Constructing a portable DC Generator
As most of my operating is either portable or mobile, using 12-14V DC, I decided to make a petrol driven generator to charge batteries.
I managed to get a second-hand petrol driven lawn-mower for £5 from a local school jumble sale. It looked in good condition, and having established the engine ran OK, I dismantled the mower and cleaned up the motor. This mower had a horizontally mounted motor, that is the drive shaft of the motor is horizontal. Other mowers have vertically mounted motors which would require a different construction method. The engine is rated at 3.5 Hp. which means it should be able to turn an alternator up to 70 Amp output without much of a problem. 1 Hp. is equivalent to 746 watts. 70 A is only 840 watts.
From a local auto-electrician I was able to get a reconditioned car alternator for £15. I could have tried a vehicular dismantler's or scrap yard for one but I thought I would get a more reliable unit.
The alternator is capable of 45 Amps output, and should be ideal for charging batteries when out on the hills for any length of time. And it should save running the vehicle engine when the battery state is low. At the moment, my current maximum required is 35 Amps intermittently. So this should be enough in hand, until I get my 500 watt HF linear running!
With my mobile/portable setup, I use an auxiliary battery supply which is 'split charged' from the vehicle alternator system. I am using a 110 Ahr leisure battery, which may not be as good as something like a wheel-chair or golf-cart battery, however it is a deep-cycle type of battery. This type of battery is not the same as the normal car battery. It is designed so it can be almost fully discharged and then recharged. A normal car battery is primarily designed to start the vehicle and then be topped up by the vehicle charging system. It would be damaged, with a shortening of it's life expectancy, if used with deep discharge cycles. Using the vehicle battery can lead to embarrassment, especially if you operate for a long time, and discharge your power to the point when it is not going to start the engine!
To save the cost of welding a frame up to support the engine and alternator, at present I have bolted them to some exterior grade ply-wood to form a base which seems to work OK.
The alternator is belt driven from the engine. With a 5" pulley at the engine and the standard pulley on the alternator. Alignment of the pulleys and belt prevents undue ware. Tension is applied using an adjusting bolt between the alternator and the base board.
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