# Thread: Measuring the real amount of AH available?

1. Registered Guest
Join Date
Aug 2009
Posts
18

## Measuring the real amount of AH available?

I'm confused yet again about amp hours this time.

I have 2 batteries, each with 100AH on them and at full charge. I've connected them in parallel to keep the 12V needed for my Protwatt Xantrex 1000w inverter. Upon testing the duration these batteries can last I put a 1/2 Amp load on the inverter using about 60 watts of power. My theory was that it could last 400 hours with that load. After only 15 hours my inverter shut off due it not having enough voltage to stay on.

What's going on here? I figure it could be any of the following:
a) DC Amp Hours is nothing even close to AC amp hours. 200 DC AH = 15 AC AH?
b) my batteries were bought used, maybe they don't hold a charge very well, not sure how to test that but I'd like to.
c) Amp Hour ratings on batteries are not measured above 11volt charge since that's when my inverter shuts off. Instead, the AH rating is probably measured through the entire life of the battery from full charge all the way down to 0v.

For calculation purposes and forcasting energy usages and such, how can I determine for certain how many AH i have available before my inverter shuts off. I'm considering buying more batteries but I need to know what the calculation is before buying!

2. ## Re: Measuring the real amount of AH available?

You're on the right track.

You left out: inverter uses power itself, and some of the power is lost in the wiring as well.

Different batteries have different recommended maximum Depth Of Discharge. In a true deep cycle, this may be as high as 80%, meaning you could expect to pull 80 Amp/hrs from a 100 Amp/hr battery. The rule of thumb around here is to not exceed 50%, and 25% is better. You're correct that as the Amp/hrs get used up the Voltage goes down. Most 12V inverters shut off at 10.5V.

So how much power do you really have with a 100 Amp/hr true deep cycle 12 Volt battery?
That's a very good question, and unfortunately there's not one answer because various brands/models of batteries will perform differently.

To get a handle on your particular batteries you need to observe their performance with a battery monitor such as the Trimetric: http://store.solar-electric.com/tm2020.html or Pentametric: http://store.solar-electric.com/pe3chbamosy.html

Once one of those is programmed with your info and the batteries are fully charged it will keep track of how many Amp/hrs are used. When the inverter shuts down you have your number.

Used batteries inevitably will not be up to manufacturer's specs as they start to decline the moment they're put into use. And are they deep cycle type? Ordinary automotive batteries may have a "100 Amp/hr" rating, but they're not designed to supply a steady current draw over a long time; they will fail sooner than a deep cycle or "hybrid" type.

3. Solar Panda
Join Date
Sep 2009
Posts
175

## Re: Measuring the real amount of AH available?

Always calculate with watts and watt hours. That means anything with amp needs to be multiplied by the voltage that goes with the amps. Doing this will simplify things and significantly reduce confusion and errors.

Lead acid batteries have about 20 - 25 watt hours per pound but only half of that is usable for planning purposes as you don't want to discharge any lead acid battery below 50% as a routine thing. (i.e. the "deep cycle" bit is misleading and misused except as a general indicator of battery type).

The 50% max use means your battery resting voltage shouldn't get below 12.0v (12v system) and not much below that with moderate loads. This guideline is to maximize battery life.

The 20 hour AH rating for these sorts of batteries means that it was calculated with about a 60 watt load (5A 12V). Higher loads will reduce available energy and lower loads increase it (see Peukert). Two such batteries in a bank would have the reference load for the 20 hour rate at 120 watts.

12v 100 AH means 1200 watt hours per battery or 2.4 kwH in the 2 battery bank. That means you have about 1.2 kwH usable. That should power a 60 watt load 1,200/60 or about 20 hours.

Many things change the available battery bank energy capacity by 10% or more. These include such things cycle to cycle variances, age, actual load and use profile, and temperature. That means the precision of any capacity measure is limited.

4. ## Re: Measuring the real amount of AH available?

Hey flight,

Allow me to chime in. I dont think the other realise your error here. You stated that you put a 1/2Amp load on the inverter. This could only mean a .5a load at 120v and not 12 volts so there is no way that would run for 400 hours. You are more likely discharging 5amp (not considering associate losses and efficiency of inveter) from the batteries. So if you are able to run for 15 hrs that would be 75Amp hours from the two batteries in parallel that should be ideally rated at 200Amp hr. So considering that they are used that is to shabby. If you go to 400hr that would have been 5A * 400hrs which is 2000Ah which is the factor of 10 that you are off by.

Cheers...
Damani

5. ## Re: Measuring the real amount of AH available?

I have 2 batteries, each with 100AH on them and at full charge. I've connected them in parallel to keep the 12V needed for my Protwatt Xantrex 1000w inverter. Upon testing the duration these batteries can last I put a 1/2 Amp load on the inverter using about 60 watts of power. My theory was that it could last 400 hours with that load. After only 15 hours my inverter shut off due it not having enough voltage to stay on.
going from 12 volts to 120 volts makes it a little easier to convert amps. Not including inverter losses, 1 amp at 120 volts converts to 10 amps at 12 volts or 10 times more which is the same amount of watts. 1amp X 120 volts = 120 watts or 10 amps X 12 volts = 120 watts. You said you used a 1/2 amp load at 120 volts which would draw 5 amps from your 12 volt battery. Multiply that by 80% efficiency of your inverter, it would draw more like 5 /.80 percent or 6.25 amps. Your 200 amp hours of batteries should never be drawn down less than about 50 % which give you 100 amp hours of usable electricity. 100 amp hours / 6.25 amps should give you about 16 hours of time with a 60 watt light bulb. I suspect your batteries may not be up to snuff as you probably depleted the battery 80% to get it down to 11 volts where the inverter shut down in only 15 hours.
IHTH.

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