Voltage drop on battery switches

Oct 29, 2006
388
Beneteau 381 Olympia, WA
Hey all, so I just completed running 8 gauge wires to our inverter, really helped make me feel better about the inverter running its own wire independent of the panel. Now in doing this I'm still trying to figure out a .2-.3 V drop and now identified that it's happening in the Beneteau Battery selectors. Our 361 has "house" "start" and "negative" battery selectors and when measuring along top (boat side) I get .2V less than on the bottom (battery side) so in the switch its dropping over 10%. I turned the switches round 10-15 times and that seems to have helped as it was .15...

I was wondering if anyone seen the same and maybe cleaned it or identified a reason. We have a combiner so never really use the shutoffs other than when working on electrical. Any tips appreciated.
 
Feb 6, 1998
11,709
Canadian Sailcraft 36T Casco Bay, ME
At what amperage? Your inverter must be very small for 8GA so that level of drop May mean a bad switch. Best way is to set the DVM to DC volts then touch the leads to the two battery switch studs with the switch passing various currents.
 
Oct 29, 2006
388
Beneteau 381 Olympia, WA
Thanks for quick reply Maine, thats with no load on the system at all. i did measure at the panel using beneteau a guessometer with load and it didn't drop much at all. This really is betwen the +- on switch out and +- on switch in. Ill look at it some more and get some real numbers but seems to me they should be the same.

Yeah my inverter is only 150w and runs a small PC with a monitor.
 
Feb 6, 1998
11,709
Canadian Sailcraft 36T Casco Bay, ME
Thanks for quick reply Maine, thats with no load on the system at all. i did measure at the panel using beneteau a guessometer with load and it didn't drop much at all. This really is betwen the +- on switch out and +- on switch in. Ill look at it some more and get some real numbers but seems to me they should be the same.

Yeah my inverter is only 150w and runs a small PC with a monitor.

You can't have voltage drop without a load.. Ohm's law still applies so I am guessing the measurement procedure was incorrect or you had a load you did not account for..

Again, under load you set the DVM to DC volts or DC mV and place the positive DVM lead on one stud of the switch and the black lead on the other stud. If it shows a voltage other than 0.000V that is your voltage drop..
 
May 17, 2004
5,694
Beneteau Oceanis 37 Havre de Grace
Was the charger on when you tested? The current from that could cause a voltage across the switches if they're failing, depending on how it's wired.
 
Oct 29, 2006
388
Beneteau 381 Olympia, WA
Yeah the charger was on, and by no load i mean all panel breakers were off so no load that I know about.

Ok I'll try to do some more testing, mark the real numbers and post, and maybe draw something up for batt to switch and charger.

Ill measure with load, without and with charger and without. My plan was cleaning boat but this is way more fun now :)
 
Last edited:
Feb 6, 1998
11,709
Canadian Sailcraft 36T Casco Bay, ME
Yeah the charger was on, and by no load i mean all panel breakers were off so no load that I know about.

Ok I'll try to do some more testing, mark the real numbers and post, and maybe draw something up for batt to switch and charger.

Ill measure with load, without and with charger and without. My plan was cleaning boat but this is way more fun now :)

A charger is a "load"... If it was pushing 40A across the switch then the load on the switch was 40A. However on most Bene's the charger feeds direct to the batts or to the batt side of the switch so it may not have been a load depending upon how it is wired...
 
Feb 1, 2017
1
Beneteau B40 Shoreline, Long Beach, CA
Did you ever figure our the voltage drop across the Negative switch? I'm having the exact same situation:(
 
Oct 29, 2006
388
Beneteau 381 Olympia, WA
Hey Joe, I have not, I'm getting back into spring maintenance mode soon and electronics is high on the list. I'm definitely hoping to get a better idea what is going on there and my plan is to start by physically disconnecting wires and see if that changes anything, this did puzzle me quite a bit as it seems an open switch should read the same voltage on sides when its connected. I'd be curious if you find more information too.