# GC2, 50% or 80% discharge level?



## JohnP (Sep 1, 2010)

How low do you discharge your golf cart batteries? I've read, never bring them down to less than 80% and I've also read bringing them down to 50% is ok.


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## TnAndy (Sep 15, 2005)

Batteries are rated for the number of cycles at a certain point of discharge. (Depth of Discharge-DOD)

The REAL no-no is close to zero.

If you look at a battery's rating, you'll find a certain number of cycles at 50%DOD.....say it's 500.

Then look at 20%DOD....you'll find often it doubles or more.....say 1200.

And IF the manufacturer does an 80%DOD rating, you may find that is 200, or less.....purely examples, not based on any one battery.

From that, you can see that the lower discharge, the less number of times the battery will take it.....so IDEALLY, you want to discharge them as little as possible. In practicality, that's a little easier said than done.

So, when setting up whatever your means of charging the batteries, you want to aim for no more than 75-80%, ideally.

IF you're using a variable system, like solar PV, or wind, you ALSO have to consider not making the bank *TOO* big.....or you will be wasting a lot of your potential energy just keep the batteries up to snuff. 

There is a formula ( I forget it right off ) for sizing a battery bank to the amount of PV watts you have, and if you exceed the bank size too much, you decrease the efficiency of the system due to excessive need to keeping batteries topped off.


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## 12vman (Feb 17, 2004)

I would work in the 20-25% DOD myself. It seems to work well for me. 7 yrs. on a set of GC batteries that still load test near 100% tells me the story.

I always figure 4 times the aHr. produced by the array in a day as for battery sizing and work within this parameter in a 24hr. period. (25% of the aHr. rating of the battery)

Hope this helps..


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## idahodave (Jan 20, 2005)

I never go below 50% and usually stay above 25%. 

I set the amp-hours parameter on my TriMetric to 50% of the battery bank. That way the % readout reads 0 when the bank is 50% discharged. Makes it easier for my wife to check.


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## JohnP (Sep 1, 2010)

How does DOD (depth of discharge) relate to SOC (state of charge)?

In otherwords, how low of a voltage can I go and be at the 75-80% DOD.


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## idahodave (Jan 20, 2005)

The battery voltage is not a good indication of charge. It varies with the load so a higher load will drop the voltage more than a small one for the same SOC. 

No load measurements are OK, but the battery needs to rest (no charge or discharge) before the voltage can be used to measure SOC.

See http://www.arttec.net/Solar_Mower/4_Electrical/Battery Charging.pdf
for voltage measurements under load.


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## trkarl (Dec 15, 2009)

Your best bet to get a true soc reading is to get a good quality temperature compensating hydrometer and learn how to use it. Then take readings at various dod's and compare to the at rest voltage. This way you will get used to how your batteries perform under load and at what resting voltage etc. will equal an approximation of the state of charge.

This would work http://www.solar-electric.com/brtecoprhy.html


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## JohnP (Sep 1, 2010)

Yes, I bought a cheapo from the parts store but will be getting a good one as well as a meter for the C40 which will give accumulative amp hours that can be reset to zero. So far the batts remain at 78 degrees temp and the specific gravity remains at 1.330, the max reading on the hydrometer.


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