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Old 01-19-2013, 11:54 AM   #1
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Join Date: Apr 2011
Location: Southwest Florida
Posts: 748
Figuring Battery Amp Hours

Please check my numbers and thought process and let me know if I'm okay or need to re-figure.

Is the industry's 20-Hour discharge rate for rating batteries figured at a constant 5 amps?
If so, 5 amps x 20 Hours = 100 amp hours ... To me, that means you can draw 5 amps for 10 hours and be at 50% depth-of-discharge.

My Interstate SRM-24 battery is listed as 76 amp hours on the manufacturer's website using the 20-hour rate. Using that same formula, I could draw 5 amps for 15.2 hours, or about 7 1/2 hours to 50% DOD. Considering all the variables in an electrical system, is that about right?

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Old 01-19-2013, 01:37 PM   #2
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Join Date: Sep 2011
Location: King George
Posts: 2,124
The way I did it just to be sure it was not going to put me in the dark was to install a good 12VDC monitor voltmeter.

Then when the DC VOLTMETER read around 12.0VDC this tells me I am very near the 50% charge state of the batteries and I need to start recharging my batteries...

Using my three standard Issue 12VDC Interstate Batteries in parallel i run around 20AMPS draw from 8PM to 11PM watching HDTV and running some other items from a 600WATT PSW Inverter and direct connections to the 12VDC batteries. Then after 11PM I go into my night mode and may draw around 1-2AMPS until the next morning at which time I want to see around 12.0VDC reading on my DC VOLTMETER PANEL. This is when I go into my battery re-charge using my trailer shore power cable directly connected to the 2KW Honda Generator which will allow the on-board trailer converter/charger unit to get my batteries back up to to their 90% charge state in as little as three hours. This is using smart mode technologhy charging from my converter/charger unit.

I built up one of the BATTERY MONITOR PANELS to read my two battery banks and also read how much current I am drawing from the battery bank. A BOGART TRIMETERIC TM-2025RV Battery Monitoring system will do good as well


Roy Ken
Roy and Carolyn
I claim Horse Creek Country in Southern Ill - Momabear is from North Texas
We live in King George VA

"We're burning daylight" - John Wayne
2008 STARCRAFT 14RT OFF-ROAD POPUP with PD9260C and three 85AH 12VDC batteries
2010 F150 FX4 5.4 GAS with 3.73 gears - Super Cab - Towing Package - 2KW Honda EU2000i Gen
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