# batteries in series

Discussion in 'General Electronics Chat' started by antennaboy, Mar 12, 2010.

1. ### antennaboy Thread Starter Active Member

Jan 31, 2008
45
0
hello forum,

a question about constant voltage batteries in series:

IF a 9 volts battery is connected via cables with some minimum resistance to a battery that is 4V (with the + terminal of one battery connected to the + terminal of the other), what would happen?

I think the 9 V battery will determine the direction of the current. Will the 4 V battery get damaged?

On the long run, would the 4 V battery break down and work like an open circuit?

If the 4 V battery was completely discharged, would the 9 V battery be able to recharge it to its maximum (4V)?

When are two batteries connected that way? Only when we need to jump start a car, or recharge one battery using the other?

thanks
antennaboy

2. ### Audioguru New Member

Dec 20, 2007
9,411
895
I think you are confusing SERIES and PARALLEL.
To jump-start a car, batteries are always connected in Parallel, never in series.

A 9V battery does not have a constant voltage. Its voltage drops when it is loaded and as it runs down.

If you connect a 9V battery in series with a 4V battery with their + wires together then the output voltage is 9V - 4V= 5V. When one battery is completely discharged then it will become charged backwards through the load from the remaining battery.

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
It is generally not a good idea to connect batteries of different voltages in parallel to each other. This causes very high current flow from the higher voltage battery to the lower voltage battery. If the resistance of the wiring/connections are low, power will be dissipated across the internal resistance of the battery, generating heat. This can cause the battery or batteries to rupture forcefully.

A 9v "transistor" battery has a relatively high internal resistance compared to most other battery types. It is likely to be overheated first, and destroyed.

If for some reason a battery needs to be recharged from another battery, it is a good idea to control the maximum current using a resistor or semiconductors and Ohm's Law.

Sometimes it's necessary to use "jumper cables" to charge one vehicle's battery from another. In this case, the cables are typically very large gauge wire. Both vehicles should have their engines off when connecting the cables, and the final connection should be a negative connection to some point that is away from the battery, preferably to the engine block. This is to avoid making a spark near the battery, which may be venting explosive gases.

A fully charged lead-acid battery may measure in the vicinity of 12.6v-12.9v or thereabouts depending on chemistry, construction, and internal temperature. A fully discharged battery may measure around 11.5v. Even with such a small difference in voltage, heavy current will flow from one battery to the other. However, automotive batteries are designed for such heavy currents.

After the cables are safely and securely connected, the vehicle with the good battery can be started to use the alternator to charge both batteries for a period of time.

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.