Slightly OT Generator 12V system

Hi All
This may be a bit OT but I couldn't think of anyone else to ask.
I have been building a race timer and starter for the sailing club
where I am a member. It runs off a 12V supply using car horns lights
etc. Ar present we use a battery which has to be taken away to be
There is no mains electricity and electricity is supplied when needed
(only a couple of hours a week) by a 3 phase diesel generator (about
I was considering connecting the race timer battery to the genset 12V
starter system using a relay so that it would be charged by the 12V
alternator when the generator is running in the same way a caravan
fridge operates on 12V only when the car engine is running.
I have had a look at how to do this and I was surprised to see that
the 240V (415V) systems are not separated at all. Terminal blocks,
PCBs etc are shared by both sytems.
It occurs to me that in a fault situation there could be up to 415V in
the 12V system so that automotive type lamps horns connectors etc
would be inadequately insulated.
Is this a real risk or is it OK to go ahead and use the 12V system.
Would it be better to plug a battery charger into the 240V output of
the generator and just leave it connected.
I'd be grateful for any comments or suggestions.
Reply to
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It all rather depends on the way the system is wired / configured and the nature of the fault.
Does the generator have separate windings for the HV and LV (which I would expect) and just the control circuitry shares a PCB and terminal strip etc? If so, a fault (eg a short) on the latter group could result in HV on an LV circuit. However, if the unit is maintained, I'd think the risk small.
A "plug in" charger would get around that problem but probably is overkill, in my view.
Reply to
Brian Reay
Have you considered using a solar panel to charge the batteries during the week or a windturbine or even a mixture of both.
Also if thinking is correct, the 240 Volts from the genny would be from any leg to the neutral point or centre tap so it would be unlikely to get more then the nominal 240 from accross the winding if the generator is wound in a star so should be OK.
Reply to
Adrian Hodgson
Hi Russell, when a large generator like this is used without a mains supply the neutral (star point) must be earthed through a reliable earth rod. The frame of the alternator and engine are all bonded to this earth rod, therefore, as you say there is common connection between the two.
This is not only normal it's essential for safety, as is the fitting of an RCD in the distribution system because of the relatively large earth fault loop impedance you get with a rod. The design of the system should be such that there is negligible risk of the AC getting into the DC system, but if it did then protective devices should operate.
Back to your battery charging issue, I would not recommend connecting them in parallel as there are too many issues which I won't bore you with, the simplest thing to do is get a mains powered charger. The problem you may find is that just two hours charging may not put enough back into the battery, so as others have suggested you may have to supplement it with solar or wind charging.
Reply to
Thanks to all for the advice.
I am reassured about the safety - as far as I could think it through the low internal resistance of the battery would be low enough to earth 240V and trigger the RCD but it's nice to be sure.
We already have a small solar charger - but it needs supplementing with something. I thought a big diesel generator would be good.
I am still inclined to use the car type alternator on the engine to charge the battery - I know there are difficulties if you simply connect them in parallel but I think I can get around them with a relay so they are only connected in parallel when on charge.
I'm also inclined to go with alternator charging because the alternator output is much better than most battery chargers and will be more effective for a couple of hours charging.
Thanks again.
Reply to
If you follow that route, you need a 'split charge relay', widely used on boats & caravans. Get the biggest (amperage) & best you can find, it should be more reliable even if you don't 'need' the capacity in theory.
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Reply to
Tim Leech

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