Batteries again

[geronimo]

There are many electric wizards on this forum, please help.

I have just come back from my boat.

The 2 domestic batteries (2x100Ah) were charged a day earlier, they were under a small load overnight, I switched off everything and got 11.9 Volts, around 12V 15 minutes later.

Switched on the charger which was charging 12.5 V to start with and after 20min got up to 13.2 V, around 7-8 Amps of current (seemed like pulsating on the Amp meter).

And while charging I could see bubbles forming in each cell.

What do you think of these batteries.

While charging with the alternator 14.4 V comes to the starting battery, 12.9 V to the 2 domestic ones. At the exit of the diode both are 14.4 (Multi battery isolator, Sure Power Industries). Where I am losing this voltage?

I am grateful for your comments and help!

 

[Malcb]

Do the batteries need topping up?

 

[geronimo]

Not with water

And the amps going in are low so it does not accept more (does it mean it is fully charged)

 

[Oldhand]

12V is very low if they have only been lightly discharged after a full charging cycle. Also the charging current sounds very low for batteries whose voltage indicates they are well discharged. The gasing while charging should only be very slight, if it's more than that, along with the other symptoms it sounds like your batteries have had it.

Also the relatively low voltage and current out of the charger is worrying. If it was just the batteries at fault, unless cells are short circuit, I would expect a much higher voltage by the time charge current dropped to 7 -8 Amps. However, you don't state the capacity of your charger. Could it be your charger is well under capacity and hasn't been charging your batteries properly and thus they in turn have become sick?

 

[DaveS]

To (sort of) answer the easy question first: if you've 14.4V at the output of your diode splitter and 11.9V at the battery you have an excessive voltage drop which is being caused by excessive resistance somewhere. Poking around with a voltmeter should narrow it down, e.g. as a first step take the voltage between diode output and battery positive, then across individual cables and joints, but I would recommend first just quickly feeling all the joints (that aren't too close to moving engine bits!) - one might well be warm! (1.5V X 8A means that 12W is being dissipated somewhere.)

Regarding the batteries, I would be tempted to disconnect them, then connect them one at a time to the charger, and see whether it will accept a complete charge, and what rest voltage it assumes afterwards. Armed with that information, things might start to become a bit clearer.