This instructable is written to describe a way to deal with a common issue in cheap vacuum robot (IRobot knockoffs) - the short battery life.

Inspired by the great instructable: https://www.instructables.com/id/DIY-Battery-Pack-f...

and this YouTube clip:

I decided to try a new version of Li-Ion recharging.

Important background reading:

1) https://www.instructables.com/id/Li-ion-battery-cha... general explanation on batteries and charging.

2) https://www.instructables.com/id/Salvage-Lithium-Io... salvage batteries

3) http://batteryuniversity.com/learn/article/is_lith... li ion explained.

What you must realize is that li ion aren't easy to deal with, they are great power packs but very sophisticated as well, require electronic control circuit and other measures to avoid fire or explosion...

Step 1: Gather Materials

The following is needed for this project:

1) 8 - 18650 cells salvaged from old laptop batteries (follow this inst': https://www.instructables.com/id/Salvage-Lithium-Io... make sure to keep the tabs whole!. careful on the plastic as well!!!

2) wires - at least 1 red, 1 black and 5 more of whatever color U got. all have to be 15 cm long. try to find thicker wires than the skinny ones. we're passing alot of current here.

3) Li-ion charging module like this one: http://www.ebay.com/itm/Hot-PCB-BMS-Protection-Boa...

***choose the cell number according to your robot's voltage!***

4) Soldering iron

5) Solder

6) Wire stripper, pliers, wire cutter, screw driver etc.

7) Electrical tape

8) Utility knife or other sharp knife

9) old battery from the robot

10) some kind of plastic card, I cut a piece of a box. thin plastic, for insulation.

A note on voltage: check your robot existing battery and see what voltage it uses, you can check with voltmeter, read a label (if there is a label), or simply count the number of existing cells (Ni-Cd cells are usually 1.2 volts and arranged in series so multiply 1.2*N - N=number of cells).

Step 2: Strip Existing Battery of Wires

Take the robot's battery out, usually pressing two tabs and pulling.

Open cover with screw driver

Count the cells, mine is 12 cells, so 1.2X12=14.4V. I usually go higher so I chose to use 4 cells Li-Ion 18650 in series with 2 cells in parallel. Total voltage 3.7*4=14.8 nominal with tons of amp-hours...

A note on parallel and series: read the link on the intro, but to make thing easy - parallel makes a higher amp-hour rating but keeps the same voltage, series connection makes a higher voltage but keeps the amp-hour same. So 4 cells in series and 2 in parallel gives us 5200 amp-hours and 14.4 volts. just to compare, a single QUALITY Ni-Cd sub-c size battery has 2200 amp-hour so we're more than doubling the charge!

Now simply cut the plastic with a knife and cut the wires about 3 cm from the cells to which they are connected. We keep the 3 cm so that we can use the whole pack in future projects.

These 2 wires are saved for last, don't use them and keep the contacts attached to the wires!

Step 3: Prepare 4 Two-celled Sub-packs

Now we're going to make the sub-packs that make the basic piece of battery in our total pack.

Take 2 18650 cells from the salvaged laptop battery, tape them together with the + sides facing the same side.

solder a small piece of wire between the terminals on both the negative and positive side, try not to overheat the battery itself. Use the existing tabs, they make it much easier and flatter. Basically what you want to achieve is a flat sub-cell made of 2 18650 cells, that can fit into the old battery plastic with ease, without shorting anything...

Now expose 2 cm and wrap one of the 5 odd-colored wires on the negative terminal and solder it to the tab/wire that connects the two 18650 cells. This wire is the balance charge cable, it is necessary so don't skip!

Repeat this step 4 times but on the last sub-cell use a black wire instead of an odd-colored one.

Finish the sub-cells by soldering a red wire to the positive terminal of one of the sub-cells you made.

You should end up with the following:

one sub-cell with red and odd-colored wires,

one sub-cell with black wire on the negative side,

two sub-cell with odd-colored wires on the negative side.

Step 4: Connect the Cells!

OK now we must pay attention not to short our wires! we have a lot of power here...

connect the sub-cell with the red wire to one of the sub cells with the odd-colored wire. first you should tape them together with a piece of tape, arrange them with the positive side of one sub-cell facing the negative side of the other one. now connect the two: use another odd-color wire for this job from the negative side of the sub-cell with the red wire to the positive side of the other one. solder the ends so it's strong and solid. Now mark the odd colored wire B2 (important for later reference.)

Repeat the step with the 2 other sub cells, note that now you connect the negative side of the odd-colored sub cell to positive side of the black wired one. label the odd colored wire B4.

I didn't take a photo but attached a sketch of electrical connections to make it easier...

you now have two large packs of four 18650 cells with many wires. it looks confusing right?

connect the last 2 cells from negative to positive, to make a series connection of the 4 sub-cells, note that you have one unlabeled odd-colored wire coming out from the connection you just made, label it B4.

Now we have finished the whole pack, it should look like the photo.

Step 5: Connecting the PCB

Look at the PCB you bought from Ebay or whatever, it has several points that can be connected to wires, depending on the voltage of the battery. two of those are gonna be P- and P+ those are the terminals that connect to the wires we cut from the old battery, and supply power to the robot. the others are marked B1,B2,B3 and +/- or B+/B-. the last two - usually on opposing ends of the board are the black and red wires from the poles of the whole cell (black is -, red is +) and the other three are connect to the wires we labeled B2,B3&B4 previously.

solder everything accordingly, make sure to cut the wires very short - 2 or 3 mm tops so they won't shorten anything. I recommend starting with the balance charge wires (odd colored) and then red and black. also you should tape unused wires to prevent electric shock or short...

That's it. You're almost done!

Step 6: Finish Him!

all that's left is to squeeze it back to the box the old battery sat in, wedge a plastic card between the 2 large cells (1/2 & 3/4) to make sure they are insulated.

if everything's connected properly the robot should recharge and start running much longer than before!

If you wanna be real cautious you can read the voltage with a voltmeter and see it's around 15V.