Simple way to measure coach battery capacity

[Gerald55]

I'm looking for a reasonably simple way to measure the current capacity of the single coach battery in my newly purchased (used) RV.

I have a basic voltmeter which I don't think will be much help, but I do also have a kill-a-watt - so I figure maybe I just charge up the battery, then turn everything off except say the TV which I plug in through the kill-a-watt and measure it that way? Any downfalls to that approach?

[MEXICOWANDERER]

Good, solid, home-brew testing. The only variable of course is battery voltage which means wattage through the light. But for comparative testing New/Now it is priceless.

[SCVJeff]

red31 wrote:
I've always got a chuckle out of this method, knowing the ever changing current of the inverter, light and clock is the issue!

analog AC clocks are extremely useful for things like this and simply looking to see how long the power was out at home. Any idea where this one came from?

[RoyB]

When we are camping off the power grid I just watch my homemade three meter BATTERY MONITOR panel (Two 12VDC Meters and one 75 DCAMP meter). My meter panel looks similar to this drawing for my Battery Monitor Panel


When the battery bank DC voltmeters drop down to the 12.0VDC level I can assume this is pretty close to being 50% charge state. This is not absolute but close enough for me. I will either start my battery charging routine or disconnecting the batteries from the system.

In my case I have planned all of this out and I usually see the 12.0VDC 50% charge state at 8AM each morning when I would normally charge up my batteries to their 90% charge state using smart mode charging techniques...


With my 255AH capacity battery bank we can run 22AMPS DC current from 6PM to 11PM including the parasitic drain and make it through the one day/night run off the batteries and just be at the 50% charge state at 8AM the next morning. We run all the 120VAC items we want to run from a PSW INVERTER and the 12VDC items direct connected to the battery system.

Then recharge the batteries during breakfast the next morning starting at 8AM in a three hour generator run time when allowed which will make us good for the next day/night run off the batteries.

This is how we do our camping off the power grid runs...

Roy Ken

[doughere]

If you just want to know what the battery SHOULD provide, just look up the size and look at any rating, you'll be close.

Your method of testing would work fine.

I don't have a Kill-a-watt, but I have a Voltmeter and Ampmeter. I am in the process of testing my 2 GC's, using an inverter and a slow cooker to load at about 11 Amps. Checking and recording voltage and current draw every few hours, will give me a good approximation. They're 8 years old, and a few years ago they tested at 80%. We're getting ready for a couple of months on the road, and rather than trust to luck, figured I ought to test them.

Doug

[red31]

I've always got a chuckle out of this method, knowing the ever changing current of the inverter, light and clock is the issue!

[SoundGuy]

Why not just look up the rating for your particular battery? :h Yes, there will be some small differences from brand to brand but within a given group size a battery's rated capacity will be similar to that of another brand in the same group size.