I am working on a wireless charging project and could use some help with controllers.
A single 21700 liion battery will need to be wireless charged from either a power bank or a DC supply.
The coils are vertical instead of horizontal (flat), the receiver coil will have a smaller diameter and fit inside the transmitter coil... like how a sock goes over the ankle.
I'm hoping the battery, placed inside the coils, will act enough as a core for the magnetic field... otherwise I'll add a metal strip on both the inside of the smaller coil and outside of the larger coil.

Any recommendations for an IC to use on the transmit/receive coils?
I also need an IC to control charging of the 21700 Liion batt, must have protections such as connection for a thermistor shrink-wrapped onto the battery in case of overheat.
Looking for hopefully a single IC that handles both the receiving coil and battery charging... and another for the transmitter coil.
I would prefer building the circuit myself rather than purchasing a pre-made board... I need to keep things small on the receiver end.
Thanks!

 

Try doing an internet search for "wireless charging IC".

 

I have but there are so many, was hoping for like a top 5 recommendation or someone who swears by a certain one.
Since this is for a narrow product application (just one 21700 li-ion batt), the number of 'best chips' should be low compared to the sea of available options.
One I have been looking at is the TI BQ51050B series... low cost and small package.
Uses Qi V1.2, the newest reg is Qi V1.3... not sure if I should look to using 1.3 (seems to only be security/authentication upgrade).

 

You understand your proposed application far better than anyone else, so you are the one most qualified to find a suitable solution by reading the data sheets. If you don't feel confident enough to do that, maybe you should choose a different project. I will try to help you if you run into a problem, but will not do your research for you.

 

Welcome to AAC.

How did you decide on the coil geometry?

What sort of PCB are you planning on using? Most charge controller boards are pretty densely populated already, I am not sure you could do too much to reduce the size. You won't find a single IC that hands both wireless charging and BMS functions, but there are plenty of small, functional single cell charging chips.

 

None of the wireless charging schemes are very efficient, They all waste power in the name of convenience. And mostly they create electromagnetic leakage fields exposing folks to what some claim is a terrible hazard.
I do not comment on any perceived hazard, but the power waste is real.

 

The one I listed above boasts up to a 93% efficiency... pretty high, im sure thats max with perfect coils and a strong ferrite core.
So my coil arrangement will be nested vertically like a spring inside a spring, wrapped using small diameter enameled wire.
Not sure how many wraps I need to do... since one coil will be larger diameter than the other, I believe I'll have to use less wraps on the bigger one. Probably just gonna have to make them and measure the voltage difference... adjust from there.

 

Regarding it as a tuned transformer will help. The primary to secondary turns ratio will determine the output voltage. Efficiency will be inversely proportional to the space between the two coils.

 

Regarding it as a tuned transformer will help. The primary to secondary turns ratio will determine the output voltage. Efficiency will be inversely proportional to the space between the two coils.

Certainly the coils are an unavoidable part of the charging string, but also the associated circuits will matter as well. and given that for most commercial products COST is the primary concern and the biggest driving force. commercial wireless charging will always be a waste of energy.
A wireless system created by an individual, not as a product for sale, is much less limited, and may be more efficient by quite a bit, possibly.

 

I am confident that I can keep the coil distance to less than 5mm... right now I am expecting 3mm.

 

How should I go about deciding how many windings on the coils?
I am looking for a 1:1 transmit/receive voltage at 5vdc... But is there an optimal number of coil windings?
For example I expect 1000:1000 windings is better than 10:10, but is 1k too much?
is there a way to determine the optimal amount of coil loops?
Or does efficiency increase infinitely (within reason, not going theoretical)

 

Magnetic field strength is proportional to the product of current and number of turns, so you'd be trading one for the other.

 

Certainly there is an optimum inductive reactance for a transformer winding. I don't have that reference book handy so I can't provide the formula. In fact, there is a whole lot of information available about efficient transformer design. At lleast it was in the technical library at the school I went to back in 1970.