5 cells have 21V when fully charged. -> I cannot use my favourite LT1763 voltage regulator. (max 20V)
How about additional balancing cirquit, transport limitations, ...?
I hear you saying: use the step-up and a 1S configuration. But a running DC/DC converter near a precision cirquit is also not a good option and will lead to strange drifts due to EMI.
And most of the weight is in the housing. -> that would be the better way to save weight.
Charging Li-Ion to 4V would only minor decrease capacitance, so 20V limit would not be that critical.
A balancer for Li cells is needed, but for NiMH balancing is needed too and charging is much more difficult (-dU/dT or better: peak detection).
Charging NiMH cells like Li cells is not recommended and trickle-charging at constant voltage or even very low current would damage the cells in a (short) period of time.
This seems to be quite critical in this application as it should be powered 24/7 and is trickle-charging most of the time.
12 cells of NiMH go down to 10.8V (0.9V/cell), far away from safe minimum 15V. Even nominal 1.2V result only in 14.4V, so not shure how you calculated the safety margin for discharge
You are right: transportation of Li cells is restricted in several forms, a powered device cannot be shipped in normal way. No option for Li cells in this project, but not shure if the restriction of powered device is only valid for Li cells
Yes, switching regulators are not suggested for such a precision gear, perhaps SC+LT1763 could be an option as the currents are quite low.
If the size of battery pack is reduced there is more room left in case, so the case could be shorter and the whole weight decreases.
But there are further limitations for cheap transport.
Within Germany we have a 5cm limit for the height of a letter (2.60 EUR up to 1000 g)
From Germany to Europe there are mainly the weight limits 500 g = 3.70 EUR and 1000g = 7 EUR.
If marked as "Warensendung" it is only 1,90 Eur/500g or 2,20 Eur/1kg within germany.
The prices to europe are the same as for (most of) the world, problem could only be the customs to pay for.
Not shure if there is an option to say it is only temporary out of germany/europe to avoid customs, as it would be if you travel with it, otherwise could be that customs has to be payed twice