Can a 3,4V (open circuit voltage) solar panel charge a 3,6V NiMH battery pack?
You would need a higher voltage as 3.6v cells usually charge up to 4.1v or there abouts. You also have to consider thermal effects on the panels(I think telefox eluded to this). 6.5v in direct sunlight on a good day...my panels would drop close to a 1 volt and sometimes more due to the panels heating....I think it's called thermal breakdown or something along those lines...I originally had the panel pushing around 5.25 but the v dropped too much for the vreg and charger to work effectivly.
To be more specific, I was thinking about using this and this (lol sorry for the quality of my English):
but I guess I'll use this with a 1N5817 diode (to prevent the batteries from discharging through the solar panel when there is no sunlight). That will give 18mA at short-circuit and the 300mAh battery will be able to dissipate the extra heat for an any period of time.
Would the 4.5V solar panel work? Or is that too much?
4.5V is not too high, --in fact I would use between 5 and 6 volt cells. After all the actual voltage output depends on the available light starting from zero at full dark and only giving full output if used under bright light.
I followed the links you mentioned and notice that with postage these solar cells will cost you about $14 USD (to the U.S.)
This is only one solar cell at too low a voltage and amperage to be useful. I personally would order from Satistronics in China. No, I do not work for them, but I have dealt with them before and was pleased with the results. The only drawback is the delay in shipping from China which has been about 2 weeks each time.
You can get TWO 5.5volt 80ma solar cells for less than $6 USD and another $4.50 for postage (to the U.S.)
This seems a better choice. If those cells are too large physically, they do sell other smaller ones also.
With two of the solar cells, you can wire them in paralell (with a 1N5817 in series with each to cover differences in the two cells) for higher current, or wire them in series with each other for a higher voltage which will help in lower light conditions.
I hope this information is helpful.
Thanks a lot for the reply. The postage isn't so bad, as I am also buying lots of other stuff and they also ship free for orders over 75$. The amperage is fine because the point of the robot is to just trickle charge while it's off (or on as well) so it doesn't run out of batteries if it's turned on like half an hour each day. I looked at Satistronics, but the solar panels are too big for what I am doing (actually a small BEAM robot that uses two pager motors). Nevertheless I will keep that store in mind and these solar cells sure do look usefull for other robots I might build. Cheers! So I guess I will wire two 3.4V panels in series and, with the 0.6V drop from the diode, it should be around 6V.
I'm guessing the drop across the diode will only be between 0.3 and 0.4 V and that low current. I have seen current drops across the 1N5817-5819 diodes less than 0.2 V (which does not even show on the spec sheets).
The actual solar panel voltage will drop (sometimes by quite a lot) when feeding the batteries, so you won't be able to charge the battery pack directly to full.
You can either add another panel in series to increase the voltage, or you can use a boost converter (AKA step-up converter) to increase the voltage output. You can probably find a product that integrates both a booster and NiMH charger.