First, a battery is basically a voltage source. The current rating is how much current the battery can deliver when the load demands it. It is not the current the battery always puts out. So, just because the 5 kV battery can deliver up to 500 mA, it doesn't mean that the coil will draw that much current. That is up to the coil. In steady state, the coil would have to have a resistance of 5kV / 500mA = 10 KOhms to draw the 500mA. More, and it will simply draw less current. Less, and the battery's current capacity is exceeded and its output voltage will probably droop, it could vanish into a puff of greasy black smoke, or whatever.
The magnetic field strength generated by any electromagnet is directly proportional to the current thru it. Let's say your coil had a DC resistance of 10 KOhms and therefore drew 500 mA from the 5 kV battery. The current it would draw from the 500 mV battery would be 500mV / 10kOhms = 50 uA. That means the magnetic field will be 10000 times weaker. The fact that the battery could have delivered 5 kA is irrelevant since the coil isn't trying to draw that much current.
Earlier I said that the magnetic field of a electromagnet is proportional to its current. That is true for any one electromagnet. Different electromagnets can be constructed with different proportionality constants. For example, it's quite possible to make one electromagnet that draws 1A at 10V and another one that draws 2A at 5V that have the same magnetic field strength. The first one would use a longer but thinner wire, but both could be wrapped around the same core.
Update from OP.
Current: Needs to be no more than 30mA.
Distance: A 4mm air gap.
Minimum data rate: Around 256Bits/second.
Size: Needs to be as small as possible.(Must fit into 5x10x5mm spot)
Cost: Looking to keep it under $1.50
Is that 30 mA receive or 30 mA transmit.
Unidirectional?
Cost of $1.50 covers what? TX & RX, just one (which?),Hall cell in that price.
How many? 1 10? 100? 100,000?
MUCH more information allows us to provide a single instant answer without playing death of 1000 cuts / iterations.
The Hall cell chosen is completely unsuitable for this task.
This is because it is a sampling type which sleeps for most of the time and wakes to take a reading occasionally.
Th data sheets hows that it has a 0.1% on time and 99.9% off time.
Cycle time is 45 to 90 ms and on time is 45 to 90 uS.
So you can only signal at most at 1 bit per on time if you are careful or at about 10 bps max and probably less.
There are many Hall cells available which are not the sampling type and low enough current.
[This is Digikeys cheapest at about 58c/1.]http://www.semicon.toshiba.co.jp/docs/datasheet/en/Sensor/TCS20DPR_en_datasheet_110207.pdf)
This has 4.4 mT sensitivity worst case.
Mutiply T by 10,000 to get Gaus.
4.4 mT x 10,000 = 44 Gauss = about te same as before.
Doable at range and size specified. Implementation details depend on all answers not yet known.
More when more known ...
This question is eminently answerable but rather than giving you a single "this will work" answer, having more information will lead to a much better answer.
What range do you want to work over from the face of the Hall cell to the face of the inductor?
Is there anything in the way obstructing, spinning, cutting ...?
Is it in seawater, embedded in a block of steel or a lava field, ...?
What maximum data rate do you require?
Be as specific as possible re constraints on cost, size, and anything else you can hink of. DO NOT have us say xxx meets your needs and then say "Oh, but it must be British Racing Green and work at 2000 feet underwater" or whatever :-)
Don't let the following worry you. The answer is a piece of ferrite and some wire - but this is "what lies underneath":
IF there is a need to wind a coil and activate the sensor at a distance, then it may come down to formulae like this:
Relating to an inductor like this:
(http://www.netdenizen.com/emagnet/solenoids/thinsolenoid.htm)
From here
Or it's big brother which has finite thickness, from here
BUT probably not.
Adding a core increases the field by the permeability of the core - but, we'll come to that.
FWIW those formulae are about the nicest I've seen for a common problem that usually get's a horrendously complex answer. This is mainly geometry. Most analyses are for the field INSIDE the solenoid and few deal with it beyond the ends.
Best Answer
One thing you could do is up your voltage, C batteries only give around 1.5, same as AA, so if you put multiple AAs in a series you can max your voltage as their resistance(0.12) is a bit lower than Cs