The little parasite in the bilge

Some of you have noticed that I am a bit of a perfectionist*about electrical systems, having been on too many boats with inadequate battery and charging systems.

Well I've been caught out and am guilty of a stupid mistake.**I recently went down to the marina and found the house batteries at about 50%. They are 4 x 105ah gel cells.

The DC genset had a cheap sealed automotive battery as its start battery and because we hardly use it in summer - we also have 2 x 80W solar panels -*I connected the genset start battery to the house battery charging circuit, through a 24v to 12v converter,*to keep it topped up.

(Most systems on board are 24V but the DC genset starter circuit is 12V)

The*issue was that the genset battery was wet-cell and the additional voltage from the house charging circuit (set for gels) boiled it dry and caused one cell to short-circuit. This little battery was thus a continuous load on the house cells and*steadily sucking them dry. The shore-power charger, and solar panels, couldn't keep up with the discharge rate!

I*have now*installed a little gel battery*for the genset start.* I've learned that I need to keep different battery types isolated from each other.

Jeff b

This may be a silly question ,as I'm not particularly electrically minded, but wouldn't a smart charger sense the generaor battery was fully charged and stop charging it?

Bendit wrote:I've learned that I need to keep different battery types isolated from each other.

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A better lesson might be to keep the genset battery completely isolated.

Do you have a fuse between the battery banks so a failure in one can't let the other see it as a dead short and try to dump the full battery into a small wire?

It sounds like you dodged a bullet this time, are you feeling lucky again?

RickB wrote:A better lesson might be to keep the genset battery completely isolated.

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I think this suggestion should be LAW! (It was on my big sport fisher.)

*

Keeping the genset battery completely isolated*was not*satisfactory as, being a flooded cell, it lost charge when the genset was operated infrequently. However, the gel cell replacement does not lose its charge so*readily and I have isolated it again and will run the genset more often.

Re fusing, the house and start banks are connected via a (VSR) voltage sensitive relay with a 100amp built in breaker.

-- Edited by Bendit on Wednesday 1st of December 2010 01:28:36 PM

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Shrimp: the genset battery*was discharging internally and never appeared full charged,*so the smart charger just kept on pumping amps in to the whole system,

My boat has two battery banks, house and start seperated by a voltage sensitive relay (VSR) and a gen-set with it's own isolated and dedicated starting battery. When on shore power, I float charge the gen-set battery with a Guest automatic marine float charger plugged into a near by outlet. It maintains a 13.2V float charge seperate from the other battery banks. Seems to work well for me as the battery is approaching 5 years old and all 6 cells are maintaining normal specific gravity readings.

The Guest automatic float charger is good for both flooded and gel-cell type batteries.

SteveH

-- Edited by SteveH on Wednesday 1st of December 2010 10:25:52 PM

Re fusing, the house and start banks are connected via a (VSR) voltage sensitive relay with a 100amp built in breaker.

This works , but for $18.00 a 75A RV solenoid off the acc. position on the start switch is one heck of a lot cheaper.

The VSR allows the solar panels, shore charger*and DC*genset to top up the start*bank as well.

Bendit wrote:

Keeping the genset battery completely isolated*was not*satisfactory as, being a flooded cell, it lost charge when the genset was operated infrequently.

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We have a 4D dedicated to the generator.* However it receives a charge from the*main*engine alternators when they're running.

We installed a Heart Freedom 25 inverter/charter*some twelve years ago.* We selected this unit not because we needed a 2500 watt inverter but because at the time this was the smallest inverter/smart charger that could accomodate two echo chargers.*

I don't know if this is typical of today's smart chargers with echo chargers but the only "smart" charge from the Freedom 25*goes to the battery it's connected to.* In our case, this is the port 8D (now a bank of* four 6vdc GC batteries).* One echo charger is connected to the starboard 8D (now a pair of GCs).* The other is connected to the 4D*generator battery.* However the echo chargers are not "smart."* They charge only at the float charge rate.

As such they will not overcharge the batteries.* However the echo charge circuits will not put out a "high" or "medium" charge rate, so they take longer to bring the batteries they're connected to back up to charge if they're low.

However we've not had a problem with this since the generator battery is never drawn on except to start the generator.* We leave the starboard bank as our "start" battery so it doesn't get drawn down much.* And older GBs like ours*incorporate a combiner relay in the engine room*that connects the two main batteries (or banks) together as long as a start button is engaged.* So the starboard "start"*battery is never worked very hard at all, and the float charge from*its echo charger*has*so far seemed to be all that is needed when the boat is in its slip on ground power.

so i am going out on a limb here..
why would you need a separate genset starting battery?
could you not start the genny from the engine batts?
what would be wrong with this.

Main engine and house batts are 24V, genset start is 12V.
Could do it by tapping off one battery, I suppose, but it's a long cable run from the 24V banks to the genset.
The cable would cost more than the little gel battery I now have.

Per wrote:
so i am going out on a limb here..
why would you need a separate genset starting battery?
could you not start the genny from the engine batts?
what would be wrong with this.

Click to expand...

Simplicity.* In an emergency, you should be able to start them using other batts, but as Jeff found, any connection between the batts exposes them to collective failure.

My Gen and ME*batts are AGM, and while they never go more than a month without starting, they could probably go 6 months and lose only 5-10% or so.* They are never charged, except when run.

why would you need a separate genset starting battery?
could you not start the genny from the engine batts?

Many folks believe the stand alone noisemaker is "insurance" , should the start batts and house batts suddenly go dead together.

"Stuff Happens"

ok, you could also just isolate one battery then, to use only for emergency starting.
by example, my engine batts consist of 2 x 8D agm batts, a bit overkill imho, considering to re-wire one of them to include on the house side. opinions?


but indeed "stuff happens"

Bendit - cannot gels safely go below 50% or at worst shorten their life by a few months if drawn down? Do you keep your vessel on a mooring vs dock? Assuming your genset battery is isolated, the right size and in good shape, it should go several months without a recharge and still start the genset. As rickB said, isolating the genset(s) battery gives lots of backup and redundancy.

Fortunately, the builder of my DeFever saw fit to install a*Newmar 60 amp charger dedicated to genset and engine starting batteries. This then allowed the inverter/140 amp charger to be dedicated to charging the house bank. Like Marin's setup, the main engines charge house and start*battery banks when cruising. I got 8 years (they were still strong) out of the first set of wet cells (60% cost of gels too) with this setup.