Seems nearly impossible without a fancy custom built circuit. I have a Black and Decker GrassHog weed whacker that uses an 18v nominal slide in pack. The original packs were high capacity NiCads and the charger was transformer based [rated 21.75 @ 0.85 amps].
15 NiCads at 1.45v = 21.75 so in theory I could leave the pack plugged in for a long time without damage. The minimum charge time was supposed to be 8 or 9 hours.
Both packs failed at about 25 cycles. In the meantime I had bought 4 more chargers and 3 more packs [standard capacity, not hi-cap]. All of those standard packs still work.
The price of a pair of hi-cap packs is 3/4's the price of the whole original machine + 2 packs, so I looked into building nicad or NiMH packs from single cells and found it was cheaper to just buy a new pack than to rebuild. Last week I saw protected 18650 lithium cells for cheap on eBay and bought 20. These can't be tab welded, as the bottom of the cell is a plated disk soldered to the BPC at the base.
I soldered a pack of 6 and charged the pack with one of the nicad chargers I had. All the remaining chargers are switching supply types. Being B&D there is not much quality control. The Voc of the transformer supply was 21.7. The switching supplies were 24.6, 28.0, 28.8 and 29.6. Only the 24.6v supply will charge a string of six 3.7v lithium cells without going over, but one cell stays at 3.85 while the others go to about 4.05.
The other chargers pushed at least one cell waaaaaay over 4.2v and would have burned up the pack if left alone long enough. The battery protector circuits don't seem to work very well at all.
I went to eBay and bought plastic holders and twin-pack wall chargers for the 18650's and intend to charge them loose and insert and remove loose cells when I need to use the weed whacker, even though it isn't exactly convenient to do so.
The lithium cells do not like the high rate of discharge the weed whacker demands, but at 3 times the voltage of a nicad they are still cheaper even if they don't cycle any more than the nicads did.
The only way to leave lithium cells in a pack and get some balance to the string seems to be a circuit that charges each one separately, but I don't know how to do that in a simple manner.