I have noticed that when the TIMER and /EN pins are grounded and the /CHRG pulled high through a pullup resistor (2K), the /CHRG pin oscillates at 10ms low and 20ms high.when a battery is absent. Can I depend on this behavior for battery absent detection? The data sheet does not describe this condition, but it is certainly useful if it is not just an anomaly.
Take a look at the battery node voltage on a scope while this is happening. You should see a similar waveform. This is because, with the timer disabled, it is relying on current-based termination which is satisfied immediately when it starts charging. This causes the output to cycle as it continuously restarts charge cycles. This behavior is benign and reliable; you are welcome to use the /CHRG oscillation as a no-battery indicator.
You can also change the blinking speed by adding more capacitance to the battery node. This will slow the battery "charge cycle" completion, resulting in slower toggling.