The battery pack of my Roomba is made of 12 sub-C NiMH 1.2v cells; two of them are down to around 0.5v, and the Roomba won't work.

Can I replace just these two cells with brand-new ones? Will this be safe (as in "not exploding or causing a fire") if I use the same tech and voltage (and perhaps a greater or equal capacity)?

Found this question which leads me to think it should be OK, although my question is slightly different. Many other questions have safety concerns, but they're usually about mixing different kinds of batteries, or mixing voltages, etc.

I don't know what I don't know, so I thought I'd better ask!

  • \$\begingroup\$ Most likley yes. Have you tried to just charge the depleated cells? \$\endgroup\$
    – winny
    Apr 12, 2018 at 8:17
  • \$\begingroup\$ electronics.stackexchange.com/questions/360987/… \$\endgroup\$
    – Oldfart
    Apr 12, 2018 at 8:19
  • \$\begingroup\$ @winny I wouldn't know how to do that - I don't have a battery charger for sub-C cells. I guess I could try a regular charger (for consumer rechargeable AAAs) that I somehow hack to make the sub-C fit? \$\endgroup\$
    – ggambetta
    Apr 12, 2018 at 9:12
  • \$\begingroup\$ They are probably dead, but if it where me, I would do just that, hack the charger with probably two aligator clip leads to the dischaged subC cell to rule it out before buying new cells or pack. \$\endgroup\$
    – winny
    Apr 12, 2018 at 10:31
  • \$\begingroup\$ If two cells in the pack are dead, there are probably other cells nearing end-of-life. I'd replace the whole pack, otherwise you'll likely have to replace another cell or two in a month or so. \$\endgroup\$ Apr 12, 2018 at 23:34

1 Answer 1


Assuming the cells are in series, it is not usually a good idea to mix rechargeable NiMH cells in a stack. Overcharging is a problem, and different cells will reach their full capacity at different rates.


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