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I know that the differences in resistance between batteries in a circuit will create different drop, and as a result draw unequally. Unfortunately, I couldn't get my hands on a parallel D-cell battery holder, so I soldered one myself. On my multimeter, I measured as much as a 0.1-0.3 ohm difference between the contacts and ends of the leads, which was more than I was expecting.

I'm using these batteries: http://www.xenoenergy.com/eng/prod/genaral06.asp , and they will be deployed outside in sealed boxes for quite some time.

My question is this: how much of a resistance difference between leads on cells is 'too much' and could yield either catastrophic battery failure or meaningful decrease in battery life?

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    $\begingroup$ The internal resistance increases as the batteries discharge, so the situation is self balancing to some extent i.e. the batteries with the lowest internal resistance will discharge fastest so their internal resistance will rise fastest. $\endgroup$ – John Rennie Oct 30 '17 at 7:40
  • $\begingroup$ Would Electrical Engineering be a better home for this question? $\endgroup$ – Qmechanic Oct 30 '17 at 9:15
  • $\begingroup$ Would the internal resistance compensate for the differences in resistance on the battery terminals? And, perhaps it would be better suited to EE. $\endgroup$ – osteichthyes Oct 30 '17 at 10:08
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In the data sheet accessible from your link, there is a diagram of discharge charactersitics at different currents (first diagram, first page), from which the internal resistance of the cells can be determined.

The 30mA and 100mA curves have a voltage difference of about 210mV, showing an internal resistance of about $3\Omega$. This will dominate your $300m\Omega$ contact resistance, which you can probably ignore. Of course, lower contact resistance is always better.

How much difference in resistance is too much in a parallel cell battery? At the current you intend to draw, what is the voltage drop? If the difference in voltage drop is a significant fraction of the change in cell voltage over its discharge life or discharge current, then it will set up a capacity imbalance between the cells, which means you will not get best performance out of your battery. In particular, it will mean that end-of-life performance will be supplied by only one or a few cells, not the whole battery.

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