The little parasite in the bilge

[Bendit]

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

 

[Andy G]

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?

 

[RickB]

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

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?

 

[Codger2]

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

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

*

 

[Bendit]

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

 

[Bendit]

*

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,

 

[SteveH]

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

 

[FF]

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.

 

[Bendit]

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

 

[Marin]

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.

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.

 

[Per]

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.

 

[Bendit]

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.

 

[Jay N]

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.

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.

 

[FF]

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"

 

[Per]

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"

 

[sunchaser]

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.