Low battery alarm ?

With regard to post #41495

I have the electronic knowledge of the average cabbage but last night I thought of a device that may or may not have been thought of before and I don't know if it is even possible to build so I am hoping someone here can either tell me it won't work or possibly produce some.

I made an assumption that all 12 volt batteries regardless of type or capacity would have a mean average voltage level for 45% depletion. Is this correct or would it differ ?

If correct would it be possible to build a device that is triggered when the battery is depleted to this level. The trigger action could light a small flashing LED and or set off some form of annoying electronic noise to alert the relaxing or distracted camper.

I imagined that such a device could be in the form of a hand held unit with alligator clips for periodical checking or hard wired into a vehicle/camper to monitor 1 or more batteries.

I realise that there are commercially available battery monitors - are any fitted with alarms ? - but I thought an alarm would be more helpful to those of us that may suffer from any of the following conditions:

1. Busy making sure kids don't drown/ kill themselves or each other
2. Busy repairing broken vehicle part
3. Busy inspecting inside of hat/ eyelids whilst waiting for a bite on the line
4. Busy appreciating nature with appropriate beverage in hand
5. Busy attending to marital duties whilst kids are unconscious and no one is within earshot or line of sight

If anyone thinks this is a good idea and can actually build such a device please let me know as I would be willing to purchase a couple.

Cheers ... Taz