Complicated Electrical Question - Good Puzzle for You Electrical Geniuses

[DeepFrz]

Originally Posted by chala

Taking into consideration Kirchhoff first law, that the shunt is big enough to carry the current and can be wired into the negative and the following, why not?

chala, just a matter of cable length.

[chala]

Originally Posted by Dockhead

It will be a minor hassle switching the banks back and forth but I suppose not a big deal; we'll see.
Attachment 16910

I would assume than a cable of a current carrying capacity of at least the maximum demand of the domestic load or the intended battery bridging circuit already connects to each other the two negatives shown on the drawing above. If not then the intended battery bridging circuit will not work. The shunt then should be installed the closer to the “service” bank if it is the bank with the highest current demand. Yes “It will be a minor hassle” reading both banks.Buying another “Victron battery monitor” will make things easier.

Originally Posted by Dockhead

That night, I smelled battery acid (one of the banks is under my bunk)

I hope you do not smoke in bed.

Originally Posted by Dockhead

This was in the middle of the night with almost no loads on, other than refrigeration.

And how much was the almost no load? I find the 28.25 volts not much of a pumping voltage, was the battery hot, gassing?

[Dockhead]

Originally Posted by Pete7

Perhaps one duff cell in one of the batteries, which because they are now all wired together .................

Pete

Yes, Pete, you got it in one. They are not all wired together, because the interconnect switch is off when I'm on shore power. But never mind.

As it turns out -- I couldn't figure it out because I was assuming that all the batteries were new. In fact I was cheated by the brokerage who were supposed to replace every battery on the boat (it's even in the contract). They chose four out of the old batteries which seemed to more or less work, and left those as the house bank. They all looked new since they were in protected battery boxes, but they were not. There were two dead cells in two of the batteries. The charger falsely sensed that the batteries were down and pumped in current to bring them back up. Since there is no temperature sensor, the charger couldn't tell that it was just burning up the batteries. Problem located.

I installed four new batteries to the house bank and voila everything works fine. I just got back from a three-day cruise with no shore power anywhere. Without activating the interconnect switch, the new house bank of 4x 110Ah 12v batteries wired to provide 220Ah at 24v were quite sufficient for 12 to 15 hours between charging (admittedly we were not using much cabin lighting because it was so light out), but we had instruments working (for anchor alarm purposes), a 25 watt anchor light burning, stereo working, and refrigeration going full blast in the heat.

I also got a note back from Driftgate saying that categorically, positively, there will be no harm to the splitter from leaving the banks interconnected while charging and no feedback or other problems.

The Newmar battery charger manual also says categorically, positively, that there is no harm in having the charger on while the engine is running and charging through the alternator.

So it looks like problem solved (other than the coming show-down with the cheating boat brokers). I don't regret the interconnection switch, because that will let me double power capacity if I need it for some reason.

Only thing missing now from my setup is the Victron charger-inverter which will allow me to (a) charge faster from the generator, (b) use AC power for short periods without cranking the genset; and (c) will prevent my flipping the isolation transformer breaker all the time, by strictly limiting AC power consumed, supplementing it with inverter power when necessary.

Oh, and figuring out how to make the battery monitor work. But as long as the interconnection switch is not used, the monitor works as normal, and I'm finding it to be extremely useful.

[Dockhead]

Originally Posted by chala

I would assume than a cable of a current carrying capacity of at least the maximum demand of the domestic load or the intended battery bridging circuit already connects to each other the two negatives shown on the drawing above. If not then the intended battery bridging circuit will not work.

The negatives are connected to the boat's main grounding system which is pretty heavy duty. But I don't think either negative or positive interconnection between the banks needs to be sized according to the peak loads. I think it needs to be enough to transfer charge between the banks at whatever average rate there is. There cannot be a big current in the interconnection between the banks unless there is a big difference between state of charge, as when one bank is completely dead and the other fully charged. But there is a fuse to prevent any overloading of the interconnection. Please correct me if I'm wrong in this reasoning.

Originally Posted by chala

The shunt then should be installed the closer to the “service” bank if it is the bank with the highest current demand. Yes “It will be a minor hassle” reading both banks.Buying another “Victron battery monitor” will make things easier.

How, exactly? I don't quite understand how you are suggesting it to be done. Two monitors and totalize myself the readings?

Originally Posted by chala

And how much was the almost no load? I find the 28.25 volts not much of a pumping voltage, was the battery hot, gassing?

Four or five amps max, at night with everything off except refrigeration.

The charger was putting 30-something amps into the batteries. They were still hot enough to cook your dinner on (as the electrician who looked at it said), several days later. Scary.

[hellosailor]

Damn, I could sure understand if you "returned" those four batteries to the broker, upside down on the hood of his car or some other convenient location.

(Hey, I'm using restraint here, the traditional maritime payback would be keelhauling, but there's EPA regulations against shredding anyone under your boat these days.)

Just to complicate life, I notice a number of battery makers are putting their LABELS on top of the manufacturing date codes, making them damn near impossible to find.

[btrayfors]

Right-side up on his head would be appropriate, IMO :-)

B.

[Dockhead]

FOLLOW UP -- THE REST OF THE STORY.

Thanks to everyone for great advice, which all turned out to be right.

After replacing the four old batteries, the battery charger works perfectly normally again. It was indeed a bad cell somewhere causing it to pump up the voltage.

I just spend a month living on board and rarely with shore power. Mostly we were living on our own power either on the hook or in municipal ports without shore power or on moorings.

We had no trouble at all with electrical power. I left the two banks (each 4x 110aH @12v for 220aH @24v) connected together the whole time, which as predicted by you guys and confirmed by the manufacturers did no harm to any of the charging systems.

This amount of power (440aH @24v) was plenty for our needs. I never started the generator just to charge batteries. The batteries were charged exclusively from genset runs initiated for other reasons or from motoring (and we did very little motoring, all but four days at sea were sailing days). They would easily go for 24 hours of our usual usage without falling below about 60% charge according to the Victron battery monitor.

This is the first time I've ever been cruising where I wasn't always worried about keeping the batteries up.

I had been planning to install a much bigger battery charger than the 40 amp Newmar I have, but now it does not seem to be a big deal. I do need the inverter, as the need to run the genset for AC power was an irritant, also the Victron combo inverter/charger will kick in to supplement shore power to cover a surge -- something else I need. But I will no longer have a fetish for large charging capacity as the 40 amp Newmar (that's 40 amps @24 volts; like 80 amps @12 volts) coped just fine.

Thanks again, guys.