Powering 24 V and 12 V device simultaneously from two 12 V batteries

Question

I have two devices which need to be battery-powered (remote area, no grid power available). One device requires 24 V, the other requires 12 V. Both are very low amperage (~50 mA).

Can I use my two 12 V batteries in both series and parallel to provide 12 V to one device, and 24 V to the other?

Alternatively, I know I could use a step-up or a step-down transformer, but at least one of the devices monitors and reports battery voltage (maybe both, not sure yet), which I would lose with a stabilized voltage output. If for example I have to use a step down converter to go from ~24v > ~12v, is there a type of transformer which simply divides the input voltage in half, instead of always producing a stable 12V output? If so, what is it called? And is it more efficient to step up, or step down in this case?

Edited for clarity: I prefer not to run the 12 V device off a single battery, because I'd like the batteries to drain at the same rate.

Here is a simple drawing showing what I'd like to do:

Answer 1

You cannot connect them as shown in your drawing. To draw equally from the pair you will need a dc-dc buck converter as shown below.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Your generally stuck with the buck converter solution, otherwise yo end up with the grounded battery going flat and the 24V side battery overcharging, leading to 2 dead batteries in rather short order (enough moving truck owners try the same for radios..)

If your device that monitors battery voltage has a separate input for it, then you can have a divider just for that, while running the device off the buck converter (Based on my assumption its a device that monitors ignition voltage)

e.g. many ignition inputs on the kind of device I suspect it is, is about 100K load, so you can compensate the divider for the rather small range of voltage you actually want to know about

There is an alternative but its less common, a balance charger for the middle battery, they are not that cheap, and rely on your device being quite low load https://www.outbackmarine.com.au/battery-balancer-victron-bba000100100

Answer 3

You can use the Buck converter to drive your loads. And install a separate 12v monitor on each battery directly to the terminals paying careful attention to not wire the battery terminals in parallel while they are also wired in series for 24V. Each individual battery monitor will report only on that battery and provided they don't share the ground to the chassis and get it directly from the battery, the intermediate ground that floats 12v above chassis will not short out the system. Also be careful to isolate the chassis of your 12v monitors because if you ground the chassis of the high side, this will short out the batteries. I would recommend installing them directly on the battery with double side tape and using very short leads so that there is no misunderstanding about the black floating grounds as might happen if you were to run long wires to a remote location.