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Wireless charging Lipo batteries
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Wireless charging Lipo batteries

by NoSpicy on Fri Nov 08, 2019 3:36 pm

Hello,

I am just starting a RC car project hopefully with Wifi camera and controls. I am trying to making the operation all hands free including charging the power source (Lipo battery). I am a complete beginner and am first tackling how to charge a lipo battery through Qi charging. From reading other forums, some people say it is not a smart idea since charging Lipo batteries has many things to consider. Some key words I saw: constant voltage/current, overcharging, heating, swelling, exploding ...etc.

From what I gather, I will need a wall adapter connected to a Qi transmitter (https://www.adafruit.com/product/2162). On the other end will be a Qi receiver attached to a Lipo battery and possibly a step-up converter for arduino boards/motor drivers/camera. Is there a board in between the battery and the receiver to help regulate the voltage/current (as a safety measure) to not overcharge the battery? From the Qi receiver pictures (https://www.adafruit.com/product/1901), it seems there are examples of it connected to a smaller lipo (with a board in between) and to a larger lipo (with no board in between).

Can these batteries be left connected to a Qi charger for long periods of time?

Any advice appreciated!

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Re: Wireless charging Lipo batteries

by dastels on Fri Nov 08, 2019 3:44 pm

I'm not 100% sure it will work yet, but I'm working on a robot that I plan to use wireless LiPo charging for. I'm using the saem transmitter and https://www.adafruit.com/product/2114 connected to a https://www.adafruit.com/product/2465 which has a 4400mAh LiPo connected to it, and powers the robot.

It does seem to change the battery, but I haven't integrated it into the robot yet.

Dave

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Re: Wireless charging Lipo batteries

by adafruit_support_mike on Sat Nov 09, 2019 2:32 am

As long as you have an actual LiPo charging circuit handling power to the LiPo, you shouldn't have to worry too much. The charger will keep the LiPo healthy, and the Qi receiver will just be a power source for the charger.

You do want to be careful about stray wires near the Qi charger, since the system's whole purpose is to generate voltage in wires near the transmitter coil. If you can put a piece of ferromagnetic material (sheet steel) between the receiver coil and the rest of your circuit, things should be okay. Magnetic fields like to pass through ferromagnetic materials the way electrons prefer to move through metals like copper. The vast majority of the field will stay in the sheet metal and won't cause problems for electronics beyond it.

NoSpicy wrote:Can these batteries be left connected to a Qi charger for long periods of time?

Again, a LiPo charger will handle that problem.

LiPos have a complex charging cycle, which is similar to filling a large sponge with water. The 'constant current' phase comes first, while the polymer is depleted far enough that it will absorb any energy it can. It's similar to pouring water on a dry sponge, trying to find the maximum rate of flow without spilling any water over the side (the LiPo equivalent to spilling is overheating, which tends to be pyrotechnically exciting). During the constant-current phase, the LiPo's voltage will rise steadily to the full-charge voltage (4.2V for the LiPos we carry).

At that point, the LiPo has absorbed about 80% of the energy it can hold. It's like a sponge that's saturated on top, but still has a dry layer at the bottom. If you stop charging a LiPo as soon as it hits 4.2V, the voltage will sag to about 3.9V over the next few hours as the charge distributes itself evenly through the polymer.

Once the LiPo hits the 80% level, the nature of the charging process changes. Instead of pouring energy in as fast as possible, you have to switch over to matching the rate that energy dissipates through the polymer. A charger does that by holding the LiPo voltage at the full-charge level (4.2V) and adjusting the current to keep the voltage from rising any higher. That's called the 'constant voltage' phase of the charging cycle.

There's no clear physical boundary between a LiPo that's fully charged and a LiPo that's about to explode, so we stop charging at a point which has historically proven to be safe and effective: when the current flowing into the LiPo drops to 5% of the constant-current level, the LiPo is considered fully charged.

A dedicated LiPo charger's job is to handle that whole process, to end the charging cycle at the correct time, and to stop charging immediately if it sees the current or voltage behave in way that doesn't fit the standard charging profile. And since most chargers are always connected to the LiPo, they know how to wait until it's time to start a new charging cycle (usually when the LiPo voltage falls to some limit, like 3.8V).

So as long as you have a dedicated charger taking care of the LiPo, you don't have to worry about the effect of a Qi transmitter/receiver pair. The charger will handle the constant-current and constant-voltage phases, will stop the charge cycle at the appropriate time, and will wait until it's safe to begin another charging cycle.

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Re: Wireless charging Lipo batteries

by NoSpicy on Sat Nov 09, 2019 3:01 pm

Thank you for information and great analogy Mike.

I guess I am confused about which product you mean on Adafruit when you say a "dedicated LiPo charger". You mentioned the charger should handle all the safety protocols of the LiPo charging process and not worry too much from the transmitter/receiver pair. I know there are chargers for LiPo batteries where you physically plug in, but I am hoping to not use any physical connections to charge the batteries (using Qi charge only). Besides the Adafruit Qi transmitter/receiver, what else would I need?

I also mentioned in my first post that I plan to run an WiFi RC car off the battery. I understand that if I use a lower voltage/mAh battery like the 3.7V from Adafruit, I would require the PowerBoost to step up to 5V for my arduino + IP camera + WiFi board (ESP8266). In this case, would it be smarter to use another larger mAh battery for the motor driver (and therefore the DC motors + servos)? Step up converters can increase the voltage but at the same time lower the current (which I could potentially use more of for motors). But, using two batteries would require two receivers and transmitters (a hassle to align both charge points if I were to control the RC car remotely). I guess an alternative is to use a larger capacity battery to run everything and step down the voltage for connections to the arduino + camera + WiFi board. Thoughts?

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Re: Wireless charging Lipo batteries

by adafruit_support_mike on Mon Nov 11, 2019 2:49 am

NoSpicy wrote:Besides the Adafruit Qi transmitter/receiver, what else would I need?

You'll need a LiPo charger like these:

https://www.adafruit.com/category/575

Those aren't optional.. a Qi charger on its own will destroy the LiPo, and probably set it on fire.

NoSpicy wrote:I guess an alternative is to use a larger capacity battery to run everything and step down the voltage for connections to the arduino + camera + WiFi board. Thoughts?

My first thought is that battery-powered designs are always a tradeoff between the size of the battery and how long it will last.

Make a list of all the devices you want to use, then find out how much power (voltage and current) each one consumes while running. Add those up to get a total, and use that to figure out how much energy you'll need per second of operation. Then you can use that number to decide how long a given battery will last.

That's called an 'energy budget' for the system. This tutrorial explains how to calculate one:

https://learn.adafruit.com/energy-budgets/overview

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