Battery wiring mystery

[rbryn]

I have a 2003 260 Sundancer using the stock battery switch and wiring. 2 batteries (12v g27 starter on battery 1, 12v g27 deep cycle on battery 2). The negative leg is shared between both battery 1 and battery 2 via a single cable that attaches inline to both batteries..

Now the backstory... we had an incident recently where the battery switch was left on "both" (by accident) and the pedestal in the marina was switched off for maintenance - the boat drained both batteries to dead (battery 1 went all the way to 1v). After getting the batteries recharged fully we were unable to get battery 1 recovered enough to start the engine. Any time we tried to start in battery 1 the power went off while the starter was engaged. Had no problem with battery 2. We assumed that we killed battery 1 and pulled it off the boat, taping up all the leads on battery 1 with electrical tape to ensure there were no accidental shorts. Knowing that the marina might pull the pedestal power off again (They warned us of upcoming maintenance) we intentionally switched to battery 2 and turned off the refrigerator/radio/cabin lights on the DC panel in the galley. We thought this would at least allow the bilge to operate if necessary.

Question #1
I thought that the bilge was directly wired somehow so that dc power was always provided regardless of the battery switch position. Is this a true statement? Is it only wired to battery 1 or both?

Mystery #1
With battery 1 pulled and the switch over to battery 2 - the boat had no DC power at all. It was like there was no battery connected at all - we tried the switch in all 3 positions (1,2, both) and the result was exactly the same. Checked battery 2 with a volt meter and it shows 12.7v across the terminals. We pulled and re-seated the cables with no change.

We went and grabbed a brand new starter battery (880 CCA g27 - same config as the previous battery) and put it in.

Mystery #2
As soon as we added battery 1 back to the circuit we were able to regain power on both battery 1 and battery 2 on the switch. Everything was working again (or so we thought). We attempted to start the boat on the brand new battery (battery 1 position only). No joy, the starter would click and the lights would dim, but it wouldn't turn the starter.

Assuming that it was just a half charged battery (it did just come off the shelf from west marine) and it being a beautiful day I decided to start the engine on battery 2, then switch to "both" to charge the battery and run around for a bit. I ended up drift fishing for about 5hrs (the engine on idle most of the time with 20min intervals at 4000+rpm). Voltage at the GPS and dash registered 14.5V all day (even at idle).

Mystery #3
At the end of the day I return to the marina and shut down. Switch the switch from "both" to battery 1 and try to start the engine... As soon as I switched to battery 1- the power cut off - not enough power in the batter to run the radio or gps. I thought the engine would run on just the juice from the alternator and when the engine was on, it had excess to recharge the batteries. In this case it drained battery 1 instead of charging it. Grabbed the volt meter and checked both batteries, Battery 2 still showed a perfect 12.7v, battery 1 showed 10v.


So now I'm flummoxed... Is it a bad switch? or bad wiring? Any ideas on how to troubleshoot and chase this one down?

 

[paulswagelock]

On mine, the bilge pumps and stereo memory are hard wired to battery 1.
If the ground is common to two batteries, and your removed one, did you break the ground while it was disconnected so no DC power?
An alternator at idle hardly can keep up with the load on a running engine. You need to go above approximately 1700 rpm for it to produce excess amperage. That said, an alternator is not really designed to charge a low battery rather just to top off batteries after starting load draws them down a bit. With the switch in both position, the charge from the alternator can go to either battery, and in your case, it sounds like battery 2 took most of the current. If you switched to battery 1 only than the alternator would have brought it up closer to charged.

 

[Bt Doctur]

1 start by checking and cleaning all the battery cable connections. at the batteries, the battery switch, and the engine .
2 locate the auto bilge pump feed from the float switch. remove it from where its now wired and wire it directly to the battery 1. install a second bilge pump and wire it to battery 2. Be sure to include a additional ground lead from the negative side of the pump.

NEVER REMOVE A BATTERY CABLE FROM A RUNNING ENGINE WITH AN ALTERNATOR.

In the worst case you will replace every bulb, electronics fuse, and maybe the engine electronics if the alt runs wild at 24-38 or more volts

 

[rbryn]

On mine, the bilge pumps and stereo memory are hard wired to battery 1.
If the ground is common to two batteries, and your removed one, did you break the ground while it was disconnected so no DC power?

The ground/negative wire that runs to the engine and the rest of the boat is one single cable with a two connections (one for each battery). It appears to be created with the battery connector in the middle of the cable very much like this one (except it's black)


In theory by disconnecting battery 1 (which would be middle of the wire) it should still ground out and conduct through the empty spot.

 

[rbryn]

1 start by checking and cleaning all the battery cable connections. at the batteries, the battery switch, and the engine .
2 locate the auto bilge pump feed from the float switch. remove it from where its now wired and wire it directly to the battery 1. install a second bilge pump and wire it to battery 2. Be sure to include a additional ground lead from the negative side of the pump.

NEVER REMOVE A BATTERY CABLE FROM A RUNNING ENGINE WITH AN ALTERNATOR.

In the worst case you will replace every bulb, electronics fuse, and maybe the engine electronics if the alt runs wild at 24-38 or more volts

We cleaned all the contacts to the battery posts and the cable connections and re-seated them making sure we were contacting with 100% surface. I didn't check the switch connections (never touched it before or where they connect to the engine). I'll check those as well.