Rechargeable AAA batteries, with a simple little trick

I recently found out about the existence of rechargeable 1.5V AAA batteries. Most people don’t know that normal NiMH AA or AAA batteries don’t output 1.5V like a normal Duracell battery would.

NiMH AA/AAA batteries are rated to 1.2V but they’ll give you 1.4V when fully charged and start dropping their voltage when they start to deplete.

The chinese company Kentli sells 14500 format Li-ion batteries that include a buck converter that will drop the 3.7V a Li-ion battery gives to 1.5V. The form factor is the same as a normal AA/AAA battery and they’ll work in most appliances.

Medtronic 722 insulin pump

This is specially useful for insulin pump users. I just moved away from disposable batteries, that last at best 4 days, to a pair of Kentli AAA cells.

There are two main caveats to this type of batteries in pumps:

  1. They show 1.5V for the full battery life, so the pump will always show 100% battery capacity remaining and die unexpectedly.
  2. They use a special charger

The positive end of the battery has a ring, protected by a piece of plastic, that will output 3.7V. This is directly connected to the Li-ion cell and it’s where the special charger makes contact.

AAA Kentli battery
Kentli special charger

The round lead on top, goes to the buck converter, giving you exactly 1.5V for the whole duration of the cell.

I haven’t found information anywhere about their use on insulin pumps, so I’ll report back once the battery depletes or if it gives me any troubles.

You can grab a pair for 15€ on Aliexpress.