What parameters do we need to predict & detect imminent danger caused by a charging battery?

More details on your design are needed to improve it.

The present version of my Flame Detector is very simple.

1. It contains an ESP32 MCU
2. 2 semi-conductor infrared flame detectors connecting in parallel to 2 potential dividers respectively.
3. Another Microchip analog temperature sensor connecting to an analog input of the ESP32 MCU.

This combination works very well.  It triggers alert when I light up a cigarette lighter near the infra-red flame sensor.

Chemistry of the batteries to monitor?

I focus on Li-batteries because its market is the biggest!

With lithium if a flame is present it's way too late.

make a silicone capsule around that battery to monitor volume changes with a strain gauge drown in it ... you'd monitor Pressure and Volume and Temperature... the IGLaw  PV = nR T .. 

make a silicone capsule around that battery to monitor volume changes with a strain gauge drown in it ... you'd monitor Pressure and Volume and Temperature... the IGLaw  PV = nR T .. 

Strain Gauge is very expensive, but can be an option for industrial end-users.

My end-users are house-wives. 

House-wives are more prone to battery fires?

They are especially careless.  We need to assist them to monitor their charging batteries.

Also lithium batteries are quite foolproof regarding end users.

Very 20th-century thing to say. I think you intend to mean your end-users are (say) less technically oriented consumers, which could be anyone.

Anyway, I'm reasonably technically oriented sometimes, but I am still trying to understand the workflow that the solution that you're creating is intended for. 

Most consumer products do not have removable Lithium-Ion batteries, so is the flame detector pointed at the product, e.g. mobile phone or laptop, while it is charging?

The most sophisticated ways to analyze the condition of the battery will be to run battery measurements/diagnostics with electronics directly attached to the battery.

What would be interesting, is if a charger (or other inline device) would automatically stop charging at night, and when there's no-one nearby, i.e. the sensor to be facing all the surroundings. That could help prevent fatalities when electric scooters etc are charging. But I bet some users would disable that anyway.

Everybody assumes battery charging works as design and users use it wisely.

and I assume misusing, low quality parts, accidents, etc. 

Yes but for what scenario? People charging mobile phones? Or scooter batteries, or laptops, tablets etc?

Is it like a fire alarm, but pointed at only one device at a time?

So I am thinking to add more sensor parameters to my apparatus, such as even more flame detectors.

HKPhysicist said:

Everybody assumes battery charging works as design and users use it wisely.

All BMS I saw take care to prevent accidents but bypassing safety features is always a possibility.