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Old 08-05-2007, 05:31 AM   #1
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Alternator off... Providing a an adequate replacement power source

I've been thinking about this, sparked by a recent purchase of some small voltage regulators from ebay for another project I was working on.

The alternator gives an output of 14.4 volts, howev the battery gives an output of 12 volts.

When you go alternator off, at least in my car, the lights go dimmer, and other things (e.g. the windscreen wipers) slow down, and so on. This is also a problem when you are doing engine-off coasting. Also, the car battery isn't really designed for this, and therefore, it will reduce the life of the car battery. (Note - I am using a replacement battery which is not faulty or worn out etc).

I was thinking of using the following alternative option:

Have two leisure batteries, which can be charged in-situ (you can get ones with small plastic tubes for gas venting), in the boot of the car. Have a control circuit, which can run in two different modes:

(1) Standard - power from alternator, electrics driven by alternator + car battery
(2) Alternator off (includes engine-off coasting). - alternator electronically disconnected, electrics driven at 14v via voltage regulators from the 24v supply from the two leisure batteries in the boot, car battery charging circuit disconnected.

The circuit could be activated on/off via a switch on the dash (so you can switch it off when you leave the car), but would only activate when it senses that the alternator voltage drops below a certain voltage (e.g. 12v), when you are engine-off coasting, or when you switch on the 'alternator kill' switch.

I think that a homebrew circuit would be fairly cheap to make, if you did it yourself, e.g. using multiple voltage regulators such as these which are cheap, and you get about 450 watts worth for $10. They can be used in parallel : and I think this idea could be used for more than two regulators. These ones are 15V but I'm sure you could find 14V ones too. You could then use relays to disconnect / connect the various power supplies too. You could design it to be easily disconnected / bypassed in case of failure, with some strong 'bypass' wires kept e.g. in the glovebox for this purpose. A voltmeter (or voltmeters) would show the voltage of the leisure batteries, so you could switch back to the alternator when you get to e.g. 50% capacity.

Any comments on this idea?

Edit - I realise that switched-mode regulators would have to be used rather than resistive ones - so maybe the ebay ones are not such a good idea.


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Old 08-05-2007, 07:09 AM   #2
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Yeah you definately want to be running in switch mode and these regulators may be able to do that - usually you add an inductor in series with the output then a cap and the feedback for the regulator goes on the cap. There are some other types of switch mode 12 and 15 volt modular regulators that can handle several amps that are 80% efficient and totally self contained except for some trim pots for output voltage control you may want to try. But a simple high powered transistor and inductor with some regulation could be built up pretty simply to output 15-20 amps at any voltage you want.

Then you may want to switch out the series batteries to parallel and charge them off the alternator on down hill engine braking.

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Old 08-05-2007, 10:51 AM   #3
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Many race cars run without alternators. You can buy 14 volt batteries and chargers. I had several hits with a google search. Your way would work too.
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