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Old 01-07-2008, 12:36 AM   #1 (permalink)
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How Low Can You(r Battery) Go?

Originally Posted by MetroMPG View Post
That said, running a vehicle off its battery alone is a pretty extreme mod. Yes it will save fuel. It will also destroy a regular starting battery in short order, since they're not designed to be deeply & repeatedly dis/charged.

This is a mod I often use in summer months. But I've got a supply of old deep cycle batteries which I can ruin at will.
This has come up a couple times, and looks like it might pop over here soon too.

However, I don't remember seeing hard numbers/curves on cycle life vs. depth of discharge for starter batteries (oem, gel-cel, etc.).

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Old 01-07-2008, 06:41 PM   #2 (permalink)
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I haven't seen many hard numbers either, mainly because the numbers vary so wildly.

Cycle live/ D.O.D. is going to depend a lot on battery design, and even more on the quality of battery maintenance that you do. For that reason I believe a flooded lead battery would be the ultimate champ (or loser depending again on the maintenance) of a cycle live/DOD shootout.

In essence, if you're using a normal starting battery for running your car w/o an alternator, you are going to destroy the battery in a short amount of time.

You (probably) won't be monitoring the batteries in real time with a voltmeter, making sure you never get below 10.5V, thereby damaging it.
You (probably) won't be, or won't be able to, carefully monitor the SOC by checking the electrolytes' specific gravity, thereby further damaging it.
You (probably) won't charge your battery with nearly as good a charger as most EVers use, thereby damaging it more.

It's kinda like looking for hard numbers on what exact gas mileage you should expect to get from your car. Sure you have the EPA numbers, but the driver will be the single largest determining factor.
"I got 350 heads on a 305 engine. I get 10 miles to the gallon. I ain't got no good intentions." - The Drive By Truckers.

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Old 01-07-2008, 10:15 PM   #3 (permalink)
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Here are some ballpark figures about deep cycling conventional starting batts: 30-150 cycles.

Golf cart (flooded lead acid) batteries are much more robust (fewer and thicker plates):

For example, golf cart batteries will average 225 cycles at 80% DoD and increase to 750 cycles at 50% DoD. - source
I don't have any gel figures, but Google is your friend!

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