Adding to BruceHoult's point...
For 12V 1.5A plus, with NiMH you will need 18 to 32 cells, which is a big number to handle. With LiIon, you could get away with 3 in series or 6 cells in 2p3s (2 parallel in 3 series). But building a 18650 type LiIon pack will require a bit of experience. Besides building the pack, you will also need to build a way of charging the pack. Building a LiIon pack (and a charging solution) is not a job for beginners. Done wrong, 3 LiIon cells in series carries enough energy to get you into serious problems up to and including the cells breaking out in flames.
With the questions you were asking, I assume you are a beginner.
The simplest solution I can think of is LiIon 14500 sized "self-charging" batteries - those with charger already build-in to the battery cell. I am thinking the
NiteCore NL1475R or similar. The cell is AA cell sized, the cell itself has a micro-usb socket to be charged via USB. No need to build chargers, just get 3 usb to micro-usb charging cable. You will need one of those battery holder that is 3xAA in series.
Link to NiteCore battery spec page:
https://charger.nitecore.com/product/nl1475rEDIT: adding this
If you do use usb-charging NiteCore, don't charge them with single charger with 3xUSB outlets
while they are in the battery holder. a 3 USB outlets on the same charger will probably have all three USB share the same ground. If you plug the charge cables into the batteries while they are in the holder, you will be shorting at least two of them.
Note also, this is version two. I removed my initial reply and rewrote it when I remember NiteCore makes a battery with build-in charger.