Two cells gives you 5.6W in full sunlight, which is enough power but not enough voltage. You need more cells, but they don't have to cost a lot more because they can be smaller. A panel with two 5x5 cells may cost less than $4, but at this price the cost of the converter could be a significant factor. It may be better to pay a little more for the cells, rather than having to use a more expensive converter that works on lower voltage.
Most boost converters require at least 3V to produce 500mA at 5V. To get 5W at 3V you need 6 cells, with each cell delivering at least 0.8W. A good 2x6 cell can generate 1W, and the cost for a whole panel's worth could be under $5 (eg. 40-2x6 Solar Cells for DIY solar panel).
Alternatively you could make a panel with 12 1x6 cells to provide 6V, then just use a cheap 5V LDO linear regulator.
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
A PV (solar) panel acts as a good approximation to a constant current source when loaded to below its optimum voltage.
Updated: With the added PV panel and battery specifications it can be seen that:
The panel will fully charge the battery in about mAh_battery / Imp
= 2300 mAh / 380 mA ~= 6 hours of full sun.
Sunshine hours (full sun at 1 kW/m^2) vary with location, season and weather conditions but typically varies from about 2 SSH in midwinter to 6 SSH in full summer. SSH for many locations worldwide can be found from The fabulous Gaisma site.
With 2 SSH/day it will take ~= 3 days to fully charge the battery, and with 6 SSH it will fully charge in 1 day.
As Voc is >> Vbattery and as the panel will deliver useful current above Vmp, then the panel will deliver ~= Imp into the fully charged cells and modern 2300 mAh cells with no readsorption of gas generated will rapidly be destroyed. A regulator is therefore needed.
As mentioned below, a clamp regulator set to about 8.7V will probably be acceptable. Current when fully charged should be checked and if necessary Vclamp may need to be adjusted. An alternative is to let Vbattery rise to somewhat above 8.7V indicating full charge and then cut off charging either until next day or until discharge is initiated.
NimH are usually regulated by detecting negative delta V at full charge or delta battery temperature rate or absolute battery temperature but none of these are reliable in most solar applications due to variable charge rate with insolation and solar heating. Absolute temperature from charging and solar heating combined is a good indication that charging should be stopped but not that charging is complete if solar heating is significant.
Older:
NimH cells need about 1.45V each for full charge at current well below C (eg well below 2A for 2000 mAh cells. This voltage varies somewhat by cell brand and model but 1.45V is a good starting point.
So 4 cells need 4 x 1.45 = 5.8V for full charge (before Schottky diodes is allowed for) and
6 cells need 6 x 1.45V = 8.7V.
So - a panel rated at 9V Vmp (voltage at maximum power) may not treat 6 x NimH too badly but will continue to charge 4 x NimH cells at nearly full panel current even when the cells are fully charged. So whether you have 4 or 6 cells matters - and panel Imp mA and battery mAh also matter.
Older NimH with say <= about 1800 mAh for an AA cell could be trickle charged at <= C/10 indefinitely. More modern cells of >=2000 mAh capacity for AA MUST NOT be trickle charged after full charge state is reached. While older cells had gas recombination systems to recombine Hydrogen and Oxygen generated by electrolysis, these systems are not included in higher capacity cells (as the room is used for additional active battery materials. Trickle charging higher capacity cells will dry them out rapidly and lead to early failure.
If a PV panel is liable to frequently fully charge NimH cells then overcharging can be prevented by adding a "clamp" or shunt regulator set to 1.45V/cell. For very low capacity panels a TL431 adjustable active zener( clamp regulator) can be used directly for PV panels with more mA output than a TL431 can handle can use a shunt transistor driven by a TL431. A conventional zener has far too soft a V-I "knee" to be used in this application.
It is also possible to use a series regulator but this needs care due to battery voltage drop when charge is removed.