That is normal, all the mkiii does that.
12 volts DC (nominal). It's the voltage of the battery which will vary from a low of about 11 to a high of about 14 when the engine is running.
Not charging. Poor battery, poor alternator, voltage regulator, slipping drive belt.
Check fuses first. If fuses are good, check the voltage from the negative battery cable to where the wire bolts onto the back of the alternator. (With the engine running). If you can't reach the back of the alternator, then measure across the posts of the battery. Voltage should be about 14 volts DC. If lower than that, replace the alternator. When you replace an alternator, it's a good idea to replace the battery, as a precaution.
this sounds too simple to be asked. Or do you mean doing something else to the battery? if you can see the battery just disconnect the terminals using a spanner and replace the battery with one of similar voltage.
Check spark plug wires, and voltage on the distrubutor.
how will I know my Voltage regulator is bad. I have replaced the starter and had the battery and alternator checked they are good. My car will start when it is colder outside but as the day gets hotter it will not start.. Can you help
Is this an automatic or manual transmission?
The battery icon is on because there is a problem with the charging system. In most instances the cause is a defective alternator. Check the voltage at the battery with the engine running with a digital volt meter set to the 20 volts dc scale. You should read 13.5 to 15.5 volts. Any less and the alternator is possibly defective.
There are two different batteries in the Toyota Prius. There is a small 12 volt battery used for powering up the computer and powering the 12 volt accessories in the car. The other is the high voltage battery. The small 12 volt battery lasts on average 4 or 5 years. This battery is usually trouble free, unless you don't drive your prius very much. I believe these run around $150 USD. The lifespan of the high voltage battery varies. I saw one go bad at around 90,000 miles. Another prius made 150,000 on its high voltage battery. Expect to pay about $3,500 USD for one of these. Both of these batteries can only be purchased at your local Toyota dealer.
if you are trying to change it while it is running it always needs an earth for the ecu otherwise there should not be any reason connected with the battery to cause idle problem
Sounds like a bad alternator. Slow windows indicate low voltage (same for radio cutting in and out), the alternator light indicates battery voltage is higher than alternator voltage (most likely meaning alternator isn't putting out anything).
Sensors of this nature normally generate their own return signals based on speed and temperature under varying conditions, they are not constant values.