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I am working on a device which needs 1000 mA current at any time. Due to space constraints, I forced to use two different size and capacity battery from the same manufacturer. One is of 270 mAh and another one is 500 mAh. Both have 3.7V but the internal resistance is different. For 500 mAh battery, it is 0.19 ohms and the 270 mAh one has 0.8 ohms. If I am going to connect them in parallel will it cause any problem? Is there any problem to charge and discharge them?

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Yes of course there is. It won't work cause if your device accept 1000ma and you consider the best case that the resistance was equal and the current is additive. So 500+270=770 ma which is less than 1000ma. So of course the circuit won't work. What you need a better current supplying power supply.

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    \$\begingroup\$ No, you have made the common error of conflating mAh (energy) with mA (current). You can't directly compare these two characteristics. It is quite possible that a 700 mAh battery can supply 1000mA, or more. \$\endgroup\$ – Elliot Alderson Mar 14 at 10:52
  • \$\begingroup\$ Oh okay I didn't know that...should I be changing my answer then?? \$\endgroup\$ – infottie Mar 14 at 11:03

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