How to best charge set(s) of 3 x AA NiMH batteries from a 12V solar panel?

Since both the power from the solar panel and the surrounding temperature can change suddenly, I suppose it would be most reliable to charge with C/10 from morning to evening, without any fancy algorithms?

I'm consider trying to use the solar charger https://www.sparkfun.com/products/12885 witch is based on LT3652. That charger is supposed to charge a lithium cell, but the charging current can easily be modified to match C/10 of the NiMH batteries. However, its charging voltage is 4.0V (not 4.2V) and that would be more difficult to change. 4.0 / 3 means charging with only 1.3V per NiMH battery. Lets say the solar panel provides power for 8 hours a day.

How would NiMH batteries react if charged with 1.3V and C/10 for about 8 hours:

- I assume they would not be charged up to 100%, but about how much of their capacity can be expected?

- Would there be any long term issues if the NiMH batteries are seldom charged to 100% of their capacity? (e.g. memory issues, sulfation like issues, etc)

- Would it be acceptable to mix batteries with different charging levels if charging only with 1.3V and C/10?

Are there anything else I should think about? (I'm more concerned about safety and number of battery life cycles, than using their full capacity.)