Here's the problem: We have a Cessna 172 that goes for months without being started, especially in the winter time. The airport charges a fee to give it a jump. How can we charge the battery? We can't drive a car onto the field, so whatever we come up with has to be portable.
Here is what I am thinking. There are 24 volt battery chargers available for sealed lead acid (SLA) batteries used in scooters. There doesn't look like there is a reason that these would not work on a the wet 24V batteries in small airplanes. They are designed to cut off if the battery has no charge and they have electronics to prevent an overcharge. They have low charge rates, but they would still work if the battery had any life at all.
I could construct a battery charger that went from a portable twelve volt source to a charger to the plane's external battery connection. Now to find the parts.
I have already started ordering them and I will take pictures as they arrive.
1. Small planes like Cessnas use a plug type AN2551. This is usually about $100 in the catalogs. I found it at www.aircraft-spruce.com. They sell one made out of ash wood. This is fine with me. If it is not finished, I will paint it to help it resist the elements. The exact page is here. The cost was $30 plus shipping and I got the plug two days later - very fast shipping.
2. Charger. What I wanted is a charger that had a cigarette lighter connection so that I could use a 12 volt source to charge the 24V battery. These are made for battery powered scooters and are not too bad in price. I found one here. The price was listed at $60, but at checkout I paid $55 plus $5 USPS Express shipping.
These seem popular in England where there are lots of 24V devices, but they are hard to find in the US.
I have not received this yet.
3. Now I need a 12 Volt Source to power the charger. I have two choices. The first is a rechargeable 12 volt battery. There are "emergency kits" that have a 12 volt battery that I can buy for about $40. The picture is from the J.C. Whitney Catalog. This unit has a car lighter plug for the charger and it has a nice strap so it's easy to carry.
I have not yet ordered this. I expect that I can pick one up at sears after Christmas, cheap.
An alternative to the 12 Volt battery source is a 12V solar cell. This could be hooked up to a female cigarette lighter plug and placed in the Cessna's window facing south. The wire from the charger could snake out the window and out to the battery connection, keeping the plane charged all the time. You can buy a VW solar cell for under $20. These were used to keep the VW batteries charged during shipping and the seem to be all over the place. They are set for trickle charge and the amperage might not be enough to power the charger, but combined with the battery pack, it might be a good thing. I am watching the eBay auctions on these and will buy a couple to experiment with.
Next, is to put it all together and try it. Keep checking back and I'll let you know how it works.