Its not a good idea to mix Battery voltages when connecting in Parallel
It is never a good
idea for you to connect batteries of different voltages in parallel
to each other. This will cause very high current flow from the higher
voltage battery to the lower voltage battery, causing you with
problems with the more powerful battery taking over from the smaller
battery. If the resistance of the wiring and connections are very
low, then the power will be dissipated across the plates and the
internal resistance of the battery, this will generating heat and
will cause you problems, it may cause the battery casing to split and
rupture.
Take for example a 9v "transistor" battery which has a high internal resistance, compared to most other battery types. This battery if fitted with a smaller voltage battery is likely to be overheated first, and will destroy itself.
If for some reason you have a flat battery on your car and it's necessary to use "jump cables" to use the power of another vehicle's battery to start your car. Then this will be fine as long as you use the correct cable to do the job, the cables are typically very large gauge wire and are suitable for doing this job. Both vehicles should have their engines switched off when connecting the cables, and the connection should be a negative connection on the failed battery to the negative connection on the donor car, this is to avoid making a spark near the battery, which may be venting explosive gases.
A fully charged lead-acid car battery may measure in the vicinity of 12.6v to 13.5 volts or somewhere in that area depending on the batteries chemistry, construction, and internal temperature. Maintenance free batteries are usually at the top end of the scale. A fully discharged battery could have a charge of around 11.5v. Even with such a small difference in voltages, heavy current will flow from one battery to the other and therefore starting the car with the flat battery from the donor battery, automotive batteries are designed for such heavy currents and there should be no problem starting the car with the flat battery.
Take for example a 9v "transistor" battery which has a high internal resistance, compared to most other battery types. This battery if fitted with a smaller voltage battery is likely to be overheated first, and will destroy itself.
If for some reason you have a flat battery on your car and it's necessary to use "jump cables" to use the power of another vehicle's battery to start your car. Then this will be fine as long as you use the correct cable to do the job, the cables are typically very large gauge wire and are suitable for doing this job. Both vehicles should have their engines switched off when connecting the cables, and the connection should be a negative connection on the failed battery to the negative connection on the donor car, this is to avoid making a spark near the battery, which may be venting explosive gases.
A fully charged lead-acid car battery may measure in the vicinity of 12.6v to 13.5 volts or somewhere in that area depending on the batteries chemistry, construction, and internal temperature. Maintenance free batteries are usually at the top end of the scale. A fully discharged battery could have a charge of around 11.5v. Even with such a small difference in voltages, heavy current will flow from one battery to the other and therefore starting the car with the flat battery from the donor battery, automotive batteries are designed for such heavy currents and there should be no problem starting the car with the flat battery.
After the cables are safely and securely
connected, the vehicle with the good battery
should be started allow
the alternator to charge both batteries for a period of time, to
help them recover the correct voltage. If the car with the flat
battery repeats the problem and will not start again, then a new
battery will be required.
The engine with the
good battery should be turned off before the engine with the
discharged battery is started. Otherwise, the alternator in the
vehicle with the good battery may be damaged.
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