Producing replacement battery pack for original model Roomba vacuum

BUILDING REPLACEMENT ROOMBA BATTERY

The battery pack for the original model Roomba vacuum is not holding a charge as well as it did when new. The pack is a specially shaped module holding 12 NiMH cells, each producing 1.2V with a capacity of roughly 3000 mAH.

They are chained together to yield 14.2V for the pack, but with no circuitry to distribute load or even out charging, which is why they degrade over time as do most simple-minded serial chains of battery cells. All but one of the serial connections are tack welded metal bridges, the remaining one is a carefully created fusible link tack welded across one pair. This provides overcurrent protection for the pack.

One final aspect of the pack is the inclusion of a varistor, a resistor whose value varies with temperature. This thermistor is wedged in the gap between four adjacent cells. It is used by the charging circuitry to lower the incoming voltage or cut it off if the cells are getting too hot. The pack as a +, a - and a thermistor contact.

Opened battery pack, white fusible link on left, thermistor next to it

My plan was to replace the 12 cells with similar or higher capacity NiMH cells, soldering the metal bridges and one fusible link between cells to place them in series. These new cells had to fit within the original plastic package, in order for the repaired battery pack to fit back in the Roomba.

I found some high capacity battery packs intended for use in remote control cars and other toys. These packs produce 7.2V at 3500 mAH capacity by bridging six cells together. These RC packs are plastic wrapped, with the series connection made inside, and two wires coming out at one end.

RC pack with six 1.2V cells inside

I had hoped to place two of these packs in the original battery package, tie one pack to the other with the fusible link, wedge in the thermistor and close it up so that it now chained 12 cells to produce 14.4V. I found that the RC packs were just a bit too large, particularly the end caps which were wider than the two columns of cells.

RC pack almost fits inside

End cap which is too wide to fit RC packs side by side inside the Roomba package

I considered various means of grinding down the inside of the Roomba package and the end caps of the battery packs, to force them inside together. Even the unwrapped cells themselves are too wide to fit side by side lengthwise. While grinding may work, the most certain method is to fully disassemble the RC packs and find myself with 12 NiMH cells of exactly the right type.

RC pack without endcaps still wider than half the package width

I can solder wires and the fusible link across the terminals to chain them in series. More work but it will fit better and more closely match the original battery construction. I began to work on the end caps of one pack to determine whether that partial method is at all feasible, since it avoids much of the soldering necessary with the full disassembly route.

RC pack with plastic removed, cell to cell connection exposed on right

I found that the cells are tack welded with strips of metal, just as with the original Roomba construction, but the strips of metal are not long enough to fold the cells in alternating directions as is required to have them sit vertically in the Roomba package.

Existing metal plates bridging cells in the RC pack

I had to break off all the metal links from the positive pole of the cells, to separate them and to test their fit inside the Roomba package. I found that they will indeed fit exactly as they should inside when the cells are each stacked vertically. Of course, they must alternate up and down pointing cells and the connections run down one column, cross over at the end and run back on the other column to end up with the correct series polarity.

Test fit of the cells sitting vertically, but not in the alternating direction ultimately needed

I then removed the other end of the metal plates from the negative can of the cells and removed the red and white wires, leaving me 12 unattached cells to begin soldering together. I will have to find and cut some flat metal strips, as the proper diameter wires would take up too much vertical space around the cells. I to source the strips and cut them to length before I can continue with this project.