Low-Budget Photovotaically Charged Drill Batteries, by C.K.

I’d like to discuss some practical aspects of power tools. Some posts in the past have been mentioned about them but I am going to discuss making solar power tools. And not just buying the pieces and making them (which you could do if you wanted) but actually making them from salvaged “junk”.
I’ll be the first to admit that while I love and own all types of traditional (non-electric) hand tools, using them does take considerably longer and more effort than the powered ones. (Not surprisingly).
My background is that of a professional tinkerer, and a trained marine and environmental biologist. I have tinkered with electronics since I was a small child and while no expert (and welcome corrections (there is much I don’t understand)) what I know is from first hand tinkering and reading.
As a professional scavenger I have come to realize that we live in an incredibly wasteful and unsustainable society on many levels (no surprise to most readers I am sure). Over the years I have found drills, reciprocating saws, circular saws, chainsaws, and all types of other power tools thrown away for various reasons. You just have to keep your eyes open.
The project I am going to describe was one in which I took an older Black and Decker 7.2 volt NiCd battery powered drill (that I found in someone’s trash), and turned into a lithium ion solar-charged drill for free. This project can be adapted to almost any brand, voltage, or type of battery powered tool.
Tools and supplies needed:

  • Voltmeter/multimeter
  • Soldering iron
  • Solder
  • Flux
  • Wire (decent gauge, larger than 3mm, preferably braided)
  • Diode (at least rated for 20v)
  • Some type of male/female plug with 2 leads
  • Photovoltaic (PV) panels (5-10 volts (peak) more than the total battery voltage.
  • A battery-powered tool of some sort
  • Several lithium ion computer batteries. (Non-Apple.)

This project assumes the reader has basic soldering knowledge and basic electrical knowledge, if not there are plenty of how-tos and tutorials online.
Diodes can be scavenged from almost any types of old junk electronics whether it’s an old television, old computer monitor, printer or many other types of “junk” electronics. Many diodes have numbers on them, just type them into an Internet search engine and you can usually find the power it is rated for. Unsolder it and you’re ready to go.

[JWR Adds This Warning: Use extreme caution whenever cannibalizing parts from any high voltage electronics such as televisions. Most of these include high amperage capacitors which remain energized with a potentially lethal charge, even when the electronics are powered down. (And in fact even after discharged they can even “bounce” building up a new charge, unless they are shunted.)]
The same goes for scavenging male/female plugs, it can be the circular types used for plugging an AC adapter into electronics, the square type or anything really that you can plug together and has at least two leads. Adequate wires are easy to find in most electronics, just keep your eyes open.

