Voltage drop when running lights turned on

[wheresbob]

Is it normal for the voltage to drop to 10 volts (from 12) when I turn on my running lights? I've got a Group 24 12V battery with 650cca. There's only an all-around light and a nav light on the bow. The outboard is a 1985 Force 50hp. After 2 summers and a few laughs from this forum, my little beast is ready... Thanks for any input on this. jt

[Quietboats]

You may have a short or the battery has a bad cell. Most likely the latter. If you have access to another battery, swap it out and see what happens. Also, take the voltage readings on the battery itself, if it reads 12 volts but you get 10 volts at the light--then look for corroded terminals or a bad battery conection. Best, Tom

[wheresbob]

Thanks Tom. The battery is brand new, so if there's a short, hmmm, where would that be... the wires from the voltage gauge run to the ignition and then the battery. The connectors at the battery look a little corroded. Could that be it? Thanks again. jt

[Quietboats]

JT, the battery connectors would be the first place to check. Clean them real good, tighten the connectors and then put on some corrosion protection--even vaseline will work better than nothing. Also, wing nuts on batteries should be more than hand tight. If you still have problems go get a digital multi meter from Radio Shack or Sears and test voltage directly on the terminals. It is not uncommon for a new battery to have a bad cell. There are (6) two volts cells in each battery. If a battery drops 2 volts when the smallest of loads is put on it--that is an indication that one of the cells has a bad internal connection. If the voltage drop is not read on the battery--then the problem is elsewhere. Good Luck. Tom

[wheresbob]

Ok - sounds like a nice Sunday morning project. I'll let you know how it goes. Thanks so much for your recommendations. jt

[wheresbob]

Update on voltage drop: I put a volt meter on the battery and it's just over 12v without the running lights on. When I turned on the running lights, it dropped a tenth of a volt. Then I put the volt meter on the back of the gauge and it measured 11.9v with the lights off although the gauge read 11v. When I turned on the lights, the gauge dropped to below 10v but the voltmeter still read almost 12v. So it looks like the gauge might be suspect... To add to the mystery, with the voltmeter on the back of the gauge, when I turned on the ignition, with the running lights on, the volt meter dropped to zero. Hmmmm. Oh well, it starts, it runs, the lights work...I'm going to bay anyway! Cheers! jt

[Tim B]

Replace the gauge with a digital one if you're concerned (and there are loads of different ones around). Also check the wiring, is the gauge wired into the ignition switch correctly? doesn't sound like it to me. I would have expected the volt meter to be give a reading whenever the power is on (as it does in a car).

Cheers,

Tim B.

[CDK]

Look at the terminals on the back of the gauge and on the ignition switch. If they look dull or grayish, unscrew and clean them. Put some vaseline or other grease on them.
The voltage drop is caused by resistance in a junction of 2 wires, often the positive ones, but sometimes also on ground leads. Because you get different readings between the gauge and the volt meter I suspect there is a salt deposit on the terminals. The test pins are nickel plated, the screw terminals are not, so the dissimilar metals and the deposit build an electrical element with approx. 1 volt potential.