16v Solar Panel
Want to power this with Solar Panels?
I want to build a standalone wifi repeater — powered by the sun. The problem is how much Solar Power and how big of a battery?
net draw 4.5v @ 0.66A with load
net draw 4.5v @ 0.60A no load
I’m assuming the best choice would be a 6v battery with a a couple diodes in series to induce ~1.5v drop. Then, I need something to charge it — I found 12v 6w solar chargers in the automotive section of Sears and 16v 2w solar chargers in the marine section of Dick’s Sporting Goods.
How many solar panels and what capacity batteries should I use? Is this the best method or should I use a voltage regulator and go with 12v batteries?
I need this to be as cheap and simple as possible…
I never said anything about using an inverter, changing DC into AC back to DC is just stupid…I’m saying to use a REGULATOR which takes a range of higher voltage, unregulated DC and outputs lower voltage, regulated DC. This is how most car-cellphone chagers work (cellphones use 4-6v to chage)
You will find a much larger selection of system components to make a 12V solar system rather than a 6V solar system. So you might consider finding a switching regulator to drop 12VDC down to 5VDC, then use a diode to further drop it to about 4.4V. A switching regulator can be had with efficiencies in the 85% range. Avoid a linear regulator; its efficiency will be less than 5/12 = 42%.
Assume the following design criteria (adjust to your liking):
1. battery to be a deep-cycle (a.k.a marine battery)
2. battery discharging stops at 20% capacity
3. battery nominal voltage 12Vdc
4. run-time on battery 3 days between charging (due to cloudy days)
5. effective charge time 6hours/day at full output (this depends on your lattitude and season).
6. 12Vdc/5Vdc switching power supply with 85% efficiency
7. Load is a continuous 0.66A at 4.5V
8. Battery charging is 78% efficient
9. Solar panel operates at 17.1V output (maximum power point)
Calculations:
The needed battery capacity (to 20% charge) in ampere-hours is:
0.66A x 24h/day x 3 day x 5V/12V / .85 / .80 = 29.1AH
The number of kWh needed per charge cycle (6 hours) is:
29.1AH x 17.1V / .78 = 638 VAh ==> 0.638kWh
The average current required during the 6-hour charge cycle is:
29.1AH / 6H / .78 = 6.22A
The nominal Solar Panel size required is:
6.22A x 17.1V = 106W
Most solar panels today are constructed of strings of 36 individual cells. These panels require that a solar charger be used to regulate the charge current (to preclude overcharging the battery). A simpler system is to obtain a solar panel with 33 cells per string. This is considered to be a self-regulating panel that can be connected via a diode to the battery, without a charge controller. It is a less expensive system, but it won’t charge as well as a 36 cell panel will on an overcast day.
As cheap and simple as possible…
Consider relaxing your design criteria. E.g., allow outages after 1 day. Power down the access point at night or during periods that it isn’t used.
Good luck! Sounds fun.