I have an inverter with 12V input voltage and one 12V battery. I am planning to increase my backup time by buying one more 12V battery. Will this setup work: 12V inverter with two 12V batteries connected in series or parallel?


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  • \$\begingroup\$ what is the inverter spec? A 12V unit is often only 10~15.5V. It may not last twice as long if you don't have twice the input power*time \$\endgroup\$ – Sunnyskyguy EE75 Aug 26 '16 at 20:52
  • \$\begingroup\$ 1200w 12vdc inverter \$\endgroup\$ – Akshay Aug 27 '16 at 9:53

Do not connect them in series. That will give 24V to your inverter which is designed to run from 12V. 24V may damage the inverter.

Connecting batteries in parallel is OK, and it is often done. Ideally you should follow these rules:

  • use batteries of the same capacity. If your original battery is 18Ah then buy another battery of the same capacity.
  • use batteries from the same manufacturer. This way the batteries should "age" at about the same rate.
  • use batteries of the same age. In your case this is not possible, and you may want to consider buying 2 new batteries.
  • before connecting the batteries in parallel for the first time you should make sure that both are fully charged. If you don't then the one which has more charge will try to charge up the second battery. In this case very large currents can flow, such as 10 to 20A, which may damage one of the batteries or the wiring between them.
  • (again ideally) once you have connected the batteries in parallel you should keep them permanently connected. Charge and discharge them together as if they were a single battery and you will get long life from them.

When you connect batteries in parallel connect the positive (+) terminal of one to the positive (+) terminal of the other. Same for the negative terminals. Get this wrong and you will soon have a fire.

  • \$\begingroup\$ can u explain what is actually meant by double capacity when two batteries connected in parallel? \$\endgroup\$ – Akshay Aug 27 '16 at 11:41
  • \$\begingroup\$ Battery capacity is measured in Amp hours (Ah). A typical small lead acid battery will have a capacity of 7Ah. Very approximately this means that battery can supply 1A for 7 hours. If you connect two of these batteries in parallel then the combined battery still has a voltage of 12V, but its capacity will be 14Ah, ie it has double the capacity of each battery. It could supply 1A for 14 hours. You may think that it could also supply 14A for 1 hour, but in practice batteries do not work like that, and you may only get 14A for about 30 minutes. \$\endgroup\$ – Steve G Aug 27 '16 at 11:59
  • \$\begingroup\$ if i connect in series, then will i still get 1A for 14 hours? \$\endgroup\$ – Akshay Aug 27 '16 at 12:01
  • \$\begingroup\$ No, you will get 1A for 7 hours, but at 24V rather than 12V. \$\endgroup\$ – Steve G Aug 27 '16 at 13:06
  • \$\begingroup\$ I got it. Both ways I will get same amount of power. If it is 24v, the device will draw less current than it would draw at 12v. If it draws 1amp at 12v then it will draw 0.5amp at 24v. Is it right? \$\endgroup\$ – Akshay Aug 27 '16 at 15:59

If the inverter is designed to have a 12 Volt input, then you must connect any added batteries in parallel with the original battery to maintain 12 volts.


simulate this circuit – Schematic created using CircuitLab

If you connected the batteries in series, they would provide 24 volts.


simulate this circuit

  • \$\begingroup\$ i understand above picture. I just wanna ask does series/parallel provide same amount of backup time? \$\endgroup\$ – Akshay Aug 27 '16 at 15:56
  • \$\begingroup\$ Yes - you have the same amount of energy in the batteries either way. To get 100 Watts (for example), you would draw about 8.3 Amps at 12 volts, or 4.15 amps at 24 volts. \$\endgroup\$ – Peter Bennett Aug 27 '16 at 16:45
  • \$\begingroup\$ and is it right if 1 battery of 12V 200ah takes 4 hours to charge then if I add another battery of same ratings, connected in either series or parallel they will take 8 hours to charge full? \$\endgroup\$ – Akshay Aug 28 '16 at 11:55
  • \$\begingroup\$ Charging time will depend on the state of discharge of the batteries and on the capability of the charger. I'd expect that adding a second battery in parallel would increase the charging time, but not necessarily double it. \$\endgroup\$ – Peter Bennett Aug 28 '16 at 15:39

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