Why can't new and old batteries be mixed

Batteries are already very common in our lives, and the application range of batteries for home appliances, automobiles, etc. is very wide. Because people don't know much about the use of batteries, some errors will occur in the use of batteries. For example, new and old batteries are used together in series, but everyone knows that this approach will shorten the service life of the new batteries. The new battery has more chemical reaction substances, higher terminal voltage and lower internal resistance, while the old battery has lower terminal voltage and larger internal resistance. Generally, the internal resistance of the new 12V battery is 0.015-0.018 ohms, but the internal resistance of the old battery is large. Above 0.085 ohm, if the new and old batteries are used in series, the charging voltage at both ends of the old battery will be higher than the charging voltage at both ends of the new battery in the charging state. As a result, the new battery is not fully charged, and the old battery is already too high. In the discharging state, because the capacity of the new battery is larger than that of the old battery, the old battery is over-discharged as a result, even the old battery is reversed, and the battery swells and causes side effects. It will consume the electric energy of the new battery, and at the same time will cause the voltage in the electrical appliance to be unstable. There is also the danger of overusing the old battery. Never mix old and new batteries, as the result will outweigh the gains.