The most energy and space efficient source of 6 V DC for a trail camera

Question

I use several low-end trail cameras for monitoring wildlife. All of them are powered with 4 x 1.5 V AA batteries. Replacing non-rechargeable cells in several trail cameras starts to generate more electro-waste than I like (and it also costs money). All cams have external 6 V DC input, so I am starting to think about options I have in order to get rechargeable source of 6 V. Regarding current, it only takes few mA (maybe les than 1 mA) in stand-by and this usually takes most of the battery life.

As first, I have asked a manufacturer of my recent trail camera (Suntek Mini300) about using 7.4 V (2S li-ion cells) as external power source. As I expected, they do not recommend it, because "the infra LED will not work" (no more details to this statement). So I will probably need source of 6 V. Below, I sum options I came with. There is a need to say, all of them will be really "external" - can not be fitted to 4xAA compartment in trail camera.

1. 5xAA Ni-MH 1.2 V cells - probably the simplest solution, but re-charging 5 cells is really uncomfortable.
2. 4xAA Ni-MH 1.2 V cells + step-up converter - I know very little about it, but stepping up 1.2 V might not be very efficient.
3. Li-ion 3.7 V cell / 2P cells + step-up converter - I have no experience with li-ion cells in outdoor conditions + uncertain efficiency. 1 cell might not have enough capacity, and connecting 2 cells in parallel may be dangerous.
4. Li-ion 2S cells @ 7.4 V + step-down converter - low voltage difference -> might not be very efficient (?)
5. Li-ion 3S cells @ 11.1 V + step-down converter - voltage difference is higher, so conversion might be more efficient(?)
6. Above mentioned with Li-po cells - no experience, aware of cold conditions
7. 6V Pb cell - can have enough capacity, but is too bulky & heavy and especially not very eco-friendly.
8. Lion 2S @ 7.4 v directly :) - should I try it despite the fact it is not recommended? It is needed to assume 8.4 V when fully charged.
9. Other suggestions?

Personally, I mostly like first 2 options, but maybe I have not mentioned some obvious solution, that is why I am asking here. I will be very grateful for any suggestions or discussion of above stated options. thank you.

Answer 1

You should try checking out LifePO4 batteries as an alternative solution. They have a nominal voltage of around 3.2V, so having 2 in series is 6.4V, which is really close to what your camera needs. You will require a (tiny) BMS circuit on it though since discharging them below its recommended minimum voltage will permanently damage it.

Answer 2

I have actually first come across similar cameras about a month ago, possibly the same model or manufacturer.
A customer was confused as to why they worked on batteries but not on an adapter. He thought some electronics failed on the board. I discovered that the camera actually needed a specific voltage range or it wouldn't even turn on. I don't remember exactly, I think it was from 4.5V to below 9V. He actually had a power supply whose voltage was outside of this range (I think 12V).

Anyway, first to correct you: connecting 2 cells in parallel can be dangerous only if their charge levels vary greatly (like more than 0.2-0.3V I think), so all you need to do is make sure their voltages are within 0.1-0.2V of each other when connecting them together and they will just work great afterwards.
If you are using a DC-DC converter, make sure that it has a VERY low quiescent current (the amount of current it uses with no load whatsoever, for running its own circuitry).

Before trying anything else, I would suggest you measure a camera's current consumption at 5, 6, 7 and 8V and see if it varies significantly (especially with the IR LEDs turned on).
Your solution might be as simple as 2 Li-Ion cells in series, without any conversion. You would just have to make sure they disconnect before the voltage of either cell drops too low (below 3.2V; depends on its chemistry). If the cells have circuitry on them, that's good, but you may want to have your own adjustable, micro-power monitoring circuit.

One tip: lithium cells can be recharged many more times if you use only 50% of their capacity (from 30%-80% charge level), so you could employ that tactic.
Another tip: You could include a ~7.2V mini solar panel to charge your batteries. It wouldn't be much larger than the camera.

Answer 3

remember you get a robust 0.6 or 0.7 volts drop across silicon diodes.

consider 1n4001 for your current level