Getting Charged (Not)

[captain49]

Late last season I replaced one of my two batteries. After sailing all season, one day I went to the boat and it was dead as a doornail. I never really thought much about it at the time as that battery was 6 years old.

I launched in late June this year and all seemed to be working well although in retrospect, one of the two deep cycle batteries did seem to charge up almost instantaneously after being in storage (heated) for the winter. I use two deep cycle batteries (1 three years old, the other 12 months old) after being advised that current technlogy has improved to the point where they can be used for 'starting' as well. In any event, I keep one battery for starting only and one for house electrics.


I went to the boat yesterday and there was no power at all...both batteries dead. I had inadvertently left the battery switch in the 'Both' position (for twenty four hours) but nothing else in the boat was switched on. The only circuit that would have been able to become active was the auto bilge pump but the bilge remains dry.

I have removed the batteries and will have them checked tomorrow...next will be the alternator/regulator. I think I will find one battery 'cooked' and the other OK but that the cooked one drained the good one completely.

Also today I disconnected the cables from the batteries and read the resistance across the cables - first in Batt position 1, then Batt position two then Both. In all cases I was reading 0 ohms in off but around 610 ohms whenever the battery cables were connected via the switch to the boat wiring. It appears there is something downstream from the battery that is loading it

With the meter on the battery cables and showing the 610 ohms I disconnected each piece of equipment - VHF, GPS plotter, fume detector, etc.;

I removed the fuses from all of the lighting circuits;

I lifted all of the positive lead wires from the +ve main electric distribution post and then all of the negative wires from their tie post;

The 610 ohms remained;

I did not disconnect the alternator or the boat starting/ignition system although I thought it was isolated by the usual switches.

Should there be any resistance between the battery cable terminals when everything is switched off.

Any thoughts on where I am getting the 600 ohms...doesn't it provide for a constant draw on the battery of approx 20 ma.

Am I reading through the winding for the ammeter?

[tenders]

Don't connect batteries in parallel for long unless they're of similar age and design capacity and ESPECIALLY when you know they aren't!

Your old battery has no more capacity in it. It probably has a short in it, and keeping the two connected in parallel drained, and possibly ruined, your new one too.

Never mind what device drained them -- probably nothing did. Take them both out. Fill the new one with water (if so equipped) and have them both tested. Expect to replace the old one, hope not to need to replace the newer one.

[Jim Booth]

I did not disconnect the alternator or the boat starting/ignition system although I thought it was isolated by the usual switches.


Any thoughts on where I am getting the 600 ohms...?

Sounds like you might have a bad rectifier diode in the alternator. (And a bad battery).

Jim

[bayareadave]

ohm reading

It is possible that one bad battery dragged the good one down with it. As for the ohm reading - I do not believe it means there is a load since ohms is a measure of resistance and not amperage. When you turn the switch on, you complete the circuit and your ohm guage would be measuring the resistance in the wire over the completed circuit.

[High Hopes]

C49,

Interesting problem. I suspect that the problem is probably not with lights and instruments which have an on-off switch. It may be the alternator as Jim suggests, but I'm not sure 600 ohms is a bad value for reverse bias resistance on a set of high current diodes. One thing to consider, if you draw 20 ma off a 100 amp hour battery, that's equivalent to 5000 hours of battery life (208 days).

As for pinpointing the drain, start at a higher level instead of one-at-a-time'ing it with every device on the boat. My battery switch has many wires connected to it. Start there and disconnect all of them except the battery cables and make sure you get an open reading. Be sure to disconnect the alternator cable which sometimes looks like a battery cable. Now you know your switch is OK. Then check each switch wire by itself to locate the circuit with the 600 ohm load and go from there. Add good wires back to the switch one at a time and recheck.

Let me know what happens. BTW, an ammeter is in series with the load. They should measure about 0.006 ohms.

Steve