Thrift Store Neato XV-21 Batteries Can’t Hold a Charge

The batteries from a Neato XV-21 were found to be flat and possibly damaged. This would certainly explain why the little robot vacuum failed to power on in the thrift store where I found it. Since the thrift store didn’t have its associated charging base, my fallback was to leave its batteries to trickle charge (at 100 milliamps) overnight on a bench power supply.

The battery packs nominal voltage was listed at 7.2 volt per pack, so the voltage limit for charging the pair in series was set to 14.4 volts. Fully charged six-cell NiMH battery packs can be up to 9 volts each, for 18 volts total, but setting to 14.4 volts leave us with a bit of headroom in case these batteries didn’t behave as expected. This turned out to be a good idea.

Neato battery pack on power supply

When I checked in on the batteries the next morning, I saw voltage sitting at 14.4 and amperage had dropped to 40 milliamps. If all goes well, the voltage would be evenly divided across the two packs at 7.2 volts each. This was not the case: one battery pack (I’ll arbitrarily name it pack A from now on) was holding at 7.9 volts and the other (now designated pack B) at 6.5. If it weren’t for the headroom, we might have inadvertently pushed pack A beyond its limits.

At this point I got distracted by other events and left the batteries sitting for a week. Checking in from time to time, I measured their open circuit voltage and wrote them down. It was an unplanned test of the batteries’ self-discharge rate.

Pack A:

  • Overnight trickle charge: 7.9 Volts
  • After one day: 4.67 V
  • After three days: 4.44 V
  • After a week: 2.0 V

Pack B:

  • Overnight trickle charge: 6.5 Volts
  • After one day: 2.77 V
  • After three days: 2.68 V
  • After a week: 2.61 V

While it is normal for NiMH batteries to self-discharge over time, this observed discharge curve was far more severe than is acceptable. Looking at the voltage discharge behavior for pack A, it appears to have two almost-useless cells, three weak cells, and one good cell. Pack B behavior hints at four almost-useless cells and two good cells.

But even if they are unusable to actually run the vacuum, they are still better than dead weight: these batteries can hold a charge for a very brief period. So they’re reconnected to the power supply to trickle charge again. I don’t expect them to actually power any motors, but perhaps there’d be enough to power on the electronics for a short while. Hopefully long enough for me to assess if the rest of the vacuum is healthy.

After another overnight charge, I plugged them into the vacuum and pushed the power button. Good news: they did power on the electronics for a short while. Bad news: It only lasted for three seconds, far too short to assess anything else about the vacuum. But the vacuum computer did boot, and that’s enough motivation to keep going.

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