BUILD A CHEAP VERSATILE BATTERY CHARGER POWER SUPPLY
By: Brian Allen Kuehn
I used this power supply at Flite-Fest 2014.
Would you like to build a 12 volt power supply that will provide clean DC power to 8 or 9 lithium polymer battery chargers at the same time? How about $7.99? (Okay, that’s a stretch but not a huge one). The basis for the power supply is a PC (personal computer) power supply. The one I used for this build I got from NewEgg.com for $7.99 plus $3.99 shipping. The price was low because it is a factory refurbished power supply. I had the rest of the parts on hand already so the total cost to me was $12.00. PC power supplies have overload protection and short circuit protection built in and they provide clean stable outputs of +12 volts and +5 volts. If you salvage a power supply from an outdated PC, it may cost you nothing. You can find one or buy one with more watts than the one I used. More watts means that it can power more chargers or charge even bigger batteries at the same time. The power supply I used is rated a 350 watts. There are plenty of more powerful units out there.
The PC power supply that I bought provides 20 amps of power on the 12 volt rail. I could charge 9 batteries rated at 2200 mah at the same time with this power supply (using a 1C charge rate). Since I own 5 chargers rather than 9, that won’t happen any time soon. With my 5 chargers, I could be supplying 4 amps of charge power to each of the 5 chargers (a couple of them aren’t capable of delivering that much power). In any event I can use all 5 of my chargers with this power supply and any combination of LiPo batteries that I currently own for the foreseeable future.
The wires going out to the various computer connector plugs are color coded. The yellow wires provide 12 volts (positive). The red wires provide 5 volts (positive). The black wires are the negative or ground wires. For each charger location you need at least one yellow and one black wire. Since there are 6 yellow wires and about a dozen black wires, I was able to join 2 yellow wires to each other and also join 2 black wires together to power each of the 3 planned charger stations. Doubling the wires provides a larger conduction path that permits more amperes to be delivered with less heat build up caused by resistance.
Illustration #1 shows the original computer connectors after cutting them off with diagonal pliers. The various wires have already been twisted together and soldered onto the metal parts of the banana connectors. The LARGE main connector that would normally plug into the motherboard has NOT been cut off. If you do accidentally cut this large connector off, don’t fret. There is ONE green wire in the bundle and several black wires. It is necessary to connect the green wire to any black wire to get the PC power supply to turn on. I have done this by creating a jumper from a section of paper clip. One leg of the “U” shaped paper clip section is inserted into the connector socket for the green wire and the other leg is inserted into an adjacent black wire socket. You could connect the green wire and any black wire with solder or a small wire nut.
Your computer uses a momentary switch (large button on the front of the case) to complete the connection between the green wire and a black ground wire to turn on the power supply. The power supply also has a built in rocker switch to turn power on and off. In a computer, the rocker switch is normally left in the “ON” position. Since I have the green wire permanently “on” via the paper clip jumper, I use the rocker switch on the power supply to turn it on and off.
Illustration #1 also shows the red wires joined together and soldered. Next to the red wires is a pair of black wires that are joined together and soldered. I later put a small wire nut over each of these soldered connections to preserve them for future use. The 5 volts they put out could be used to power a servo tester or a receiver.
Illustration #2 shows a close-up of the soldered banana plug metal connectors. These connectors also have color-coded plastic outer sleeves that are normally held to the metal cores with a small screw. I discarded the screws as they would have interfered with the system of mounting the banana plugs that I used. Red and black heat shrink have been added to the various connectors to reinforce identification of positive and negative connections.
I used part of a paper clip to "jumper" the green wire to a black ground wire. This is necessary to switch the computer power supply on. You normally do this by pushing a button on the front of the computer case. With the paper clip "jumper", the rocker switch on the power supply itself will now function as the on/off switch for the power supply.
