OK, you want the longest life C cell and worried about leakage. I truly don't think you will have any leakage problems with changing out alkalines C Cells every year. And life…If the load is very low, why not just use them. I don't think you can improve without going to a larger battery. I know climbing on top of a bus isn't the most fun thing to do….But there are people that climb all over trucks and airplanes to do maintenance also…..Try going out on top of a Boeing 747 airplane….or even worse, that Airbus A380 thing….even going out on the wings are up there.
A single C cell….1 time/yr change out…I'm assuming Alkaline & you want something that will last longer and be rechargeable???? You stated that they are under extremely light load……I don't think your gonna do any better than what you doing now…In fact, rechargeable batteries probably will require more changes/year.
You stated that you would like to go with Lithium size C….If you were referring to Lithium Ion type batteries, those are called 26500 cell in the Lithium Ion world….or sometimes 25500….the first 2 numbers are the diameter of the cell. the next 2 numbers are the length. All in millimeters…..so a C cell is roughly 1 inch in diameter (~25.4mm) and 2 inches in length (~50.8mm)….The problem with LIthium Ion type cells is that they have a very high voltage of 3.2 to 4.2 volts. I would never put those into an application where a 1.5 volt battery is called for…..And you are correct that there are not many out there. In fact since the end of Covid, I'm almost sure there are 0 because they just don't make them like they use to…I don't know the reason for this. There are many Lithium cells that in short supply post Covid.
There are also 1.5 volt USB rechargable lithiums. These are lithium ion type batteries in a AA, AAA, C, or D size case. In other words, these contain a transistorized regulator to drop the 3+ volts of the internal lithium battery down to 1.5 volts. These are usually rated in Watt-hours. I guess they are playing games with us at to make us think they have more capacity than other chemistries..These are lower capacities because the internal battery is always smaller than the case they are in due to the recharge port (USB) and the regulator.
You can go here (HKJ's web site) and select 26500 and you will find 1 cell…from 2016. (Probably not around to purchase anymore)
https://lygte-info.dk/info/batteryIndex.html
So according to HKJ….at
https://lygte-info.dk/review/batteries2012/CommonCDcomparator.php
And putting in some C cells….at 100 mAh loads. The upper cells in the selection pull down are NiMH type…the Lower selections are Alkaline (Can't select loads less than 100 mAh on the website)
I'm thinking you are better off with the alkalines that you currently use.....But.
Looking at your location…I'm seeing Rosemount…..where it probably gets cold….and Alkalines don't like cold…especially below 0 cold….They tend to go dead….
Now for the best tested rechargeable C battery is probably the Turnigy Sub C 5000 mAh….You can get them at Hobbyking when they have them (and elsewhere)….I believe they are made for the power tool industry and have a higher voltage as compared to other full size C cells. They have been optimized in design because of the millions used in the tool industry….Almost all other full size C cells have a lower voltage when discharging. If you consider using these, you will have to also use a copper, aluminum slug, or a magnet to take up the 7 or so mm's in length to make connections. I don't think I would use this because of the vibration of the bus.
Now we get to non-rechargeable Energizer Lithium AAA's to C adapters…..According to HKJ at
https://lygte-info.dk/review/batteries2012/CommonAAcomparator.php
At 100 mAh load, these have a capacity of around 1.25 Ah…or 1,250 mAh….and the adapters hold 4 of them, which adds up to 5000 mAh….They hold their voltage all the way out to 1.25Ah each and drops like a rock….And these will work where the temperature drops to below 0….But now you have to install all of them correctly and the contact pressure of the adapter has to be almost all the same…lots of variables.
Your adapters only holds 1 AA battery. You can go to the above site and select the Energizer Lithium AA...That battery has little over 3 Ah or 3000 mAh...
Now if you are really want rechargeables, I have had very good luck with Amazon Basics NiMH batteries. They always test out above their stated capacity of 5000 mAh…In fact they charge to 5500 to 5800 mAh, and discharge to 4900 to 5200 mAh discharge. I am on my 3nd year of using them in an old 1970's Sony portable radio. I get around 9 months at 30 min/da before it's time to recharge. This radio is a power hog. When I first got this radio, alkalines back in the 70's would last like 2 months...(much better than those crappy carbon/zinc batteries)..I eventually bought an AC adapter.
Over time the alkalines did get much better and would last at least a year today…
You will have to get a charger that will recharge the Amazon ones. I use an AccuPower 338XL. Those are the only rechargeable C cells I have tried. Don't forget to break them in….like at least 2-3 discharge/charge cycles….and they are almost back down to their pre-Covid prices. I have also had very good luck with the Amazon Basics size D's.
https://www.amazon.com/dp/B07PHCVSSS/?tag=cpf0b6-20
1 other thing about rechargeable batteries….if you discharge them to much, they die.
I think I'm stating all of this correctly….Lots of numbers, lots of different battery chemistries, lots of different things….
oh...as Columbo (and Steve Jobs) were to say....just 1 more thing. If that Ubisense location device sends out a low battery signal, then I'm betting all bets are off and your gonna be using alkalines. All the other chemistries would surely fool the low voltage alarm. That device was made for alkaline batteries. The curves are that different. NiMh batteries are closest to alkalines.