Engine start battery losing power

[sabmd42]

I have had my 1999 Jeanneau SO 45.2 for just over a year. I replaced all the batteries at the beginning of last season (2012). The engine start battery kept being discharged after sitting at the mooring for 1 week, even though the battery switches were all off. The 3 house batteries were fine. I took the start battery back and replaced it, but the same thing kept happening. The batteries are at home for the winter and the start battery is holding a charge normally. I brought my altimeter to the boat today and checked the resistance between the positive and negative terminals. It should be 1, but set on the smallest scale it was reading around 30 to 60, even when the battery switches are off, and was the same with the switches on. I checked the resistance at the battery switches, and they are all reading 1 when off and 0.005 to 0.007 when on, as they should be. I assume there must be a slight short somewhere. Any clues where to look next?
Steve

[Zanshin]

It is going to be a bit of work to track down this problem. There should be just two places where you should be seeing a current drain off the charging battery, the first is the starter mechanism itself and the second would be the charging system. I suppose that you are charging the starter battery off the main engine alternator through a splitter. When you say that you turned off all the switches, do you mean that you turned off the big main red and black circuit breakers for DC power? If that is the case and you still have less than infinite resistance then you have a serious problem (you need to turn off both + and - master breakers) since these should be wired directly to the main battery cables. Do you have other wires coming from the starting battery or just the 2 main cables?

[windward54]

I had a similar issue on our boat. You have to take a voltmeter and start at the starter battery and work back. Depending on your battery type, if the battery is being charged, the voltage should be between 13.6 (float charge) and 14.6 (charging) volts. You just have to track back from there and keep checking until you find the problem.

My problem turned out to be a battery isolator failed and wasn't allowing the charge voltage to pass through to the starter battery. I replaced it with a Blue Sea System ACR and no problem since.

[cpetku]

I had a similar issue and hunted it down this weekend. In my case it turned out the charger for the motor battery had been improperly installed nine years ago. the po never figured it out and had a switch installed between the house and motor batteries to allow starting from the house bank when necessary. this would also finally connect the motor battery to the charger when at dock if left in the connected position (bad idea).

[Trevor]

Hi Steve,

I would disconnect the cables from the battery isolator switch until the resistance between the two cables going to the battery terminals (no battery connected) is infinite. You should see no resistance when the isolators are off and any resistance you do measure is leakage. This leakage is the reason the battery goes flat over time.

I hope this helps,

Regards,

Trevor

[sabmd42]

I did some more checking over the past weekend. I discovered that the leak is in the shore power battery charger. When I check the resistance between the negative cable and the positive cable to the starting battery, there is not infinite resistance. (There is infinite resistance between the negative cable and the positive cable to the house batteries.) I never use shore power. I am at a mooring. I don't feel the need to repair or replace the battery charger. It is very hard to access. The battery charger is connected on the battery side of the master switch. If I connect it to the non-battery side of the switch, there should be no leak when I have the master switch in the off position, which is how I normally leave it when I leave the boat. My question is: Is it safe to have it connected like this, or should I just leave it disconnected?
Steve

[Don Reaves]

Clearly, when the master switch is in the on position, it is just the same as having the charger connected to the non-battery side of the switch. (With the exception of the resistance of the switch, which ought to be negligible.) Connecting it like this would mean that you could actually use the charger if you find yourself visiting a marina someday.

On the other hand, if you disconnect it, you must make sure it doesn't inadvertently connect to something it shouldn't. I had a friend who did something like this to his car. After all the major wiring bundles burned up due to a short, the car was junk.

Don

[Trevor]

In my view putting the charger on the load side of the battery switch would resolve the leakage problem but I would lose sleep not knowing why it was leaky. It could be some moisture across the terminals so I would have to find out why.

I would not leave the charger in that state because if that leakage worsened it cause a significant current draw = heat.

Good luck with it. If you have an investigation you may find it is a simple case of cleaning around the terminals of the charger.

Regards,

Trevor