http://www.trojanbattery.com/Tech-Support/BatteryMaintenance/Testing.aspx

This site has some good information for battery use and testing. The 10.5 volt cut out is usually to protect devices such as inverters. To use a voltmeter to determine a battieries state of charge, the measurement needs to be done open circuit, or "0" load. A battery is 80% dischrged at ~ 11.66 volts open circuit. Now, with regards to usable A/H capacity, here are some considerations.

Using float or alternator charging, it is not likely to ever get a battery to 100% state of charge, (SOC) unless you can float it for a relatively long itme. This is why deep cycle batteries really need inteligent 2 or 3 stage chargers.

Once the charging stops, the top 10%, even with no load will dissapate in a few hours. This is known as the surface charge. It will then stabilize and remain at 85-90% SOC for many days or weeks, but the top 10% goes quickly.

Total A/H capacity is effected by current draw, thus different ratings for different discharge rates. A battery has resistance and there is also a recovery effect. As the battery is discharged, the resistance increases which causes a voltage drop. E=IR The chemical reactions are also time based so at a higher rate of either charge or discharge, the chemical resistance gets in the way. This is why you really want to charge a battery at an 8 to 10 hour rate. Yes, the alternator can charge smaller batteries too fast. 20 A/H charge into a 60 A/H battery is too much for long battery life.

A battery should always be charged after use, even at 50%, sulphation will start to form quickly reducing the batteries capacity.

Hopefully, some of this will help you.

Dennis

----- Original Message -----
From: Loren Busch <starwagen@GMAIL.COM>
Date: Tuesday, June 20, 2006 11:43 am
Subject: Understanding Amp Hours

> The answer to another question here on the list raised this
> question.  Given
> that we have the amp hour rating for a battery and also given that
> we don't
> want to draw it down below about 10.5 volts, about 80%.  Does that
> amp hour
> rating assume draw down to the 80% level, or draw down to 'dead',
> zero?I've always assumed that if I have a 50 amp hour battery that
> I can draw 1
> amp for 50 hours before reaching the point that will require
> charging, or
> will I only get 20% of that, 1 amp for 10 hours, before reaching
> the point
> where I need to recharge?
>