For those who don't mind some non-factory modifications:
The 212 (and similar) Tek miniscopes use nickel-cadmium cells charged with a non-isolated line-based charger. (The isolation occurs in the DC-DC converter that the cells power.)
For me, nicads are a pain: They tend to be discharged when you need them, and they sometimes leak and/or salt, making a mess.
I noticed that the compartment that holds the AC power plug is (with some filing and cutting) the right size for a single 18650 Li-Ion cell. I cut apart an old battery holder to get the spring and "+" contact and mounted them in that compartment.
The 18650 cell goes to a single-cell protection circuit removed from an old cell phone battery. (You could use a protected 18650 cell.) No charging is attempted in-circuit here, but the protection circuit does prevent over-discharge.
I used a boost converter (cheap on eBay) to convert the 3.7V to the 11V the 212 power supply needs.
I stripped out the original AC charge circuitry and re-purposed that pole of the switch to go between the (protected) battery output and the boost converter.
I found I needed to put a 0.1uF capacitor paralleled with a 10 Megohm resistor between the Bat- and Pack- terminals of the battery protection circuit. This gives a momentary kick to the circuit to convince it to release from "overdischage" mode when a cell is first connected.
The added circuitry easily fits into space within the old battery mounting frames. The frames are retained and fitted with some internal rigid struts since they form part of the mechanical support for the scope circuit boards.
The Li-ion cell holds a charge well, and the nice rear-door battery holder makes swapping in a spare very easy.