Yes you can connect several Li-Ion cells in parallel but before you do so, check that they have (almost) the same voltage.
If you buy several at the same time from the same supplier, changes are this will be the case.
If the voltages are more than 0.2 V (I just sucked this value out of my thumb !) different, you have to balance the cells. Either charge them fully with the same charge, after charging they will have the same voltage. OR you connect them in parallel but with a small value resistor between both + poles. A 100 ohm resistor will do. This resistor will limit the current flowing from one cell to the other while they balance themselves. When there's 0 V across the resistor left, the cells are balanced and you can connect the + poles also without the resistor.
If the 2.5W solar panel will be enough, depends on your patience ! Using 2 cells will double the charging time. Using a solar panel with double the power will halve that charging time again. But the charging circuit you're using can only supply up to 1 A so it makes no sense to use more than 2 2.5W, 5V (so 0.5A) solar panels.
Panasonic are excellent batteries ! Also Samsung and LG make excellent batteries. I would not recommend most cheap UltraFire. Either get cells with "solder tabs" to connect them in parallel and to connect wires to them OR get cells without "solder tabs" but then get a battery holder. You should avoid to solder directly on the battery.
I would recommend getting "protected cells" (these have a small battery protection circuit) without "solder tabs" (most protected cells do not have these anyway). And to use a battery holder, if you ever need to replace the batteries, it will be easy.
This answers many but not all queries. Look at this and ask re what is still needed.
These SE questions which I have answered are liable to be useful
As others have said a series parallel arrangement (1S + 2P)is not at all good.
Capacity is that of the single cell.
I have a 3-AA battery holder and I am looking to build a solar battery charging circuit. I am planning on using 3 AA NiMh 1.2V batteries to be placed in the battery holder.
At least consider using a single Lithium Ion cell (or LiFePO4). These will prove easier to manage well. You'll need a control IC made for the task but thgey are low cost and make the charging task very easy. There is NO REALLY good way to solar charge NimH with small solar systems (Ask me how I know :-) ).
I am planning to use a 1N5817 blocking diode, so current does not get drawn back to the solar panel.
1N5817 is good. Except where panel voltage is critical (see below) a std 1A silicon 1N400x is fine.
If the batteries are placed in parallel, then the total voltage for the battery pack will be 1.2V, 2300mAh * 3 = 6900 mAh. I have concerns about losses at such low voltages ?
Vseries = V1 + V2
mAh series = mAh of lowest if non identical.
Vparallel = MUST be the same (some exceptions but don't do it)
mAh parallel = sum of two
I could also wire two of the batteries in parallel and 1 in series for 2.4V, 4600mAh. But I would have to balance the cells somehow ?
Very bad idea.
As above, mAh = that of lowest series item = here 2300 mAh.
No advantage in doing this.
According to this page: http://www.amazon.com/Sunnytech%C2%AE-100ma-Module-System-Charger/dp/B00HQ9CUMO, it says that for a storage battery of 1.2V, you need a solar panel that outputs 2-2.5 V, and for a storage battery of 2.4V, you need a solar panel outputting 3.5-4V.
Those are quite good figures. Because:
NimH AA at about C/10 (eg 230 mA for a 2300 mAh cell) have a fully charged Vf of 1.45V. This varies somewhat with temperature and manufacturer but not vastly. As C rate rises V rises. Charts are available. IF you are aiming at C/10 per cell use 1.45V/cell for fine design and 1.5V for rough basic design.
1N5817 at 200 mA drops about 0.35V at 25C - falling with increasing temperature. See fig 13 in datasheet
Even at 600 mA (3 x PV panels) Vf typical is under 0.4V (fig13) .
Assume 0.4V diode Vf for design.
So battery needs 1.45 + 0.4 = 1.85V to fully charge. If wiring and connections etc drop 0.15V at 600 mA then 2V is fine. In practice a bit more doesn't hurt SO their 2.0 to 2.5V is about right. Note that the V specified is USUALLY loaded voltage in full sun. It is "not unknown" for small Chinese panels to be somewhat on the low side of their specification. The max current is at midday with the panel pointed at the sun. Output drops essentially linearly with light level. Voltage drops off much more slowly with decreasing light.
