Ive got this large-ish battery . 130ah. its pretty new havent used it much. Im trying to charge the thing and it wont seem to go above 12.9V. I know nothing about batteries. I think i left a radio on for a few days and this de charged it a bit - to a bout 10V No panels to get it back up. Probably stuffed it? i would have thought a few days with the radio on would be ok?

It may take a couple of days to recover and fully charge it so I would not worry too much yet. Just leave it on charge (assuming you have a decent charger). Just check it every couple of hours to ensure it doesn't overheat.

the charger doesnt have a volt meter on it. battery is currently at 12.8 V. ive been trying to get this charger up above that for a few weeks. I think the charger is pretty crap. just has the , low, med, high, full lights which doesnt really tell me much!

The "open cell" or "resting" voltage of a fully charged 12v lead acid battery is around 12.8-12.9 volts, and flat dead ones are at 12.0 volts, so 12.4 volts on a resting battery means it's about 50% charged. Charging voltage will be 14.4 - 14.8V during the bulk charge, depending on your type of wet battery. Be sure to select the correct type of battery ie gel, lead acid, AGM or calcium, which will be stated on the battery label.
When you finish charging you will need to let the battery settle for a few hours as it gasses up and heats up during charging before you can measure it accurately as it will drop after you turn the charger off.

If it's presently sitting at 10V it is very, very sick and you will need to charge it hard to try to recover it. If it has a dead cell it's a goner. You can test if a cell has died with a Specific gravity tester which are are cheap as chips at Super Cheap Auto or wherever.
Just make sure before you charge that the electrolyte levels are right in each cell by topping up with distilled water if necessary so the plates are covered and then use a quality 3 or 4 stage smart charger. These chargers analyse the state of the battery and then ensure the right voltage is applied for each stage of charge. The bigger ones ( that charge quickly because of their output power) aren't cheap though!

With the greatest respect, a decent multimeter allowing voltage testing and current testing up to 10 Amp DC would be a good investment and they are not expensive nowadays. $20 will buy a basic one and $80 a pretty good one. Amaxing how many times it will sort out what's going on.
If you have a fair bit of electrical gear try to source a 2nd hand copy of Nigel Calder's "Boatowner's Mechanical and Electrical Manual". It is awesome, as are all his books, and you don't need the latest edition either!!! Don Casey and John Payne are similarly excellent boat maintenance authors.
Hope it goes OK

Hmmm, I checked the water in our battery bank today and was shocked at the amount of water that was needed. I don't keep a log on that sort of thing but maybe I should ! The volts have been looking good but not many useable amps. Volts unfortunately don't tell the whole storey .

Thats a passable voltage like Wazza mentioned but also like Woko said the voltage might be OK but the battery may have no remaining capacity.

To explain that, imagine if you had 9 AA pen light batteries connected in series (like Xmas tree lights) you would get 13.5V (9 x 1.5) which as a voltage looks very nice but wont start a motor! You've got volts but no amps.

The next thing to do to decide if the battery is dead is put a load on it and see how long it takes to discharge. You could use you nav lights as a load. Put your meter onto its 10A range and connect it in series with the battery and the nav lights and see how much current they draw. Say the current is 3A. If the battery really is 130AH in capacity and its OK it should keep those nav lights on for 130/3 hours - ie. the battery should stay above 12.0V for around 40 hours.

Ive got all kinds of battery testing stuff at my office in Parramatta, you could bring it over and we can test it with a 30A electronic load we use which would be quicker. (130/30 = 4 hours).

I threw out two 130AH AGMs a couple of years ago because they had the same symptoms your batteries got. I bought two new brand name ones on Ebay and they have been fine ever since.

Trek said..
Thats a passable voltage like Wazza mentioned but also like Woko said the voltage might be OK but the battery may have no remaining capacity.

To explain that, imagine if you had 9 AA pen light batteries connected in series (like Xmas tree lights) you would get 13.5V (9 x 1.5) which as a voltage looks very nice but wont start a motor! You've got volts but no amps.

The next thing to do to decide if the battery is dead is put a load on it and see how long it takes to discharge. You could use you nav lights as a load. Put your meter onto its 10A range and connect it in series with the battery and the nav lights and see how much current they draw. Say the current is 3A. If the battery really is 130AH in capacity and its OK it should keep those nav lights on for 130/3 hours - ie. the battery should stay above 12.0V for around 40 hours.

Ive got all kinds of battery testing stuff at my office in Parramatta, you could bring it over and we can test it with a 30A electronic load we use which would be quicker. (130/30 = 4 hours).

I threw out two 130AH AGMs a couple of years ago because they had the same symptoms your batteries got. I bought two new brand name ones on Ebay and they have been fine ever since.

Thanks Trek. I might take you up on that offer if you dont mind in about a week. Ill be working in Dundas - nearby.
Ive just hooked the battery up using jumper leads to a motorcycle headlight. ive got the volt meter on as well. Instantly the battery dropped from 12.8 to 12.2 where its sitting now, its dropped about 2 amps in the last 30 min.

THanks alot everyone for your words of wisdom, experiences and advice. Think im on top of it now.

Initially i couldnt get the battery above 12.8. well it did, but then it sat for a bit, with no load and i have been charging it again over a few days and it wouldnt go above 12.8. I put the motorcycle headlight which is about 60w on the battery and over about 3 hours the battery dropped from 12.20V to 12.09.
I then recharged recharged the battery and got it fully charged at 13.8 which is were it is now. But the battery loses some amps if it sits for a day. Is this what happens usually? Do batteries just drain if theres no trickle charge , or panel or something on them?
The battery seems fine anyway and im guessing that when i get a full time panel on the boat the battery will stay up. Hopefully :)