Proper LiPo 3.3v Charge/Regulator Circuit

I’ve been working on project for a while now where I am creating a custom PCB Tracker. The board runs on 3.3v and I’m using the MKL26Z64 Microcontroller (Found in the Teensy LC). I would like to be able to power my board from a single LiPo battery, so I wanted to include the circuitry for that. My circuit needs at least 900 mAh because of the transmitter module, which is too much for the microcontroller’s in built 3.3v regulator to supply. I came up with a 3.3v regulator circuit, but I haven’t been able to find any good information on good charging/safety chips to use, and how I would also interface this with the micro USB port.

My schematic is below.

NoahSchematic_APRS_tracker_Teensy_2021-05-01_20-37-15.pdf (103.9 KB)

If your device is working stationary without permanent attendance and must not be optimised for lowest possible weight I would use not a lithium-polymere battery. (LiPo)
If you overcharge any Lipo the lipo wil blow up in fire.

I would use a Lithium iron phosphate battery.
A Lithium iron phosphate-battery is safe in itself and does not go on fire.

If you still insist on using a LiPo I would buy a ready to use charging module To have some safety. I guess you don’t want to go through a certification process that your DIY-PCB fullfills all safety-requirements professional charging circuits have gone through that were used in smartphones.

There is a reason why cheap chinese hoverboards go up in flames.

best regards Stefan

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I would concur about the use of LiPos, it does take quite a bit of care to do it properly and load sharing when the transmitter is taking 1A or so is not going to be so easy.

For the regulator, the AMS1117 can do 1A, but not when providing 3.3V from an input of say 3.5V when the LiPo is mostly discharged. Finding a very low droput requlator (200mV ?) capable of 1A is not going to be easy.

Why do you need such a powerful transmitter (1W ?) in the first place, trackers can do real long distances on just 10mW.

Indeed so, highly recommended.

No need for a regulator either.

As for charging circuits go, this is a safe option, using TP4056 (taking care of constant current) and a FS312-F and a FS8205 to protect from over discharge, or current spikes. This circuit uses a T3608 doubler to provide 5v, but i suppose you could discard that. or find a different regulator option.

That is your main issue ! Almost all 3.3v regulators require about 4.0v , you might actually be better of using a current limiting resistor in combination with a Zener diode (btw once a LiPo is at 3.5v it is going to be a problem to properly charge again)

They use the same circuit as the link i posted (a TP4056 circuit), but do not come with discharge protection.
But they are a good starting point, as usually all you need to do is replace the resistor that sets the charging current to the one applicable to your battery.

TP4056 is a poor choice. The issues around its lack of load sharing capability are discussed here;

https://forum.arduino.cc/t/solved-usb-5v-1a-lipo-battery-charger-with-load-sharing-and-protection/569726/16

In addition the so called ‘cut off protection’ is at 2.4v, way to low to be safe in my opinion, unless you are throwing the batteries away each time this happens.

The TP4056 just does the charging, the FS312-F does the low power cutoff

Have you looked at Li battery packs that have built charge/discharge circuitry?
These batteries for example, will turn themselves off at 3V to prevent overdischarge. Then all you need is a charge circuit, perhaps one that use MCP73871 to control and monitor charging.
https://isino.en.alibaba.com/product/62222818380-812820569/Rechargeable_Lithium_Polymer_Battery_Cell_705060_3_7V_2700mAh_for_Lighting_Products_Lipo_Battery_Customized.html
“Discharge cut-off voltage 3.0V”

Use a Step up/Step Down regulator to keep the output at 3.3V while the battery varies from 4.2V down to 3V.

That’s a nice unit ! not so cheap, but a good solution.