Need help choosing a device for this project

[edavid]

I think you are worrying a little too much about the automotive environment.  It's harsh, but not that harsh.

However, you are not worrying enough about transmitting radio from the engine compartment to the cabin.  The battery is surrounded by grounded metal = Faraday cage.  Why bother fighting that, when the voltage is right there at the lighter plug?  Your professor sounds like an idiot.

[Aldietur]

I think you are worrying a little too much about the automotive environment.  It's harsh, but not that harsh.

However, you are not worrying enough about transmitting radio from the engine compartment to the cabin.  The battery is surrounded by grounded metal = Faraday cage.  Why bother fighting that, when the voltage is right there at the lighter plug?  Your professor sounds like an idiot.

In upper Canada it can be very harsh with very cold winters, and this summer has been one of the worst on record. Faraday cage isn't even something I know to account for. Maybe he just wants to recreate a device that already exists, or at the very least add a way to transmit it with a wireless signal to a phone, but that would mean adding something directly to the battery.

[Pack34]

If you just use the ODBII connector, a lot of the temperature issues should be mitigated since it'll be in the climate controlled portion of the car and not in the engine compartment.  Then just use an ADC to measure the voltage.

Get the prototype working first before worrying about making it robust.

[rrinker]

 He keeps saying he doesn't want to reinvent the wheel, but when you can buy a $10 adapter that gives you the voltage via BT (and a whole lot of ADDITIONAL operating parameters) then that is EXACTLY hat he's trying to do. How cheaper can it possibly get to make a voltmeter that transmits via BT and mounts under the hood? A device that is fully inside the cabin wouldn't have to worry much about operating outside human compatible conditions - non op conditions may be extreme (closed car, hot sunny summer day, or car parked outside overnight on a cold winter night) but the typical cabin environment would be far more stable than under the hood.
 Maybe I misread part of this, but I see no point in locating the measurement device under the hood like that, and especially not when any relatively modern car already provides this info, even if it is not natively displayed on the instrument panel. ANd mine would have to go in the trunk - that's where my battery is. Unless it's not battery voltage that is really being measured but some other parameter.

[David Hess]

For amateur radio equipment, I install fusible links at the battery and bring a separate set of 10 gauge wires through the firewall to the radio directly from the battery and any other equipment.  This also works fine for monitoring the battery voltage precisely and a cheap digital voltage meter is all that is needed.

[Blastcap]

Ive seen a finished gadget that does exactly that. (I have to dig around)
Some sort of overpriced gadget with a smart app, telling you live the battery voltage, and when to replace your battery.
But i find it silly to spend 50$ on a gadget to monitor your battery, when you can put those toward a better battery and not having to monitor at all.

If hes worried about the car not starting in the cold weather, then he could invest in a starter module.
http://www.aepint.nl/hybrid-power/products/engine-start-modules/powerstart-100/