I made a simple BMS and placed 3 series of li-ion cells, voltage, and current sensors to read data but I have actually two problems.
1. I need some sort of sensor or something to detect if the charger or load is connected to the battery terminals so I can control the high-side charge and discharge N-channel MOSFETs (back to back).
2. I need a drive signal to control these MOSFETs. There are a lot of ICs especially designed for charge and discharge switches but they are expensive and I don't want to place such expensive IC on my simple BMS.

It is a simplified schematic of the circuit:

 

Trying to avoid an "expensive" IC that does exactly what you want is usually a waste of time. A DIY approach makes sense when you can't find an IC to do what you want.

Your schematic makes no sense to me because the MOSFETs are in series.

Regarding your item 1, I've used comparators to detect whether a battery is connected to a charger. A voltage of just 1mV or so above ground tells you a cell is present.

 

Trying to avoid an "expensive" IC that does exactly what you want is usually a waste of time. A DIY approach makes sense when you can't find an IC to do what you want.

Your schematic makes no sense to me because the MOSFETs are in series.

Regarding your item 1, I've used comparators to detect whether a battery is connected to a charger. A voltage of just 1mV or so above ground tells you a cell is present.

So just because Mosfets are in a series it doesn't make sense to you? Search for "Bidirectional switch" or "charge and discharge switch". I placed a comparator but it doesn't tell if the load is connected but I solve that issue by placing a current source.

 

Back-to-back FETs "can" work just fine .............

So now You have a "bi-directional" FET,
but the rest of the required Components will cost much more
than a dedicated Chip designed for the specific purpose,
and the dedicated Chip will probably have better performance, and more useful features.
.
.
.

 

I need some sort of sensor or something to detect if the charger or load is connected to the battery terminals so I can control the high-side charge and discharge N-channel MOSFETs (back to back).

Exactly what are the criteria for this "some sort of sensor" to turn the MOSFETs on and to turn them off?

 

Exactly what are the criteria for this "some sort of sensor" to turn the MOSFETs on and to turn them off?

I solved that issue by placing current source. So if the load available the voltage sensor senses the voltage on the load but it is low voltage around 100mV but if the charger is connected the sensor senses the charger output voltage.

 

I solved that issue by placing current source. So if the load available the voltage sensor senses the voltage on the load but it is low voltage around 100mV but if the charger is connected the sensor senses the charger output voltage.

So what's the purpose of opening the MOSFETs if there is no charger or load?

Note that the N-MOSFETs you show will require a gate voltage higher than the battery voltage to turn on.
If you use P-MOSFETs, then you can turn them on by grounding the gates.

 

So what's the purpose of opening the MOSFETs if there is no charger or load?

Note that the N-MOSFETs you show will require a gate voltage higher than the battery voltage to turn on.
If you use P-MOSFETs, then you can turn them on by grounding the gates.

If there is a charger or load then the switch closes, if there is nothing connected MOSFET is open.

 

If there is a charger or load then the switch closes, if there is nothing connected MOSFET is open.

Again, so what is the purpose of the MOSFETs?
Obviously if nothing is connected, then it makes no difference whether the MOSFETs are on or off.

 

I made a simple BMS and placed 3 series of li-ion cells, voltage, and current sensors to read data but I have actually two problems.
1. I need some sort of sensor or something to detect if the charger or load is connected to the battery terminals so I can control the high-side charge and discharge N-channel MOSFETs (back to back).
2. I need a drive signal to control these MOSFETs. There are a lot of ICs especially designed for charge and discharge switches but they are expensive and I don't want to place such expensive IC on my simple BMS.

It is a simplified schematic of the circuit:

View attachment 292910

Do you just have to turn the transistors on and off or control the current so you can control the discharge and/or charge current level?