Add another battery (cell) in series and you need another 1.5V giving Vpv ~= 3.5 - 4V = again jut what they say. !!! 3 batteries = 5V - 5.5V.
So a 6V nominal panel is a good choice for 3 x NimH. This allows it to still be useful in somewhat lower light conditions.
My questions are: 1) What voltage of solar panel do I need?
As above
I did some research online and I can only find 2.5V panels from China
Almost all small panels are from China.
There are many many many brands and many many voltages and current levels available.
Do I need a charge controller? If so, why? According to this link: http://www.solar-electric.com/solar-charge-controller-basics.html/, if the panel outputs 2w or less, I wouldn't need a charge controller.
Web advice on battery charging is often poor.
Older NimH allowed trickle charging at <= C/10
Modern higher capacity NimH allow NO trickle charging. For reasons why see other SE answers.
DO NOT trickle charge a fully charged modern NimH battery.
The best controller for solar charged NimH is a voltage limiter that limits the voltage that the per-cell voltage can rise to, to 1.45V/cell. Ideally this is applied per cell, but for not too many cells in series it can be applied to a number of cells in series. This can either be a voltage clamp which dissipates all PV energy once battery voltage is 1.45V/cell, or a series switch whih cuts off feed to the battery when Vbattery is high enough. In the latter case the battery voltage will change when Vcharge is removed and this needs to be accounted for with hysteresis. A good and cheap means of sensing battery voltage is to use a TLV431 reference zener, but this is by no means the only method. The TLV431 can be used to drive a voltage clamp or series switch.
3) If I use a 2.5V, 200mA max panel with a 1.2V, 6900mAh battery pack, I would plan on using 3 of these panels in parallel for a max output of 600mA. Does this seem reasonable? Or should I use more panels? 600mA would be roughly 10% of the battery capacity, which I have read you shouldn't exceed.
3 batteries in parallel give 6900 mAh as you say.
C/10 = 690 mA = OK.
NimH may be charged at up to C/1 (2300 mA for 2300 mAh cell) with no problems provided a robust charge termination system is used. This is difficult (or worse) with solar chargers as voltage and available energy may vary at any time and temperatures tend to be high and unpredictable. These factors greatly disturb standard algorithms.
Best Answer
The in-battery protection circuitry is usually intended to act as a gross fault protector and it is strongly recommended that it not be relied on as a means of charging control. As a means of gross short circuit protection it may be suitable as long as the values they choose for max Iout are acceptable to you.
For charging, use of one of the large number of LiIon charger ICs is recommended.
A major factor is that the over voltage circuit does not remove the applied voltage when the CC charging current falls to a low value. This means that the battery is "floated" indefinitely with the risk (I'm told) of plating out metallic Lithium.
A PV panel (solar panel) that is nominally 12V rated and intended for charging lead acid batteries, will have a loaded Vout of about 18V and an O/C or light load Vout of over 20V. The maximum voltage that you need AT the battery pack is 4.2V/cell or 12.6V in your case. PV panel available Iout values are a reasonable approximation to being linearly related to isolation (sunlight level).
However, Vout is not related to light level in the same way. A PV panel will produce over 90% of its full power voltage for light levels of a few % of maximum and above - say at 10%+ to be safe. If you want the panel to charge the battery to fully-charged even on a low sun day, if necessary, then you need a panel that is full load rated at at least 12.6V/90% = >= 14V. As above, as an SLA targeted 12V panel makes about 18V at full-sun full-load, such a panel will provide more than enough voltage under all practical light conditions.
You will get substantially longer cycle life from a LiIon cell if you terminate discharge at a slightly higher voltage than allowable absolute maximum. With LiIon , below about 3V under medium loads you have used the large majority of the stored energy.In-battery low voltage cutoff circuitry will probably allow discharge to about 2.6V/cell, which is lower than is wise for good battery lifetime.