Battery Recharging Research

[digitaldanny]

Is your feature request related to a problem? Please describe. This circuit is required to make the glove a wireless system.

Describe the solution you'd like Research the following..

• Battery University
• How do recharge circuits work?
• What components will I need?
• Approximately how much power does my system need to support?
• Does the TMS320 have full functionality while being powered by a 3.3V power supply?

Describe alternatives you've considered N/A

Additional context N/A

[digitaldanny]

Notes on "Battery University"

• "Primary" non-rechargeable batteries hold more energy and self-discharge is lower than "secondary" rechargeable batteries.
• Lead/Lithium-based chargers operate under constant current constant voltage (CCCV). The charge current is constant and the voltage is capped when it reaches a set limit. Reaching the voltage limit, the battery saturates; the current drops until the battery can no longer accept further charge and the fast charge terminates. Each battery has its own low-current threshold.
• Nickel-based batteries (NiMH, NiCd) tend to get hot when charging. Discharging pulses between charging pulses is called "burp" or "reverse load" charging can result in a cooler and more effective charge.

[digitaldanny]

Power Gauging Methods/State of Charge Notes This comment was moved to issue #26

[digitaldanny]

This YouTube video provides a good overview of lithium ion battery charging. Below are some of my notes on the video.

• Recharging is a 2-step process called CCCV or "constant current, constant voltage." There is also a 3rd optional "Pre" step that can rejuvenate fully dead batteries.
• During the CC step, the recharge circuit must supply a constant current of 50% of the max current while the battery voltage builds back up. At this point, the battery is 80% charged.
• During the CV step, the battery voltage must stay at its max value and the recharge circuit's input current will begin dropping. Once the current has reached a battery-specific threshold (commonly 5% of the max current), the battery is 100% charged.

Need to do some additional research to find a circuit design I can experiment with.

[digitaldanny]

Note about purchasing a Li-ion battery: Make sure not to purchase "bare cell" batteries. The Li-ion cells must be integrated with appropriate safety circuitry to avoid overcharge or over-discharge.

[digitaldanny]

A common unit of measurement I'm seeing is "mAh" (milliamp hours). This refers to the amount of energy the battery can store. For example, a 400 mAh battery can power a load drawing 40 mA for 10 hours. This will be more important later on when I have a better estimate for the amount of current my device is pulling.

To be on the safe side, I will pick a battery with large storage capabilities. Once most of the hardware is completed, I could determine the average current draw and (hopefully) pick a smaller battery that takes up less wrist space.

[digitaldanny]

Some notes comparing buck converters and voltage regulators that might be relevant. Buck converter vs linear voltage regulator YouTube video.

Voltage Regulator

• Cheap (~$0.50).
• Wastes a lot of energy since it "burns off" the extra energy by turning it into heat.
• Due to the way it works, the component can get hot to touch.

Buck Converter

• More expensive (~$2).
• Much more energy efficient.
• Can cause interference in the circuit since it uses coils.
• Takes up more space.

A voltage regulator would save some PCB space, but I will need to make sure it doesn't burn through battery life too quickly - the glove should last 8-10 hours at least before requiring a recharge. Heat is probably the biggest drawback since this is going to be a wearable glove. An alternative to using a buck converter could be just to add a heat sink to the voltage regulator. My biggest concern with the buck converter its inductors will mess with the magnetometer measurements.

[digitaldanny]

As noted in this StackOverflow topic, there may be some power distribution issues when trying to enable normal load operation while re-charging the batteries. Basically, the battery may draw more current than necessary and could unexpectedly cause the device to power off because it is not being provided enough power.

A solution that should be explored more in a different issue would be to create a load balancing circuit that guarantees both circuits are provided sufficient power. This Charge Circuit with Load Sharing site shows a load sharing circuit that only requires 4 additional components - a P channel MOSFET, a schottky diode, a resistor, and a bypass capacitor.

[digitaldanny]

This allaboutcircuits link shows that USB-C has VBUS and GND pins that act as power and ground. The default VBUS voltage is 5V, which can be dropped down to 4.2V in a voltage divider for charging the Li-ion battery during the CV phase.

This Digikey link has a breakout board for USB-C.

[digitaldanny]

This MCP73831 seems to be a commonly used IC for this type of application. May be able to make a circuit based on the functional block diagram in this datasheet.

[digitaldanny]

Below are the expectations for the charger's output.

• CC circuit when V_battery < 4.2V
• CV circuit when V_battery == 4.2V and I_charger >= 0.01 * I_batteryMax
• Switched off otherwise