Changing Battery With Engine Running

1. I own a Toyota Vios 1.5E auto (2004 model). I have just replaced the wet battery with a dry battery. But I saw the mechanic change the battery with the engine running at a idle speed. He said if he changed the battery after the engine is switched off, the car would have engine tuning problem and other minor side effects. He added that even on changing a wet battery, he would also have the engine running at idling speed. It seems the latest cars, which have many of their system running on computer circuits and electronic sensors, must be treated this way.

2. I used to disconnect one of the battery car terminals (wet battery) when I did not use it for more than two days (to avoid depolarisation and damage to the battery). Which terminal should be disconnected in term of safety? I always disconnect the (-ve) side.

3. Now that I am using a dry cell battery, and will not be using my car for 10 days next month. Do I need disconnect one of the battery terminals? If i do it, will the problem as in question no.1, surface again after I reconnect the battery?

4. What type of battery (in term of quality and maintenance) is more suitable for cars - wet or dry?

Reply:
1. The standard rule is to turn off the engine and switch off all accessories whenever removing or replacing batteries. One should not have the engine running because if the positive terminal is to touch any part of the battery being replaced, it would short out the alternator and cause severe damage to the coils and diodes, let alone the possibilty of fire.

2. Two days are a short period to disconnect the battery. While I do not disagree with the practice, it is mainly to prevent discharge through the clock and some equipment that may have small current draw and if if left uncharged, the battery could become sulphated. By the way, it makes no difference whether it is a 'wet' or 'dry' battery because there is still sulphuric acid in the battery that would cause sulphation. I think by 'dry' battery, you mean a 'no maintenance' battery. Generally 'no maintenance' battery are sealed so that you do not need to top up with distilled water but if you notice that the electrolyte level is very low, the cells can be opened and topped up.

3. You can remove the battery connection while you are away or even leave it connected because I do not think it would be so weak when you get back as to be unable to start the engine. What is important when you get back is to run the vehicle for some time to charge back the battery.