Ive never quite understood how that works. I always think it would just be showing the current running into the batteries from the charger when it’s turned on. How does it know. I love this kind of stuff, but I really need step by step visuals to try it myself.
It uses voltage not current. I'll explain exactly how later. :)
I wish I could stick one of these on my SRK
Here a diagram on measuring charge voltage v/s battery voltage is. I hope this helps. :)
It could hypothetically be done, you may need a CNC milling machine to mill the slots in it but not impossible. ;)
2 new pics with my 2 redilasts. This really shows that we get the level of the battery at a glance.
We can see right away if we are closer to 25, 50, or 75%. I can’t imagine an iphone showing the battery voltage instead of the remaining level, it would be very awkward…
Yeah i can see what you’re saying. Much nicer than a voltmeter, i think you would have to be a real nerd to have memorized what voltages equate to percentage of battery. I’ve got battery discharge graphs but my brain is to far gone to remember them. Where as that gives a instant read out.
I like the idea but I have memorized battery graphs and find it more useful to know the voltage. Also your thing doesn't quite show the full amount of power as yours only works down to 3.4 volts while a lot of 18650s go down to either 2.5, 2.75, or 3.
In response to the iphone thing one of the main reasons I jailbroke my iphone was so I could do that. What I learned is interestingly they (apple) charge it up to 4.2 volts but only discharge it to 3.5 volts.
Only under load. As soon as you unload the cell, it’s open-circuit voltage jumps back up to 3.3V-3.5V when fully discharged.
So when measuring unloaded voltage on say a 3400 mah cell I should consider it dead at 3.4 volts?
Pretty much.
If you use HKJ's image of estimated remaining capacity you could also provide a link to his article. 
Hey Silicon, do you still have any of those 4.2V-3.5V fuel gauges for sale? I have a two cell 18650, 3400mAh battery pack I’d like to use it with. I’d prefer the five level version.
Now thats a hopeful necrobump