Solar Cell - Preventing overcharge of an NiMH battery

I am trying to design a circuit and was wondering how to design it that the 12V 800mAH NimH battery I am using will not overcharge. I have researched trickle charging and as far as I understand, a zener diode that regulates the voltage down to about 13.3-13.7V should do. I chose a 14V Zener, taking into account the .6-.7V drop.

I have heard that trickle charging is not the best for the battery, but am not sure how much more work designing a smart charger would take.

I was also thinking of using another diode to prevent flow back to the panels from the battery.

^{simulate this circuit – Schematic created using CircuitLab} The three voltage sources on the left represent the solar cells, with the NiMH battery on the right.

I was wondering if I needed any additional components if I were to use three 5.5V solar cells in parallel outputting 40mA (16.5V total at 40mA, regulated to 13.3V by the zener) in order to prevent overcharging.

A similar question to mine was asked at How do I avoid overcharging a battery? However, this dealt more with a larger lead-acid battery and used a voltage regulator with a schottky diode.

Smart charging circuit for NiMH battery pack This link discusses almost my exact same question, but I was confused to the answer given which said that up to 1.45V per cell could be charged, taking my max charge voltage up to 14.5V.

Considering I am only outputting 40mA to an 800mAH battery, it seems that I may not need any more battery protection, but would love any input.

Answer

First of all, you have the three 5.5v solar cells in parallel. They need to be in series to generate 16.5v.

NiMH batteries are usually charged using special IC's made for this purpose since they require special monitoring. However at the low level of current you have available (40 mA) you can get by with a much simpler charger as mentioned here. You need to make sure the current doesn't go above this level (it needs to be below 0.05C, where C = 800 mA). If the current does go above this, up to C/10 (800 mA), then you need to add a timer so the charging cycles is not more than 3 hours.

You need to replace the 5.1 Zener with a 12v one, such as the BZX85C12. In addition, you will need to add a 110 Ω resistor between the output of the solar cells and the Zener. The value is calculated by dividing the voltage drop 16.5v - 12v by the full-load current of 40 mA. At no load, the Zener will dissipate 0.04A * 12v = 500 mW, or half the maximum rating (1W) of the diode.