Hi guys.
I have these solar cells that give me about 4v and 50mA to 80mA depending on the environment. They can go up to 200mA but that is not were I am using them. I need a circuit made of discrete components to start charging a Lipo battery (inside a wall clock) rated 1.2v and 800mA when the voltage goes down to 1 v and stop when it is 1.2v. Is it achievable? I dont mind using op_amps but I am afraid that the circuitry will hog all that tiny current.

 

What type of battery has a fully charged voltage of 1.2V?
For example, NiMh batteries go to about 1.6V fully charged.

Below is the LTspice sim of a window comparator circuit using the dual LM393 comparator IC driving an N-MOSFET to control the battery charging:
It turns the MOSFET ON when the voltage is 1V and OFF at 1.2V, as determined by the TLV431 1.24V voltage reference and the voltage dividers R1, R2, and R3.

The total circuit current (red trace) is <1mA, not including the battery charge current.

The transistor can be just about any logic-level type P-MOSFET with a Vgs max threshold of ≤1V.

 

Thank you! Is it a good practice to charge Lipo batteries to maximum? I hear a lot about that 60% to 85% is the best range to keep but I am not sure if it is a myth or truth. I just thought since it is a set-up-and-leave design, it would not be a hassle to keep the battery in that range, but with a cellphone, it is hard to keep the battry in that limited window. Thanks again for the design.

 

What type of LIpo 1.2V battery is rechargeable?

 

Sorry I meant the Ni MH. It is rated 1100mAh. Mixed with phone-type of battery

 

I do not have LM393 in my LS. All I could come up with from Texas Instruments was what boiled down to LM2903B. Could you please share the file or verify if this is the good?

 

You can use the LM2903B.
It's a newer version of the LM393 with a wider temperature range.

But you should charge a NiMH battery to at least 1.4V for a decent charge (below is the circuit modified for that):

 

Only 1.2V cell I know of is a Nickel Cadmium (NiCad). And I don't know if that's full charged.
EDIT: Found this: https://en.wikipedia.org/wiki/Nickel–cadmium_battery

 

Only 1.2V cell I know of is a Nickel Cadmium (NiCad)

You aren't familiar with NiMH (Nickel Metal Hydride) cells?

 

Thank you for design. I do not know where the issue is in my simulations, but I dont get that sharp corner when the voltage is at 1v. That may not be an issue in practice, but when I change the battery with any fixed voltage (1v, 1.2v 1.5v) I get the MOSFET's gat ON all the time

 

I dont get that sharp corner when the voltage is at 1v.

If you mean the slow rise-time of the signal, that's not a problem in this application.

when I change the battery with any fixed voltage (1v, 1.2v 1.5v) I get the MOSFET's gat ON all the time

The voltage needs to go slightly below 1V for proper switching.

 

Thanks. I meant if the voltage is above 1.2v, supposedly the battery inserted has more juice that 1.2v, say, 1.3v, then the circuit will glitch? Because that is what I see in my sims. In the first snippet I put a 1.5v battery and the MOSFET gate is 4v

 

Thanks. I meant if the voltage is above 1.2v, supposedly the battery inserted has more juice that 1.2v, say, 1.3v, then the circuit will glitch? Because that is what I see in my sims. In the first snippet I put a 1.5v battery and the MOSFET gate is 4v

Post your .asc file for that sim.

 

https://www.instructables.com/How-to-Make-Lipo-Battery-Charger-Circuit/

 

https://www.instructables.com/How-to-Make-Lipo-Battery-Charger-Circuit/

He actually wants to charge a NiMH battery, not Lipo.

 

He actually wants to charge a NiMH battery, not Lipo.

my apologies, read Lipo in #1
https://www.instructables.com/DIY-a-Smart-NiMHNiCd-Battery-Charger/

 

Post your .asc file for that sim.

This is the noodle I made

 

my apologies, read Lipo in #1
https://www.instructables.com/DIY-a-Smart-NiMHNiCd-Battery-Charger/

Thanks! that looks like a custom tailored IC for the job but I do not have it im my stuff. The other thing is that those ICs normally need a wall charger. I am not sure if they can function when the battery is being charged , which I suppose sinks that tiny voltage and current of a solar cell to a level that the IC wont operate at that range.

 

This is the noodle I made

So what is the reason for adding the diode, resistor and capacitor to the battery circuit noodle (not shown in any of your posted schematics), as that is fouling up the circuit operation?

 

So what is the reason for adding the diode, resistor and capacitor to the battery circuit noodle (not shown in any of your posted schematics), as that is fouling up the circuit operation?

Oh sorry, those things were added later to see if it makes any changes. Ok then I breadboard it with LM393 and will keep you posted. Thanks again