How can you tell if the cell has gas recombination capability?
I don't know of any way to tell without asking the manufacturer. If you know of a way, please tell me: I'm curious. That's not something I've looked into. Behaviour at end of charge may well provide some clues.
Anyway, as for traditional NiMH batteries:
The advice from most major NiMH suppliers is that traditional NiMH batteries over about 1800–2000 mAh should not be trickle charged at all. A few suppliers say you may trickle charge at low mA for a time after charging—but only for hours, not days.
A few suppliers tell you to trickle charge at C/10. Run away fast from their products.
As for LSD NiMH batteries:
- LSD cells are tricky. They have lower capacity per size, but this does not prove they're safe to trickle charge. I'd guess that, at rates well below C/10, Eneloop Lite cells are OK to trickle charge, Eneloop standard may be OK, and that Eneloop XX are not OK to trickle charge. For cells which are safe to trickle charge, C/40 sounds like it's probably an OK figure.
tl,dr: In your case, you don't seem to need charge control at all, and it is more complicated than you seem to sink because of various reasons. Just put a 1N4001 between the solar cell and the battery.
The NiMH battery is a quite low impedance, pulling as much current from the solar cell as available at the cell voltage. During charging, it is likely around 1.4V/cell. This is an output voltage of 4.2 Volts, so in bright sunlight, you likely exceed the 170mA quoted for the maximum power point, expect something like 200-220mA, which is below C/10 of the cells. I have no idea how you want to do NiMH charging control at widely varying charging currents that never exceed C/10 (no -deltaU, no dT/T seems applicable, but let's ignore that).
First thing to notice: Your circuit will not work as drawn. You drew a NPN transistor. It needs a voltage at the base that exceeds the emitter voltage (by around 0.6V) to make it conductive, but the uC has no access to provided the needed 4.8V (battery voltage + 0.6V). You need an PNP transistor instead. In that case, you need to to provide current from the base terminal to a more negative sink. Also, you would connect the emitter to the solar cell in that case. Note that to turn off the transistor, you need the base voltage to go up to 0.5V below the unloaded solar cell voltage, this is around 8V.
Start with the desired collector current (220mA maximum cell output) and take a look into the transistor data sheet. Let's choose a BC327 for it's higher current rating compared to the typical 100mA transistors. Take a look at figure 4 (Saturation region) if you want to have low losses on the transistor, which seems like a good idea now (but see later). They have curves for 100mA and 300mA collector current. As we don't have a lot of energy to waste, choosing a base current at the low end of the neary flat end of the saturation voltage curve is a good idea, which yields something around 4mA if you interpolate between the 100mA and the 300mA curve. To turn in the transistor, at 4.4V solar cell voltage, 200mV transistor drop, and 4.2V battery voltage, you need to sink 4mA at 4.4V-0.6V (emitter-base voltage). To turn it off, base must rise to 8V (see above). This means, you need a pull-up resistor between base and emitter, providing the turn-off voltage from the cell instead of the uC and a diode (1N4148 will do) to protect the uC from that high voltage.
So the circuit looks like this: PNP with emitter to solar cell, and collector to the battery. A resistor connecting emitter with base (the value does not matter much, and something around 100k will provide enough pull-off effect without disturbing the circuit while the transistor is on), a diode and a resistor in series to the uC. The uC needs to sink 4mA to turn the transistor on. This results in an output voltage of 0.4V. So the resistor has to sink 4mA while having 0.4V above ground on the uC end and 3.8V above ground on the diode end. 4mA at 3.4V is 850 ohms. So 900 Ohms in your circuit does not seem off that much.
You will not be happy at all with that circuit for different reasons, though: Constantly sinking 4mA takes too much of the charging current (getting 220mA outdoor on a bright sunny summer day is one thing, on cloudy days, being inside, expect something like 10mA maximum), and you are wasting 4mA of that just to turn that transistor on. Furthermore, when it gets dark, the solar cells draw power due to their leakage current and discharge the battery by operating the transistor in reverse mode (collector acts as emitter, base current is provided by the solar cells and the 100k resistor meant as pull-up, and the emitter acts as collector). Common wisdow is you need to protect against that, for example by putting a diode between the collector and the battery. You will lose voltage there, though. Another possiblity would be a "secondary pull-up" that will pull the base up, even if the emitter supply (the solar cell) fails to provide power, by connecting a resistor from collector to the base terminal. This resistor has to be low enough to pull the base up against the 100k that pulls it down, so go for something like 20k there. Of course adding all these resistors doesn't make the thing more efficient.
Actually, your charging current is very low compared to cell capacity. As estimated above, you only get to C/12 on extraordinary good conditions, you the charging current can safely be considered as "trickle charging" unless you put the device into the beam of a bright headlight 24/7. You don't need any charge control. You just need to prevent discharge through cell leakage, and this can be povided by a single diode between solar cell and battery.
Best Answer
Well I am not sure how much my answer will help you, but I am working on similar project of NiMH batery charging for last 2 months and putting what I have learned till date.
Trickle charge, as you said C/10, C/40 is usually very low charging current and is good for long battery life and mainly used in applications where you are not expected to the replace battery for months.
Fast charge is can be between C, C/2, 2C also and is used to revive the batteries quickly and come into action, but with this charging the battery life is reduced.
You can look into this link for specifics. So it depends on your application what sort charging you should go for. Tricke or Fast. And there is another term also called as Top-Off charge which is basically a maintenance charging.
For fast charging we tested TI part BQ2002 which will only do fast charging and is good for 5V wall supply.Here is a 12V application circuit using BQ2002.
Then comes the trickle charging we tested BQ24400 and this IC controls the charging current based on the Rsense resistor and you can manually set the charging current based on your requirement.
I understand you are actually looking for a combination or merged properties of both the ICs I suggested, but if you are okay to go with some charging current between Fast and Trickle charging current, BQ2002 may be useful for you.
Now another important point is to take care for both the ICs is there charge qualification stage, they actually scan the battery voltage and temperature regularly before deciding which charging need to go. And when to terminate the charge, will again depend on the battery voltage and temperature of the battery. If you are not willing to use the thermistor for temperature sensing(like me) make sure to fool the ICs by providing the expected voltage on the Tsense pins to qualify for the charging. We have tested the fooling option it works fine.
And do not misunderstand me as TI agent, due to availability of free quick samples, we tested their parts as of now.