I have a 3.7v 100mA/h battery that i want to charge with a (very) small solar panel. The panel has an output of aprox 5V and at full sun, the current is about 1mA when charging the battery.
However i have my doubts that the battery is being correctly charger. The power that the battery outputs after a long charge with the panel seems much lower that the (theoretical) power the solar panel gives to the battery. I know that some power is lost during charge but even so...
Anyway my question is:
Is there a minimum current to charge a battery?
Yes, because a battery has some internal leakage current that tends to discharge it over time. What I would suspect is that your panel is not optimised for maximum light collection and you are getting much less charge than you think you are getting.
Open circuit voltage of a PV cell can be up to twice the output when under load. Therefore it may be that your PV cell simply isn't giving enough volts to effect a realistic charge current. I'd suggest you are looking for a cell that can deliver at least 15mA under load. So either your cell is damaged or simply not up to the job ("very small")
Duh, that's not much help. 100mAhr is a tiny battery. Is it even rechargeable?
What you should do is to search the web for info on the specific battery chemistry
of what you have you have, and see what they say.
You'll probably find something like Q/10 [whatever] minimum, to keep the battery
from self-discharging. So your solar cell is probably too small, even for your tiny
maybe-it's-rechargeable battery.
Well to answer your title question, there is no minimum charge current rating for any battery. As long as the charging source voltage is higher then the battery's voltage some current will flow and cause energy to be stored in the battery. However if charge current is only flowing say a few milliamps that would effectively be a 'trickle charge' more aimed at maintaining an existing charge rather then actually charging the battery, as it could take days depending on the MAH rating of your battery to reach full charge. Your stated 3.7vdc sounds like a LI cell, and what you have to be concerned about when charging it is that you cannot allow the cell's voltage to exceed 4.2 vdc no matter how small a charge current you happen to have flowing. Most proper LI cell chargers switch from a current control charging method to a constant 4.2vdc charging method when the battery reaches full charge to prevent damage or even fire. By the way most LI cells are rated for a maximum charge current of C/1, so a 100 mah cell could handle a 100ma charge current. There are higher performance LI cells that can take much higher charge rates, but you usually have to pay extra for that capability.
If it is a Li-Ion or Li-Po battery, you will need to make or get a special charger for it. You can't just hook up a solar cell and hope for the best.
Generally for these two types of battery you want a charge current of 0.1C, in other words one tenth (0.1) of the capacity (C). For a 100mAh battery, this is approx 10mA.
You use a CC mode (constant current) until the cell voltage has reached 4.2V, then you switch to a CV mode (constant voltage) until the current flow has tailed off to near 0. At which point it is fully charged.
For CV mode, a constant voltage of 4.2V from your cell and appropriate circuitry is required.
For CC mode, a constant current of around 0.1C or 10mA for your case is required.
It should be possible to charge it at a slower rate (say 1mA without a problem). However, consider this: If you charge at 0.01C, it will take a VERY LONG TIME - say over a hundred hours at least.
I question your 0.1C charge rate, most are rated at 1C charge rate. Most cell phone these days use Li cells and they certainly don't take 10 hours to fully charge.
You can charge it much faster yes, but 0.1C is a good rate for prolonged life of the battery. I have seen 0.3C given as a good charge rate as well. Perhaps modern batteries are more resiliant - the last time I played around with Li-Po batteries was about 3 years ago so lots may have changed since then (Model helicopters used to be one of my hobbies).
You need a blocking diode between the Solar Cell and the battery to prevent the battery discharging back through
the Solar Cell when its dark.
So without knowing the power rating of the Solar Cell, and if its a small cell, its quite likley that the charge current is
very small, maybe only a few milliamps,which means a charging time of weeks.