The Ghetto Bank

A little while ago, the first battery of my trusty Makita power drill died, and the second battery is in not that good shape either, nothing wrong with the drill itself though.

A little shopping and googling tour revealed that a spare battery for my drill is almost as expensive as a brand new drill with two batteries. So, if I wish to replace both of my drill batteries with new ones, the most ecomical way to do that is to buy a new drill set, and discard the drill. In my honest opinion, that is beyond wrong!

Therefore, I had to try to find some alternative way to power my drill, and to resist the Man.

Well, what is a drill battery?

It is some metals and electricity producing chemicals wrapped into a plastic shell, carefully designed to be incompatible with anything but one particular brand and model of a drill.

How is the battery attached to the drill then?

Naturally, with at least a pair of complicated clips, incompatible with any device known to man, except one particular brand and model of a drill.

Well then, how is electricity transferred from a battery to a drill?

Via two pieces of metal, of course!

So, the world is full of cheap batteries and drills, but only very expensive ways to attach them together.

There are no complicated control circuits or anything like that between the basic drills and the batteries. Therefore, the only possible reason for the ridiculous prices of the spare batteries must be an international conspiracy by the secret order of evil incompatibility design engineers. (Some say that they may have the influence of the Stone cutters and the No-Homers, combined!)

OK, which batteries to use in an attempt to resist the Man?

The main criteria, price and availability, leave only one option; 18650 Li-ion cells from “dead” laptop batteries.

Most of the modern households have at least one “dead” laptop battery, and most of the people are only happy if someone takes out their trash.

I happened to have an old HP laptop with a battery that was able run the computer for only some tens of seconds, and, according to the info on the screen, refused to be charged above 1% of the total capasity. So. I carefully cracked open the battery case with a knife and a screwdriver. Carefully, because there is a true risk of turning a Li-ion cell to a torch by puncturing it and causing an internal short circuit.

Next I separated the six cells by ripping off the connecting strips of metal, and measured the voltage from each of the cells. From a battery without any signs of life in last five years, I did not expect too much, but to my great surprise, the voltage in all of the cells was between 3.4 and 3.5 V, i.e. they were all “empty”, but not dangerously over-drained. Depending on the source, the dangerous level of voltage, leading to irrepairable damage, is somewhere around 2.5 V (The protection circuit cut-off value varies between 2.2 and 2.9 V, depending on the manufacturer.)

So, it was possible that my batteries were not quite dead yet, and I continued my project by testing whether the batteries could still be charged. Problem with Li-ion cells is that in addition to being dangerous in low voltage conditions, over charging can, and will, transform them to a torch and/or a low power hand grenade. In order to avoid unwanted fires or explosions, most of the commercially available Li-ion batteries come with an in-built protection circuit, which takes care of over and under voltage situations. However, in case of laptop, and many other battery packs, a single protection circuit protects the whole package, and individual cells are un-protected, prone to explode or catch fire in inexperienced hands. To keep my house, eyes, hands, and other body parts intact, I test-charged the cells one by one with a protected Li-ion battery charger in relatively fire proof environment, never leaving a charging cell alone (just staying far enough from possible shrapnels). After a good two working days of charging, all of the cells were happily charged to the chargers cut-off level of 4.2 V. No smoke, no fire, no explosions, and the cells seemed to keep their charge pretty well, at least without any load applied. After about two weeks on the table, the voltage of the cells varied between 4.18 and 4.19 V. Then it was time for some capasity testing; all of the presumably 2200 mAh cells were able to run a 10 W led lamp for 40 minutes ( to avoid over dis-charging the batteries, I did not continue any further). After the test all of the cells still had at least 3.6 V in them, and although the test was not very accurate, it proved that the cells still had decent capasity in them.

In the end, I came to a conclusion that all of the cells from my dead laptop battery were in good working order, and that the failing part of the original battery pack must have been in the external circuit board, not in the cells.

Now I had six 2200 mAh cells to play with, and I decided that instead of just trying to replace a battery for my power drill, I’d try to build a bit more versatile power pack with a smallest possible budget, a true ghetto bank.

