Solar Charger Tutorial – Part 3By Phillip Stearns
Part 3: How (and why) do I store power?
- a solar panel, 6 Volts or higher (we use our 2 Watt solar panel)
- solder-less breadboard
- a few different NiMH or other battery packs
- diode (Schottky or other rectifier)
b. calculate the power capacity of our battery pack
1. Measure Power into NiMh batteries - Connect solar panel to pack of 4 NiMh AA2 and measure the Voltage and current at each step (where applicable). WARNING: DO NOT use Li-Ion batteries for this activity.
|Description||Voltage (V)||Current (A)||Power (W)|
|At panel – Unconnected||6.44||-||-|
|At battery – Unconnected||5.12||-||-|
|At panel – Connected||5.79||.19||1.1|
|At battery – Connected||5.22||.19||.99|
|Drop of diode||0.53||.19||.|
2. Calculate Capacity – We just measured how much power is flowing from the solar panels to the battery. The next ingredient we need to determine charge time is capacity of the battery pack.
There are two types of capacity that we should be aware of: charge capacity and power capacity. Batteries have a rating that tells us the charge capacity or how much electric charge they can store: the ampere-hour (Ah) or milliamp-hour (mAh) [note: 1000 mAh = 1 Ah]. However, it’s much easier to think about the power capacity or watt-hours. Power capacity can be calculated by multiplying the charge capacity of a cell by the voltage of the cell: amp-hours * volts = watt-hours or A * V = Wh (also: mA * V = mWh). We’re using 1.2V AA cells with a rate charge capacity of 2,700mAh. The power capacity for each of our 1.2V cells with 2,700mA would then be 3.24Wh (1.2V * 2,700mAh = 3,240mWh = 3.24Wh).
To find the total power capacity of our battery pack with 4 AA batteries, we simply multiply the watt-hour rating of one cell by the total number of cells. It doesn’t really matter whether we have all four in series, parallel, or two parallel sets of two in cells in series; our 4 AA batteries in series has a total power capacity of roughly 13Wh.
Now can we calculate how long it’ll take to charge? Yes!
Looking at our data we can finally estimate the time it will take to charge: divide the total power capacity by the amount of power flowing into the cells. But there’s a catch! The average efficiency of NiMH batteries is 65%, meaning 35% of the power put into them is lost as heat. We much multiply the power being put into the cells by .65 (efficiency coefficient) to get a “real world” estimate of charge time for our battery pack, which looks like this:
.99W from our panel is flowing into the battery pack (yes this factors in a .1W loss from the diode!)
.64W (.99W * .65 efficiency coefficient) is being stored by the battery.
If our batteries were being charged from a completely discharged state, we have the whole 12Wh capacity to charge.
12Wh / .64W = 20.25 hours
Assuming that the power transferred into the batteries would double if we added another panel in parallel with the first (a total of 4W), we can get that charge time down to about 10 hours.
- using more efficient batteries (i.e. lithium-ion)
- using more efficient charge circuitry
- make the panels more efficient
Charge Smart Battery Packs – Our V11 is a smart battery pack with electronics designed to optimize solar power to charge lithium-ion cells. This energy is provided at a regulated USB 5V standard up to 650mA. Additional electronics also offer protection features: thermal protection, short circuit protection, overcharge protection, and over discharge protection.
Connect the V11 to a 4W (2 x 2-Watt panel) and measure the voltage and current to calculate the power.
What!?! Only 2.27 Watts out of 4 Watts of panels? It’s still a little better than the estimated power transfer of only 2 watts directly charging the NiMH battery pack. Those panels have been hard at work all afternoon, let’s cool them down with a nice ice bath and see if that helps…
With the panels cooled down, the output to the V11 increased to 2.73W, a 20% boost! This is because solar cells are more efficient at a lower temperature and it was a very hot day out there.
Let’s see that in a table:
|4W Panel Setup to V11||4.54||.5||2.27|
|4W Panel Setup to V11 (after ice bath)||4.71||.58||2.73|
Calculate Charge Time: The V11 has a rated power capacity of 11Wh. 2.27W was being transferred from the panels to the V11, and assuming that 75% of the power from the panels (under normal conditions, not iced) is stored in the battery, we can safely assume that 1.7W is used. The charge time for a completely drained battery would then be 11Wh divided by 1.7W or about 6.5 hours. This is consistent with our field tests.
Coming up in part 4 a look inside the circuitry that is designed to protect the battery.
Review Part 2 of our Tutorial – How do I measure total output? Connect solar panels to loads and measure how much power is being generated.
Review Part 1 of our Tutorial – How do solar chargers work? Understanding the basics of generating solar electricity and how to measure Voltage and Current in different conditions.