Cheap Homebrew Vehicle Charger

I was just reading the post about the AC wall adapter USB charger. I have 2 PC’s at home for charging my Clip on the USB port. I wanted a charger to use in my pickup. I found a couple USB lighter plug style car chargers at Best Buy. I think one was $20 and the other was $25.

I did some research on USB specifications. All you need for charging is 5.0 volts DC, that is the standard voltage from a USB port. I checked my Clip and it only draws about 65mA (0.065 amp) while charging. All you need to do is provide 4.75 to 5.25 volts DC to the red and black wires on the USB cable. Of course red is positive, black is negative.

I made up a little charger with stuff at had at home. I used a lighter plug from an old cell phone charger. I opened the plug and installed a 7805 intergrated circuit linear voltage regulator. The regulator is very easy to use. It has 3 pins, voltage in, ground, and 5.0 volts DC out. Just connect the ground from the lighter plug to the center ground pin on the 7805. The +13.8 center pin from the lighter plug connects to pin 1 on the 7805 (voltage in). My lighter plug had a 3 amp fuse, so I left it in for added safety. This voltage can be anything from 7.5 to 35 volts, so 13.8 is fine. Cut the USB cable, feed it into the lighter plug, connect the black wire to ground and the red wire to pin 3 on the 7805 (+5.0 volts DC output). Don’t connect the other wires from the USB cable, you only need the black and red for charging. I had a mini USB cable from a cheap digital camera that fits the Clip. I also installed two 0.1uF 50 volt capacitors for stability. One cap from pin 1 to ground, the other from pin 3 to ground. This probably isn’t necessary, but won’t hurt.

The 7805 is available from Radio Shack for about a dollar. I had a few in my parts drawers. The 7805 doesn’t need a heat sink, it will only dissipate about 1/2 watt of heat while charging the Clip. MIne works like a champ and only cost me about an hour of time I would have wasted watching TV anyway. Go ahead, build one, it’s fun!

Happy Soldering.

When the battery is fully discharged the charge current will be at least 100 ma,  so make sure you plan for that.  I would make sure you can support 200 ma for margin.

Message Edited by sansafix on 12-19-2007 12:34 PM

Yeah, I wondered if it might draw more when the battery was low. I measured the current after playing about 2 hours from a full charge. I believe the lithium ion batteries are similar to lead acid in the way they are charged. Don’t they need a constant voltage applied? I know if you put 13.8 volts on a nearly dead 12 volt lead acid battery, it will draw a lot at first and the current will taper off as it charges. That must be similar to the battery in the Clip.

I felt the 7805 and it was barely warm at 65ma.The 7805 IC is rated for 1 amp, but at 1 amp it would need a heat sink with the 9 volt input/output differential. I imagine it will be OK at 200ma if the Clip is almost dead, the 7805 might just get a little hot initially, then cool off as the Clip battery charges. With a 200ma load, the 7805 would need to dissipate about 2 watts. That’s about the max for a TO-220 package without a heat sink, but allowable. The lighter plug never seems to get warm on the outside. I glued the lighter plug together, put I might crack it open and mount the 7805 to a little aluminum heat sink.

MinimaxMan, Your charger seems like a nice cheap way to be able to charge from your vehicle. Thanks for sharing! My only question…when the Clip is fully charged, can it be damaged by being continually plugged into your charger? I have no idea how someone might modify your design to detect a full charge and shut the juice off as needed. Thoughts? -Mike

Clip will stop drawing current once its fully charged,  no worries.

Thanks sansafix! Good to know.