Wiring up an alternator

Such a simple subject. Yet so often it is done incorrectly.

Typically the alternator is left as it comes wired up from the engine supplier. This will mean the output of the alternator is connected, via a cable that is too small, into the wiring loom which will eventually meander it's weary way down to the starter motor. From there it will run via the main starter cable to the engine start isolator switch (if fitted) then eventually to the engine start battery. This is fine if the alternator only has to recharge the engine start battery. The engine battery doesn't take much to recharge it. Starting the engine removes very little power from it (assume 500 amp starter motor current, for 5 seconds = 0.7 amp hours! - even Peukert corrected for a typical 650 CCA engine start battery it only comes out at 1.4 amp hours - clearly this isn't going to take much power from the alternator in order to recharge the battery).

Often, however, it is not the case that the alternator only has to recharge the engine start battery. The alternator will be expected to charge another auxiliary battery bank via some from of split charging device.

I'm going to analyse exactly how the alternator power works it's way to the engine battery because it is one of the most important points in this issue.

Firstly one has to accept that each and every connection introduces some losses. No matter how good, there will still be losses. A typical, well made, connection in a 35mm cable (say a lug onto a bolt) will have a resistance of around 0.0002 Ohms at the actual connection interface, that is to say, at the point of contact between the lug and the bolt, and then this same resistance between the lug and the cable. This, admittedly, sounds like nothing. But lets count up the number of connections between the alternator and the battery.

Alternator output bolt to lug = 1
Lug to cable = 2
Other end of cable to lug = 3
Lug to starter motor terminal = 4
Starter motor terminal to lug = 5
Lug to main battery cable = 6
Other end of battery cable to lug = 7
Lug to isolator switch = 8
Isolator switch to lug = 9
Lug to cable = 10
Other end of cable to lug = 11
Lug to battery post = 12

So just to get back to the battery, the alternator current has passed through 12 separate connections, each one with a resistance of around 0.0002 Ohms.

On top of this we have the 2 contact points in the isolator switch (yes, most isolator switches have two separate contacts points) each introducing around the same resistance, perhaps slightly more. So that is a total of 14 connections of 0.0002 Ohms each, or a total of 0.003 Ohms. This still sounds like nothing. But at 50 amps charge rate that translates onto a voltage drop of 50 amps * 0.003 Ohms = 0.15 volts. That can make a huge difference to charging efficiency.

So clearly, the more connections we can remove from this chain, the better the charge results will be.

Now consider the cable from the alternator to the starter motor. This is usually 16mm cable or thereabouts. A 1 metre length (typical distance through the engine wiring loom from alternator to starter motor) of 16mm cable has a resistance of 0.0012 Ohms.

And finally the other 2 metres of 35mm cable from starter motor to engine start battery has a resistance of another 0.0012 Ohms.

So we have a total resistance from the alternator output to the battery terminal of 0.003 + 0.0012 + 0.0012 Ohms = 0.0054 Ohms giving a voltage drop from the alternator to the battery post at 50 amps charge current of 50 * 0.0054 = 0.27 volts. So although the alternator is correctly regulating the output at 14.4 volts, the batteries are seeing 14.13 volts.

Now we still have to split this charge across from the engine battery to the auxiliary battery, introducing say another 8 connections and another metre of cable. This could introduce another 0.15 volts of drop. So now the auxiliary bank is being charged at 13.98 volts instead of the 14.4 that we thought.

If we disconnect the output of the alternator from the engine wiring loom (and insulate the cable into the wiring loom for safety) then fit a new cable (correctly sized - say 50mm or more) from the output of the alternator directly to the auxiliary battery isolator switch we instantly bypass about 90% of all these loss inducing connections. Thus the losses between the alternator and the auxiliary bank will be reduced by the same 90%

Thus the axiliary battery bank gets a substantially improved charge with greatly reduced losses.

An added benefit is that the split charge device now only has to pass the relatively small charge current into the engine start battery instead of passing the relatively large charge current into the auxiliary battery bank.

Now it is true that an external alternator controller will compensate for the voltage drops through all these cables and connections and ensure that the voltage at the batteries is correct, but it cannot do anything about the wasted power that all these extra (insufficiently sized) cables and connections introduce. This is power that could otherwise have been going into the batteries.

In the example given above, there is a total loss between the alternator and auxiliary battery bank of 0.42 volts at 50 amps charge rate. That is 21 watts of wasted power. All it is doing is heating the connections and cables up instead of recharging the batteries, which is what it should be doing.

There is one important point to be aware of if this is done.

If the engine is started with the auxiliary battery isolator switched off then the alternator will have no load on it. This could easily damage the alternator.

If you can be certain never to do this then clearly it is not a problem. However if you feel you may forget, or the installation may be used by other people who may forget, there are a few tricks that can be done to prevent this happening.

One way is to power absolutely everything, except the starter motor, from the auxiliary bank. This way, if the auxiliary bank is switched off, then the engine will not start. This has the disadvantage that the domestic bank must always have sufficient power in it to energise the starter relay and heater plugs (if fitted). If the auxiliary battery is totally flat, then the engine will not start. However, if the auxiliary battery is so flat that it will not energise a starter relay, then the battery bank is probably destroyed and the fact that the engine will not start might be the least of your problems.

Another way is to use the starter switch that normally energises the starter relay, to energise another relay powered by the auxiliary battery that then energises the starter relay. The starter motor and starter relay will still draw their power from the start battery but the auxiliary battery will power the small relay. Again sufficient power has to remain in the auxiliary battery to energise one small relay. This system has the advantage that the engine will still start with the auxiliary battery in a much lower state of charge than the first option (all it has to do is power one very small relay), but has the disadvantage that it is much more complicated to install.

If you still want to rewire the alternator to get improved charge into the auxiliary battery bank, but this problem concerns you (i.e. the inability to start the engine if the auxiliary battery is totally flat) there are still a few more ways this can be guarded against.

One way is to add a system to manually connect the two battery banks together. This could be another power feed to a split charge relay (SmartBank has this function designed in as standard) or a manual isolator switch connected between the two battery bank positives to connect them together (obviously this would normally be left switched off).

Another option is to fit a switch so that in an emergency the starter relay is energised from the engine start battery instead of the auxiliary battery. This has the disadvantage that if this is done with the auxiliary battery isolator switched off then the alternator is still at risk of damage. Hopefully if you have to resort to this switch, then you will remember it is because the auxiliary battery is switched off (or flat) that you are having to do this in the first place.

Finally, you could fit a SmartGuard battery protection system to prevent the situation arising in the first place.


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Page last updated 02/04/2008.
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