Help with LiPo/solar panel/motor circuit

For a school project I am working on, we have been instructed to use the following circuit design:

If you can't see the image, here's the general idea: The solar panel connects to battery via SW2. The battery connects to the motor via SW1

[Solar panel--Switch 2--Battery--Switch 1--Motor]

The general idea is that when SW2 is closed and SW1 is open, the battery will charge from the solar panel, and when SW2 is open and SW1 is closed, the battery will power the motor.

I'm concerned about this as the lipo has no boost/charge circuit, no undercharge or overcharge protection or anything like that, just directly connected to the solar panel and battery.

Will this work at all? If so, how well? If not, what's the simplest way to make it work (is there a way without using ICs or complicated circuits?).

Any help would be appreciated.

EXTRA INFO: The battery is a 3.7v, 1S lipo battery. This is the solar panel, this is the motor.

• Lithium ion batteries with no protection circuit? You should confirm that with your teacher as it could be quite dangerous. Lithium ion batteries should be charged with a circuit specifically designed to do so.
– K H
Mar 19, 2020 at 1:14
• What is the battery mAh rating?. Spec sheet? Mar 19, 2020 at 2:26
• @RussellMcMahon I don't know but it looks similar to this one: core-electronics.com.au/… (When I got it it was covered in tape so I don't know the product number) Mar 19, 2020 at 2:44
• THat one is 400 mAh. Ones of similar size will USUALLY be of similar capacity. 50 mA charge can easily be accommodated. Mar 19, 2020 at 4:24

Congratulations on asking a good question that should have been covered by the instructor.

The battery will be damaged or destroyed if left on charge when the battery is fully charged.
A 1S LiIon cell must NEVER be charged to above 4.2V.
It MUST NOT be floated at 4.2V It can be floated at 4.1V or less.
4.1V max still gives most of capacity,
4.0V is lower capacity but still useful,
3.9V is getting "a bit low".

An easy solution is to add a zener diode across the PV panel rated at a voltage that clamps the panel without battery connected to below 4.2V under maximum light level.

The actual voltage will depend on the zener diode specifications. A 3.9V 250 mW or higher power zener is probably a bit low - higher power zeners of the same voltage will clamp the about 50 mA max panel output at somewhat higher voltages.
The 1 Watt zener listed below will probably give a safe and usable Vout at 50 mA.

1N5228 1 Watt zener datasheet here
Vout is 3.9V at 20 mA.

A better solution is to clamp the panel Vmax with a shunt regulator. An eg TL431 plus 2 resistors will allow you to precision clamp the panel voltage. Max clamp current depends on the version used by 50 mA is within the spec of many versions.

An alternative is to limit panel output using a conventional LDO (Low Dropout) regulator.
A good example is an LP2951. Datasheet here
This is the adjustable version - you do NOT want the fixed 3.3V or 5V versions.
See fig 19 for adjustable output circuit.
The DIP LP2951ACN package

Are you in Australia?
If so Jaycar has LP2951ACN's available.
See Here \$A2.50 each in stock.

Jaycar appears to NOT have TL431's available.

Jaycar 3.9V 1 Watt zener

I've read up on the correct Li-ion battery charging procedure, and I'll see if I can get a proper charge circuit, but if I can't, what do you recommend as an alternative?

A proper charger is a good idea BUT as long as VPV abs max is under 4.1V you do not need a "proper" charger.
However, the widely available low cost TP4056 charger modules would work well in this application - but need a PV panel zener clamp to ensure that the modules Vinmax is not exceeded.

For a basic system I recommend the 3 methods I gave in my answer.

What's the best way to both buck and boost the battery input voltage so that the battery still charges when the solar panel voltage drops?

You do not need a buck boost converter. The panel claims to be rated at 5V at 50 mA. Under no load it should give about 6V+. In any sunlight conditions at or above medium overcast it will give more that the 4.5V or so that you need for your battery + LDO. For a zener or TL431 you need only 4.2V or less before the regulator is applied. So a boost regulator is not needed. A buck regulator is usually under 90% efficient so will give you little or no energy over just using a clamp regulator (zener or TL431) or an LDO - as per my answer.

• Thanks! What's the best way to both buck and boost the battery input voltage so that the battery still charges when the solar panel voltage drops? I've read up on the correct Li-ion battery charging procedure, and I'll see if I can get a proper charge circuit, but if I can't, what do you recommend as an alternative? Mar 19, 2020 at 3:03
• 2. I recommend the 3 methods I gave in my answer. 1. You do not need a buck boost converter. The panel claims to be rated at 5V at 50 mA. Under no load it should give about 6V+. In any sunlight conditions at or above medium overcast it will give more that the 4.5V or so that you need for your battery + LDO. For a zener or TL431 you need only 4.2V or less. So a boost regulator is not needed. A buck regulator is usually under 90% efficient so will give you little or no energy over just using a clamp regulator (zener or TL431) or an LDO - as per my answer. Mar 19, 2020 at 4:06
• Are you in Australia. Mar 19, 2020 at 4:06