Re Phillishave HQ7780 Why does battery fail completely when switched off at the end of each shave and then has to be recharged before it will operate again.

A compatible battery can be found at shaveroutlet.com using the link below

http://shaveroutlet.com/shaver-replacement-battery-aa-nimh-p-108.html

Panasonic ES-RT51 battery problems?
If your razor battery runs down after only a few shaves e.g jumps from 60% to zero Let the razor completely discharge by leaving it switched on until the blades stop moving completely. (This can take over 24 hours so be patient) When they are stop moving recharge the battery and you will find the battery will charge and discharge just like new!
(This may take a couple of discharge cycles). Only charge the battery when it is flat after this and the problem SHOULD not reoccur.

Terence Saunders

If your solder joints are good and no solder has slopped over to short it out, the only thing left to check is the polarity of the battery. Positive end goes toward the motor

When the battery is almost empty (when there are only 5 or fewer shaving minutes left), the battery symbol lights up red.

Make sure that the battery does not have a memory effect. What I suggest is taking the shaving head unit off and running the shaver till empty, then recharging. Check if the red light is still flashing.

There's also a head indicator at the bottom of some shavers.

To remind you to replace the shaving unit, the shaving unit symbol flashes (all types) and you hear a beep when you switch off the shaver (RQ1090 only). This happens at the end of 7 successive shaves to remind you to replace the shaving unit.

Without knowing what model you have it's difficult to diagnose further.

I believe that you are referring to the heads. You will find replacements for model HQ7780 at shaveroutlet.com. See link below.

http://shaveroutlet.com/norelco-philips-hq177-cool-skin-replacement-heads-p-245.html

My store carries the heads for your Norelco shaver. I can ship them to you. Please call toll free 1-888-643-9806 and ask for Action Mike

the steady red light showing when in charger shows that
it is charging. the blinking red light when taken out of
charger or turned on and off means it is not fully charged.
leave it in charger till red light turns green.

Apparently you need to change the heads within two weeks after the red LED first starts to flash, otherwise the memory chip inside needs resetting. This also poses a further question. How does the unit know the heads have been changed or not ?

You neet torx T9 , quite cheap.