Hi all:

Requirements:
A BMS for a small solar panel (3.2W) and a 12v SLA battery.
Need to be able to monitor the battery voltage without loading the battery at night.

Proposed circuit, first draft
MOSFET
-powered from the battery which is not a problem at night due to its high off resistance.
-starts conducting via R3 when the solar panel wakes up.
-puts a voltage on Q2 base.
Q2 inversely controls Q1 providing voltage regulation.
Zener protects MOSFET gate: __ v
Battery diode probably won't be necessary.

 

The polarity of your battery is backwards. The long lines are the positive side.

 

The P-channel MOSFET will be switched on when the solar panel is in the dark. Is that what you want?
How big is the battery?

 

The polarity of your battery is backwards. The long lines are the positive side.

Oops (I keep doing that)
Tks.

 

The P-channel MOSFET will be switched on when the solar panel is in the dark. Is that what you want?
How big is the battery?

No I want the opposite.
4.5AH battery, (now correctly oriented)
Thanks

 

[QUOTE="The P-channel MOSFET will be switched on when the solar panel is in the dark." [/QUOTE]

Ian:
Can you suggest a better circuit?
Tks

 

That's it ??
No one can suggest any corrections/improvements?

 

Assuming a lead-acid battery will self-discharge at about 5% per month. That's 225mAh per month. Divide by 30 and divide by 24, that's a self-discharge current of 312uA. If your sampling circuit uses <30uA which is about 400kΩ it will make no difference to battery life - why make things complicated?

 

" why make things complicated? "
So what are you saying? I don't need the MOSFET?
Without it, R1+R2 will draw > 1mA.

 

Increase the values of R1 and R2 so that they take <30uA.
The base-emitter voltage of Q2 is nowhere near good enough to use as a reference to regulate the battery voltage. You will need an op-amp.