Super Simple battery charger...

[dcole]

I've recently switched to using a 12V SLA battery for my DAC (defintely worth it for my system). But now I need to build a battery charger, here is what I was thinking of, but I thought I should run it by a few people first to make sure there wasn't something I hadn't thought of. I don't have a schematic, but it is simple enough that hopefully if I explain it, it will make sense.

I want to use a 317 chip adjusted to 13.8V with R1 and R2, but also include a small resistor connected directly between Vout and the voltage divider that will limit the current that the 317 can put out. For example if I used a .5 ohm resistor it would allow a maximum current of ~270mA @12V to be applied to the battery. This would keep the charger from applying too much current to the battery during initial charging, of course I would need a higher value resistor if I wanted to set a reasonable absolute maximum current, but my plan is to have the battery connected to the charger all the time that it is not playing, so I don't think the battery will ever drop below 12V.

Does this seem like a reasonable approach for a dirtbag battery charger? Anything I've overlooked or suggestions for a different approach? Thanks,
Doug

[crown300]

You mean using something like Fig. 19 here:

http://www.power-sonic.com/techman.pdf

[Davey]

Doug,

The circuit you suggest (it's straight out of the LM317 data sheet..I assume you knew that) is about as simple as it can get, and should provide an excellent float charge on the battery while limiting the initial current. I've used this myself a few times.

Cheers,

Davey.

[Terry Demol]

Originally posted by dcole
I've recently switched to using a 12V SLA battery for my DAC (defintely worth it for my system). But now I need to build a battery charger, here is what I was thinking of, but I thought I should run it by a few people first to make sure there wasn't something I hadn't thought of. I don't have a schematic, but it is simple enough that hopefully if I explain it, it will make sense.

I want to use a 317 chip adjusted to 13.8V with R1 and R2, but also include a small resistor connected directly between Vout and the voltage divider that will limit the current that the 317 can put out. For example if I used a .5 ohm resistor it would allow a maximum current of ~270mA @12V to be applied to the battery. This would keep the charger from applying too much current to the battery during initial charging, of course I would need a higher value resistor if I wanted to set a reasonable absolute maximum current, but my plan is to have the battery connected to the charger all the time that it is not playing, so I don't think the battery will ever drop below 12V.

Does this seem like a reasonable approach for a dirtbag battery charger? Anything I've overlooked or suggestions for a different approach? Thanks,
Doug

Keep the 13.8V 317 reg, and feed it with a 317 set up as
current source. The voltage reg can only pass as much current
as is supplied to it and will only come into voltage regulation
when battery is charges up to 13.8V

Cheers,

Terry

[dcole]

I just went back and looked at the data sheet...ouch. I was sure it wasn't a new idea, but I hadn't read far enough into the datasheet to realize they had the exact same circuit in the datasheet...maybe I'll just take it as a sign of good designing on my part (instead of bad research) that they suggest the same circuit.

Doug

Terry: Is keeping the current constant during the initial charging stages important enough to use this technique as opposed to what I suggested?