Re: Split charging.... yet again... and boring... yet again

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"Gibbo" <gibbo@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:1124914604.658567.278540@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Ok....
>
> Time to spout some opinions :-)
>
> Whatever electrical system is installed, however it is wired, whatever
> charge splitter is installed, it is always going to be possible to pick
> holes in it. Nothing is perfick.
>
> It's generally agreed that 2 battery banks is best. One for the engine
> and one for everything else. No doubt someone will try to pick a hole
> in that. Try by all means.
>
> We are then left with how to split the charge, assuming only one
> alternator.
>
> Some people like diodes, some prefer relays, some prefer 1-2-both-off
> switches. All have their disadvantages. But for every hole someone can
> to pick at a split charge relay, I can pick 2 each in a diode and a
> 1-2-both-off switch.
>
> So I reckon relays are best (I tell myself I *know* they are best).
>
> So now, how to wire it up?
>
> Firstly, use a relay that will handle starter current. Yes it might
> cost you 10 quid more. Big deal.
>
> Alternator to domestic bank, split across to the engine with the relay.
> I know it can be done the other way round, but for every hole someone
> can pick in this way, I can pick 2 in the other way.
>
> The engine is energised from the D+ terminal on the alternator. NOT
> from the ignition switch. Though of course that is possible, and means
> all starting will come from both batteries, which isn't always going to
> be a good idea.

I recal some alternators are/have had their main output terminal markes as
D+ (if they have no B+ or BAT terminal then D+ may well be the main output
terminal) and wiring one of these as stated would have the relay energised
all the time.

>
> Also install a push button switch (non-latching) that manually
> energises the relay. People get scared at this and think they have to
> run wires all over the boat. In fact it's very simple and can be done
> on the engine panel simply by shorting across the charge warning light.
>
> If 2 alternators are fitted. Then still fit the relay anyway. This
> allows the spare capacity of the engine battery alternator to be
> diverted to the domestic battery.
>
> It also builds in redundancy in that either alternator can fail, and
> both batteries still get charged. If the relay fails, both batteries
> still get charged. 2 failures are required before a battery doesn't get
> charged.
>
> (This system is possible 99% of the time. Very occasionally the
> alternators will not allow it. But it's very rare)
>

On this group we hear a lot about using ex vehicle alternators and many of
those have no sutable energising terminal so there may be a slight problem
with these. I am also trying to think where we could get a suitable relay
feed from on a standard Lucas ACR unless we fitted phase tap.

In these situations the push button switch would be the best way - as long
as someone remebers to push it. Or you could use a rising oil pressure
switch and hope the engine can not produce sufficient oil pressure on
cranking.

With could feed some relays from the alternator side of the warning lamp,
but I know from experience that sometimes the warning lamp current is high
enough to energise the relay, on other relays it is not, so all works as it
should.

> It also enables emergency starting from the domestic bank.
>
> If you want to protect the alternators from running into an open
> circuit this can be done in many different ways (as started in that
> other thread). The simplest is to power *everything* except the starter
> motor and heater plugs from the domestic bank.
>
> The above system *is* the best. Of course that's just my opinion.
>
> But I'll put my money where my mouth is. I'll *give* a brand new, free,
> very advanced split charge system (relay based of course) to anyone who
> can come up with a better system for the same money as the above.
>
> By better it has to perform better and have less inherent problems.
>
> The matter will be settled by picking holes in each other's systems :-)
>
> Finally, can someone please tell me something good about split charge
> diodes. I've been trying for over 15 years to think of something good
> about them and I can't.
>
> Gibbo
>

I know something good about diodes. They must be good because they are high
tec and thus can cause customers to pay big money for them, they also give
the manufacturers the opportunity to cause customers to buy diodes that look
as if they will do the job until you look at air flow and ambient
temperature requirements, so the customer either keeps replacing them or
buys another more expensive one ;-)

As far as I am concerned the whole thing should be as simple as possible and
I do not like the sound of additional "protection" relays or mixing engine
and domestic electrics up. I am still trying to work out a simple way of
using an "ignition" switch to operate the starter and any glow plugs from
the starter battery and the instruments and alternator energising from the
domestic bank - unless another relay, inline diodes (not charge splitting
diodes) or a rising oil pressure switch is used - more complication. (A bit
like 24 volt domestics and 12 volt engine systems - but we had better not go
there).

The whole electrical thing is a compromise and I find it interesting that
both myself and a canal hire fleet independently came to the same conclusion
about how to wire such charging systems.

The important thing is that we both agree that big red switches and diodes
are not the best solution to split charging. I will accept some reduction in
the maximum domestic battery charging current and Gibbo (and others would
rather not). As voltdrop is proportional to current flow, the voltdrop
across the main relay circuit and terminals will reduce as the current
falls. If you have an ammeter see just how quickly it does fall. In my case
it will drop from 70 to 40 amps in about 15 minutes. Not all of that fall
will be felt by the relay contacts, but some will.

I do agree that with very high current alternators (90 amps +) one has to be
much more careful about such things and that the relay contacts (and
internal wiring) will run cooler if the high currents can be kept away from
them.

Tony Brooks

Tony Brooks


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