Homebuilt 12 Volt Generator Part II

48$ NZ got me a 6/12 V battery load tester including delivery, and it finally arrived from Christchurch a couple of days ago. These units perform both a simple voltage and a discharge load test on a battery. They do a pretty good job of identifying weak lead acid batteries and I thought it would be quite handy to have one.The inside of the unit contains a big resistor; basically, you connect the clamps to a fully charged battery (red to positive terminal), check the voltage and then activate the load test switch for around 10 seconds. The front plate gets warm during this process, and the analogue scale gives a reading of the battery’s capacity. It indicates red for bad, yellow for okay and green for good. In addition, the green area is divided into Cold Crank Amps partitions up to 1000 CCAs. A test of a 3 month old, unused 350 CCA rated battery gave a reading of 12.6 V and between 200 – 400 CCAs.

12 V Batter Load Tester

Armed with this tester and some tools, Gary and I drove down to Pick-a-Part in Mangere to get some second hand 12 V car batteries and an alternator. You pay 2 bucks to enter the yard – they won’t let you in wearing sandals – and yeah, walk around, find the car and part you want and remove it. There is a section with batteries; they seemed all topped and charged up, giving readings of 12.5 V and more. 5 also passed the load test (around 200 CCA), and I ended up taking them at 35$ each.

We spotted a couple of big, solid alternators in a Volvo and Audi, and had a good go at removing them; in the end we took one out of a 1992 BMW 318i. It came off easy, had the required V-belt pulley and looked good enough. We also took what seems like a water pump. The alternator went for 46$ and the pump for 20$. What was amazing is that it took more than 4 hours to drive down to Mangere, get the parts, get a fan belt, some battery clamps etc. and back (80 km drive in total), but it only took 20 minutes to put everything together for a test run. The next picture, however, shows the batteries hooked up in series (using hose clamps) and to the alternator via jumper cables. We did this once we had a successful test run, assuring the alternator was rotating the right way and was operating properly.

14.0 V on the multimeter; battery bank is being charged up.

There are three connections on this alternator, its metal case serves as ground and must be connected to the battery’s negative terminal, there are two wires at the top of the alternator. Both must be connected to the battery’s positive terminal; the current flows through the thicker wire to the batteries when the alternator is operating and the other wire connects the regulator and the field coil to the battery. Alternators do not possess permanent magnets, therefore they can only generate electricity if their rotor coil (field coil) – the inside coil that rotates at the same speed as the alternator’s pulley – is excited with some sort of initial current. It is important to disconnect the batteries from the alternator when not operating, otherwise the batteries will run themselves down powering the field coil. The alternator has a regulator that controls the amount of current flowing to the field coil; with this wire it checks the voltage across the batteries and maintains the current to the field coil so that the voltage stays at a constant 14.4 V (the optimal voltage to recharge car batteries).

I charged the battery cluster for about 90 minutes, initially at 12.7 V and 13.4 V after charging. Will find out soon enough what this means in practice, in terms of how often I have to recharge them. After all, they are second hand, in different conditions and it is winter over here. The MEM said to charge the batteries when they drop below 12.0 V. The pulley ratio between the crank shaft and the alternator is slightly greater than 2:1. At idle, the alternator puts out 14.0 – 14.1 V across the battery bank and we assume it runs at about 2000 rpm. Diesel consumption is approximatively half a liter per hour, and considering that most petrol generators operate at 3000 rpm and that petrol is more expensive than diesel, we’ve certainly got a nice little setup here. The next task is to build a housing for the battery bank and a mobile, weather-proof casing for the generator.

3 thoughts on “Homebuilt 12 Volt Generator Part II”

  1. Nice setup. Do you plan to add a voltage converter for your 220V appliances into the box? Also, what about a basic battery status LED that shows when you need to load the batteries again? Letting them drain completely might cause trouble with the alternator that needs minimum current to work.

    1. Cheers Lars, I’ve got a modified sine wave 300W inverter that I’ll use for 220V appliances to start with. For now it should do, as the laptop and lighting will work on 12 V. You don’t want to leave the batteries drained, that will harm them, so yeah a Volt and an Ampere gauge would come in handy. For now, I’ll have to use my multimeter.

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