Battery voltage under load

What is normal for the voltage drop of a battery under load? I have a 100Ah SLA battery in my camper trailer and if it is sitting at, say, 12.8V when the 40L Engel switches on, the voltage will drop to 12.1 or 12.2. Is this normal or is the battery on the way out?
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Reply By: Allan B (Member, SunCoast) - Saturday, Jan 04, 2014 at 19:30

Saturday, Jan 04, 2014 at 19:30
Without getting into maths and battery specifications, the Engel will draw about 4 Amps and a voltage drop of 0.6v sounds a little high if measured at the battery terminals.
If however, you are measuring closer to the fridge, then the cable resistance comes into play and 0.6v drop may be quite normal.
Cheers
Allan

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Reply By: Member - Phil G (SA) - Saturday, Jan 04, 2014 at 20:15

Saturday, Jan 04, 2014 at 20:15
Where are you measuring the voltage?
If you are using a multimeter at the pos and neg of the battery, then yes it is pretty low - I'd expect it to be around 12.5ish while the engel is cycling.

If you are measuring anywhere else, voltage drop in the wiring etc will be affecting the result.
AnswerID: 523880

Follow Up By: Member - Phil G (SA) - Saturday, Jan 04, 2014 at 20:19

Saturday, Jan 04, 2014 at 20:19
I hadn't see Allan's post at the time I posted, so I apologise for repeating his good info. Good to know we said the same thing though!
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Follow Up By: Allan B (Member, SunCoast) - Saturday, Jan 04, 2014 at 20:43

Saturday, Jan 04, 2014 at 20:43
Is orrite Phil, I'm a slow typist and sometimes several other posts are published whilst I'm tapping on the keys with one finger. Then I look a right idiot! --- Again!!
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Allan

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Reply By: The Bantam - Sunday, Jan 05, 2014 at 00:11

Sunday, Jan 05, 2014 at 00:11
If you are at all concerned about the condition of your battery, you need to test it properly under known conditions and with a known load.

When you have the time, take it out of the vehicle and get it fully charged with a known good charger.

Check the terminal voltage immediately you take it off charge and over the next 24 hours.

If you do not have a "proper battery tester".
Improvise yourself a load...one or two 100 watt driving lights and a reasonably accurate clock, used with your multimeter will surfice.

This will allow you do do a number of tests.

You should be able to get a full spec sheet for your battery off the manufacturers web site.

AND as others have mentioned, you are bleep in the wind if you are not measuring directly at the battery terminals.

As another matter...you can use your multimeter to check the voltage drops in your wiring.

ALL too often, wire used is too light.

cheers
AnswerID: 523898

Follow Up By: Sigmund - Saturday, Jan 11, 2014 at 21:34

Saturday, Jan 11, 2014 at 21:34
Laboriously I did a load test like this with my two 100 amh Hour CT batteries, following instructions on the ABR Sidewinder site. Running down to 50% SoC showed 70% of orig. capacity. In practice with our normal load (other things equal) that meant no more than 24 hours of power compared with the new batteries' 5-6 days.

Well, we'd had nearly 5 years out of those units (20 weeks in the bush) so nothing to complain about.

Except the cost of replacing them.
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Reply By: Yabbo - Sunday, Jan 05, 2014 at 08:34

Sunday, Jan 05, 2014 at 08:34
Thanks for your responses. Yes, I am measuring the voltage at the battery. I will follow Bantam's advice and do some load testing to compare with the manufacturers data sheet.
Cheers
Yabbo
AnswerID: 523907

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