Moderated Discussion Areas
  ContinuousWave: Small Boat Electrical
  Parasitic Current Battery Drain

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Parasitic Current Battery Drain
skred posted 05-11-2012 12:38 PM ET (US)   Profile for skred   Send Email to skred  
My 1996 Johnson 50-HP tiller model has electric start and trim-tilt. The only thing connected to the battery has been the engine cable. Lights not connected. Within seven days, a fully charged battery is drained. The motor has not been run--just sitting in the garage. What could cause the current drain?
jfortson posted 05-11-2012 01:40 PM ET (US)     Profile for jfortson  Send Email to jfortson     
The first obvious question is like asking a driver who's car has stopped if there is any gas in the tank.

Is the battery good? Does the battery stay charged if unhooked from the motor?

If that's not it, I'll leave it to the more knowledgable people on the site.

skred posted 05-11-2012 01:57 PM ET (US)     Profile for skred  Send Email to skred     
Yes, the battery holds full charge if not connected to the motor. Also, the battery is less than a year old.
contender posted 05-11-2012 03:25 PM ET (US)     Profile for contender  Send Email to contender     
You have a short or a open circuit some where (or something was even left on). I would try and trace all of your wires see if you can find something, also when the boat is sitting its always best to disconnect the battery (battery kill switch is nice I like the one made by Hella) this makes sure there is no current running from the battery. Good luck
skred posted 05-11-2012 06:00 PM ET (US)     Profile for skred  Send Email to skred     
Thanks for both responses. I have already planned on installing the Hella battery cutoff switch. But, somehow, I feel this is bypassing the source of the drain. After looking over the motor, I see on the tiller handle, about in the middle on the side of the handle, is a 1 to 1-1/2 inch opening molded into the handle that looks suspiciously like it may have contained an ignition keyswitch. Nothing in there now. If that is so, then, the ignition would have to always be in the "on" position electrically, and that could be the cause. Anyone know if this is so?
jimh posted 05-12-2012 08:19 AM ET (US)     Profile for jimh  Send Email to jimh     
If the only device electrically connected to the battery is the outboard engine, and if the battery remains charged when disconnected from the outboard engine, then the source of the parasitic current discharge must be in the outboard engine or the wiring to it.

It would be unusual in my opinion for the outboard engine to be designed with a parasitic current drain, so I would presume that the parasitic current drain must be caused by some sort of non-standard condition. A couple of things come to mind:

If the outboard has power trim-tilt this usually means it has an alternator for charging the battery. The output of the alternator is always connected to the battery. In an alternator are electrical rectifiers or diodes. Typically in smaller outboard engines there is no voltage regulation, and the alternator diodes are in direct connection to the battery. A diode that is working properly has a very high back-resistance, that is a high resistance to flow of current in the reverse-biased direction across its junction. However, it is a common malfunction that a diode breaks down, and the back-resistance becomes much lower than normal. Such a bad diode in the alternator will create a parasitic current drain on an attached battery. This should be detectable by temporarily disconnecting the rectifier--usually contained in a small assembly--and testing the forward-resistance and back-resistance of the diodes with a test meter.

Another possible cause for a parasitic current could be a build-up of corrosion at a terminal post or other connection in the positive circuit that has bridged over to the engine block or negative circuit. This should be detectable with a close visual inspection.

skred posted 05-12-2012 09:41 AM ET (US)     Profile for skred  Send Email to skred     
Jim, you're probably onto something. Looking at the parts blowups, I cannot find a diode shown or listed. I don't see an alternator listed, either, unless the starter serves as a charging device. In any case, my rudimentary test of continuity from hot to ground on the battery cables(disconnected from the battery) shows there is current flow from positive to ground. I'm not informed enough to know if that would be a normal state or not. I tested what I suspect might be the part containing a diode. I have continuity even when I reverse the polarity on the test leads, but one direction shows a significantly higher reading (4-5 times greater at least). I suspect it's time to have a technician look at it...
jimh posted 05-12-2012 04:44 PM ET (US)     Profile for jimh  Send Email to jimh     
I looked at SHOP.EVINRUDE.COM for the parts breakdown, and I did not see a rectifier listed, either. I expected an engine with power-trim and electric start to have a battery charging circuit. Maybe I looked at the wrong model. What is your engine's exact model designation?
davej14 posted 05-12-2012 07:40 PM ET (US)     Profile for davej14  Send Email to davej14     
Check for capacitors. A failed capacitor could allow current leakage.
acseatsri posted 05-13-2012 04:17 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
The other cause of a parasitic drain could be a GPS being hooked up to it. The antenna on my old GPS used to kill my battery, even when the unit was off. Stopped when I put a battery switch in.
skred posted 05-14-2012 11:58 AM ET (US)     Profile for skred  Send Email to skred     
Thanks for all the advice. Before I could trace the problem, the motor was sold. The buyer is aware of the parasitic drain issue...

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.