Charging Circuit

[redhot]

I have a problem that has me foxed and may confuse you all.
You know the alternator is supposed to charge the starting battery for 10 mins or so and then switch to the domestics?
Well mine charges all at the same time but at about 12.4 and 12.7 volts, so they are seperate circuits but they charge simultaneously.
If I disconnect the split diode, it sends about 13.8 volts to the starting battery and none to the domestics.
Can anyone explain in laymens terms how this should be wired or better still, if you are local to Reading, pop round and take a look and see what you think.

Oh, I forgot to say, I have changed the alternator and diode and it has made no difference.

Help PLEASE!!

Thanks
Mike.

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[johna]

Mike
There are other contributers to this board more able to answer your questions but it sounds to me as if the split diode is doing exactly what it is intended to do and that is to separate the positive sides of the two batteries. There is no switching per se. What you may have been told is that after 10 minutes the starter battery will have regained its charge lost from starting the engine and from then on the alternator output will more or less be soley directed to the domestic battery.

Now from the readings given I would say that your diodes have a relatively large voltage drop across them (1 volt+) and also it sounds as if your alternator is machine sensed rather than battery sensed. For use with diodes the alternator needs to be battery sensed in order to overcome the voltage drop across the diode. The output from the alternatot should be about 14.4v with 13.8 at a fully charged battery

I think you need a boat electrician to look at the arrangement to make sure all the kit is compatible.

I trust this is helpful.

John

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[johnsomerhausen]

Any diode will cause a voltage drop of at least 0.7 volts. That's the cause of your problem. I suppose you have a general switch with "off, 1, both,2", so the simplest sol;ution is to take off that diode and start the engine on "both" and switch to the house batterey when the engine is stopped. Some perople will say that putting a partly discharged and a charged battery in parallel is damaging them, but I've been doing it for two seasons and have not noticed any harm. As a matter of fact, my alternator is not working at present and the house battery is being charged by the solar panel a; I top up the engine batterey from th house batterey.
john

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[ccscott49]

For a start the alternator charges both batteries at the same time, through the split diode, the engine batteries will come up very fast and then the alternator continues to charge the domestic batteries. The alternator does not know however, that the diode is dropping the voltage by .7 to 1 volt, so your batteries are never getting fully charged, the only way around this, is to have a smart regulator, which has a voltage sensing wire, after the diode, the regulator then senses the volt drop and increase alternator output. Otherwise the system is working exactly as it should, albeit very inefficiently. I hope you kept the old alternator and diode, because there is probably nothing wrong with them. I cannot reccomend fitting an "smart" alternator controller strongly enough. With some you will not even need a splitter diode. Hope this helps.

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[oldharry]

Agree with all the above - there is nothing wrong with your set up except that the voltage drop is too great to allow proper charging, a common fault with diode splitters.

A cheap alternative (around £30) to a smart controller (£150+) is a caravan charge relay: this is simply a relay arranged so that when the ignition is switched off the domestic battery is disconnected from the chargeing circuit thus preventing the starter battery being discharged by excessive 'domestic' use. This does not of course uprate the charge output from the alternator, which is held down by its onboard regulator to a trickle charge, but Caravanners dont seem to find this a problem.

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[saturn]

for a start, disconnect diodes and wire alternator output directly to batteries but make sure no one switches the selector switch to off,with engine running.
you should have increased charging voltage.
if that does happen, best bet is to buy a regulater ( i have a stealth type).and find someone competent to wire it into your circuit,i do not bother with a splitter.

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