Parasitic Electrical Loads

mvn

I was helping a neighbour with his 1999 Mastercraft X-Star.   He keeps draining one of his batteries very quickly (only after a couple of days sitting in the driveway).   Looking at his setup,  he's got 2 Mercury 650 MCA Group 24 batteries hooked up through a standard Perko battery switch.  When I asked him what position he leaves the switch in,  he said both,  even on the trailer. 

This is the problem,  right?   With different internal resistances,  one will always drain the other? 

I also noted that his Leece Neville alternator with external voltage regulator was drawing 180 milliamps with the engine off.   Is this normal? 

Thanks, 

Mark

212rowboat

Dang man.. I am a chemical, biological, radiological, nuclear, and high yield explosives detection mitigation and response technician by trade, which forces me to at least grasp computer, electronics, electrical, and mechanical systems by just glancing at them... Maybe it's the lack of crude/coffee this morn, but I'm baffled..

It stands to reason that two batteries on the same channel will stabilize between themselves... Unless one is jacked up with dead cells.. or, unless there is a ground hiding out somewhere that slowly sucks the life from one or both.. I may be wrong, but unless that alt has a wingdingyfancy minder, it ought not draw anything unless it is in fact part of the conduit for that ground..

I could be way off, though...

mvn

I think one of the batteries has a pooched cell.   Told him to get it checked out at the local auto parts place.  The alternator's behavior has me baffled.

Mark

PS Holy sh!T. .. nice career.... you must have ice water running through your veins! Closest I get to that is working on MRAPs and Strykers. 

212rowboat

If it's one of the 3200 mraps sold to homeland, I wouldnt mind if you forgot something, like, say, a transmission missing from them.. :-) I kinda fell into this career about 12 years ago, now.. just got lucky, I guess.

A dead cell will certainly do that.. if he were to simply isolate the suspect battery with the switch, he could likely isolate the problem to that bank, which makes troubleshooting a lot easier..

mvn

I disconnected everything and found only two parasitic loads... the alternator at 180 mA and the stereo equipment at 200 mA.

Thanks for the advice Al,  I'll pass it along. 

Matk

mvn

Well,  as it turned out,  the alternator was the culprit.   It was drawing current even with the engine turned off.   Bad external voltage regulator. 

All is well now.  Also trained him on the proper use of a battery switch. 

Mark

Michael T

Here's another parasitic culprit. As more boaters wire NMEA 2000 backbones into their boats to facilitate communication between electrical devices remember to wire your backbone to a switch. My harness came with a warning that if I failed to switch it that it would drain my battery "over time". How long "over time" meant I have no idea, but wired it to my ignition switch. MT