Voltmeter question.

[Tdub]

After all these years (5 Nautiques) I still don't quite understand this gauge. I plead ignorance. So, we here in the north sometimes in the Spring or Fall are forced to have the boat set for a week or two between sessions. So as not to be disappointed when I get to the lake, I will throw the charger on it. Do I need to do this? If I check the gauge, what should it read so it will start the boat?

 

Cheers.

Tom

 

[BraceMaker]

Voltmeters tell you the effective voltage at a given point in time. So if you get in the boat and turn the key to "on" it should read roughly the voltage of the battery, it isn't perfect but if you took a digital multimeter and read the voltage across the battery terminals it should be close to the number on the gauge. It should be somewhere around 12.5 volts.

 

However it really cannot tell you the boat will or won't start as 12.5 volts is with no load on the battery, a battery load tester will tell you the condition of the battery, this is basically a voltage gauge that has a heating coil that draws amperage, if the battery still has good voltage while you are putting a draw on it the battery is good.

 

What a voltage gauge is better for is to watch for it to change during operation, so for instance it reads ~12.5 volts when the key is on but the engine off, if you crank it and its running it should go up to around 13-14.7 volts because the alternator is charging the battery. If though you are cruising down the lake and that gauge drops down to 11 volts its a sign that either A- you are running too much stuff like the radio, bilge pump, electric heated seats, cell phone chargers etc. B- your engine tossed a belt and the alternator isn't making juice or C- actual problem with alternator or charging circuit.

[jpwhit]

It's true that a reading right around 12v with the engine off, after sitting a while, won't guarantee that the boat will start. But a reading much below 12V will be a good sign that the battery has discharged and it's likely you will have trouble. Especially anything around 10V or lower is very good indication of trouble. A 12V car battery is made up of six 2V cells. So when a cell goes bad, it'll often read right around 10V.

 

If your battery is in reasonable shape, and you don't have any issues in your electrical system that causes loads on the battery when it's off, you shouldn't really have to put a charger on it when it's not used for a few weeks. If you do, then it's a sign of a problem that should be fixed.

[BraceMaker]

@jpwhit - very true - also my voltmeter in my older prostar reads lower than a good multimeter across the gauge reads, ie battery 12.5 at terminals, gauge reads about 11.5, but multimeter reads ~12.5 on the prongs - that's lead me to run down several rabbit holes searching for voltage drop when its just a somewhat inaccurate gauge.

[Tdub]

Thanks to all. Question answered. Another quick question. How long do most of you feel comfortable playing the radio without running the engine? Just curious.

[oldjeep]

You should be able to play the radio for hours and hours without the boat running. I've got 2 batteries, but the second is always off. We regularly anchor and listen to the Sirius in the boat for hours and have no issues starting it up to leave on the same battery that we were listening to the radio on.

[floridagm]

I use a $70 insurance policy for cold starts, radio use, inflating tubes, under inflated trailer tires, those that aren’t as maintenance oriented, etc., a jump box from Costco. Nice peace of mind and have scored more than a few beers helping someone else get started!

[LeonL]

@Tdub for extra level of comfort, turn of the master battery disconnect when you leave, assuming you don't already do this.

[Tdub]

A jump box is a great idea. Yes, I do turn off the master switch. And about the radio, that (pardon the pun) sounds good. When not skiing at our lake, we often times hang out in Sandusky Bay at a sand bar.Thanks @oldjeep Radio blasting, beers flowing. Thanks to all.

[oldjeep]

@Tdub - our radio is never blasting, just loud enough so that we can hear it in the boat ;) Blasting for all to hear might take more battery power.