Connect solar cells in series to add their voltages, because one silicon cell gives only about 0.5 V, and in parallel to add their currents. A panel meant to charge a 12 V battery needs about 32 cells in series to clear the charging voltage plus losses, and 36 cells, about 18 V, give a margin. Match cells by measured current before you join them, because the weakest cell in a series string sets the current of the whole string.
Part of Solar Panel Construction
Joining solar cells into a panel whose voltage suits a battery, and the order of checks that keeps a string from underperforming.
A Single Cell Gives Half A Volt
The carrier gives about 0.5 V per silicon cell. That is useless for charging a 12 V lead-acid battery directly, so cells are joined end to end: the top of one to the back of the next, so voltages add and the same current flows through every cell. The carrier’s example is a string of 36 cells giving about 18 V, at about 0.5 A. To raise current instead, strings are connected side by side in parallel. Series vs Parallel Configuration covers the choice; Tabbing Wire and Bus Bars covers the joints.
The 18 V is higher than a 12 V battery needs for a reason. A lead-acid charge ceiling is 14.4–14.8 V (Charging Circuits), and between the panel and the battery there is a blocking diode and some wire. The panel’s voltage while it is delivering power must cover all of them.
What You Need
- Cells — salvaged and tested, or scratch-made
- Tabbing ribbon, flux and solder — Tabbing Wire and Bus Bars
- A multimeter — voltage and current ranges, see Metering
- A soft padded surface — silicon cracks easily
- A steady light source — full sun or a fixed lamp for matching cells
Method: Build A String
Step 1: Test every cell, in the same light and at the same angle. Record open-circuit voltage and short-circuit current.
Step 2: Sort by short-circuit current and group together cells that read within a few per cent of each other.
Step 3: Lay the cells in rows face down on the padded surface, in the order they will be joined, with a small gap.
Step 4: Solder the string (carrier: top tab of one cell to the back of the next).
Step 5: Test each cell as you go, not only at the end; a failed joint found early is cheap to fix.
Step 6: Measure the finished string in sun and compare with the arithmetic below.
Worked Example: How Many Cells For A 12 V Battery
Work out the voltage the string must deliver while charging:
- Charge ceiling: 14.8 V (top of the 14.4–14.8 V range)
- Blocking diode: about 0.6 V for silicon (Diode Construction, the germanium and silicon diode figures in the electronics folder)
- Wiring drop at 2–3% of the charging voltage (Voltage Drop): 0.02 × 14.8 = 0.30 V to 0.03 × 14.8 = 0.44 V
Total: 14.8 + 0.6 + 0.3 = 15.7 V to 14.8 + 0.6 + 0.44 = 15.84 V. At 0.5 V a cell, 15.84 ÷ 0.5 = 31.7, so 32 cells at least. The carrier’s 36 cells give 36 × 0.5 = 18 V, a margin of 18 − 15.84 = 2.2 V.
If the loaded cell voltage is only 0.45 V (an assumed figure for a hot or poor cell), 36 cells give 36 × 0.45 = 16.2 V, still above 15.84 V, but 32 cells give 14.4 V, which cannot reach the ceiling. So the margin matters.
Mismatch. If one cell in a string delivers only 0.40 A where the others deliver 0.50 A, the string is limited to about 0.40 A, a loss of (0.50 − 0.40) ÷ 0.50 = 20%. On 18 V this is 18 × 0.5 = 9 W against 18 × 0.4 = 7.2 W, 1.8 W lost. The estimate is approximate, since real cells give a curve rather than a single current.
Common Mistakes
- Joining unmatched cells — the worst cell sets the output
- Too few cells for the battery — the battery never reaches full charge
- Forgetting that heat lowers voltage — a panel in a hot summer delivers less voltage than in the cold (general knowledge, not from the other articles), so keep the margin
- Testing only the finished panel — one bad joint hides among 36
Safety
Cells make voltage whenever light reaches them, and strings add up
A single cell is harmless, but a series string raises voltage: 36 cells at 0.5 V make 18 V, three such panels in series make 54 V, and six make 108 V, in the range the lethal-current article calls potentially serious (50–120 V with wet skin). A solar string cannot be switched off. Cover it with an opaque sheet or work with it disconnected at the array, and treat it as live regardless. Soldering irons burn, and flux fumes irritate the lungs (Soldering Basics).
What’s Next
- Learn next: Tabbing Wire and Bus Bars — making the joints
- Combine with: Series vs Parallel Configuration — choosing the string layout