Please check if my plan will work, or what you would change so I can avoid horrible fire in moms basement.

Parts:
(1) Raspberry Pi 0 W
https://www.amazon.com/Raspberry-Pi...52111509&s=gateway&sprefix=ras,aps,152&sr=8-6

(2) 5v 500mA Solar Panels
https://www.amazon.com/gp/product/B00CBT8A14/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

(1) RC car with 3.7v 400mAh Lithium Iorechargeablele battery pack.
https://www.amazon.com/gp/product/B07CLPLYHP/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

(1) 5v DC-DC Buck Converter fixed 5v output.
https://www.amazon.com/Regulator-DR...=gateway&sr=8-1-spons&psc=1&smid=AFHAE9RJVUMB

(1) L293D DC Motor Driver - runs two motors, forward/back and left/right
https://www.amazon.com/PIXNOR-16-pi...keywords=L293&qid=1552105249&s=gateway&sr=8-1

***** HELP HERE *****
(1) TP4056 Charging Module 5V - I am not sure this is ok to use for my 3.7 Li-on battery?
https://www.amazon.com/Makerfocus-C...s=TP4056&qid=1552108464&s=gateway&sr=8-2&th=1

***** HELP HERE *****
(1) Transistor? Mosfett? Something so I can turn on and off solar panel power to charging the unit while in operation. I don't know what to use here.

***** HELP HERE *****
(1) Schottky diode? Something to prevent back flow to solar panel from battery pack?

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I am using two 5v 500mA Solar Panels in series to generate an approximate 10v @ 500mA. This Solar Array connects to my 5v DC-DC Buck converter which turns the power into a fairly clean 5v. I attached that 5 volts to my Raspberry Pi zero W which idles around 5v 130mA.

Next, I will connect the L293D motor driver up to the Pi and the cars 3.7v Battery pack, so when the Pi tells the L293D to power the motor, the L293D chip uses the 3.7v Battery for power.

Next step, punch myself in face because here is where it gets way to hard.

1) The Pi will disable the solar panel from charging the car's 3.7v battery to avoid overdrawing power and shutting off the Pi.
** Problem 1 **
I think this is done through a transistor, but which type? I imagine there is a wire coming from the 5v Buck converter into a transistor that connects to the TP4056 Charging Module and then to the battery and the other transistor wire to a pin on the Pi. When the Pi pin turns on, I want the transistor to stop letting power pass between my buck converter and the charger. Also the transistors I saw allow 1A of current, but I probably shouldn't allow more than 400mA for battery charging as 1A would cut voltage and possibly shut down the Pi.

2) Next, the Pi will tell the L293D chip to engage one or two of the motors, and the L293D will automatically use power from the 3.7v battery as that is the connected power source.

3) After the driving maneuver is complete, the Pi will re-engage the power from the buck converter to the charging IC so the battery can recharge.

** Problem 2 **
If all that looks doable, the only other question I have is, should I put a Schottky diode between the buck converter and the charging module? If so what kind / size?

Thanks for your help, I know it's a long read. Let me know if you need me to give more information.

I don't know how to make a real schematic of this for you, so some imagination is required.

 

** Problem 1 **
I think this is done through a transistor, but which type?

The N-MOSFET that you choose doesn't suit for your application, because it was used for high voltage, the 1.9Ω Rds resistor is too high and it will be get too much Vds voltage drop when the MOSFET is turn on.
IXFHFT6N100F_1KV6A_Vgs10V_1Ω9.pdf

You can buy a IRF540, although it is not enough, but it should be OK for your application, or you can buy some others which have logic level Vgs=4.5V, what you need to choose like the one is low Rds<20mΩ, IRF54 has 77 mΩ, and the second is Vgs=4.5V logic level, or you can check here.

** Problem 2 **
If all that looks doable, the only other question I have is, should I put a Schottky diode between the buck converter and the charging module? If so what kind / size?

1A or 2A.

 

Do you think with my set up I will need a MPPT instead of my buck coverter + current choice of charging unit? Or would an MPPT be unnecessary? Trying to save a few bucks.

For example, maybe I should be using this unit instead of my buck / charger unit.
https://www.amazon.com/Converter-DR...uck+converter&qid=1552153175&s=gateway&sr=8-3

 

About the first DC/DC converter, although the seller claimed that it can be output 3A, but I will divided it by 5 as Iout=3/5=0.6A or less, and it also mentioned that it was designed for the transformer, the second one mentioned that it was designed for the MPPT Solar Controller Module, so it is better than the first one.

 

Thanks ScottWant,

So if I understand, the MPPT module works as a Buck Converter & a charging uint all in one, and if I use the $16 MPPT module it will provide a steady 5v to power both my Pi and charge my battery.

Does that sound right?

 

Normally we don't use power and battery for the same device with the same time, like the TP4056 Charging Module provided the power for the RC car, it also can be charge the battery when its current is enough, but the battery should be only provided the power when there is no power from Solar power, so here is the issue, the one is that you use the Pi to control the N-MOSFET to cutoff the Solar power and another one is that when the battery is full charged(4.2V) then it should be cutoff from the Solar power, when the Solar power is cutoff then it have to provides the power to the RC car, here has a block diagram similar with yours.

 

The charger circuit is designed to provide 1000mA but that is too much and it will explode the little battery.
Reviews say the battery runs the car for only 20 minutes but it takes 3 hours to charge. It is similar to my RC airplanes so I have a bag full of charged batteries.