LM317 Charger Problem

Hey, Garth, thanks for the reply. Since the cordless driver is rated/marked at 2.4 volts I would think that the vehicle's power (12v battery) would be more than enough to handle charging the thing. The only reason I want the driver is to remove my front license plate which obscures access to my front-mount receiver on my truck (where I hook up things like tow hooks and trailer balls), so 1) it won't get used very often and 2) even when it does, it can take an overnight to charge up, no problem. The problem is that my current charge setup is only putting out about 75 mA so it actually will take a fully charged driver and, over about a week, discharge it to where it barely runs (as opposed to if I just disconnected the driver from any charge source in which case it would go dead in about 2 days). :(

Using an inverter is not a solution I'm willing to entertain because 1) space (where to mount it?), 2) cost, I don't want to spend any money to buy an inverter that will only be used to charge a 10 dollar driver, and 3) it's not an elegant solution like the homemade charger would be. :-/

I'm kind of obsessive about things. :) It's hell being me. Thanks again for your help.

--HC

Um, Hello, my name is HC and I'm an Idiot.

If you hook up the LM317 just like the technotes document from National says to, running Vin to 13.8 volts, and Out to the load, but you MISPLACE the wire on the breadboard to the Adj pin, so that the connection that is supposed to exist between the Adj pin and the load does NOT exist, it runs basically straight 13.8 volts (Vin voltage) to the load. :-/ I went to start on my constant voltage regulator and was pulling wires off the breadboard and saw that I had missed placing the Adj wire connection in the right row, it was one row off...and I had not seen it. I hooked it up right and it's putting out 120 mA like it should. Now the question is whether or not that'll burn that battery out because 120 mA is more than it draws at idle...but at least I'm now in the right range.

Sorry folks, sometimes I just screw stuff up. :)

--HC

Now that I've (hopefully) eradicated sheer stupidity from my project I have it charging the driver but the problem is that it's putting 120 mA across the driver even though the driver is fully charged (the same driver on the factory wall wart only draws 110 mA). I'm afraid that difference may mean that I'm going to boil/burn/ruin the battery. Plus, I have had to put a heatsink on the LM317 because it's puttin' off some heat (enough that it's painful to touch it). Can anyone shed some light on what I'm doing? Am I doing (yet again) something stupid?

The constant voltage regulator tried to push 260 mA over the same driver and got hotter than all git out, too. :-/

Thanks, all.

--HC

Hey, Garth, thanks for the info, again. I got the thing charging now with the output at 122 mA, and I'm running it all through a green LED in parallel with a 22 ohm resistor (to keep the LED from burning too hot) and it seems to be charging the driver fairly rapidly. The time limit you mention makes sense (since the current draw is 122 mA (give or take 1 mA) whether it's the drained driver or the charged driver that I'm currently testing), but I want to avoid that since I intend to leave it in my vehicle and I want to be able to use it at a moment's notice. With the darned thing discharging itself after a day or two it would be useless to have to leave it unplugged after X number of hours and then plug it back in for Y number of hours; 1) I have three vehicles (not braggin', and they're not all new) and I use whichever suits my needs or tastes on any particular day; the one in question here is my truck and I try not to use it for short trips (4 miles) to town to check the mail; so it may be days between when I drive it, and

Maybe this stuff about the drivers draining themselves in a day or two is Ryobi's cheap way of circumventing complicated (I think the stuff would be) circuitry to check if the battery is full...just use a load resistor across the battery so it always eats current, that way you could never over charge it. Maybe? I'm just not very well versed or knowledgeable about electronics. :-/

I'm testing the rig now with the LED parallelled with the 22 Ohm resistor, it's across my meter putting out 122 mA, and it's charging the deader of the two drivers (the one I've had in my truck on that system for over a year). I had to put a heatsink on the LM317 (I had several I robbed off an old VCR that I was trashing) and we'll see how that's doing shortly.

Thank you again, I appreciate your help and thoughts.

--HC

You need a constant current at more than 12 volts to charge the battery. The LM317 will lose a couple of volts across itself and that alone puts you below what is needed.

You need to boost the voltage first and then regulate the current. There are many IC's that can boost the applied voltage but their output current will be limited. I doubt you'll achieve a rapid charge though you will probably be able to trickle charge. That could take about 10 hours which I'm sure is unacceptable.

How about going the easy way and get an inverter...this will create 120 VAC and you can use your home charger to do the job. This is not efficient so watch the drain on your vehicle battery.

For some reason I got the idea that this was a 12V driver. Sorry!

