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New River, Arizona
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I tried it with as high as 20V and plenty of current availability.  I think you're right about the current to ground being a possible problem; I'll have to check that....as soon as I replace the 7805.  I lost it by having the output voltage higher than the input voltage and no diode to protect it.  Knew better, still made the stupid mistake.  But, that's what experimenting is all about.

Right???
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I've done exactly this.  I have 5 LM317s set to 13.4V  (I'm pretty sure 13.9V is too high) and power them off an old 10 amp charger with a 4,700 uF cap across it.  Well, it's just a transformer with a full wave bridge.  In reality, more like a battery killer.

The set-up works like a charm and has for the past 7 years.  I can float charge 5 batteries all winter.  They range from a 4AH dirt bike battery to a 40AH garden tractor battery.  The present batteries range between 5 and 8 years old, so it seems to be working well.

I just give each battery a good, full charge with a good automatic unit, which brings them to 14.5V, keeps them there for about 3 hours then switches to a 13.4v float.  Once they reach that, I move them over to one of the LM317s.  The LM317s will limit current to about an amp and will actually charge the batteries quite effectively (if slowly for the bigger ones), but once the batteries settle in at 13.4, all five together only draw about 220ma.
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That's the kind of information I've been searching all over the web for.  Information overload keeps getting in the way.  I was going to start at 13.9 and work down because I've lost many more batteries to undercharging than over charging.  The idea was to make sure the water didn't go away too fast.  13.4 sounds like a good number to start with since it should be a full charge; if I haven't misunderstood something along the way.  I only had one 317 to test with and it seemed to have the same problem with open circuit vs load regulation the 7805 had.  I only tested this a little though.

I'm trying to keep this as cheap as possible and the 317 has a different pinout than the 7805...right?  That would mean some creative mounting inside the case.

So, if I read your note correctly, you have one big DC supply and you charge all the batteries from it by putting a 317 in the middle for each battery.  Right?
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..... I only had one 317 to test with and it seemed to have the same problem with open circuit vs load regulation the 7805 had.  I only tested this a little though.

I've not seen this problem with the LM317.  Using the ground pin for adjusting the voltage on an LM78XX the way you have done it would make it more vulnerable to load regulation problems as that ground input is quite low impedance.  You would normally employ a op-amp as a buffer, but neither method is ideal.

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I'm trying to keep this as cheap as possible and the 317 has a different pinout than the 7805...right?  That would mean some creative mounting inside the case.

Yes, it's about as different as you can get.

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So, if I read your note correctly, you have one big DC supply and you charge all the batteries from it by putting a 317 in the middle for each battery.  Right?

Right.
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Hi there

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so I've been looking into a float charger that doesn't let the battery discharge or boil the water out over time.

If this is important you should reconsider the price issue.

I have good experience with chargers from ctec. I know they are expencive but they will keep your battery in good condition.

-Fletcher
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New River, Arizona
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Thanks Fletcher, and I've reconsidered my thoughts on this many times.  Each time I go out to the barn and try to fire up my tractor and the darn battery is dead.  I just can't bring myself to crank out several hundred dollars to put one on each of the batteries.  I have a nice float charger that I move from one machine to the next, but that sucks over time, and I have a real good high current charger, but I hate to use it because that means I've already screwed up and let a battery go dead.

My neighbors all tell me to just spend the money and shut up about it.  I'm going to show them......I hope.
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Hi Draythomp

If it's a question about DIY pride - I rest my case  smiley-razz

I fly gliders and from time to time we forget to turn off the radio in the cable retracting cars (is it called that? - and they are also used to tow the gliders on ground) and the next weekend the battery is flat. We can start the engines with aid from a power source but at the end of the day the batter is still more or less flat. It's nice to know that we can put the charger on the battery sunday and the better will be OK the next saturday - no water level drop due to electrolysis.

Btw, nice blok about the dessert house. Have you considered using PV panels to lower the electrical bill? How about insulation the house? In my part of the world we have 20 mm (or more) on the sealing to keep the heat in  smiley-cool

-Fletcher
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I'm not familiar with insulating ratings anywhere but here in the US, so I'll do the best I can.  I have R39 in the space between the ceiling and the roof.  The roof itself is white to reflect most of the incoming heat.  This makes it a problem to work on the roof at certain times since your pupils slam shut and you can't see when you climb down.  Yes, this is with sunglasses.  Maybe I should use a welding helmet?  The walls are around R30, but I can't give an exact number since I had them beef up the insulation a lot during construction.  The floor is a concrete slab setting right on the dirt.  That may have been a bit of a mistake, but it does hold the cool for a long time during the day.  However, in the peak of summer I can tell about a 10F difference from the center of a room to the edge where the sun is shining.  The floor near the outside walls actually feels warm.  Its funny how the floor actually feels warmer than the walls, but I didn't insulate the darn floor.  Granted, it's probably 160F or so on that wall, so I have to make some allowances.

I did look into solar cells.  I really, really looked into solar cells.  The thing is that the lease option we have here is carefully sized to give the maximum benefit to the solar company, not the customer.  If I buy them outright, it costs U$5 a watt.  So a 60 watt bulb will cost me U$300 dollars in solar cells.  And that 5 bucks is AFTER all the rebates and crap.  I'd love to do it, especially here in Aridzona, but I just can't see giving them that much money.  See, my A/C unit runs around 6800W in the summer (and cold winter); do the math and you'll see how long it would take to break even on a purchase like that.  Plus, I have a lot of little loads like freezer, refrigerator, you know, the usual stuff.  I do plan on using solar cells to power my battery float chargers (when I finally get them working).  I'll get about 50W worth and set them up on a pole by my barn.

My latest energy related project is drop down shades on the patios to cut down on the sun hitting the windows there.  Did you know a basic 10' roll up shade for outdoor use can cost as much as U$120 at one of the local home improvement stores.  The price depends a lot on how much shade (percentage of sun passed through) and material, but they're expensive.  Adding insult to injury, if you want a custom size, like 11 feet, the price shoots up like a rocket.  Naturally, I need 9 feet 6 inches for one of them and 15 feet 4 for another.....sigh.  So, shade cloth, PVC tubing, polypropylene clothes line, and pulleys here I come.

Meanwhile, my 317s are on order.  They're cheap enough to try and see what happens.  When I have conquered the float charger, I'm going to start on a larger one to recover a battery that has been mistreated.  I'll get one of those great big chargers and smarten it up a whole lot.  I thought about that for the float chargers, measure the voltage with a divider hooked to an arduino and adjust the output voltage to supply the current in stages.  That seemed overkill for a float charger, but I may revisit that if the 317s give me too much trouble.
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