I have a cheap solar powered battery charger. The solar panel puts out approximately 7 VDC, not sure how many watts it can handle.

The charger can be used to charge 4 AAA, AA, C, or D batteries. I have attached a basic schematic.

I want to add some sort of indication to let me know that the batteries are charged. What do you suggest I do to accomplish this?

 

The cheapest and easiest would be to get yourself a digital multimeter for a couple bucks and just monitor the voltage on your battery pack. The exact voltage you're looking for will depend on the chemistry and specs of your batteries.

The next approach would be to use a comparator to show when the battery voltage has risen above a target. You'll need a voltage reference - such as a zener diode - to compare the battery voltage to, and again you'll need to know what the target is. You can light an LED with the comparator. Keep in mind that the circuitry and of course the LED will waste power.

 

Yes, any comparator circuit will use power, but the use of a micro power band gap voltage reference and a micro power comparator should keep the current consumption to a matter of perhaps a few tens of microamperes. Be sure of course to use reasonably high scaling resistors, consistent with the input bias current requirements of the comparator, which again should be as low as possible.

Finally, it may be best to arrange that the LED comes on after charging is finished, unless the reassurance of a "Charge On" indication is preferred. In any case, very low current indicator LEDs are available which will do their stuff for just a few mA, much better than some older types that need 20mA or so th produce a glimmer!

 

All true.

Another way to eek out a bit more power is to select a Schottky diode for the blocking diode. You want the voltage drop across the diode - which is unavoidable - to at least be as small as possible. A "normal" diode with a 0.7V drop across itself is wasting over 10% of the power when you're charging batteries at 6V.

 

The "charger" circuit (only one diode?) is much too simple:
1) Nothing limits the charging current.
2) It continues to over-charge the battery cells when they are fully charged.

You should learn about your rechargeable cells (each chemistry is different) then buy or make a charger that works properly.