Starting the genset and engine with the same battery?

[poiu]

On my boat I have 3 independent battery banks. House, engine start and genset start. An independent house battery makes sense, but I can't understand why there are two sets for engine/genset starting. I can manually combine the house and engine batteries so I have starting battery redundancy. The engine and the genset each have alternators for charging their own starter batteries.

Is there a good reason for having the third battery bank and couldn't one set of batteries work for both?

[Dockhead]

My boat is set up exactly the same way, except that both main engine and genset on my boat are 12v, so I can't jump the engine directly with the 24v house bank. IIRC, the Oysters with Perkins engines have 24 volt starters.

Of course you could get rid of one of those banks, but why would you bother if it's already there? This redundancy is great.

I have had to jump my genset with the engine start battery and was really glad that these two batteries are adjacent in the same battery box. In an emergency (say, both engine and genset start batts dead), I could take one battery out of my house bank and use that to start the engine.

One time I arrived on board after being off for six weeks to find every battery on board dead. The house batts died because I had left the batts unisolated -- I had just installed a wind generator and thought that they would keep up with the small drain. My secondary bilge pumps are not directly wired so I though I could just leave them on. Big mistake!

Why the engine and generator start batteries were dead also I have no idea to this day. Bizarre. It was a cold, dark night when I came on board my totally dark and cold boat with all dead batteries after a long flight. In desperation, I took the tiny start battery out of my dinghy (!), the only non-dead battery on board, and used that to jump the main engine. Incredibly, the little bit of extra power did it, and I got my Yanmar started, and then set about gradually bringing the ship back to life.

Moral of the story -- sometimes that little bit of extra redundancy may save your azz.

[djmarchand]

You loose a slight bit of redundancy. Sometimes you forget to isolate the starting battery and it runs down with the house battery at anchor.

But even a 90% depleted house/starting battery will probably start the genset as it doesn't take much current. Then you can recharge your starting battery and get home.

OTOH it doesn't take much of a battery dedicated to the genset. A Group 24 will work for sure. If you have the space then I would keep it.

David

[poiu]

I already have probably unnecessary redundancy as all battery banks are 24v and I have 3 banks, so I reckon on needing only 2 and am planning for the demise. I have a quadruple redundancy in the dinghy battery, which as you point out can be used in an emergency. Redundancy also is less likely to be needed in my set up also as I have a low voltage cut out for the main battery.

I am also unlikely to forget to isolate the starter battery as I never connect unless in an emergency. The connect is there only for that purpose.

I have two parallel 100Ah batteries for each of the engine and gensets. About 50kg of unnecessary ballast in a pair and wasted storage space and maintenance cost.

I'm wonder if it is OK to feed them occasionally with the alternators feeding from both the genset and the engine if they are working at the same time or if there might be a problem if I started the genset when the battery was getting a full charge load from the engine. Voltage spikes maybe? There must be some reason why the electrician wiring it all in did it that way.

[Fuss]

Originally Posted by poiu

I already have probably unnecessary redundancy as all battery banks are 24v and I have 3 banks, so I reckon on needing only 2 and am planning for the demise. I have a quadruple redundancy in the dinghy battery, which as you point out can be used in an emergency. Redundancy also is less likely to be needed in my set up also as I have a low voltage cut out for the main battery.

I am also unlikely to forget to isolate the starter battery as I never connect unless in an emergency. The connect is there only for that purpose.

I have two parallel 100Ah batteries for each of the engine and gensets. About 50kg of unnecessary ballast in a pair and wasted storage space and maintenance cost.

I'm wonder if it is OK to feed them occasionally with the alternators feeding from both the genset and the engine if they are working at the same time or if there might be a problem if I started the genset when the battery was getting a full charge load from the engine. Voltage spikes maybe? There must be some reason why the electrician wiring it all in did it that way.

I would leave it as it is as it not really much extra maintenance cost in the grand scheme of things.

If I was doing a fully optimised power system with solar etc and I was not happy with the current engine charging system for my house bank then I would consider some changes that would involve removing the generator batteries and connecting the generator to the engine start batteries. I might even increase the size of the generator alternator and connect the generator to the house bank starting it from there. The house bank needs all the amps it can get doesn't it. The main engine needs its own batteries as a matter of safety. The generator not.

On the question regarding problems/spikes if the 2 alternators are feeding into one set of batteries at the same time.....
This depends a bit on the regulators on the alternators and the state of charge of the 24v start battery. Of course one alternator will end up doing more of the work, but not all of it.
An example.... If the battery is in bulk mode and the voltage is at 29.6 (which I think is what you boat has as its absorbtion voltage) and you now start the generator, then it might happen that the generator alternator for a short time, maybe 10 seconds, increases the voltage to 30-31 before it or the other regulator reacts. I've seen this happen.
However, if the voltage is under absorbtion voltage, say 28, then its no problem.

However, why would you start the generator when the engine is on as I assume that you have a big inverter that should also run most things while motoring.