Hello I have These small units that monitor the state of a battery or packs.
The way I use them is to sodder on the red leds leeds 2 wires to the exterior of the sub or boat in any location so that when it goes out I have still enough juice to make it to port and since it turns off at 7.5 volts it is a great way not to deep descharge the batteries.Also if there is a short or unusual load the led will dim or go out during the run telling you to come in.
About an inch squared and very light,open pc bourd construction makes it eaqsy to modify for any voltage!
Reverse voltage protected.
I am taking orders on these units for $15.00 each.Plus SH
E mail me for more info at firstname.lastname@example.org