I made the base from 2 pieces of wood. I have a shop in my basement and always have plenty of scraps of wood on hand. The main base is pine cut from a 1x6 board. See illustration #4. The final size I used is 5.5 inches by 10 inches. Wood is priced and sold based on the wet or green dimension. When this wood was cut and milled, it was actually 6 inches wide by 1 inch thick. When it was done drying out, it had shrunk to 5.5 inches by about 13/16ths of an inch. After planing off the rough surface, there is a thickness of ¾ of an inch left. The second piece of wood is about 3/8ths of an inch by 1.5 inches by about 8 inches. Exact size isn’t critical. It serves as a place to glue the plastic shells for the banana plugs. I used a scrap of walnut because it’s pretty, but any piece of wood will do.
I used Thin CA [cyanoacrylate] to glue the two pieces of wood together because it is fast. Wood glue or Elmer’s glue wood have worked fine too. CA can be encouraged to set by using an alkaline chemical as a catalyst. Baking soda works great. On hard balsa or most species of wood other than balsa, I rub baking soda into the wood surfaces to be joined then brush or blow away the excess. The tiny amount of baking soda that remains on the wood surfaces is adequate to facilitate the chemical reaction. Hold the two pieces together and let some thin CA wick into the joint.
I don’t recall the size of the holes that I drilled to fit the plastic banana plug shells. Drill bits come in sets that usually step up by 1/64th of an inch. On a scrap piece of wood, drill test holes until you get one to fit the banana plug shells that you are using. You may well end up with plugs that are different from mine. The shells were a somewhat loose fit in the holes that I drilled (the next smaller size was too small to fit them through. I used thin CA (super glue) to secure them in the holes.
Illustration #5 shows a close-up of the plastic shells placed into the wooden holder for gluing. The banana plugs that I used will “nest”; that is one banana plug can be plugged sideways into a hole in another banana plug (see illustration #5). To permit this, be sure to space the shells far enough away from the wooden mount to permit another plug to nest. The hole that is peeking out from the wood is the hole where the discarded screw was fitted. It’s okay to partially block the screw.
Thin CA (cyanoacrylate) will wick into the tiniest crevice or space. Carefully put a small drop on the plastic shell at the junction with the wood and it will soon be fixed firmly in place. Use accelerator if the glue’s curing time exceeds your patience. Any other glue sufficient to adhere plastic to wood can be substituted.
Now its time to mount the PC power supply on the base that you have prepared. Going for speed again, I used the same double-sided foam tape that I use to mount receivers and ESC’s into R/C airplanes. It’s strong and provides vibration damping. The two fans in the power supply are smooth and quiet, so they don’t really need to be vibration damped but it doesn’t hurt. I could have run a sheet metal screw through the wood and into the bottom of the power supply’s metal case, but that might have cause a short circuit inside the power supply. I could have used 5 minute epoxy or hot melt glue or JB weld or a host of other glues. Use what you have and what you like. I like the double-sided foam tape so I used that. See illustration #7.
The banana plugs that I used will “nest”; that is one banana plug can be plugged sideways into a hole in another banana plug (see illustration #8). To permit this, be sure to space the shells far enough away from the wooden mount to permit another plug to nest. The hole that is peeking out from the wood is the hole where the discarded screw was fitted. It’s okay to block the screw hole.
The metal part of the connector must be carefully located so that the holes line up to permit another banana plug to nest. I used another banana plug nested in the holes to hold the two pieces in alignment. Then I placed a dab of glue from a hot glue gun between the back end of the plastic shell and the heat shrink material to lock the metal connector in place in the plastic shell. I chose hot glue for speed. Use the adhesive of your choice. See illustration #8
Plug the black A/C power cord into the PC power supply, plug the other end of the black cord into a wall socket, turn the rocker switch to the on position, and you are ready to do some charging. Use cable ties to makes any unruly wires neat.
Illustration #9 shows power being supplied to three of my chargers. Charger #1 is charging a 3s 1000mah LiPo battery pack. Charger #2 is charging a 4s 1500mah LiPo pack. Charger #3 is charging a 3s 2200mah LiPo battery pack. The power supply was unperturbed by the demands of these 3 chargers, remaining quiet and cool. It’s been handling all of my charging needs for several months now. Your mileage may vary.
This photo shows the power cord attachment and the rocker switch that now turns the unit on and off.