Howdy all!
This will be my first time making a wireless/inductive charger.
I need to charge two 21700 Liion batts wired in parallel (each batt is 4.2v 5Ah rated).
Not 100% sure yet if two will fit, but planning for it... need contingency in case only one fits.

Every inductive charging system I have seen uses two flat coils placed on top of each other like this:


It would be really difficult to use two flat coils stacked like this for my project... actually it's not really possible due to the small diameter.
I would like to be able to drop a cylinder into a tube and charge from the sides... like a drinking straw in a cup, or a cap on a pen.
The smaller coil (inner, connected to batts) will be receiving, and the larger coil (outer) will be transmitting.
The coils would be wrapped around the outside of the rod and the inside of the tube, overlapping with minimal spacing (<6mm) in charging position like this:


The only examples I have seen using this same method involve solenoids... and I definitely dont want it to pop the rod out of the holder.
Can I simply re-spool a wireless charging induction coil in this configuration? just match the number of coil wraps?
I absolutely need all the protections such as thermistor, trickle.... I don't want the batts exploding from cutting corners.
It would also be extremely important that it can detect whether the receiving coil is in fact receiving rather than blindly trying to charge, in case the coils are not aligned properly, I do not want it to energize a circuit board or anything else conductive.
The system also needs to work while being charged and from what I have read, the NVDC method used in chips such as the MP2731 is best for?

 

I may be able to put a ferrite core inside the receiving coil to aid the EM field... iron will be too heavy though

 

Should work ok in that configuration. It's much the same as a normal transformer. But do use a proper BMS for safety, and make sure the two batts are at equal terminal voltages before connecting them in parallel !

 

The only examples I have seen using this same method involve solenoids...

That is because any coil of wire conducting electrical energy is called a solenoid. Solenoid has a larger meaning than something the is used to move a rod or other shaped armature in the coil.

 

That is because any coil of wire conducting electrical energy is called a solenoid. Solenoid has a larger meaning than something the is used to move a rod or other shaped armature in the coil.

In fact, the solenoid was the original form factor of transformers before other geometries were devised. They were invented for the purpose. So, inductive transfer of energy started with solenoids.

 

Awesome! thanks guys.
I am hoping for some specifics on how I should go about this.
Any recommendations on a pair of sending/receiving chips & schematics which have all the needed features (thermistor input, sensing, tuning to my specific charging needs of two liion batts...) which I can build my own coils to attach to them?
I really would like to avoid modifying a pre-made system. Size is important so I would like to make my own pcb's

 

Awesome! thanks guys.
I am hoping for some specifics on how I should go about this.
Any recommendations on a pair of sending/receiving chips & schematics which have all the needed features (thermistor input, sensing, tuningprocess would be the to my specific charging needs of two liion batts...) which I can build my own coils to attach to them?
I really would like to avoid modifying a pre-made system. Size is important so I would like to make my own pcb's

You might investigate the designs of the charging systems for battery-powered toothbrushes. An interesting document that seems to fully describe the requirements, design and an actual product example here: https://www.holtek.com/documents/10179/5905185/WAS-20A1EN_ReferenceDesign.pdf. There might be other examples such as this on the web; you just have to search for them. Google is your friend.
Of course your design will probably be different, due to your battery and load requirements, but the process would be very similar.
I have a Phillips ultrasonic toothbrush that uses this type of charging system. The toothbrush sits on a short post on the charging base, which is where the "transmitting" coil is; the receiving coil is, of course, in the bottom of the toothbrush.
Good luck with your project.
Cheers,
DaveM

 

What about using a battery for a core instead of ferrite?
Would that work or is that going to be dangerous?
Im working with a 21700 li-ion

 

Should be safe provided the coil current doesn't cause significant heating.
Unless the battery casing or contents are ferromagnetic they won't provide much/any magnetic flux concentration.