Wiring spotties

[Boltaction Billy]

I have just finished making myself a light bar and have mounted
4 Rally 4000 (100 watts each) spotties and have been fed a lot of different stories which way I should go about wiring them up. Is it better to wire from the battry (knowing that a battery is trickle fed and lights will drain battery quick) or from the alternator (they will only be on when car is running). Can someone let me know what would be better and weather my alternator (GQ) will be abele to handel it ?

[drivesafe]

Somebody else may be able to shed some light ( pardon the pun ) on the exact reason why NOT to tap power at the alternator, but to my knowledge, only one manufacture actually produced a vehicle that sourced it’s headlight power direct from the alternator, but it was only done for a very short time and they when back to the standard set-up of getting all power at the battery.

It was a VW Polo, ( from memory ) back in the 80s and as I posted, VW only did this for a short time.

Again, there must be some reason for not tapping power at the alternator as no manufacture does it today and based on this, I always recommend that additional lights source their power at the battery but it’s based on the fact that, even though the manufactures could save money because tapping at the alternator would reduce the amount of wire needed, they don’t.

Cheers

[chimpboy]

Somebody else may be able to shed some light ( pardon the pun ) on the exact reason why NOT to tap power at the alternator, but to my knowledge, only one manufacture actually produced a vehicle that sourced it’s headlight power direct from the alternator, but it was only done for a very short time and they when back to the standard set-up of getting all power at the battery.

If Drivesafe doesn't know, then I would say I don't know either

But if I were guessing, I would think it has to do with the alternator's voltage sensing. Even though this is meant to be a separate wire, it is usually just hooked into the main one pretty close to the alternator.

That's the only vaguely possible reason I can think of, apart from just the keep-it-simple factor where it is easier to find problems if everything at +12V gets picked up from that big red battery terminal.

I have 4x100 watt spotties running from my alternator... in fact I have had six of them running from it, plus headlights and a/c fan, and the alternator was fine with this. But GQs come with different alternators, and some are gruntier than others. Better check what you've got; the amp rating should be on there somewhere.

[-Scott-]

I can think of a number of minor reasons NOT to tap directly from the alternator, which all point towards the battery being the "better" place to connect accessories. Some of them relate to the Chimp's guess of voltage sensing, but none of them are particularly compelling reasons to NOT connect your spot lights direct to the alternator output.

Biggest potential problem of this arrangement would occur if the main line from alternator to battery fails. If this happens, your alternator will probably run flat out trying to keep the battery voltage up, and end up producing a very noisy output up around 16V. This could burn out your filaments.

Some late model Pajeros have a nasty habit of doing something like this; if one particular fuse blows the alternator regulator doesn't receive it's sense voltage, and starts charging the battery flat out. It will kill the battery quite quickly.

[drivesafe]

Hi chimpboy, I’ve researched it and researched it and like boltaction, I have not found a good reason why the manufactures don’t do it or why VW tried it for a short time and then went back to the battery.

I’m making a guess here but I would imagine that having all your loads source their power at the battery terminal, lets the battery act like a massive big capacitor and filters the sudden current ( and voltage ) change, easing the effect on the alternator.

Just a guess.

[-Scott-]

I’m making a guess here but I would imagine that having all your loads source their power at the battery terminal, lets the battery act like a massive big capacitor and filters the sudden current ( and voltage ) change, easing the effect on the alternator.

Just a guess.

Yep, one of the reasons. The alternator outputs a half-wave rectified AC signal, which the battery "capacitor" filters. The difference in voltage ripple from one end of the cable to the other won't be huge, but it is there.

A few minor technical benefits in connecting to the battery, not too many reasons to connect to the alternator, so the battery connection seems to win.

[Boltaction Billy]

Hay thanks heaps fellas you have all been a great help thanks

[chimpboy]

I’m making a guess here but I would imagine that having all your loads source their power at the battery terminal, lets the battery act like a massive big capacitor and filters the sudden current ( and voltage ) change, easing the effect on the alternator.

Just a guess.

Yep, one of the reasons. The alternator outputs a half-wave rectified AC signal, which the battery "capacitor" filters. The difference in voltage ripple from one end of the cable to the other won't be huge, but it is there.

A few minor technical benefits in connecting to the battery, not too many reasons to connect to the alternator, so the battery connection seems to win.

Makes sense to me, thanks.

[Doc_Acos]

Once it was explained to me like this:

The power supplied from the alternator is completely dependant on the rev's of the engine, if you wire something directly to it, such as lights for example, at base idle or slow driving the lights may dim, etc.

The battery is a constant power source, whether it is only getting the charge from the alternator at base idle or high rev's. It will constantly supply the required power.