• Question: What happens when you put batteries of different voltages in parallel? Please explain!

    Asked by chemistryisthebest to Chris, Josh, Rebecca, Rob, Susan on 18 Jun 2015.
    • Photo: Chris Armstrong

      Chris Armstrong answered on 18 Jun 2015:

      Ooph this is a throwback to my GCSE in electronics, first off avoid doing this.

      Secondly the explanation, between different voltages a current will form, meaning you’ll be feeding the lower battery with the higher one. Now unless its a rechargeable (even then its not advisable), batteries are meant to discharge, pumping a current into them is a bad idea, they’ll heat up and likely burst given time.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 18 Jun 2015:

      It’s not good for the lower-voltage battery!

      Basically, in a parallel configuration, the voltage across all the branches of the parallel bit should be the same. So if you had an “ideal” battery, i.e. just a voltage source, you get a circuit whose equations don’t make sense, because you’re trying to equate two voltages that aren’t the same.

      In practice, a real battery isn’t a pure voltage source: you can’t get infinite current out of a battery, even if you short-circuit it. In circuit theory, this is represented by putting a resistance in series with the voltage source – this is the “internal resistance” of the battery. This does give you a circuit you can solve. The result is that some current flows the “wrong way” through the battery with lower voltage – how much current depends on the difference between the voltages, and also on the difference between the internal resistances. Ordinary batteries do not like this at all. Even rechargeable ones may object, because if the internal resistance is small the current flow will be large.

      Batteries of the *same* voltage are often connected in parallel, because it increases the lifetime (you get the same number of volts as with one battery, but more ampere-hours of charge). But having one battery of lower charge is likely to result in serious damage to the lower-charge battery, and greatly reduced lifetime for the higher-charged one (it’s trying to charge the other battery instead of running whatever you’re trying to power). This is why the instructions on equipment that uses batteries in parallel always tell you to replace all the batteries at once, not one at a time.

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