The battery has gone flat.

[Crocodile]

Hello to all you electrics gurus,

checked the boats battery today, flat, when I tried to start it, the donkey turns over too slowly, instruments light up OK.

Boat ran OK on Monday gauges on dashboard read 13.5V.
Bilge pump not working.
Fixed bilde pump on Tuesday, seemed like corroded connectors.
Boat sits unused until today, Friday.
Pretty sure nothing left switched on.
15 year old boat, wiring is in general a shambles.
Battery is 525cca wet cell marine battery, plenty of water in the cells.

Multimeter now says 11.8V across battery terminals.
With the battery disconnected, reads 20 ohms across the +- connectors(this is just the wire connectors that usually are on the battery.
If I switch on the nav lights, this reading goes to zero.

I am guessing that I have a stray current causing the battery to go flat.

The big marine battery wont fit into my car so I can't re-charge it that way.
Are those $50 chargers from SupaCheap up to the job of re-charging the battery?

Any advice is welcome.

[Fed]

With the battery disconnected, reads 20 ohms across the +- connectors(this is just the wire connectors that usually are on the battery.

You will need to find where the 20 Ohm reading is coming from.

Any battery charger will fully recharge it but small cheap ones take longer.

[Noelm]

if you put a multi meter on ohms across a battery (even a nearly flat one) you will get a reading alright, but only one, from then on the meter will be shot! not sure what you measured!

[JollyDave]

Hi Crocodile, just for info the current draw is voltage divided by the resistance in this case you would have been drawing 0.6 amps. you now take the number of hours it was left like this eg 4 days or 96 hours, therefore you draw down 0.6 X 96 = 57.4 ampere hours. Your battery is probably rated for about 100 ampere hours when fully charged and new - if old probably not that good if longer hours as you can see very easy to end up flat. The CCA rating is only in relation to a short burst for starting current.
Hope this is not confusing

Regards
Dave

[WalrusLike]

if you put a multi meter on ohms across a battery (even a nearly flat one) you will get a reading alright, but only one, from then on the meter will be shot! not sure what you measured!

I think he measured the resistance of his switches... He measured the resistance of the two leads that normally are attached to the battery... Not the battery.

When the lights are switched on there is a circuit made so resistance is zero(ish).

To my mind that means you have a near short somewhere.... Because there should be enormous resistance when switches are off... Not just 20 ohms.

Warning.... I am a amateur with no real idea... That's just my limited understanding.

[JollyDave]

Hi Walrust, not bad for an amature, I am sure you are correct, would not need to be a very big light as the power in watts is volts times current, therefore if we assume 12 volts and 0.6 amps the light would only need to be about 7.2 watts. If the wiring has some bad connections due to corrosion etc the actual globes could be higher than 7 watters but the bad joints would restrict the current flow (amps). Pretty sure that some lights or some other medium powwr user was left on ( maybe a dicky switch).

Dave

[johncar]

Yes it would seem like a good going over the wiring is needed, you must have some power leakage there or the battery is at the end of it's life. If you measured 20 ohms across your boats wiring circuit with everything off and battery disconnected, then you indeed have a problem as a good wiring system should be pretty much open circuit ie thousands of ohms, so long as there are no power indicator LEDs or such running. Yes turning your Nav lights on would give you a close to zero reading, that is normal.
You should also have a battery isolation switch anyway for safety and making sure your battery can't be accidentally drained.
The cheap battery chargers should be fine to recharge your battery.
You will have to just do some type of fault finding with your ohm meter to determine where the stray load is coming from by isolating different parts of your boats circuit until you narrow it down. But is seems as though it is related to your bilge pump work perhaps a good place to start.

Good luck with it..

[Crocodile]

Thanks for the ideas gentlemen,

I have bought a small charger, battery is up to 12.8V and now starts the motor OK.
Work(money work) will prevent me from working(fun work) on the wiring for at least a week.
I will start by isolating the bilge pump circuit and testing for ohms.
I think that I will need to re-wire properly to resolve this issue.
As this is an aluminium boat stray currents could be causing corrosion so I can't ignore this problem.
A battery isolator sounds like a very good idea, in the meantime I will open the battery box and remove the connections.

[johncar]

Yeah if she's a tinnie then definitely full isolation is always a good idea regardless