Adding a rechargeable battery pack to existing device.

Hello, I am working on a project right now that runs off of four 9v batteries. Instead of having to burn through 9v's - I'd like to install a rechargeable battery pack to this device, I would like to be able to leave the pack in the device and charge it simply by connecting a power cable to a port on the chassis of the device - rather than have to remove the batteries and charge them in a standalone charger.

I'm interested in any pre-assembled devices that might serve this purpose - but if anyone knows how to accomplish this a diy route I'm also interested.

I've tried researching this on my own but haven't found anything conclusive.

Thanks!

Try finding a schematic of something that works this way already, find out what they're doing and how they're doing it, and copy that..

Jakob E.

Note that most recharge 9 v's really put out about 7.2 volts or something like that. Unless the new ones are better. This might not matter to the circuit.

Depending on your circuit, you might want to think about using a TL499 switching regulator. With a few external parts you can build a circuit that will charge your batteries, regulate the input voltage and step up 3 volts to up to 20V when running of batteries (or up to 30V when running from a 9V battery). You can download a data sheet from here. There was a project I saw once too somewhere...I'll try to dig it up if this sounds interesting to you. I think one of the stores here sells a kit based on this chip too...

Dan, if your still around, you can use the LM317 voltage regulator as a battery charger. Most people think of it as an adjustable voltage regulator, but it can very easily be used as a current source for supplying a constant current for recharging your batteries. You would set it up to deliver 1/10 th you mah rating, which means milliamp-hours. This is how much power you batteries can deliver. It is usually stamped on the side of most rechargables. You could use a wall wart with a jack to plug in to the device your using, which would have the small transistor like 317 inside with a single resistor.

MAH rating:fFor example, a 100 mah battery could deliver 100 milliamps for one hour, or 10 milliamps for ten hours, etc. Just divide the mah rating by the number of hours you will have the bettaries on to give you how much current they can deliver. So if you had a 100 mah battery on for 5 hours, it could supply 100/5 = 20 milliamps per hour for five hours.

So if you have a 100 mah battery, select a adjustment resistor to put out 1/10 th of that, which is 10 milliamps.

Ok , alright, here are some graphs I plotted out for some real data on the LM317. You get your current output by dividing the internal ref volts by your adjustment resistor. Here is a batt charger circuit along with the current output versus resistance.

Perhaps the LM317 could be used as a current source in a discrete op amp or similar audio project. It is very stable. It does level off at about 2.6 ma, with the resistor open.
cj




Current in ma on the left, ohms on the bottom.