Charging 12V Battery While Attached to the Car

I believe Toyota are covering all the bases by warning to disconnect the battery before charging.

It does help protect the ECU's from voltage spikes and to be fare that is a good thing to do.

The problem arises with some modern chargers with a desulphator systems built in. These often work by pulsing high voltage spikes into the battery to brake up the sulphation and these could easily damage an ECU.

Having said all that I cannot see a small battery tender doing any harm. Others may disagree.

 

I've hooked up a CTEK 3300 multiple times, without first disconnecting the battery. It has a desulph... phase in it's charging cycle. So far so good.

 

I would like to see the output of that on a scope.

 

Supplementary question, would you top-up the battery while it is in-situ? Under the covers? Is there sufficient ventilation?
Frustrated by 2 weekends in a row with a weak battery I invested in a Ctek MXS 5.0 charger and a 'Comfort Indicator' with green, yellow and red lights to indicate the SOC (>80%, 60-80%, <60%). I attached the leads to the battery terminals and positioned the connector stuck to the inside of the boot so that I can see the flashing light from outside, even with the tonneau cover on.

To charge I plug the charger into the connector without removing any of the covers. I'm using the charger with the hatch open, but all covers in place. I'm considering a 3.3m COMFORT INDICATOR PANEL that would give me and indicator and socket on the lower dash. I could see the SOC and plug the charger in easily through an open front window.
BTW if I was performing an annual "recon" charge then I'd take the battery from the car and charge it on the bench.
EDIT: Based on BritPrius' advice, I'll leave the battery in at all times, if there's no dissenting opinions?

UPDATE: The indicator is yellow every morning, indicating between 60-80% SOC. Either it's not getting near 100% during driving since its last dip or there's a reasonably big drain overnight.

 

The Prius battery is designed to be charged at 4amps maximum. At this charge rate the battery should not gas externally, but recombine any gasses produced internally turning them back to water, stopping the battery drying out.

There is a vent tube from the battery to the underside of the car in case the battery does vent. If you do an annual charge on the bench it should not be at a rate above 4amps as this will cause the battery to vent starting the drying out processes.

This only applies to the OEM battery, other batteries such as the Optima usually have a higher charge current limit "10 amps for the Optima with temperature monitoring".