

however, a hydrometer only works on batteries that have removable caps so you can get at the electrolyte. Many newer car (but not motorcycle!) batteries are sealed.

-- biker (, August 03, 1999.

Thanks for the good tips.

Is there some kind of charge monitor that is sold to do this? (I suppose I should search the Sunelco or Jade Mountain catalogs.)

Also, I picked up two magazines today, both of which have a lot of good info on dc power systems: September 1999 issue of Popular Communications, and the August 1999 issue of Cruising World, which is running a 4 part series entitled "Got Juice?"

-- robert waldrop (, August 04, 1999.

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