Is my cab battery toast ?

[Figaro]

Last week my cab battery failed to start the motorhome, this was after it had a charge on a 3 stage charger (directly on the terminals) a few days earlier.
I then put it back on charge overnight and the following day was showing 12.8v resting.
After four days it's now dropped to 12.3v with no load or solar input.
I've not tried starting the engine yet.

The battery is under guarantee but involves a cost to return it for testing by the retailer Tanya.
So my question is does this drop off in voltage look like a failed battery ?

I don't want to pay the return charge to have them tell me it's not faulty.

TIA

 

[Lenny HB]

Possibly bin fodder, it shouldn't drop that fast but will depend if there is any load on it.

Have you checked to see what load is on the battery as it could be going down due to current draw.

It's dropping about 25% assuming a 90ah battery that would be about 5ah a day that's a draw of 200ma,.
Alarms and trackers don't normally draw that much but Pioneer head units draw a lot more.

 

[fishplug]

Unless you have something causing a parasitic drain on the battery then I'd agree with Lenny HB

That's more of a drop than your alarm/immobiliser etc would be depleting it by so unless you've recently added something that might explain an increased draw then more likely than not you've got a duff battery.

If you have a clamp meter that measures DC A then you could test if there's anything much being drawn from the battery with the ignition off just to be sure.

 

[autorouter]

Any garage will have a battery tester that can check your starter battery if you think it might have a problem. The battery will have a 'CCA' specification, which gives the maximum Cold Cranking Amps it can supply to a starter motor for a defined number of seconds. The tester puts a typical starter battery load on the battery and measures its voltage drop and internal resistance. The result is a percentage state of health.

These testers are a bad idea for leisure batteries, which are not designed for sudden high amps loads like a starter motor. But they do the job for starter batteries.

 

[Guigsy]

A tired battery might charge to a nice healthy voltage. But as soon as you put any load on it, it voltage sags. And it will have a significantly lower capacity, so even mild parasitic loads will flatten it in a few days.

You can get battery testers that tell you the Cold Cranking Amps for as little as £20.

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