Battery charging generator or mains

[mituw16]

We've had historically high water on Lake Lanier in Atlanta, and due to this, many marinas have had the power turned off for weeks. We took out our 2004 Sundancer 360 yesterday for a few hours to charge the batteries some (and it was a nice day!). The port motor fired right up, but the starboard motor required the emergency start switch. Starboard batteries were definitely lower.

I know that the mains do charge when they're above idle, and this might be a silly question, but when both mains and generator are running, which is charging the batteries?

 

[Lazy Daze]

They'll charge at idle, too.

If you are running the genny AND have the battery charger "on", the engines and the genny/charger will be charging the batteries.

If you weren't able to start the engine after a few weeks of sitting unused, the battery is on it's way out. This is assuming you weren't running 12V accessories, other than the typical "power off" items (CO detector, bilge pumps, ECM, radio memory, etc). But those items draw an extremely small amount of current and won't discharge a healthy battery in a few weeks. Unless the bilge pump was running a lot - but then there are other issues.

 

[mituw16]

I see, thanks! The little bit of voltage drop that I see when the mains are idling is normal then? When the mains are idling, the volts for both batt banks are at 12. Anything above idle and the volts go up to slightly less than 14.

No other DC systems have been on, but I'm not sure if the bilge pumps have been running heavily or not. There's never any water down there, but could that mean they're doing their job.

 

[Lazy Daze]

"The little bit of voltage drop that I see when the mains are idling is normal then? When the mains are idling, the volts for both batt banks are at 12"

No, this is not correct/normal.

The volts should pretty much always be around that 14 mark - it's the amps that increase (by a good amount) as RPM increases. But it will still charge at idle - just not as fast.

Since you're only seeing battery voltage at idle, something is amiss. At this point, it could be any number of things... weak/dying battery, electrical connections, slipping belt, alternator/voltage regulator, etc. BUT, I would start by fully charging the battery overnight (use the onboard charger), then take the battery out and get it load tested at a local auto store. A good, healthy battery is always a good place to start.

How old is the battery and have you been replacing them as a set or individually? Is the stbd battery a single or a bank of two? These are important to know.

 

[mituw16]

Both sides are banks, and they're new at the start of last summer. I'm guessing that would point to something else based off what you said.