Ammeter reading question

[Tarantelle]

First, when it comes to electrical stuff I’m absolutely ignorant. It’s just never been anything I’ve been able to learn.

My question has to do with the readings on my ammeter when I switch between start, all, and house. When I am on start, the ammeter is barely off of zero. When I switch to both, the reading is about 35. When switched to house, it reads about 15. Seems like the reading when both banks are on together that the reading is high. I would think it would be closer to the sum of start and house readings. Am I missing something?

Thanks for your help in advance!

 

[High Wire]

First off, if that is a dash ammeter they are not very precise. Charging a little or charging alot sounds about right for batteries that have been on a charger at the dock then engine started. The best way to confirm what you are seeing is to use a "clamp-on" type DC Ammeter. There are many out there. Look for one with 0-40 Amp DC and 0-400 Amp DC ranges.

Second, switching the battery switch with the engine running risks blowing out alternator diodes. So only do that with the engine off. The battery switch should state a similar warning.

 

[Ka_sea_ta]

There are too many unknowns in your post. First is the ammeter analog or digital? If it's digital there is most likely a shunt in the battery circuit, chances are the shunt is located in your house battery bank ( the starting battery (ies) should be isolated either by a switch, ACR or a diode based isolator. If the meter is analog it's most likely in series with output of your alternator.


So I'll assume that it's measuring the current flow in and out of your house bank (most common application for an ammeter) so when you switch to both you are charging both the house and the start batteries, the current will be higher 20 amps flowing into start bank 15 into house. When the battery switch is in start the shunt is just measuring house current which is nil. This is just a guess without knowing how your boat is wired...