TP4056:Disconnect load while charging and avoid battery discharge below 3v

[Danirov]

Hello everyone. I would be very grateful if you can help me with this circuit I am working on (sch.png).

It consists of a TP4056 charger/controller (Photo 2) connected to a 3.7v/3Ah Lithium Ion battery to supply a low battery level indicator, and a load (three white leds and two fans) through a MT3608 Step Up to raise voltage from 3.7v to 9v.

When the battery is feeding the load, P-MOSFET is ON (Photo 3). That's OK.
When the battery low and being charged by the T4056, the P-MOSFET is OFF (Photo 4).
Here comes my problem.

As it is in my schematic, I have a Logic Level P-Mosfet connected to Vin to disconnect the battery from the load while charging (Vin=5v). Also, a diode was added between Gate and Source to make Vg and Vs almost the same (Vgs=0), so the transistor is not polarized.

1) Because of this diode, the load now is powered by Vin (USB power supply) while charging the battery (Vin=5v, Vg=5v) as it is shown in Photo 4. I don't want that.
Is there any trick that I can do with that P-Mosfet or anything else to prevent the load from being powered by the USB power supply while charging the battery?
(Assuming the first switch is closed)

2) While powering the load with the battery (3.7v). If I choose a Logic Level P Mosfet with Vth=-2.4v (considering: Vgs=0-(Vbat - 0.6v)), can I stop battery discharging at 3v to avoid reaching a lower voltage level and damage it?. By default, TP4056 cut-off discharge at 2.4v, but it is too low for a Li Ion battery.

Thank you so much!

 

[rjenkinsgb]

Can you just use a lithium cell with integral protection? eg. 18650 cells with built-in protection are readily available.

eg.
**broken link removed**

Or add a single cell protection board to your existing cell?
**broken link removed**

[Though some of the "protected" cells do seem a bit steep! - **broken link removed** ]

 

[Danirov]

Can you just use a lithium cell with integral protection? eg. 18650 cells with built-in protection are readily available.

rjenkinsgb, thank you for your reply. Yes, that is a good alternative. I think it doesn't solve the problem but I will get those and try them to analyze my point (2).

I am using a Li-Ion flat battery with a protection board. The problem is that Li-Ion batteries shouldn't be discharged below 3v to prevent damaging it. The TP4056 stop discharging at 2.4v; that is too low. For this reason, I am trying to cut-off discharge at 3v considering a Vth=-2.4v (because the parasite diode of the P-Mosfet consume 0.6v, so when this mosfet is ON, at the Source pin I will have (3v-0.6v) and at the Gate 0v, that means Vgs=-2.4, and that turn OFF the Mosfet, disconnecting the battery from the load. That's what I'm not really sure if it will happen).

However, it is a little confussing because it os recommended to discharge Li-Ion batteries up to 3v , but the TP4056, a Li-Ion controller, stop it at 2.4v. Maybe it is not as critical as I think.

Do you have any idea regarding the point (1)?

Thank you!

 

[Nigel Goodwin]

rjenkinsgb, thank you for your reply. Yes, that is a good alternative. I think it doesn't solve the problem but I will get those and try them to analyze my point (2).

I am using a Li-Ion flat battery with a protection board. The problem is that Li-Ion batteries shouldn't be discharged below 3v to prevent damaging it. The TP4056 stop discharging at 2.4v; that is too low. For this reason, I am trying to cut-off discharge at 3v considering a Vth=-2.4v (because the parasite diode of the P-Mosfet consume 0.6v, so when this mosfet is ON, at the Source pin I will have (3v-0.6v) and at the Gate 0v, that means Vgs=-2.4, and that turn OFF the Mosfet, disconnecting the battery from the load. That's what I'm not really sure if it will happen).

However, it is a little confussing because it os recommended to discharge Li-Ion batteries up to 3v , but the TP4056, a Li-Ion controller, stop it at 2.4v. Maybe it is not as critical as I think.

Do you have any idea regarding the point (1)?

Thank you!

2.4V seems to be the standard cut-off point, unless you've had problems with it?, I wouldn't worry about it.

Have you considered using the TP4056 LED charge indicator pins for providing a signal to show if the battery is on charge or not?, and using that to switch the load.

 

[Danirov]

Hi Nigel. Yes that could be a good idea.

At the end I solved both issues:
1) I removed the diode between G and S, and reversed the mosfet orientation.
2) After reading your responses again and thinking, I replaced the DW01 voltage protector from the TP4056 by another one with a higher Overdischarge Detection Voltage.

Thanks!