# Rechargeable 26AmpHour Battery Performance.

Discussion in 'General Electronics Chat' started by foolios, Jun 21, 2014.

1. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
I recently bought a Duracell 600 power pack to use with a solar panel for some outdoor use.
I want to test that the battery is in good condition as some reviews have noted the battery they received isn't holding a full charge.

I used some battery calculators and they are telling me that I should get around 30 hours of use running my laptop on it. My laptop generally pulls about ~80 watts as shown by a killawatt meter that it's plugged into.

I am wondering if I am using the wrong calculations. I assumed that I should be using 12v as that's where the power is coming from but I wasn't considering that the laptop is plugged into the inverter on the power pack via an AC cable.

So now I'm wondering if I'm suppose to somehow do a conversion from AC values to DC values before using the battery calculator.

I was thinking that it shouldn't matter what voltage as long as I know the actual amps the killawatt is displaying.

So, what I figured out was:
Using formula 26Ah/0.75Amps(to include 25% inefficiency/0.6amps*.25=.75amps) gives amount of hours = 34hrs

Another version I tried was:
Take the loads watts and divide by 120volts to get amps required.
Divide the battery's Ah by the amps required to get amount of hours the battery can sustain that load minus any inefficiencies and scaling, as higher current draw and conversions will reduce duration.
Watts/volts=amps then Ah/amps=hours
80watts/120volts=0.67amps : 26Ah/0.67amps=38hours

According to the calculator at http://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html
the battery should last 21 hours at a constant rate of .6amps drawn

But my battery was fully charged and lasted 2 1/2 hours before the 20% alarm went off. So, I thought maybe I was suppose to use 120v in the calculations and that give me more of a ~3 hour runtime.

Any help in determining whether the battery seems to be ok or not would be appreciated.

Last edited: Jun 22, 2014
2. ### inwo Well-Known Member

Nov 7, 2013
2,435
315
Seems like the battery is fine.

Draw will be 10 times more at 12v than at 120v. .67 X 10 = 6.7

It would be best to actually measure the current when running on 12vdc.

I didn't see mention of how you have it connected.

If it's 80 watts = 6.7amps = 3.9 hrs expected.

I check a lot of batteries and over 1/2 is good.

I believe the rated/stated AH is under ideal conditions running battery to near zero.
Or maybe most just lie!

foolios likes this.
3. ### foolios Thread Starter Active Member

Feb 4, 2009
160
1
I have the laptop plugged into a killawatt meter and then the killawatt is plugged into the AC port(the pack has a built in inverter) on the battery power pack.
The killawatt is reading 80 watts and .67 amps.

So take 80watts/12volts = 6.67amps * .25 ~7.5amps
26Ah/7.5amps ~3.5hours - 20%(0.7hours) = 2.8 hours of runtime

I got about 2 1/2 hours out of the battery so I guess it's pretty close to being at full performance.

Thanks for the response.

Last edited: Jun 22, 2014
4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Not sure if the Killawatt does very good averaging on pulsing waveforms. It does ok on sinusoids.

5. ### wayneh Expert

Sep 9, 2010
12,405
3,256
Don't forget that the inverter is not 100% efficient, meaning your battery is powering the laptop and the inverter losses, so it's even better than measured.

6. ### BobTPH Active Member

Jun 5, 2013
807
121
Also, the Amp Hour rating of batteries is usually for operations at C/20. When drawing more current than that, you will not get the same Amp Hours.

Bob

7. ### wayneh Expert

Sep 9, 2010
12,405
3,256
On my Mac, I can get real-time current monitoring and I can even tell which apps are using the most power.

I would assume there are similar utilities for a Windows PC, if that's what the OP is using.