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|
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|
|RC pack almost fits inside|
|End cap which is too wide to fit RC packs side by side inside the Roomba package|
|RC pack without endcaps still wider than half the package width|
|RC pack with plastic removed, cell to cell connection exposed on right|
|Existing metal plates bridging cells in the RC pack|
|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.