Introduction: Give New Life to Your Drill
Like a good maker I would never leave an old quality battery drill in the trash can. So when I found it I decided it was time to improve my refurbishing skills with last (at least for me) technology.
The main purpose of this instructable is not to explain all the steps needed to complete the project, but actually I feel that I will better help if I succeed in convincing you that it's not difficult at all, and a project like this makes sense. Other makers (many of them, really) wrote very good instructables explaining every step. I red some ibles, and I referred particularly to this (which I suggest you to read too):
DIY Professional 18650 Battery Pack
But there are many others, like:
DIY 4S Lithium Battery Pack With BMS
Step 1: Remove Old Cells
Opening the battery pack should not be too hard, just try not to brake the enclosure, as you will try to assemble it again with new cells inside.
Mine is a Ni-Mh 18V battery pack. That cell technology is now outdated, like Ni-Cd batteries, and now some types of Lithium cell have been improved. They have a power density much higher, and they last longer if you have some caution in charging and discharging them.
Step 2:
The Lithium Ions cells have a nominal voltage of 3.7 Volt (with a maximum voltage of 4.2V), so I decided to buy on eBay 10 Li-Ion cells, and make a more powerful renewed battery pack. Indeed doubling the number of cells needed for the right voltage (5cells x 3.7V = 18.5V) will double the autonomy. It also double the electrical power, but the motor would probably not adsorb so much.
There are many different qualities and sizes of Li-Ion cells, I opted for the 18650 since it's cheaper and easy to find, although I decided not to buy the cheapest batteries, in my opinion around 4-5$ each for 2800 mAh cells is a good price to try.
I bought batteries from this seller and I'm very satisfied. All cells were exactly at the same voltage (3.81V if I remember right) when they arrived from China, so they had probably been grouped together with some care, and they were ready to be assembled in a single battery.
Step 3: The BMS
Since Li-Ion cells have to be charged under some conditions, you cannot charge them directly with a PSU, but you need to connect a circuit which take care of it. This circuit is the BMS (Battery Management System), and you have to buy one according to the number of batteries in series. I used 5 groups of batteries in series, each group is made by two batteries connected in parallel (plus to plus, and minus to minus), and my battery pack will be named "5S2P". I looked for a 5S Li-Ion BMS.
As you can see you have a wide choice of shapes, dimensions, and amperage (which depends by the number of parallel cells). The more power you need the higher will be the cost. I paid my BMS (the blue one) around 10$.
Step 4: BMS Connections
Here you can see the schematic I followed for my BMS, but I connected groups of two batteries like the second image. You see that each board has different positions where to solder the wires coming from each cell. Just follow the diagram relative to your BMS, or the labels on the board (voltage starting from zero and ending with full voltage, with a number of steps equal to the number of batteries in series).
BMS needs to know the voltage of each cell, to equalize the voltage, this is the reason why you need all those wires. Look the first schematic, note that internal wires are thinner, but power wires have to be thicker, this is because the power will pass through them, so remember to use a good section copper wire.
Step 5: An Open-source BMS
If you're a real bad-ass maker, you'll probably want to build your own BMS, and this instructable is for you:
Open-source 3/4/5S Lithium BMS
(thanks lutkeveld from Netherlands)
Step 6: Fitting Everything
Just make some try to fit everything inside the case. I still had some space free... I could had added a voltage regulator or a 5V USB plug... so to use the battery as power supply... You can let loose your creativity here!
Step 7: Glue Them All
If you're satisfied with position of cells, BMS, and everything else you decided to add, check that the case will close and glue the cells together with small pints of hot glue.
Step 8: Spot-welding
When I informed about the Li-Ion cells dangers, and the processes to avoid to keep them in good conditions, I discovered the a little higher heating (lets say 100-150°C) will damage the cells, decreasing the capacity. So I would never solder a wire or a metal strip on them, as tin alloys tend to melt at 200°C, and you can be fast, but metal shell conduct heat very fast.
I decided to buy a spot-welder, as I found a cheap one at around 130. There are also good projects on Instructables, where you can learn to make your own spot-welder from a microwave oven, which is essentially what you find on the cheapest Chinese spot-welders like mine (the blue one, a Sunko 737U).
Step 9: Welding Strips
In the Sunko box I found also many nickel strips (probably nickel-plated), so I was ready to try it. I made some test with other metals and with nickel strips I removed from old cells, and the only problem was that occasionally, turning on the machine, the power absorbed was so much that my home circuit-breaker shut off. Other than that I found the process very fast, simple, safe (but never touch the electrodes and always wear safe-glasses!).
Also avoid to touch battery poles other than the poles you are welding, especially with something conductive, it can be very dangerous since the voltage and the power involved are quite high.
Step 10:
As you can see I first connected the positive poles together in each group of two, then the negative poles, and then I could connect all the groups in series (plus to negative, and going on). Double strips between each group of two will reduce electrical resistance and improve the efficiency.
Step 11: Soldering
You can solder the wires to the nickel strips, keeping far from cells extremities. Then glue the BMS over the battery, connect it, and solder thick wires to plus and minus poles of the battery.
Step 12: Battery Connection
I retrieved the original battery plug, so now I can solder thick wires from the BMS to it. If you notice there are two other connections, and a thermistor, which was used by the original charger. I left it there but I don't need it anymore. I painted under the battery pack the new voltage and power.
Step 13: Powerbank Connectors
Just drill holes and solder two thick cables from the P+ and P- pads on the BMS.
Step 14: Charging
To charge the battery you now only need a PSU with the right voltage, the same of the battery. Anyway I have an iMax-B6 charger, so I set the 5S Li-Ion charging function and I connect it with crocodiles to the same terminals which power the drill, or with banana plugs to the front terminals.
Pay attention to the polarity! I really don't know what would happen if you mistake!
Step 15: Other Projects
Now that you learnt how to upgrade your drill with new Li-Ion battery, you can go on with other electric tools!