batteries......very confused

[tsekul]

Hi There,

I am hoping for some advice as I am very confused !!!!

bringing our boat back to Falmouth after wintering in penzance. I noticed the the vhf indicating a low battery level. We put the engine on for a hour or so to recharge the batteries. Turned it off and an hour or so later the low battery
indicator came on again. We turned off all electrics except the radio and made it home.
so I thought I would get the multimeter out and check the voltage of the house battery . It read -0.12 . I thought I must be doing something wrong so tested the cranking battery and it read 12.67.

I then turn on the engine and checked again it still read -0.12 .

So now I am really confused, surely there should be a charge going in from the alternator ?

Also I thought a fully discharged battery read around the 11 volt not -0.12 !!!!

so I take the battery out and take it home to charge and see what happens. unfortunately the trickle charger only kicks in at
6 volts.

I take it the battery is dead ?

any advice is much appreciated

Thanks

 

[BarneysDad]

By no means an expert but a freshly charged but dud battery can show 12.6 volts on the multimeter. Once a slight load is placed on it for an hour, say, eg a vhf transmitter, it quickly discharges and all output potential is lost. Therefore, a dud battery will not hold its charge. This happened to me last year. Any battery, new or old requires charging every 2 or 3 weeks. A battery left uncharged for months can be ruined. Solar panels/wind turbines can help keep a trickle charge going into the battery providing that they have a big enough output -- & sunlight/wind. Try giving it a full charge on your charger (not just trickle). But your battery sounds severely damaged to me.

 

[GrahamM376]

Sounds like it's totally dead, as you say.

 

[TradewindSailor]

Even a dead battery would give 8V + unless it has been shorted out.

I would first check the multimeter against a good battery. Perhaps the multimeter's batteries are flat or there is a bad connection.

Always check across the poles of the battery, otherwise you could be measuring through a bad connection that would cause a voltage drop.

Most of the time the fault is with dirty connections.

Is it possible that you need to switch the battery selector over to charge this battery .... or does it charge through a VSR?

 

[tsekul]

That's why I cant figure it out.
I went straight to the cranking and got 12.67 then straight back and got -0.12. disconnected it and tried it again making sure I had good contacts same again.

I am thinking of sticking it back on the boat starting the engine and switching the isolator to" no2" and not" both" and seeing if I can get 6v then take it back home and use the trickle charger (as it only kicks in when there is a reading of 6v)

My only worry is I have read you may damage the alternator when switching the isolator when the engine is actually running , so I will have to check the exact wiring of the house and cracking ?

 

[tsekul]

As a side note I usually leave the selector on "both" while the engine is running and only switch to "no2" (house) once I have shut the engine off.

 

[VicS]

That's why I cant figure it out.
I went straight to the cranking and got 12.67 then straight back and got -0.12. disconnected it and tried it again making sure I had good contacts same again.

I am thinking of sticking it back on the boat starting the engine and switching the isolator to" no2" and not" both" and seeing if I can get 6v then take it back home and use the trickle charger (as it only kicks in when there is a reading of 6v)

My only worry is I have read you may damage the alternator when switching the isolator when the engine is actually running , so I will have to check the exact wiring of the house and cracking ?

You should normally be able to safely start on the starter battery, switch to both and then to the house battery alone without any damage to the alternator.

Dont attempt to go via the Off position.

Personally I'd be very cautious about switching to just the house battery in case there is a wiring fault, a dodgy switch or an open circuit battery resulting in all the load being removed from the alternator.

At -0.12 volts I dont reckon its worth the bother of doing anything with the battery.

 

[William_H]

Clearly the alternator has not been charging the house battery. This needs to be sorted out. However if the battery is that low then I would suggest using a light bulb ina connection engine battery to house battery positives. To just connect them together would eman a large inrush of current so best done via a resistor or light bulb.
The battery is not necessarily dead but will have suffered some from the deep discharge. On the other hand the low voltage may indicate totally dead. olewill

 

[tsekul]

Thanks for all the replies.

Here is my plan
Use jump leads on the battery from my cars battery whilst it is running and see if I can get some charge into the battery.
I have a spare new battery which I will fit to the boat and monitor it's level and check if it is a slow loss of capacity over a week or so.

how does that sound ?

 

[CAPTAIN FANTASTIC]

It is most likely that the battery has been badly damaged by now and even if you manage to charge it up, it will never hold more than say 50% charge, still useful for domestic battery but you need to be prepared.

 

[ghostlymoron]

Just make sure that the new battery is getting charged - you don't want to wreck another battery. Some kind of automatic charge system would avoid this.

Thanks for all the replies.

Here is my plan
Use jump leads on the battery from my cars battery whilst it is running and see if I can get some charge into the battery.
I have a spare new battery which I will fit to the boat and monitor it's level and check if it is a slow loss of capacity over a week or so.

how does that sound ?

 

[tsekul]

Thanks,
I have been studying my Nigel Calder, /manual and shall be doing some tests !

The battery was installed in 2011, I'm not sure how long they are supposed to last, if looked after. We only got the boat last year . This boating malarkey has a steep learning curve that's for sure !

 

[Tintin]

Thanks,
I have been studying my Nigel Calder, /manual and shall be doing some tests !

The battery was installed in 2011, I'm not sure how long they are supposed to last, if looked after. We only got the boat last year . This boating malarkey has a steep learning curve that's for sure !

If buying another, check this out (no connection) - at £60 and a 3 yr warranty it is hard to go wrong.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=291117769204