Is this driver using sealed in rechargeable C cells? Perhaps they are sub C cells. In any case, you still need a greater voltage than the cells in order to charge them. Based on the voltage, these are NiCad batteries. The voltage needed is related to temperature but something close to 1.4 volts per cell at about 68 degrees F. Your two cell screw driver is going to need 2.8 volts to do the job. the required current should be one tenth the cell capacity for no more than 16 hours at a time. More than this will cause the cells to heat and reduce their capacity. The lesson is to charge it and then don't charge again until it is empty.

A high capacity sub C cell might be rated at 2Ahr but this is a guess.

The NiMH battery in the phone is a higher voltage than you need but a few series diodes will reduce that to a suitable value or better, use a low dropout regulator to precisly set 2.8 volts. Measure the current with your discharged screwdriver connected to the modified phone charger. If it falls within the range above, happy days! if it is too high, control it with a resistor ( but watch for the voltage drop ) or shorten the recharge time. Just remember that a hot battery isn't a happy battery.

Good, now knock the voltage down to 2.8V and you are good to go! Yeah, I have seen 1200mAH batteries too so 120mA is good .1C rate to charge your batteries. Just keep that time limit in mind, 16 hrs max or stop when the cell warms up.

Very good but what is the charging voltage?

A NiCad can develop an internal short, they do this more easily with elevated temperatures. That short can make the unit hot. There are methods to restore NiCads but I can't vouch for their effectiveness.

Hey, Garth, the voltage across the positive and negative leads of the charger (while actually charging) is 2.6v (which I think means that I'm sinking all available voltage EXCEPT 2.6 volts. The voltage across the leads of the device with the driver NOT attached, so it's open circuit potential is 13.6 (approx.) volts. I ran the unit for a while last night without the housing of the driver getting hot or even warm, but the LM317 sure gets hot. I set one device up as a constant voltage device, as I think I mentioned in one of my posts and it got hot, too. I still don't know if I should be setting this thing up as a constant voltage or a constant current device, or if I should be looking for some different technology to do this. If it's constant current, won't it do just that, remain constant so, say the driver's battery is getting fully charged so it's current tries to drop (it's drawing less current), won't the setup of the LM317 as a constant current device just increase voltage to still manage the same amount of current output? With the constant voltage device it would seem like it couldn't increase the current because it can't increase the voltage, so that might be better, but it was getting hot, too.

The heat tells me that maybe the problem is inherent to this device trying to dissipate/sink the difference between a 13.8v car system and a 2.4v driver. It would help if I knew a lot more about electronics, and it's frustrating not knowing this stuff any better than I do; I fear that my ingnorance may be preventing me from seeing or understanding some simple thing that would make this clear.

Anyway, thanks for your help and time.

--HC

The chargers output voltage is not 'stiff' so you need to measure close to 2.4 volts open circuit. That means without anything attached to the output of the voltage regulator. Given an input of 13.8V and an output of 2.4V, that means the 317 is going to loose 11.4 volts across itself. It will need a heatsink since that is quit a bit of difference. Make it reasonably large.

Now that you have created the proper voltage, you can regulate the current. The second regulator will loose some voltage across itself as well so be prepared to go back and adjust the voltage regulator to make up for the loss on the current regulator. Ultimately, you should read the output of the open circuit current regulator to be close to 2.4 volts and if connected to a battery, the current should be limited to 120mA for a .1C assuming 1200mAH sub C cells.

You need both conditions to do this properly.

Too high of a voltage applied across the battery will damage that battery. The way one sets up bench power supply is to adjust the open circuit voltage and then set the current limit to zero, short the supply and SLOWLY turn up the current limit until the desired level is reached.

Hey, Garth, what I've got right now is the circuit as I've described it with decent-sized heatsink on it and it's working well. I'm leaving it on the bench setup running almost 24x7 (well, as much 24x7 as there is in between yesterday sometime and today ) with the only exception of that I shut it off when I leave the house (since I work out of the house that's not much down time). So far it's charged the battery quickly and yet the battery/housing doesn't seem to be getting warm (which it would, I think, if it was hurting it). The LM317 get's warm, but with the larger heatsink it's not unpleasant to touch (like it was by itself).

If I understand your most recent post, though, you're suggesting that I run two LM317's in series? One as a constant voltage supply and one as a constant current supply?

Thank you again for your time and help.

--HC

Sorry, I'm running today...

Yes, that is what I'm suggesting...

Hmm, okay, I'll try that. I won't be able to do so for a couple of days (other problems/projects), but I'll let you know when I do.

Thanks again for your time and help.

--HC