Can/Should batteries be switched when motor's on?

Status
Not open for further replies.
Jun 4, 2004
35
Catalina 27 Rochester, NY
The other day I had my Atomic 4 running while tied in the slip. I had just reconnected the exhaust and was testing a new CO2 detector. I have two batteries on my boat, a new one (no. 1) and an older one (no. 2). The new one I use for cranking the engine. The other day, with the engine running, I moved the battery switch from No. 1 to No. 2, thinking I could add charge to the older battery while there in the slip. Then engine died. I switched back to No. 1 and she fired right back up. I thought that once the engine was running, the alternator provided the current for spark. Following this logic, shouldn't the engine have kept running and simply started charging No. 2 battery? I'd appreciate any insights you can lend.
 
Jun 4, 2004
56
- - Sasafrass
Sounds Odd

You can switch between 1-both-2 with the engine running but NOT off as that will toast the alt. The fact that the engine died seems odd, a diesel, once running, should not need electric given it has no spark plugs. You should find out what is going on due to the fact if your ability to run is dictated by one battery you need to know this. Is there an electric fuel pump connected to just the one battery?
 
Jun 7, 2004
70
- - Deale, MD
Yes

It is odd. IT indicates that you need to do some wire tracing. I suspect that your coil/distributor is wired to the 1 terminal of the 1 - 2 switch. IF your switch has a "both" position the engine should still run with the switch set to both. The suggestion that you may have an electric fuel pump is a good one. Many A-4s have been fitted with electric pumps when the original mechanical pump diaphragm failed. If this is wired to the 1 position on your switch the pump will quit and stop the engine even if it still has spark. A digital multi-meter will help you figure these things out. Connect the ground probe to ground and touch the input side of the coil and the power side of the electric fuel pump to see when they are or aren't getting juice as you switch between 1 and 2 (and both) on your switch. If you don't have a "Both" position on your battery switch you will fry the diodes in your alternator by switching from 1 to 2 unless the switch is a "make before break" switch. Never turn the switch to off while the engine is running. IF you do so, almost all A-4s have single wire automotive type alternators and a rebuilt replacement can be purchased at you local auto parts store. As you surely know the Atomic 4 isn't a diesel so it won't run without electrical power to the ignition.
 
Jun 4, 2004
35
Catalina 27 Rochester, NY
Gas, not diesel...

Bill: My engine is gas (with manual fuel pump), but I would think the alternator warning still applies. Thanks for the tip. The switch does have a "Both" setting, which I think saved me. I have heard elsewhere, since posting my first message, that switching the batteries while the engine is running could toast the alternator. So, if a battery's charge is low enough that it can't turn the engine, then there is no way to charge it with the alternator as there's no way to get the juice back to the battery. Right? As such, the only way to charge a battery that can't turn an engine over is to use a battery charger. Is this correct?
 
Dec 2, 2003
480
Catalina C-320 Washington, NC
Grain of salt on toast.

I am not certain of the accuracy of the following statement, but have a strong belief in its correctness arising from highly fallible memory. I don't think switching batteries off toasts the alternator itself. I believe it only toasts the diodes that convert the AC output of the alternator to DC current. On some alternators the diode is external and easily replaced. On many, if not most, it is internal and replacement is best done by a sevice/rebuild shop. I certainly wouldn't let someone sell me a replacement alternator until this was verified.
 
Dec 2, 2003
480
Catalina C-320 Washington, NC
On low batteries.

If the battery is not damaged and will accept a charge, the alternator will do the job, it just takes longer than with a high amp charger. However, if you only have one battery and it is discharged too low to turn the engine over, it may be too low to allow the engine to run. Alternators do not typically have sufficient output to supply the necessary current to an engine's ignition system and charge a heavily discharged battery. I also have some fuzzy recollection of alternators requiring some minimum 'exciter' current in order to provide any output. But that is probably mere confusion with principles governing the operation of some other device.
 
May 23, 2004
117
Catalina 30 Stockton, CA
Zap-Stop

There's a product called Zap-Stop ($29.99 from West Marine) that protects you from damaging your diodes in case someone (not you of course) turns your battery switch off while the engine is running. It's easy to install (5 minutes at most, once you identify the right terminals) and is cheap insurance.
 
Status
Not open for further replies.