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Posted by D Sherman [72.47.9.228] on Sunday, November 27, 2011 at 21:39:59 :
In Reply to: Re: Pressurized systems posted by Kaegi [24.16.253.154] on Sunday, November 27, 2011 at 19:19:44 :
I have several 3 kW 24VDC MEP series military units. They are intended to not only charge a battery but crank the starter. The trouble is that despite tinkering with them they're virtually impossible to start with the pull cord, which is just a rope wrapped around a pulley. They do have a magneto, but to start them you need a 24 volt battery. If I had that, I wouldn't need the charger! They don't even start very well with the the starter motor. Also, they're huge and it takes 2 strong guys to lift one into a pickup. To charge 12 volt batteries, they need modifications to the regulator circuit, which are pretty simple but still need to be done.
So, my idea was to make a small unit that I could easily pick up and carry myself and that would have a fully-adjustable output like the variac-controlled plug-in battery chargers that I like so much. It's a bit of a kludge, and heavier than it ought to be because of the complication of coupling an automotive alternator directly to the engine (rotation is backwards, but I think it should work) and having to build my own frame and control panel. If they were made in quantity in China, they could probably shave 25% off the size and weight, and retail it for around $300 with gauges and full controls, or $200 without.
I don't know what the real market for them would be, though. I have a fairly unique situation of having lots of vehicles at 3 different sites, many without an outlet nearby, and half of them being 24 volt and the the other half 12 volt. Also, I use a 2kW inverter and marine battery to run power tools out in the boonies a lot, so I don't have to leave a generator running all day, and it would be nice to have a way to quickly recharge the battery if it runs down in the course of the day's work, without having to jumper it to my truck and leave the truck running.
I cobbled together a test stand and a crude control panel for "proof of concept" and I'm finding that with the "2.5 hp" engine (probably more like 0.5 real hp), I can get about 15 amps at up to 30 volts out of it, dumping into a load bank. That's as much as I like to put into a regular battery anyway, since "quick charging" is hard on a battery. I'm including an output voltmeter and ammeter, and will set it up with a "field kicker" pushbutton that will give the field enough kick from a couple of nicads to get it started even if the vehicle battery is 100% dead. I'll use AAs so I can swap them in from a flashlight or walkie talkie in a pinch. I've found that alternators sometimes fail to kick in if the RPMs aren't sufficiently high and they aren't connected to a battery that has at least a little bit of juice left in it. Yes a permanent-magnet alternator would avoid this problem, but I already had the 24 volt wound-field alternator on hand. Just to be even more robust, I'm also including a switch to bypass the adjustable electronic regulator and power the field through a simple resistor so that if for whatever reason I'm way out in the boonies and the electronics gets fried, I can still use the thing, adjusting the output current crudely via the engine throttle. The electronics are designed to survive 100 volts continuous and 500 volt spikes. With the output open-circuited and connected directly to the field (worst-case combination), this alternator puts out 80 volts, so that's what I designed for.
Will post a picture when I get it done, if everyone promises not to criticize my welding.