I wish to charge a battery of 7.2V 1250mah and 9.0Wh
I have a solar panel which gives a max output of 15.4V 600mah and 10Wh.
So, a) What do I need to convert 15.4 to 7.2v (a small lightweight solution)?
b) presuming the solar panel is at max output, how long would it take to charge the battery?
Ta Rich
Assuming you mean 600mA cell output... Pretty hard to tell to be honest, PV characteristics are not so simple - attach a large load and they'll drop voltage and current fairly quickly and depending on what level of lighting they have. In the UK you'll be unlikely to get even close to max output. But you'll need to limit your voltage so as not to cook your battery. So in essence you need a charge controller and depending on how that handles the charging and voltage step-down it might charge at anywhere around 1000mAh but that's a pure guess depending on your particular solution, some may be very ineligant and just burn off the extra voltage in a resistor. So in essence it could take <2hr to charge, it could take >10 hrs if you take into account the lack of light and real values of power output rather than max.
So any idea what I need to control the voltage step down?
bump
You need a charge controller, as I said, like this (but smaller obviously, this is overkill as it's designed for a 12v battery not your 7.2) 
http://www.solarequip.co.uk/acatalog/Sunguard_4_Controller__SG-4_.html
I have a solar panel which gives a max output of 15.4V 600mah and 10Wh.
1) Wh (Watt-hours) is a measure of capacity ie volts * Amps * hours and has no meaning for a solar panel
2) if the battery is NiCd or NiMH then you can just connect it directly so long as you don't leave it connected too long - the panel won't be harmed, though the battery will be eventually if overcharged - possibly unlikely in our current climate 
at full output will charge nickel types in about 3 hours
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