Is there an 18v battery about the size of a C cell? A zener tester, probably 1970s, that goes upto 17v max, takes what looks like a single C cell. There is no circuitry that could up the v as far as I can tell.
I have a few pieces of older gear that takes batteries that are no longer on the market. If there is room I use a 9 volt battery and one of the boost converters from China. You can get them to boost or lower (buck) the voltage.
It all depends on how much room you have. I don't recall how much but some boost converters have a minimum input voltage. I did use 4 AA batteries in a device that had plenty of room and a boost converter.
Not counting the loss for efficency I have found the converters to be almost like a transformer. The power out is about equal to the power in, so if going from a low vlotage to a high voltage you draw more current and if going from a high voltage to a low voltage you draw less courrent.
That has a minimum input of 5V so a single 3.7V Li isn't going to do. Several years ago I used one of these:
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"DC-DC Auto Boost Buck step Up step down Converter Module Solar Voltage LM2577" so I could use a single
18500 Li Ion rechargeable to replace a pair of AAA cells. The input ranged from 3.7ish down to 2.7ish as the Li Ion cell discharged (beating the input spec of 3.0V so I was happy) while the output stayed stable at
3.0V where I set it. No, the board and cell didn't fit where the AAA's did, but I was tired of recharging NiMH AAA's every morning after a night of logging pulse and pO2. Anyway, there might be a more modern/cheaper version available now. Oh, the output spec is 1-30V and I chose this model because the input and output ranges can overlap, but with your 22.5V output that isn't a consideration, you just have to find the room for the little board and an 18650 Li cell (unless you want to run it from a 2032 :-)).
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