The battery pack of the NAO robot has “intelligent” charging circuit that prevents the battery from charging when the battery’s voltage falls below a set value. This would deem the battery useless and call for replacement. Our NAO was idle for about two years when restriction on physical activities was in place due to the Covid. This had caused the batteries to discharge to a very low value and we couldn’t charge them when we were back to the lab. Apparent, this is one of the major issue for the owners of NAO around the world impacted by Covid. Fortunately, thinkers community found how to hack the battery pack to revive the battery.
We managed to hack the two battery packs we have and revived them. The battery pack that have been used has deteroriated and can only last for less than 20 mins. The new battery pack that was intended as spare remains like new running beyond an hour.
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Ref: https://robohack.org.uk/viewtopic.php?f=5&t=69, https://www.eevblog.com/forum/repair/nao-robot-from-aldebaran-robotics-now-softbank-robotics-and-battery-pack-issue/, https://www.bartneck.de/2021/02/03/repair-of-broken-nao-robot-battery-pack/.