I am working on a system that uses a AAA alkaline battery but I am concerned that AAA lithium batteries that run at 3.7V exist (Amazon link). Can anyone suggest simple circuits that have a low leakage current and could protect the system in case 3.7V is applied at VBAT (designed for alkaline 1.5V)?
Either of the circuit shown below would meet the requirement.
It would be extremely uncommon for anyone to insert a 3.7V AAA lithium ion cell into a "1.5V" AAA system. The opposite would be more common.
A general solution would be to have a "switch" such as a MOSFET with suitably low Rdson (on resistance) which is turned on by a correct battery voltage but which has its gate drive disabled by a "comparator" which is triggered by an excessive input voltage. The 'comparator could be as simple as a bipolar transistor with a resistive divider supplying its base from Vin and the transistor collector shunting the switch gate to source. N or P channel FETs could be used as the switch.
If fuse blowing is acceptable a transistor could be shunted across Vin and turned on by excess Vin, thereby blowing a fuse.
Other methods exist but either of the above would work well enough.
General examples of the above two concepts. In the left hand case the MOSFET switch can be placed in the -ve supply lead if desired by using an N channel MOSFET and an NPN transistor and mirroring the circuit vertically.
M1 has to have a suitably low Vgsth rating such as to be fully enhanced (turned on) by the correct minimum battery voltage (1V or less). M1 Rdson (on resistance) has to be acceptably low at minimum battery voltage. R1 R2 chosen to not turn on Q1 at Vbat_max_allowed. An alkaline cell will often have Voc ~= 1.65V when new. In the right hand circuit Q3 MAY be able to be a MOSFET if VGSth is suitably chosen as the range of voltages between OK and bad are reasonably wide, but a bipolar transistor has a more controlled operating voltage range in this role.
Q1 could be a voltage supervisor IC - giving more precision of boundary voltage, or a comparator IC and suitable reference. A bipolar transistor is liable to be acceptable as Vbattery max acceptable is say 1.7V while VGbattery_min_agh! is say 2.5V and a Li cell will have minimal current capability at 2.5V.