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I have a question related to small battery connections.

I am working on a project where the design has to be kept as small as possible. I need 3.3V regulated supply and run two small coin vibration motors rated at 3V.

I built this network of coin batteries comprising of 3V button cells and 1.5V button cells. I want to use 4.5V to be regulated down to 3.3V (which can be used for digital circuits). And I want to run vibration motors of the same battery network but use only the two 3V cells. I have connected the two cells in parallel. By doing so would it affect my 4.5V supply? I am not sure about how effective this network would be. Hoping I will get some answers as to how efficient this could be or any recommendations.

enter image description here

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  • \$\begingroup\$ That's not how capacities work. Also, uneven draining is bad. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jun 11, 2014 at 6:42
  • \$\begingroup\$ @IgnacioVazquez-Abrams, you mean to say that I should not use 3V from the circuit but instead use the whole thing for only 4.5V supply? \$\endgroup\$
    – David Norman
    Commented Jun 11, 2014 at 6:43

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What you have drawn will work, but the effective capacities are not what you show.

The 4.5V supply will be limited in capacity to that of the 1.5V batteries, or 300 mAh total. At that point, the 1.5V batteries will be depleted, and you'll no longer get 4.5V.

Furthermore, this will also consume 300 mAh of the capacity of the 3V batteries, which means that you'll only have 900 mAh of capacity remaining to run the motors at 3V.

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  • \$\begingroup\$ Thanks for your answer Dave. Just to clarify, using this network is a bad idea? Because if 1.5V batteries deplete then the whole supply will be ruined. Can you suggest a way around this problem? \$\endgroup\$
    – David Norman
    Commented Jun 12, 2014 at 3:16
  • \$\begingroup\$ No, I didn't say it's a bad idea. But you need to understand how the capacity works out for each voltage, and you need to match the battery capacities to what you actually need in terms of what the logic and motors actually consume in the intended application. I think the cleanest solution would be to use a single battery that contains the total energy you need, and use swichmode converters where necessary to supply different voltages. Match the battery to the heaviest load; then the conversion losses for the other loads will be minimized. \$\endgroup\$
    – Dave Tweed
    Commented Jun 12, 2014 at 4:15
  • \$\begingroup\$ I cannot incorporate Switch mode as the size of my board is 50mmx50mm \$\endgroup\$
    – David Norman
    Commented Jun 12, 2014 at 5:33
  • \$\begingroup\$ I don't understand why that's a problem. You can build a converter in about 5% of that amount of space -- i.e., about the same amount of space that a button cell would consume. \$\endgroup\$
    – Dave Tweed
    Commented Jun 12, 2014 at 10:54

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