zarck wrote:

Missed that it was the Powerbank version, it might be charged in circuit, but hard to balance without individual control...

The 2 slot 18650 battery charger/power bank Xtar PB2C has an IN/OUT balancer.
They write: Another special feature is the built-in balancer, both when charging and discharging in the function as a powerbank, the two batteries are always used equally.
My understanding: with IN/OUT balancer I can put an old and new battery together into the device.
And it is easy to see: each battery has an individual slot.

It seems it is the same construction with these 4 slot devices (with small flashlight):

Same construction with 6/8 batteries would be nice.
Keep the older batteries full charged and have an emergency flashlight/power source.