dual lm317 battery charger

I'm a beginner in electronics but I want a better nimh charger without having to spend alot of money. I came up with a simple cirtuit using 2 lm317.
supply voltage goes into input. battery plugs into output. I would manually time it based upon mah.
I could also plug the battery into aux and use the second half to drain battery and measure mah.
Is there anything I'm missing? I'm mainly going to be using it for 6v battery packs, but occasionally higher. I'm not planning on charging over 500ma. I am curious on how could I know if it's charged not based upon time.
picture showing example. the little boxes are circuits

The best method I know of for detecting a fully charged NiMH cell is Delta V. The pack needs to be discharged bellow 80% or so or this method won't work. Basically you apply a constant charge current and simply monitor the voltage until the voltage goes down. As a NiMH charge the cell voltage rises steadily to it's peak charge point and then drops just slightly, detecting this drop in voltage can be a little tricky, because it's not much of a drop. The higher the charge current the more pronounced this dip is. Typically after this chargers go into a trickle charge of 1/10th to 1/20th C

By the way, give some thought to using Lithium batteries. They're even easier to charge than NiMHs, you just apply constant current till the cells reach 4.2volts and then they can be used right away, they considered 80% charged when charged that way, if you want a 100% fully charged pack once the pack reaches 4.2V per cell you constant voltage and monitor the current till it's 1/10th of the intial charging current. However if Lithium packs are only ever charged to 80% and discharged to only about 3.2 volts they will last a LONG time.

Sceadwian said:

The best method I know of for detecting a fully charged NiMH cell is Delta V. The pack needs to be discharged bellow 80% or so or this method won't work. Basically you apply a constant charge current and simply monitor the voltage until the voltage goes down. As a NiMH charge the cell voltage rises steadily to it's peak charge point and then drops just slightly, detecting this drop in voltage can be a little tricky, because it's not much of a drop. The higher the charge current the more pronounced this dip is. Typically after this chargers go into a trickle charge of 1/10th to 1/20th C

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with a constant current I still set the charger to 1.5v per cell right? I was thinking of having an led that lights up once it gets close to 6v indicating its pretty much done. How would I trigger such a voltage cutoff?
I would use lipo except the higher inital cost and the battery "pack" is actually 5AA that I will also use in other devices if needed.

But does the design look good otherwise?

Thanks

Your circuit is missing a battery charger IC that has "smarts":
1) It detects a fully charged battery and refuses to over-charge it.
2) It detects when the charging battery is fully charged then shuts off.

lol I just want something thats a bit more customizable that the dumb charger. I might get a bc-700 smart charger. But if you know of any good ~$30 chargers could you please link to them. I searched ebay and all I got was thousands of cell phone chargers and china crap

The MAX712 and MAX713 are good Ni-MH charger ICs.

1.5 volts per cell is about normal. Try reading the Wikipedia entry, it'll explain more than you ever wanted to know, and it also shows a voltage curve of a dead NiMH being charged so you can see how the Delta V method works. If you don't mind charging them by hand and watching a multimeter, or have some way to graph the voltage change all you have to do is wait for the voltage to drop, which indicates the cell has been fully charged. I've seen commercial chargers that charge NiMHs at up to 4C (IE 15 minutes) they're delta V, but they're heatsinked fan cooled and probably have a thermal monitor as well.

The voltage of a fully charged Ni-MH battery cell depends on the charging current, the manufacturer and the temperature. The voltage drop when a cell is fully charged is small and might not be noticed unless you use an electronic circuit like in a battery charger IC to detect it. Here are the voltages of an Energizer Ni-MH cell being charged: