Do not connect the "negative" terminal of the car battery directly to the "negative terminal".
Whether you are the one helping or the one being helped.
It's safer to know in advance. Prevent problems later

The reason why it is prohibited to directly connect or jump the battery power is because Battery with more power Current will be sent to the battery that has less power. That reaction will cause The sulfuric acid in the battery boils and expands, releasing hydrogen gas through the battery's vent holes. If there is a spark from connecting the battery and there is enough gas, it may cause sparks. It may explode or catch fire. To prevent sparks from spreading or damaging the battery. We recommend connecting the battery according to the following steps.

How to properly connect a car battery
1. Start by taking the red wire and clamping it to the "positive terminal" (the car whose battery is dead).
2. Take the red wire and clamp it to the "positive terminal" (cars with normal batteries).
3. Take the black wire and clamp it to the "negative terminal" (cars with normal batteries).
4. Take the black cable and clamp it on the metal frame of the engine (car with dead battery).
(Shouldn't connect the negative pole directly) because the car frame will be ground in many places, which can be used in place of the car's negative pole.
5. The car that comes to help should start the car. Leave it on for at least 3 minutes before and after connecting the battery so that the power can cycle fully. Turn off the stereo, air conditioner, or any unnecessary electrical systems first. You should not turn off the engine immediately after towing to allow the drive to charge the battery.
6. When starting the car, the dead battery is already on. Let's accelerate the engine a little. If the car doesn't have a power surge. Remove the cables in the reverse order.
7. If you clamp the battery and it won't start. You should not force the starter to continue or leave it for a while before trying to start again.

Because inside the battery there are positive plates and negative plates. Made of lead metal (pb) and has sulfuric acid (H2SO4) acting as an electrolyte. (electrolyte) The electrochemical reaction of the electrode with the electrolyte creates an electric current. This electrochemical reaction creates "hydrogen" inside the battery, so it is designed with vent holes to vent the gas to the outside. To prevent the battery from being damaged when the gas increases in number and pressure. This hydrogen gas is flammable. The battery may explode. Therefore, it is strictly forbidden to connect the "negative pole" and "negative pole" together. Instead, clamp the metal parts of the engine at least 50 centimeters away from the battery to prevent sparks and other problems.

If there are any errors in the information, we apologize here. If anyone has additional information or details, You can share your experiences under the comments.