Because your also sending that power down the electrical system and ecu. Trickle charging, etc. Runs the risk of damaging the electrical system. Much safer. Same reason why you disconnect the negative before you start disconnecting electrical connections.
I am not an electrical engineer but i know an auto charger and certainly another car puts out about the charged voltage of a battery so I do not know why that would damage anything. Also the amperage output should be about right as is is controlled by a regulator. As far as the point of introduction, you have a ground and the battery + terminal which normally is directly connected to the alternator output. 200amps max?
Like I said I do not know why this could cause problems.
Reatta Man's Avatar
Reatta Man said:
January 26th, 2010
Re: OK to charge battery with cables attached?
ANY time you disconnect battery cables, just make sure you do it with plenty of ventilation.
Charging batteries produces hydrogen gas. VERY explosive when concentrated in a small contained area, such as around a battery in a fender well or on an enclosed engine compartment.
On any computerized car, you can charge the weaker battery with the cables connected and the charging car running. But, as was previously suggested, only do that when you are keeping an eye on the car. When the car starts, turn off the engine of the charging car before disconnecting the cables; this will help prevent a spike to that car's computer.
Finally, I would NOT use a high-end Japanese car with a coded key as a charging car. On several Toyota models (Lexus) if you spike the computer and fry it, you have to get a new computer from the factory that is coded with the keys---ranging from $1200-2000. And no, on several models, the factory will NOT let a dealer or distributor recode a computer and keys.
People...removing the cables while charging is for safety. Just because it never happens 95% of the time, do you want to risk $$$ for that one time the charger surges your system? Auto insurance, most of us never use it...but we have it. If you don't want to remove the cables, then don't.
You are very likely correct, most of the time, no problem. I was hoping someone could enlighten me as to what exactly would cause a problem. As was said, a surge could cause a problem but what causes surges?
It's good and safe advise to disconnect the negative terminal but being careful and using a smart charger should minimize the risk.
In addition, it's worth the extra safety of not turning anything on while charging.
Yes, I know some circuits are always hot. This leaves me wondering about reprogramming the car if you disconnect the battery.
I use a Vector but I'm sure there are many other smart chargers that check the circuit before applying voltage and step the amps up carefully.
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