Battery Charging - when is it fully charged?

[seaesta]

Dear All,
Sometimes I bring home my boat battery to charge it up. Can anyone tell me how I know when it is fully charged?
Thanks
Martin

 

[jeremyshaw]

Sadly no simple answer to this question. Depends what type of battery it is and how you are charging it. I recommend you refer to the excellent Battery University in particular
http://batteryuniversity.com/partone-13.htm

 

[shmoo]

This may help too. You will need a very accurate voltmeter and you can really only tell if you know the current into the battery. The values are temperature sensitive too, so see the full article. The "C" refers to the battery's capacity.

For example if you are charging at C/40 (say 3A for 120Ah) 100% full would be 13.4V at 78 F battery temp. Excuse degrees F: the source is American!

www.arttec.net/Solar_Mower/4_Electrical/Battery%20Charging.pdf

 

[Refueler]

[ QUOTE ]
Dear All,
Sometimes I bring home my boat battery to charge it up. Can anyone tell me how I know when it is fully charged?
Thanks
Martin

[/ QUOTE ]

In fact there IS a simple answer to this ... and it means a walk to local Car Accessory shop. You can buy Hydrometers in a glass tube with rubber aspirator bulb on top to draw fluid sample out of each cell of battery to test density. All Wet normal batterys are using same acid / water combo ... so this little gem works for most. No need to think about capacity, voltages, temp ... just suck up ... let hydrometer settle in tube ... bingo instant reading.

Tip : If you use one of these - have a beaker of water ready to flush the instrument after - the acid is highly corrosive.