[SOLVED] Q: LM317T and Solar Panel - Recharge Nickel-Metal Hydride Batteries

Ok, so use resistors to drop the voltage for output from the LM317T.

But since I am using a 1.5W solar panel, do I need to use a resistor with the LM317T?
The solar panel has typical output voltage of 4.5V and output current of 334ma.

Can I wire solar panel’s positive(+) to pin 3(Vin) of LM317T
the LM317T pin 2(Vout) to the battery adapter’s positive

then ground pin 1( ADJ) of the LM317T, as well as the negative of solar panel, and the negative of the battery pack?

The intent is to keep the battery pack from overcharging since the LM317T has a limiter in it. (not sure)


The batteries (how many in series? 2?) will draw probably try to draw too much current which will drop the voltage from the panel to below the dropout voltage of the 317. I tried this a while back.

Also you are using a linear regulator (the 317). This means that any over-voltage from the panel will be expended as heat from the regulator. If you used a switching regulator (~$4), that power would be converted into current for charging the batteries. Much more efficient and cooler too.

Also for efficient charging and long battery life you should use a charger chip to ensure the correct charging regimen.
http://para.maxim-ic.com/en/search.mvp?fam=batt_chrg&tree=powersupplies. There are simple & cheap reference designs in the data sheets.

linear regulator (the 317)

Yes, the LM317T.

used a switching regulator, use a charger chip

Ok, I will give that a shot. Thanks for the extra info.

Use the LM317 in a current-limiting configuration and it should work, but you still need to make sure you don’t charge the batteries past their max voltage.


It’d also be a good idea to read up on “Maximum Power Point Tracking” to understand how drawing current from a panel reduces the output voltage.

Thank you for the wonderful read.