Thread: Ignition/Charging Light

Have the 24V ignition activate a relay. Feed 12V from battery positive to one of the switched terminals of the relay. Take the other switched terminal of the relay to a ~2 watt 12V bulb, and then to the excitor terminal of the 12V alternator.

How is your 12V alternator currently self exciting?

Aaron.

your first warning is your battery gauge, if it drops below 13.8v then you are no longer charging the battery.

no alternator is self exciting they require an input voltage weather that comes from an internal link from the main power feed or a separate feed depends on the alternators design every alternator Ive ever come across has either an external excitation feed or and indication terminal.

either way the hook up is the same wire a small 2-3w 12v light from that terminal to a switched positive 12v supply (thats on the same 12v battery that the alternator in question is charging) and if the alternator output dies then the idiot light comes on.

The difference between the 2 being that the alternator that uses this feed for excitation will stop charging if this circuit is broken and the alternator that uses it for an indication only will keep charging so long as the alternator is spinning (assuming of course that the alternator was excited in the first instance)

Thanks for the information. I had always understood that alternators needed an external power source hence engines that have a completely dead battery and require power to start the engine could not be jump started. However I was reading about self exciting alternators and took the explanation literally as written:

" Self-Exciting Alternator:
Is an alternator that uses a special voltage regulator that doesn't need an ignition wire to activate it. This type alternator only requires a battery wire hooked to it. The voltage regulator contains circuitry that uses the residual magnetism in the alternators fields to determine when to turn the alternator on. The regulator does this by sensing the RPM the alternator is turning, when it gets to a certain rpm the voltage regulator "turns on". Typically you start the vehicle, rev the motor slightly then alternator starts charging. "

Clearly it is the switch regulator that does not need external power but the fields are energised by the single power wire when the regulator switches on.

There must be something odd about the power in my 101 then, as the alternator will run without the 12v batteries installed (the LPG is 12v off this system and runs) (the voltmeter shows about 14v) - so all I can assume that there is a small 24v to 12v reducer somewhere to energise the alternator - so more investigation is needed.

So back the initial problem - it is just a simple matter of hanging a light off the alternator with a couple of relays to activate it through the 24v ignition and another to have the light glow when power is off and to be off when the alternator is generating power.

Thanks

I appreciate your help.

Garry

if it NEEDS the lead to the battery its not self exciting.

take an old landrover genny and spin it, without anything connected it will start making volts. do the same with an alternator and you usually get diddly.

in your place I would wire up thusly.

go and buy a 100A 24v constant duty solenoid (assuming your 12v alternator is an 85A alternator)

wire it thusly

+ 12v battery terminal
|
switched side of the solenoid
|------------------------------12w 12v light---Alternator IND/EXT terminal
12v alternator main power

and then switch the constant duty solenoid on something thats 24v switched.

Garry,

The way a normal alternator works is that you feed 12v battery power (really 13.8V on a charged battery) via the ignition switch, through the alternator charge light and then to the alternator exciter input. The ignition light glows because there is 12v on one side and the windings of the alternator on the other, from the alternator lights point of view it is an earth.

When you start up the alternator starts to generate voltage and that means you now have 12 volts on both sides of the alternator light, no potential difference, no glow. As soon as the alternator stops spinning the light will come on. If it is spinning but starts to loose generated voltage the light will vary in intensity depending on potential difference in the voltages either side of the globe.

My 101 has two batteries in series, so for a 12 volt feed use the positive terminal of the battery that has its negative terminal connected to physical earth.

I have found 2 x 24v-12v convertors on mine so far.

Cheers

Peter

Thanks Dave - I will follow those instructions of yours seems quite simple - I am still trying to workout the that has been put in on the 12v side - usual backyard job using all the same colour wiring - I am tempted to pull the lot out and start from scratch.

Originally Posted by PeterP

My 101 has two batteries in series, so for a 12 volt feed use the positive terminal of the battery that has its negative terminal connected to physical earth.

I have found 2 x 24v-12v convertors on mine so far.

Cheers

Peter

Peter - generally it is a "no no" to do a 12v take off two 12v batteries in series making a 24v battery. I have been told the differential between the charge levels between the two batteries can cause one to fail - battery equilisers used to counter this problem are very very expensive. I looked at all of this when I first got my 101 but gave up on this idea.

The second alternator is a cheap and effective way to go - I am still trying sort out whats there and what works.

The 101 has some sub-systems that are actually 12v and have converters - the obvious one is the fuel pump (12v) and the low tension side of the dizzy is actually 12v (so I have been told).

Thanks for the information.

Garry

Originally Posted by garrycol

Thanks Dave - I will follow those instructions of yours seems quite simple - I am still trying to workout the that has been put in on the 12v side - usual backyard job using all the same colour wiring - I am tempted to pull the lot out and start from scratch.

Thats what I'd be doing...

Garry,

That is right, I was only thinking warning light for water pump not spinning.

When charging like that with two devices a few h/d diodes would protect the alternators and in theory the batteries could last anywhere from five minutes to five years.
A separate battery bank for 12v system is the way to go, switched on by a 24v relay connected to standard ignition.

Cheers

Peter