How long to charge a battery?

[Brian]

How long does it take to charge a flat battery, on a boat, to about its maximum?
I know, I know-depends on output of your charger, the capacity of your battery etc etc zzzzzzzzzzzzzz.

Go on-have a guess as to how long to charge up a flat battery, on a boat, assuming you can start the engine and run the boat out to sea continually at about 25knots.

All your assumptions are average or normal.

[Searider]

So Brian,

120Ah battery
100A alternator

I reckon a couple of hours to get to about 80% and then at least a couple more for the rest.

Close?

Duncan

[AMac]

For the battery to last longer take it out and charge it as slow as possible with a battery charger
If you only charge it enough to fire up the engine you are running the risk of not knowing if it will hold a charge
If it doesn't hold charge and you are at sea how are you going to restart the engine?

[Erin]

Originally Posted by Searider

So Brian,

120Ah battery
100A alternator

I reckon a couple of hours to get to about 80% and then at least a couple more for the rest.

Close?

Duncan

Your alternator may well be able to put out 100A (more likely 60A on most large outbards), but a flat battery has high internal resistance so won't accept high current to start with. I reckon it could be almost measured in days on a boat to get it fully charged, although 70 - 80% might be achievable within a few hours..

[captnjack]

If its totally flat what did you start the boat with?

Does this alternator have a smart charging regulator or is it constant voltage? That will make a big difference.

[doggypaddle]

The answer is a blinking long time, the bulk of the charge will take place in 4 or 5 hours, but to get to 100% on a battery that's been left flat for a while with a voltage regulated taper charge may take days to break down all the lead sulphate on the plates.
A bench charger is the best bet, charge until the terminal voltage is over 14 volts,(charging) then monitor the SG of each of the cells with a hydrometer. Keep charging until either they all come up to 1260-1280 and gass freely, or you decide one cell or more is significantly lower than the highest reading cell and take it to the tip!! If the battery is 5 years old then take it to the tip anyway

[Pikey Dave]

Originally Posted by captnjack

If its totally flat what did you start the boat with?

easy, bump start it

[Brian]

Well, thanks very much chaps.
The answer seems to be 4-5 hours to get it up to 80-ish percent.
Thats good enough for me.

[rjbathgate]

I let my (now old) battery go completely and utterly flat, and on a proper battery charger it went from about 5% to 7% in 56 hours..... then I gave up and went to the battery shop.

[BrAinZ]

Remember that most modern batteries they HATE being left completely discharged.
Also sounds backwards, but most "good" (ironic!) battery chargers also won't charge a completely flat battery as they need to sense that they are connected to a battery before they start giving any output.