A power box from a cracker box
The plastic soda cracker boxes from asia are just about the right size to accommodate up to a 12 A⋅h SLA battery and some components. The photographs here show a 7 A⋅h battery that is compatible with my UPS backup system. The purpose is to have a single container that can be used while both charging and discharging during use. A 12-V cigar-lighter plug is used to plug in the solar cells while charging. Then, while in use, an inexpensive inverter can be plugged in to provide 110 V and 5 V USB. The meter can display the battery voltage or the current in or out. The switch positions are
- V: Voltage, A: Current — Determines whether meter reads the voltage or current
- D: Discharge, C: Charge — Allows the meter to read positive current while Charging or Discharging.
- ON, OFF — Disconnects battery from the meter and panel input.
The circuit diagram shows how the switches determine the meter reading. The volt meter is used to measure the current by putting a 1 Ω resistor in series with the battery. The current, in amperes, is numerically equal to the voltage across this resistor.
The wiring diagram. One concern was to limit the number of connections to each contact on the switches to two.
- Soda Cracker Box (Asian, square), with lid $free
- Digital voltmeter (0-99.9V, 3 wire) $10.40
- DPDT switch (Charge/Discharge) $4.20
- SPDT switch (Voltage,/Current) $2.10
- SPST switch (off/on) $2.50
- 12 V cigar-lighter socket (chassis mount) $4.10
- 1 Ω, 10 W resistor $1
- Battery terminal spade contacts (F2/ quarter inch or F1/ 0.19″ as required) pennies
- hook-up wire pennies
- 110V/USB, 12 V inverter $30
The open-circuit voltage gives some indication of the amount of charge left in the battery. It is recommended to recharge the battery when it falls below 50% capacity in order to preserve the life of the battery. From the graph of the battery charge vs output voltage below, the 50% level occurs just above 12 V.
The input/output from the cigar lighter should also be switched. In the current design, when solar panel is plugged in with the switch off the meter turns on and gives the impression that the battery is charging. The SPST switch can be replaced with a DPST switch with the other pole switching either the positive or negative connection to the cigar lighter.