Car battery charger

I'm a student of electrical high school. I have tried this project, to make a car battery charger with an Arduino-powered voltage meter. My problem is that I made some research about making an Arduino voltage meter and I found that I need to connect the v- of my charger to GND on Arduino. I'm not a perfect student and I've been searching for it for an hour now. The question is, can I connect the GND pin to v-, or how can I overcome this problem? Please let me know your honest opinion about the fact if it will work. Here is my schematic. I've highlighted the problem area. It is a car battery charger, so the wanted voltage is somewhat more than 12V.

• Even if asking for opinions is off-topic, the circuit is not a good battery charger. If you connect that to a battery that needs charging, the LM317 would give as much as current it can into the battery and it will overheat and go into thermal shutdown. Please note that the LM317 datasheet do contain example circuit for a battery charger, sometimes even on the first page. Mar 14, 2021 at 15:49
• You don't say how much voltage there will be on the LM317 input so we can't answer how many seconds it will last before melting. Mar 14, 2021 at 16:18
• I think you should be aware that if the transformer output is 12 volts AC, the voltage at LM317 input would not be about 12V. It would have least 16V and could exceed 17V under light load. Mar 14, 2021 at 16:26
• The differential is not the problem. The power consumption is. And that the output voltage is set to 12V. If the battery is below 12V, like 11V, the LM317 would try to give as much as current it possibly can to lift the voltage back up to 12V, but the battery would keep the voltage at 11V. It will hit the current limit, and if we say it is 1A, then 16V-11V is 5V, and 5V at 1A is 5W. The thing will dissipate 5 watts. If the device has thermal resistance of even 25 C per watt, at 5W the temperature would rise by 125C and it would go to thermal shutdown. Mar 14, 2021 at 16:36
• The problem is not cooling. The problem is you are misusing the regulator and short circuiting the output into the battery. There is a difference between a power supply and a charger. A charger can be connected safely to a battery, but a power supply can't. Mar 14, 2021 at 18:08

There’s a few improvements that need to be made.

• 12V transformer is not enough it needs to be >16V due to the 2x diode drop at high peak currents for a 12V car battery that needs 14.2V to be fully charged.

• the battery is like 10 kFarads or more so it must be charge with very low DCR transformer secondary resistance as the resulting charge only happens on peaks of the rectified sine wave and in cars they use 3 phases with 6 diodes for smoother current.

• However this is doable but your diodes must be rated for >20 times your average current expected. This is because this will be the actual peak current for <=5% of the peak of the sine and bigger diodes have low resistance and may need a heatsink (insulated)

• rather than a LM317 to produce voltage with massive linear power drop, needs a big heatsink. a far better solution is to use a 10 mOhm power NCh FET on the ground side and a 5 mOhm current sensing resistor or >=10A Ammeter to measure the current for 50mV drop to be amplified to 5V to avoid getting hot. Thats a piece of solid copper wire.

• you then monitor Current until voltage is reached to regulate or cutoff.

• I attempted to show a more powerful idea of what I wrote above >100 W with an interactive switch to precharge the simulation of a battery as a “super cap” tinyurl.com/yfg95v53 Mar 14, 2021 at 18:10