How come the 9V battery in the multimeter doesn't work when it measures 9.3V?
This is going to be covered in two ways. Because 9V laminated batteries are divided into carbon and alkaline batteries!
1. Carbon batteries have a high instantaneous current, but they don't last long! The point is that they are cheap! You don't use them, leave them for a month or two, and they basically consume so much power. When you run out, the power is not enough, change the spare battery, found that not long after the use of the power is gone again. This is evidenced by the multimeter displaying a bit, and then no words. Off and then on again can display a little. Remove and measure the voltage, there is 8V voltage.
2. Alkaline batteries have a stable output and a long life, making them ideal for multimeters. I used to use carbon batteries, several pieces a year to use. Since I changed to a well-known domestic brand of alkaline batteries, there has been no such failure. Think about it, you are testing the voltage and resistance, the multimeter is not enough power, there is a large error. What a mess! Alkaline batteries have a low discharge capacity, which allows them to be stored for a long time. They are guaranteed for 10 years, so feel free to store them.
To see if a battery is charged, it is not the voltage, but the current, as long as the battery can pull a certain amount of current, the battery will still work, regardless of the voltage.
Voltage is just a potential difference, it just shows that there is a potential difference between the two places, there is a potential difference does not mean that there will be a current, for example, when a battery is empty, the two electrodes of course, there is a potential difference, but there is no current, because there is no conductor to form a circuit.
What happens if there is a circuit? One of the most extreme case is directly with a wire (such as copper wire) to the electrodes connected to the ends, according to this time there should be a lot of current, after all, the resistance of the conductor (copper wire) is very small, according to Ohm's law, the current must not be small, but the actual situation is not that way, this is because the battery as a power source it has a resistance, this resistance (internal resistance) is very large (relative to the wire), the battery can provide current is also subject to the resistance constraints, but also to the battery's ability to provide current. The current supplied by the battery is also subject to this resistance.
When the battery is used or put for a period of time, the internal resistance of the battery will increase, the final result is to pull out much current, although this time in the unloaded time can still measure the voltage, but when you connect the load to pull out the current, due to the reason of the internal resistance of the voltage will fall rapidly, ultimately leading to the battery can not supply power normally.
In order to solve this problem, some people have used the method of injecting salt water into the battery to reduce the internal resistance, which will restore the battery's power supply ability at a certain time, however, when the battery's internal potion or electrode reaction has been consumed, it is useless to inject anything into the battery at this time.
If your 9V battery is running out fast, it is recommended that you use rechargeable batteries, even if you use regular batteries, it is also recommended that you use the freshest batteries available, all batteries have a factory date, although some people will re-date and repackage them with old or even used batteries, so the location where you buy the batteries is also important.
