I was watching a UK video today and the guy was using PP9 batteries (carbon-zinc, 9 volts, 5Ah using the familiar clip connectors). I have never come across one myself, but they are still available through element14 ($10) and RS Components ($20!)

https://en.wikipedia.org/wiki/Nine-volt_battery#/media/File:PP_series_batteries.JPG

Didn't know about all those battery shapes. I think it's a good thing that there's only a few types today.

Arguably, there are far more. Watch cells, 12 volt N-cells, lantern batteries, both the screw terminal and dolphin types, D, C, AA, AAA cells, sub-C cells, type 216 (same as the second from the left in the photo) and a few I can't remember but have seen in places like Jaycar and Altronics.

I think the big difference with batteries these days, aside from smaller sizes is that they no longer cost a king's ransom.

Along those lines, I recently upgraded the battery situation for the camps I go on sometimes. I purchased two 12V 100Ah LiFePO4 batteries and one 200Ah one of the same type. I got them all at less than half of what retailers sell them for.

The thing I like about these batteries is the lumpy voltage curve they produce. They keep up above 13 volts until just before they go flat. I can run by Waeco 35 litre camp fridge for 13 days on the 200Ah battery before it requires recharging.