Identifying a dud battery

[JumbleDuck]

Final - probably - update.

With a continuous load of 3A the aft battery declined slowly from 12.8V to 12.5V, then precipitously - when I caught it it was at 10.4V and falling at around 0.01V/s. After the same time (about an hour) and the same load the forward battery is steady at 12.5V.

My conclusion: the aft battery is down to 10Ah capacity, or thereabouts, and was taking less charge because it was effectively almost full. I have disconnected it and we'll cope with the other until the cavalry (or, this being Mull, the Calvary) arrive.

Many thanks for all the help and suggestions. Au revoir.

 

[AntarcticPilot]

Thanks. Perhaps it's just demonic possession.

Having had a nice walk on Ulva and come back to find the forward battery fully charged, according to the charger, I have done a test. Each battery in turn was given full responsibility for everything, then asked to heat the glow plugs for 20s and start the engine.

Not much difference.. Both happily supplied the 20A needed, falling to 12.66V (aft) and 12.62V (forward) as they did so. I have left forward battery disconnected and am using the aft to charge my tablet and a phone with the gallery fluorescent tube and instruments on. Solar charger running, because this is to find out if using that battery is a viable proposition.

Oddly, the multimeter tells me that the battery is at 12.8V, both at its terminals and at the PV regulator, but the regulator remote display says 12.5V. Some attention needed there too. The regulator is brand new from Photonic Universe.

I think we've got the same controller, and I have noticed that mine also displays a slightly lower voltage than the independent panel-mounted voltmeter. Perhaps the calibration is a little off? It's in the region of 1% difference (around 0.1-0.2 V). Given that these are fairly inexpensive units, I doubt they are individually calibrated.

 

[Boater Sam]

You scrap batteries are worth £10 each in a scrap yard.