Well, the cells I had, and they were free. I also had a container, a 0.5 liter plastic freeze box from my kitchen (value about 40 cents), and some short pieces of wire, scrap plywood, screws and such from my scrap box (combined value about 10 cents). The only things I had to buy were the trays to hold the cells, and the protection circuit for my ghetto bank. I ordered those from one of the Chinese web shops (Trays 4.34 $, protection circuit 2.74 $, including shipping).

The total cost of the basic ghetto bank was less than 8 $, that is about 7 €. For the subsequent connecting gadgets, I was prepared to spend anything up to 4 $.

Building the ghetto bank was relatively easy. First I wired the trays so that they can be used either in 3S1P or 3S2P configuration, and attached the trays to a small piece of 4 mm plywood. Then it was time to solder six pieces of wire to the protection circuit board. With proper tools that is not difficult at all, but it was quite tricky to handle microscopic connections with my snow shovel size soldering iron. Miraculously, the circuit board survived the soldering, and I was able to cover the connections an loose wires with generous amount of hot glue. As universal power outlet, I used two strips of metal attached to the plywood frame.

The whole package fit perfectly into my sophisticated tacticool container, a 0.5 liter freeze box. A freeze box is actually very good power bank container. It is very compact in size, it gives pretty good physical protection, it is practically water proof, and if you happen to fall from a boat, your ghetto bank floats!

Weight of the whole thing with six cells is 410 grams (about 14.5 oz), and with three cells 280 grams (about 10 oz).

Additional equipment I’m going to carry with my ghetto bank are a cigarette lighter plug with 5 V USB output, a small universal plug, which fits to the charger of my video camera, and a pair of jaw clip test wires to connect everything together. Combined weight of these is 40 grams (about 1.4 oz), and the combined cost of them is 1.2 $, which brings the total price of the ghetto bank to a whopping 8.9 $. Even if we add a price for all of the consumables, such as 2 bars of hot glue and 5 cm (2 inch) of soldering wire, we can still safely declare that it is possible to build a 45 Wh power bank for less than 10 $. (If you are on a really tight budget, you can make your own battery trays and reduce the total cost to about 5$).

Then to the testing of the ghetto bank.

Initial readings from the multimeter were healthy 12.5 V with 3S2P as well as with 3S1P indicating that the wiring and the connections were all good.

Tests with plain wires, various plugs, and 12 V to 5 V USB converter were all successful, Tablet, Phone, action camera, and video camera were easily charged, and 25 W lamp shined nice and bright. Everything else but the original target, the power drill, worked beautifully. When I connected the ghetto bank to the drill, the drill gave some signs of life, but refused to catch any rpms. Suspecting the protection circuit being a bit over protective, I bypassed it and connected the cells directly to the drill, and what do you know, the drill was spinning without any trouble.

After a little study session, it became obvious that due to the protection circuit’s maximum continuous operating current of 2.5 A, the max output of the ghetto bank with fresh cells is about 31 W (12.5 V X 2.5 A), where as the Makita drill would prefer 35 W. Oh, so close!

Well, my bad, but I cannot be that sorry. After all, I managed to build a dirt cheap power bank good for a lot of things, and if I can find a protection circuit with maximum continuous operating current of 3 A or above, for another investment of 8 $, building a drill specific battery is a piece of cake.

Finally, to another great feature in the ghetto bank, relatively flexible charging.

The ghetto bank can be charged as a whole pack using practically any power source providing 12 V DC, including automobile cigarette lighters. The protection circuit manufacturer recommend 12.6 V meaning that a cigarette lighter with occasional 14-15 V is a bit of an overkill, but the charging seems work with them anyway.

Another way to charge the ghetto bank is to use a proper Li-ion battery charger for individual cells. This way it is possible continue using the ghetto bank with three cells while the other three cells are charging.

In conclusion, I can say that I’m pretty proud of being able to build a versatile, flexible, small, light weight, water proof, floating, and butt ugly ghetto bank with almost zero budget.

If you are familiar with the Li-ion related risks, poor, and adventurous, I can strongly recommend the ghetto bank design. All of you normal people should stay away from it.