How to set up an alternator with a stationary engine

A few people have asked how to wire up an alternator to run a few lights or charge a caravan battery etc. With DC dynamos getting hard to find, many people are starting to use car alternators instead.

Below is a diagram of my set up, which is a Lucas alternator from an old Landrover 2.25 engine. However I believe similar xxACH series alternators were fitted to many other vehicles.

How to connect at Lucas alternator to charge a battery.
How to connect at Lucas alternator to charge a battery.

The main difference between an alternator and a dynamo is the alternator needs a small electrical current to power the field windings, which in turn creates magnetism and generates the charging voltage. The built in voltage regulator then senses the outgoing charge voltage to make sure it’s within a given range. If the charge voltage gets too high then the field reduces so less volts are produced. If the charge is too low then the field increases so the charging voltage also increases. A 12v battery will normally charge at about 14v-16v, give or take a volt or two. The key thing to remember is if everything is working correctly then the battery will charge at a few volts more than it’s normal working voltage. ie a 12v battery should charge at around 14v but a 6v battery (which would require a 6v alternator) would charge at about 8v.

If you place a meter across the battery terminals when the alternator is running and only get 12v then either the battery is fully charged and the alternator has sensed this so is not delivering a charge, or the alternator is not working correctly for some reason. The most likely cause of an alternator not charging is that it’s simply not spinning fast enough. Remember a car engine is normally running at around 2000RPM Assuming the alternator runs at the same speed as the car engine crank (they often don’t, they spin faster) then your 750RPM Lister D obviously isn’t going to turn it fast enough. The simple solution to this is to put the alternator belt around the stationary engine flywheel, not the pulley. As the flywheel is normally much bigger, this will speed up the alternator and you should get a charge.

In the diagram above, the whole set up is mounted on a metal frame. I actually used a plywood board with two pieces of dexion running down parallel edges to mount the alternator to with a couple of bolts. The battery also stands on this board to add a bit of weight to stop the whole thing moving around and the belt becoming loose.

The -ve terminal of the battery is earthed to the dexion mounting bracket. The alternator itself is also earthed to this bracket by being bolted to it using the normal bolt hole in the casing which would normally hold it to the car. This -ve earth provides the earth return side of the circuit.

On the back of the alternator there are also three spade terminals, which are normally connected as one unit by use of a plastic 3 pin connector. The smaller of the three spades should be connected to the battery +ve via a small charge light bulb. This is the equivalent of the charge light on your car dashboard and will light using power from the battery when the alternator is not charging. This light will come on when the alternator is not running.

The large connector at the opposite end of the block of three is the +ve charge to the battery. This should be connected directly to the battery +ve terminal and should be a large enough wire to cope with the potential maximum amps the alternator can produce. If you’re lucky enough to have a bit of wire still attached to the alternator then use this as a guide for how thick the new one needs to be. The third pin of the multi-pin connector, which is the middle one of the three, should be left blank. Remember the -ve to the battery connects directly to the frame of whatever you’re mounting the alternator onto.

With your three connections made that’s it! Put the belt around the alternator pulley, connect it up to your stationary engine, make sure the alternator is turning the right way, and away you go!