Why is battery voltage important?

Dear reader,

I am doing a project in which I make a wireless band aid full with sensors.
These sensors work on the following voltages: 1.8V and 3.3V.
I want to make use of a voltage regulator in order to ‘feed’ these sensors with the right voltage.
The battery MUST be rechargeable.

I now have the choice between Lithium technology (3.0V-3.7V) and Nickel technology (1.2V-1.6V), since these are rechargeable.
What is a reason to choose which technology? It is possible to boost a voltage with a boost-converter, but that takes up a lot of space…

Sincerely,
A student from Delft University of Technology.

Both can deliver a good amount of Amperage and have comparable energy density based on volume, but Li-Ions are lighter weight so they have higher specific energy (mass based). NiMH has a nominal Voltage of 1.2V under load and a typical Li-ion 3.6V under load. NiMH are generally safer and more forgiving of abuse (over charge/over discharge).

You can use a Zener diode or a Voltage Reference chip to create the 1.8 from the 3.3 supply; also create the 3.3 from a VR chip using a small lithium battery source (look inside a vape stick for tiny battery, some have BMS). Lithium is a better technology of battery due to size and weight, and in your case voltage.

You may be able to skip the 3.3V regulator and use a LiFePO4 battery directly. This would mean less capacity and also that the 3.3V would vary a bit (Check data sheets if that is acceptable).

Normal LiIon is 3.0V to 4.2V, if you decides not to use full capacity, you may be able to get 3.3V from a low dropout regulator.

LiFePO4 is 2.9V to 3.6V, but you can charge with slightly less than 3.6V and reduce maximum to about 3.4V to 3.5V.

NiMH requires either a boost regulator or 3 batteries in series and you will get less energy and more mass for the same size requirement.

I have done discharge curves of all types of batteries.

Have you looked at using a LiPO battery pack instead. What are your size constraints here. An alternate form factor would be a small cell phone battery, Very thin, multiple sizes available, and there are a lot of easily available charger/controller circuitry to handle controlling the output and charging.

Lithium goes up to 4.2, not 3.7V.

As HKJ says the use of a LiFePO4 could be 0K. Linking the main discharge graph of his Enerpower 18650 3.2V 1800mAh (Blue) test here:

Very clearly it can be seen that under a low load voltage stays at or above 3.2V most of the curve, not going below 3V until the last moment. So, with just a 1.8V linear regulator those sensors could be fed.

If voltage stability is super important, a li-ion cell with an ultra low dropout 3.3V linear regulator will deliver 3.3V until quite close to the end of the discharge, presuming a low load again of course. Here is the main discharge curve of the LG 18650 MH1 3200mAh (Cyan) test from HKJ, a typical energy cell, for illustrative purposes: