Hoover batteries, sensors and charging

Question

I have a battery for a robot hoover (aka vacuum cleaner) (Ni-MH) that seems to be on the blink (very low run time). I'm trying to understand how it works in case I can do anything to fix it or if I should get a new one.

The battery is this one: http://www.alibaba.com/product-detail/Ni-MH-14-4v-sc-rechargeable_1528323749.html

You can see two pairs of contacts on the top -- one set is clearly for the power, and I can measure the expected 14-15V with a DMM. The other is possibly a temperature sensor so that the thing knows when it's charged up. But it's a bit strange -- at room temperature, I measured a resistance of 7.8kOhm across this pair, and as a test, I put it in the fridge for an hour and it came out at 11.8kOhm. Does anyone know what kind of sensor this might be?

The other question I have is how the robot knows when the battery has run down and it needs to go back to its dock for a charge?

Answer 1

It's common for chargers/batteries to have temperature control, this one is an "NTC" or "Negative Thermal Coefficient" because when it is colder, the resistance increases, and when it's hotter, the resistance decreases (thus the "negative" part when relating to the behaviour during an increase in temp)

They are simply NTC resistors and the intelligent charge controller will be able to use it to help it's charging profile and protection circuits.

The robot's power management ICs or controllers will be able to monitor the battery voltage using a wide range of techniques - the most common I use in my designs is a simple resistor divider straight off the battery positive terminal, taking the voltage range for minimum and maximum expected charge to something safe for my microcontroller, and simply reading the voltage using an ADC (Analogue to Digital Converter) on-board the microcontroller to read the battery state, and this may trigger a "We need to charge our battery!" state within the robot's brain.

Some battery management ICs will have serial or other forms of communication to a host microcontroller for the same kind of battery voltage reporting, and also indicating the state of the charge cycle once it begins. They might do it internally in the chip as part of the charge control anyway, so it's handy information. The IC designers may make use of resistor dividers or more likely some kind of difference amplifier or more complex op-amp setups.