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I've seen some questions about connecting batteries in parallel to increase capacity, and the answers all indicate that it's fine to do. But why? Isn't that creating a short if the batteries aren't at the exact same voltage? I understand there's some internal resistance in the batteries, but is that enough to make this safe?

Is there any benefit to isolating the batteries with diodes? E.g., give each battery it's own diode, with the positive terminal connected to the diode's anode, and then putting each of these battery-diode pairs in parallel?

I'm concerned entirely with off-the-shelf alkaline batteries, like AA, AAA, possibly 9V.

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No, it's not ok in general, for the reasons I described in the question (creating a short circuit).

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  • \$\begingroup\$ Take a look at the questions under the "related" column in this question, and read about situations where it's not a great idea \$\endgroup\$ – Scott Seidman Dec 2 '15 at 20:44
  • \$\begingroup\$ It's not advised to connect two batteries in parallel which are far apart in state of charge as the equalizing current will be huge and might damage the cells and connecting wires. (depends on the cells used) \$\endgroup\$ – Arsenal Dec 2 '15 at 21:02
  • \$\begingroup\$ it's "fine" to connect two (or more) batteries in parallel that are identical in model (design and construction) and state (one should not be used more than the other). about the only way to do that is to buy the batteries together and never use them for anything other than being connected together in parallel. if they're rechargeable, charge them together, also. then you'll be okay. \$\endgroup\$ – robert bristow-johnson Dec 3 '15 at 3:27
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Nothing is "fine" with connecting two batteries in parallel if they aren't the same (bought the same day also). With diodes you get additional loss, because there is a voltage drop across diode 0.7V.

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  • \$\begingroup\$ Or 0.4 to 0.5 if you use Schottky diodes. \$\endgroup\$ – WhatRoughBeast Dec 2 '15 at 21:59
  • \$\begingroup\$ Ok, well that clears it up and makes me feel better about my understanding, but a little concerned that so many people are suggesting that it's fine. I understand the loss from the diodes, but what is the alternative? Assuming I don't want to design my circuit with the assumption that the user will always use matched batteries. \$\endgroup\$ – brianmearns Dec 3 '15 at 1:27
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Making the assumption we are talking about non-rechargeable batteries here, then I would say No, don't do it.

However, if you are building a bank of batteries that will be recharged (as in a UPS, or a Solar backup), then you can, and may often do, add batteries in such an array.

Here is a link that may explain more and give some examples of good ways and bad ways to put a battery (which is, after all, a collection of cells) together.

https://batteryguy.com/kb/knowledge-base/connecting-batteries-in-parallel/

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