How should I ground a totally seperate battery for toilet?

[ChasB]

I currently have a regular battery setup for my boat - engine and house (which are interchangable).

A mickey-mouse boatyard recently wired in a 30A electric toilet rather badly with inadequate guage wiring, so I'm going to re-wire it myself.

I have a surplus battery, and a spare output on the mains battery charger, so I thought I'd put them to use by giving the toilet it's own dedicated battery. That way I don't have to worry about it draining the other batteries when I'm out and about. I won't connect it to the engines' alternators -that would make it all too complex- but will live with being only able to charge it when a mains supply is available. We only ever hop from marina to marina.

I am not sure if any conductive parts make any contact in the toilet with the water, but it's safest to assume they do, so I'm worried about creating a circuit somewhere that may create corrosion.

So do I wire it like this?
- A pair of leads go from the battery to the toilet.
- A lead goes from the battery's +ve terminal to the charger's (currently unused) third output
- Another lead goes from the battery's -ve terminal to the boat's electrical common earth?

Many thanks.

 

[philip_stevens]

[ QUOTE ]
So do I wire it like this?
- A pair of leads go from the battery to the toilet.
- A lead goes from the battery's +ve terminal to the charger's (currently unused) third output
- Another lead goes from the battery's -ve terminal to the boat's electrical common earth?

[/ QUOTE ]

Yes, but make sure you have a (toilet motor current size) fuse in the positive line to the toilet from the battery, and for safety's sake, another smaller (maximum charging rate) fuse in the positive line from near the charger. This is to protect the cables.

If you want to charge it from the engine alternator, fit a splitter in the line to the main batteries, and then take one feed to the main batteries, and another to the toilet battery. This way, you will be able to recharge it, from the engine, between marina berths, or if you happen to go to anchor for a day or so. If you run it from a splitter, you will have to uprate the fuse in the line from the charger.

 

[ChasB]

Thanks.

I've already got the fuses - I didn't bother mentioning them.

My worry with adding to the batteries already charging on the two alternators is of exceeding their capacity to keep the batteries charged. Worse still, if one engine failed (it's happened!) then one alternator would be left charging 3 batteries. Probably not a problem, but if it turned out to be, I'd be stuck out on the water with two dead engines.

Still, it may be worth looking at. I could always put a cut-off switch in. And all I need is a (big chunky) splitter? Wouldn't I also need a diode somewhere?

 

[DaveS]

It would be wise to put a fuse or circuit breaker at each end of the charging wire since in the event of a fault you want both sources of energy to disconnect.

 

[ChasB]

I don't get that. If it's on mains you turn off the mains power. If it's charging off the engine, you turn off the engine. There's a label on my stitching panel that says to NEVER disconnect the electricals while the engine is running or you'll fry the alternator.

What kind of fault?

 

[DaveS]

As one example, consider the insulation of the wire getting nicked on earthed metal, and so creating a short circuit. This is, after all, the main reason for protecting any wire at its source. The point with a charging cable is that there is an energy source at both ends. If you only fuse at the battery charger end and your charging wire shorts, the fuse will blow, protecting the charger, but the battery will contine to feed current - possibly lots of current - into the fault.

 

[ChasB]

Got it.

 

[philip_stevens]

Noted, my omission.