Battery voltage / amp. readings

[boy boater]

I would be grateful if someone could explain to me what the readings on the voltage meter and ammeter should be for my domestic battery when the engine isn't running, i.e. no charge from the alternator. Should the voltmeter show a steady 12v until it is fully discharged, or does the voltage drop off gradually? About a week ago I turned on some cabin lights, they were quite dim and the voltage meter showed 10v. At the same time the ammeter needle was flicking from side to side (+ / - 25 amps about). I put the battery on charge, it charged at about 25 amps initially, dropping to about 3 amps after about 5 hours when I had to disconnect the charger. The voltage shown on the voltage meter was then 12v. I have not been to the boat since to see if the battery has held its charge. As a final observation, there would appear to be a slight discharge 0.5 amp from the battery (as seen from the ammeter) when no domestic circuit is switched on, e.g. fridge, lights, etc. Is this in order, please?

 

[sarabande]

some questions first which might help.

Are the meters separate or a combined one, and where do they take their readings from ? Are the meters digital or analogue ? (I suspect analogue from your readings)

Are you on pontoon mains to run the charger ?

When you leave the boat, do you consciously switch everything off, or is there anything ( e.g. alarm, auto bilge pump) which is tapped off before the domestic circuit ON switch.

What size is/are the batteries and how old ?

0.5 amp discharge will bring a normal battery (that's 12 amp/hrs each day) to a danger level in less than a week.

 

[Medskipper]

If after charging the battery it only gets to 12v. its time to replace it! A good battery should charge up close to 12.6v or more = fully charged. 12.4v = only 75% charge is held. 12.2v it is only holding a 50% charge. 12v reading means you only have a 25% charge. 11.7v or less less = no charge at all!

Barry

 

[hlb]

You should be seeing about 14 volts in your battery, 12 volts is actually getting quite low. Get a hydrometer for a couple of quid and test your batteries, it wont answer every thing but a start.

 

[markdj]

It sounds like you are reading the voltage from the engine instrument gauge and these are notoriously inaccurate.

First off, get a voltmeter or battery monitor and measure your voltage at the battery terminals. You will find that your engine is charging at 13.8 volts (just a guess).

It also sounds like you only have one battery onboard - is this correct?

Proper charge voltage should be 14.4 - 14.8 volts and if yours is less I would suggest fitting a smart regulator e.g. Merlin or Sterling Power.

If the battery is fully charged, run a light for a few minutes (to remove surface charge) then turn off all power, and check your voltage, your battery should be 12.8 volts or higher.

50% charged is 12.2 and it is recommended not to go below this level for a domestic battery but if you are using a start battery only then I would change it for a leisure battery or fit a second battery with a VSR charging system.

Hope this helps


Mark
www.boatdoctorni.com

 

[Piers]

From memory, each of the six cells in a lead acid battery is between 2.2v and 2.4v when charged but after it has had time to 'settle' after being charged.

Hence, when all charged up and ready to go, you should expect to see 14.4v.

If one of the cells has failed (this is typical occurrance), the voltage will be 12v, and will drop quite quickly when under load.

Solution? Change the battery.

0.5A discharge? Chase the culprit circuit. The multi-meter is your best friend in these cases.

 

[boy boater]

Thank you all for your most useful advice and facts.
I have 2 identical 100AH batteries, one for domestic and one for starting, supplied new with the boat about 6 years ago.
The batteries are charged from mains, via an onboard charger, or from the engine alternator.
The integral charger ammeter shows the charging rate when running on the mains, this varies depending what domestic circuits are switched on.
A voltage meter on the dashboard shows the alternator output, approx. 14V (can't be sure exactly, as analogue dial).
It looks like the charging side is ok?
There are two seperate analogue meters, one voltage and one ammeter, showing the state of play with the domestic battery.
The 0.5 discharge (with all the domestic circuits switched off) only shows when the isolator switch is ON, it goes to zero discharge with the isolator switch OFF.
I always for sure switch both (dom. and start.) batteries of when leaving the boat.
The bilge pump and heater are connected directly to the dom. battery, the pump for safety and the heater to ensure it completes its cool down cycle.
Can anyone throw light on my 10V and wildly fluctuating ammeter readings?
Is my best course of action to get the battery tested by a battery supplier?
What readings, in all regards, should a good battery show?
I have noted your 100%, 50% and 25% voltage charge readings, are these what the tester will go on?
Is it possible for the tester to show a failed cell when in fact all is ok?
I would hate to be sold a new battery when I don't really need one!
Thank you again for all of the help.

 

[carlton]

I have 2 identical 100AH batteries, one for domestic and one for starting, supplied new with the boat about 6 years ago.

I think that could be the problem - 6 year old batteries. I'd start by changing them for new.

 

[Richard10002]

The 0.5 discharge (with all the domestic circuits switched off) only shows when the isolator switch is ON, it goes to zero discharge with the isolator switch OFF.


Can anyone throw light on my 10V and wildly fluctuating ammeter readings?
Is my best course of action to get the battery tested by a battery supplier?
What readings, in all regards, should a good battery show?

1) I used to have a 0.5A discharge, even when the isolator switch was off. Turns out that the Navtex is connected directly to the batteries, and doesnt have an on/off switch so, if you want one, you have to fit one in the circuit.... which I have now done - no more discharge. I dont know of anything else which might not have an on/off switch, (where you would expect one), but there must be something, which isnt obvious.

My bilge pump is connected to the battery, so this will discharge when it activates - which is how it should be.

2) Before you pay anyone to test your batteries, if you get yourself a multimeter, (Maplins or somewhere for £10-£20, and very useful in lots of situations), and check the voltage at the battery terminals, this will confirm the true voltage, rather than the voltage which may be misread by the built in voltmeter. I tend to get 13.2V at the battery terminals, (and other places), whereas the Sterling monitor reads 12.5V or so, (these figures change as the batteries discharge, but the Sterling seems to underread consistently).

If your batteries are reading 10V at the terminals, and 12V max, something is wrong.

 

[rafiki_]

I think 6 years from batteries is outstanding. Would be amazed if these were not the problem.