how to charge NiMH with a 5v/100mA solar panel

[mikael1985]

Hello,

I turn to you Again as I have some question regarding charging of NiMH cells with a small solar cell. I want to install some LEDS etc. in my daugther playhouseto make the house more fun. But as I want to make some of my prjects off grid thought that all this could be driven by a small solar panel.
I have educated myself a Little on the various battery types, and cannot see how I can charge the large capacity of the cells properly with a trickle of 5V of 10-100mA. and also I want to use NiMH and the deltaV will probably be undetectable in this way.
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

I then considered using the Circuitboard from a solar lamp, but think it will be underspected and inadequate and quite frankly I fail to see how these solar lamps charge the cells probably.
http://www.talkingelectronics.com/projects/SolarLight/Solar5vSupply.html

Can any one point me in the right direction?

Thank you in advance
Mikael


Copy paste from my "technical" document.
Idea:
Wire a playhouse with some different leds, contacts etc. and use a solar panel to charge a battery cell pack which drives everything. Include timers for cutoff to save power.
•   Solar panel > circuit > battery > switches and timers > load

Desired Equipment
a 5v/100ma solar panel
•   0.5W/ panel in full light pr. panel
4 NiMH batteries in series (AA) (use other chemistry?)
•   AA = 4.8v * 2.6Ah = 12,5wh / 45kw
•   Can drive 125leds for 1hr, so plenty of power
Timers (lights 30min, oven, dishwasher etc. 5min)
•   MCU atmega168, latching transistors – probably the least complex solution
•   NE555 delay circuits – might use many as individually wired, but easy to expand.
•   capacitor to transistor base(darlington), cons: graduate turn off.

Problems to be addressed
How to make a circuit that don’t kill the batteries.

[Audioguru]

A simple charger will kill the batteries because it does not detect a full charge then shutoff or reduce the trickle charge current. Energizer recommends a trickle charge not more than C/40 which is 65mA for a 2600mAh cell.

What is the charging voltage of 4 cells? 1.4V or 1.5V each which is 5.6V to 6V for four cells in series. What is the input voltage to the charger when its output is 6V? A 5V solar panel will not do it.

If you want the LEDs to light for 2 hours then the LED current is 2600mAh/2= 1300mA. 2.6V to 2.8V white LEDs can be used with two in series and in series with a 20mA current-limiting resistor to make one string then the total number of these strings is 1300mA/20mA= 65 strings.

If the battery is dead and you want it to be fully charged at the end of one day then it needs 2600mAh x 1.4= 3640mA for one hour or during the day an average of 3640mA/12= 303.3mA for 12 hours. But a solar panel usually does not "follow" the sun, it points at the sun only at noon when the sunshine is the strongest. The average current should be 303.3mA but the solar panel rating must be about 1200mA to do it. Then of course a battery charger IC must be used to sense a full charge then disconnect the charger.