Using a car battery to run Arduino for about six thousand years, need to design

No arduino, this is the entire circuit. That's why I said it was off topic.

Well that's easy to fix. Change thread title to:

"Using a car battery to run Arduino for about six thousand years, need to design stabilizing charger"

I've posted far more off topic stuff, :slight_smile:

heh, heh, done.

Now, how about it? How the heck does this thing work? I understand the voltage control and I added a diode to keep it from running a battery down if the wall plug gets kicked out of the socket. I can set the voltage to darn near anything (within reason) I want with a little 25 turn pot. I just can't figure out why they use a transistor in series with the ground return and have an LED and diode in the base circuit. It also misses the target voltage by a half volt or so when in circuit on a battery.

What the heck?

You've contributed significantly to the forum. I think we can cut you some slack.

I'm moving this topic to General Electronics to give it more attention...

Why not just use a lm317 as a constant current source? I made myself a charger for my 12v 4.5a batteries, set for 13.5v,add a heatsink and let it go,

The objective here is to have a number of cheap float chargers for several vehicles that may set around a while without being started. Things like tractors, generators, quads; that kind of thing. Also, there are often reasons to set a battery aside for months before using it like some people do with boat batteries. I have this problem with some of my vehicles and I'm getting tired of the batteries being dead, or worse, bad when I want to use them. So, I've seen these little harbor freight chargers on sale for as little as 3.99 (US) and I can't possibly buy a wall wart, circuitry, components, etc for that little amount of money.

However, I also want to know how they work and what improvements need to be made for different batteries. For example, the float voltage recommended for AGM batteries is lower than flooded cells, which is different from gel cells; and I have all three kinds in various vehicles. Good float chargers can run as much as $50 and not so good ones are $20. I want to repurpose these little things to be a good one for less than that.

And, yes, I've considered just cutting the circuitry I don't understand out and relying on the 7805 wired up as a variable supply to do the job. But I keep wondering what they intended with the other components in the circuit. There must be some purpose that I don't understand completely. It's not like engineers to add stuff they don't need (well, not usually).

Assuming you traced the circuit correctly, the only purpose I can see of the 2-transistor circuit on the right hand side is to cut off the current (and turn off the LED) if the battery takes too much current or is shorted. However, I don't think it is particularly good at doing that, because the level of current at which it will cut off depends on the gain of the TIP41.

Thank you. That's exactly what a little testing indicated. If I just short the output leads, the device shuts down; it also shuts down if I connect it to a battery that is in bad shape and has discharged below 9V or so.

It also misses the target voltage by a half volt or so when in circuit on a battery.

Could it possibly have something to do with the voltage drop across the diode that you have added to the original circuit?

Don

A bunch more testing in circuit led me to look at the regulator. I'm not totally sure about this, but it appears that the 'good' regulation of a 7805 isn't as good as I thought it was. With a supply a couple of volts higher than the regulator I get about a half volt deviation between no load and an amp of load. That's not a bad thing necessarily, but it does make fractional voltages a bit of a problem. This problem is increased a bit by using the 7805 as a variable supply. The variable version (317) does the same thing, so no help there. Yes, I could stabilize it with an op amp and temperature compensation, but that would be defeating the purpose (remember, cheap).

So, my plan is to get a good battery that is fully charged (have a couple of those) and set the thing to crank out 13.9 volts on float. If the battery water drops because I'm putting too much in, I'll drop it a tenth until the water loss is minimal. Then, at least, I won't have to constantly worry about the water level in the darn batteries dropping below the plates and destroying it. It should avoid most of the sulfation due to loss of charge also. I'll just ignore the open circuit voltage like I never noticed it.

Thanks floresta for the diode suggestion, that got me to measuring things under load at the circuit level. There was one other thing that I noticed, the wall wart voltage drops down to around 12V when a load is put on. That made things complicated until I used my bench supply and cranked it up to 20V to be sure the supply was strong enough.

Of course, I blew up a 7805 testing....got some to replace it with though.

draythomp:
A bunch more testing in circuit led me to look at the regulator. I'm not totally sure about this, but it appears that the 'good' regulation of a 7805 isn't as good as I thought it was. With a supply a couple of volts higher than the regulator I get about a half volt deviation between no load and an amp of load.

The dropout voltage of a 7805 is quoted as 2.0v typical. The worst case may be more. So you should test with a higher input voltage, or else use a low-dropout regulator in place of the 7805. Also, if you don't have an adequate heatsink on the 7805, it may be going into thermal limiting.

I wouldn't use a 7805 in that application, I'd use either a regulator intended for variable voltage (such as the LM317), or a 7812 so that the adjustment needed to the output voltage is smaller and the effect of the ground terminal current is less.

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???

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.

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?

draythomp:
..... 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.

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.

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.

Hi there

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

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.

Hi Draythomp

If it's a question about DIY pride - I rest my case :stuck_out_tongue:

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 8)

-Fletcher

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.