[JWR Adds This Caution: I recommend using dedicated DC connectors with red and black polarity markings, such as Anderson Power Poles for all of your DC lights and appliances. This minimizes the risk of confusing the correct input voltage an type. You may know how it is intended to be used, but friends and relatives might be confused by a familiar-looking plug and do a Very Bad Thing. Inadvertently applying 120 VAC power could cause some smoke and/or fireworks.]
Solar panels can be a little more difficult to find but with the massive influx of cheap solar junk from China if you know where to look they aren’t too hard. I got mine from the solar patio lights you see everywhere and only last about a year before they break. I am sure if walking around your neighborhood you’ll see some that no longer work, and offer to take them off your neighbors hands, or just look in the trash you’ll find some eventually. You can usually find adequate diodes in them as well. It is important to test your solar panels to make sure they function. Most patio light solar panels output around 4-5v or so at peak, but by linking them together in series (+ to -) the total is the sum of each panels voltage (e.g. 4v+4v=8v).
Now for the batteries. Most laptop computers made in the past 10 years use round lithium ion metal encased cells. The only exception I know of are Macs which use lithium ion polymer cells which don’t work so well in this application just because they are rectangular and flat. The large battery packs you see actually contain several smaller “cells” inside and when linked together (in series again) provide the voltage required, the same applies for power tools. When your battery no longer holds a charge it is usually because a single cell has “died” and no longer functions whereas the rest of the cells still function albeit at a lower efficiency then they did before. So you can disassemble an old laptop battery pack and test the cells with a multimeter. If all the cells show around 3.7v and one shows 0v you found your culprit.
 Most cells are rated for 3.7v, so when deciding how many you will need for your project just make sure the sum is over the rating for your tool. In my case I needed at least 7.2v so I used 2 cells to power the drill (3.7v + 3.7v =7.4v). If you want your battery to last twice as long you can connect some of the batteries in series-parallel ( which produces sum of the current in milliamphours)  (+ to + and – to –) but it is important to balance the batteries out. So if I wanted to double the run time of my drill I would have first made 2 sets of 2 cells together in series, (+ battery – to + battery –) then connect the sets together in parallel by connecting the positives on each end together and the negatives on each end together.
If this seems confusing read more about it online and get a solid grasp of the theory behind it before connecting batteries together causing a fire or worse yet, an explosion.
The same goes for your solar panels wire them together to produce around 5-10 volts more than the sum of your batteries so that even when a cloud passes by you are still able to charge your batteries. For my project I wired 3 panels together in series (4+4+4=12v) and this works fine for me charging to 7.4v . Once again if you want to charge twice as fast just wire two sets of 3 panels (4+4+4=12v) together in parallel, and now you’re charging twice as fast.
Now in comes the diode. The diode functions like a one way valve, allowing electricity to travel in only one direction in your circuit. This is important because without it every time a cloud covers your panels, the electrons stored in your batteries will seep back out into the panels possibly damaging your panel and draining your battery. Diodes typically have a single white or black band indicating the orientation of it. The band indicates the negative side, so current flows from the banded side to the other, not vice-versa. The diode should be soldered between the + of your solar panels and the + of your batteries. If you have a multimeter you can set it to test your diode and make sure it is in the correct orientation, and working.
So let’s finally put everything together.
1.      Determine some method to attach your batteries to the drill and that they will fit.
2.      Determine if there are leads coming from the drill and which are + and – make sure they are long enough to reach the batteries.
3.      Solder the drills + lead to the + side of the batteries as well as another wire to attach to the diode and male/female connector later.
4.     Solder the – lead to the – side of the batteries as well as another wire to attach the male/female connector later.
5.      Solder the diode banded side lead to the extra wire we soldered to the + side.
6.      Solder one lead from the male/female connector to the non-banded side of the diode.
7.      Solder the remaining – lead to the remaining lead of the male/female connector
8.      Determine a way to mount the connector to the drill either drill a hole and glue it in or some other secure method.
9.      Solder the other half of the male/female connector to your solar panels (making sure your orientation matches up to the other half attached to the batteries)
10.  There are many ways to make this project look nicer and neater, some possibilities include encasing the back of your solar panels with fiberglass resin, to create a larger single panel, or using an old battery case (Dewalt etc) and taking out the old NiCd cells (please recycle, cadmium is deadly) and placing your new batteries, wires and diodes into the case, and closing it back up.
11.  Remember this should be fun, educational, and there is always a way to build a better mousetrap. I always love to hear others ideas and criticisms. If you have the skills (and time) you could include a charge control system or other features.
A few tips
1.      I always first just “soft wire” (as in, I twist the wires together) making sure none are touching and determine if my circuits work (like seeing if the drill works) before soldering things together.
2.      Double check the orientation of your male/female connector using your voltmeter before plugging them together and [with reverse polarity] possibly destroying your batteries.
3.      Make sure that any and all bare wires or soldered joints are not exposed. I use electrical tape, hot glue, and heat shrink tubing to cover all bare metal surfaces. If you don’t when you shove it all together then things will touch and short circuit.
The solar powered drill I made for free is still running strong after a year, and I’ve only had to recharge it once! Next solar project is a reciprocating saw!
Important safety note: Lithium ion batteries can explode if improperly recharged. (Read the pertinent news headlines.) Make sure you know what you are doing before attempting this project. There is no charge controller in this simplified solar system so it is important to make sure not to over charge your batteries! I would monitor the voltage of your batteries the first time charging in full sun and determine the time it takes to reach full charge. That way you have idea about how long to keep it connected to the panels in the sun. Also note that the quality of the batteries you start with will largely determine how often it needs to be recharged. So if your batteries are on their last legs expect the same from the drill.