12 And 24 Volts From One Easy Portable Package Dan Lepinski copyright 1991 Dan Lepinski Ever need 24 volts when you only had 12 volts available? Or maybe you wanted 12 volts when the system ran 24 volts? Here is a handy little circuit that can provide portable power of either voltage at the flip of a switch. Better yet, you can recharge the batteries from either voltage level as well! The Basic Circuit In its simplest form, only two batteries, one switch, and a fuse are needed. The switch must be rated to carry the current that you will draw from the batteries. I never needed over 4 amps at either voltage level so the switch I used (and shown in the parts list) is rated for 6 amps. You may substitute a heavier switch. The batteries should have an ampere-hour rating suitable for your anticipated load. Also, the batteries themselves should be of identical capacity. Rate the fuse to pass approximately 20% more current than will be drawn by the heaviest load. Construction There is nothing critical about any of the components used in this little battery pack. A single plug or connector may be used both for power output as well as charging, although both should not be done at the same time. The wire used to connect the batteries to the outside world should be capable of carrying the maximum anticipated current without undue loss. If you would like to add a visual indicator of the output voltage level, an optional circuit to perform that function is shown. For minimum current drain from the batteries, this circuit may be omitted. If you would like the best of both worlds, you could use a "center-off" version of the switch. This will allow use of the visual circuit and also provides a means of disconnecting everything. Battery Charging To recharge the batteries, select the voltage that matches your charger's output and connect the charger to the battery pack. You may use the output connector or install a separate one just for charging. By wiring the circuit with the 3-pole switch (as shown), you will only be able to charge the batteries with the voltage selection switch in the position that matches your charger voltage. Caveat If you cannot locate a 3-pole double-throw switch, a double-pole double-throw switch may be used IF certain precautions are observed; you MUST place the voltage selector switch in the proper position BEFORE connecting the charger. Exciting times can result if you forget to perform this simple step. A fundamental warning is in order here: Exercise care when using or charging this battery pack. Expensive repairs or injury can result if you get careless. Final Note Tri-Tek has a limited supply of surplus 6 volt 10 AH sealed lead acid cells available for $6.95 each or $24.95 in groups of 4. These are ideal for a portable power supply of this type. Basic Parts List: Two 12 Volt or four 6 volt batteries (sized per your requirements) 3- pole Double-throw switch (optional 2 pole may be used) One fuse holder One fuse (appropriate for your load) Basic Power Supply Parts & Suppliers: 3-pole, 6 Amp double-throw switch Tri-Tek, Inc. 12 Volt batteries sized as needed Tri-Tek or HP advertisers Double-pole double throw 6 Amp switch Radio Shack 275-652 (optional) Double-pole double throw 10 Amp switch Radio Shack 275-1533 (optional) Fuse holder & fuse Radio Shack Surplus Panasonic 6V, 10 AH sealed lead-acid battery from Tri-Tek, Inc. (while supply lasts) Miscellaneous hardware as needed Visual Voltage Level Indicator (Optional): D1 18 Volt, 1 Watt Zener diode 1N4746A D2 Red LED (or color of your choice) D3 Yellow LED (or color of your choice) Q1 2N2222A or similar NPN switching transistor R1, R2 1.2K ohm, 1/4 watt resistor R3 820 ohm, 1/4 watt resistor Access: Author: Dan Lepinski, 4631 W. Marlette Ave,, Glendale, AZ 85301 Tri-Tek, Incorporated, 9014 North 23rd Ave., Suite 11, Phoenix, AZ 85021 ¥ 800-528-0183 ¥ 602-371-0446. Minimum order $20.00