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I'm building a battery pack that will be hooked up to a buck converter for switched output. To increase the pack's mAh capacity, I can hookup a second array of similar cells in parallel to the converter's input. However, since the cell output is passing through a switch-mode regulator, can I accomplish the same increase in capacity by connecting all the cells in series?

Conceptually, the regulator will switch on half as often (when the input voltage doubles), so the cells should discharge at the slower rate. I may be able to simplify some of my connections and packaging by using the series connection, and thus I'm wondering if there is any practical difference to connecting the cell arrays in series or parallel.

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  • \$\begingroup\$ Both ways have pro's and con's, so it depends on protection for voltage balancing in series and current balancing in parallel and degree of mismatch between cells. Parallel has more advantages but also more risk if unbalanced cells. So arrays offer the best balance when large and series strings when small just to reduce excess current risk. \$\endgroup\$ Nov 29, 2017 at 19:54
  • \$\begingroup\$ A precision microfuse helps balance ESR and add safety for parallel strings or cells. \$\endgroup\$ Nov 29, 2017 at 20:01
  • \$\begingroup\$ While this seems like a good question at first, this is actually a really broad question. There's so many engineering tradeoffs to look into. Are you after efficiency? Simplicity? Cost? What does your recharging circuit look like relative to this battery pack? This also would have to assume your buck converter can handle double the voltage. \$\endgroup\$
    – horta
    Nov 29, 2017 at 23:00
  • \$\begingroup\$ @horta is correct. Your converter efficiency will drop as the voltage ratio input/output increases. You will not necessarily switch on half as often, a PWM scheme will modify the duty cycle, not the frequency (though hysteretic controls will change frequency). Recharging cells in series can require cell balancing. You will have to make all the tradeoffs to see which is the better approach for your specific application. \$\endgroup\$
    – John D
    Nov 29, 2017 at 23:25
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    \$\begingroup\$ Efficiency of the converter should be your first thing to look into. Getting one meant for higher current will likely offer better efficiency. Then for simplicity, I would keep all the batteries in parallel to help keep the batteries balanced. Generally in SMPS, the closer your input to output voltage is, the more efficient it'll be so doubling your voltage won't help here. \$\endgroup\$
    – horta
    Nov 30, 2017 at 4:30

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Consider two batteries, that can be connected in either series, or parallel.

Each battery has the same voltage, and if we assume each delivers the same power, has the same current being drawn from it. The total output power for either connection will be the same.

Each will have the same power lost to internal resistance, as the current in each battery is the same, so the efficiency will be the same.

Given that they are driving an ideal SMPS, it doesn't matter whether it is fed its 2VI power as 2VxI (series), or Vx2I (parallel), the SMPS output power will be the same.

There is no difference between series and parallel connection for power or efficiency. There may be some practical differences however.

The wiring between the batteries and the SMPS can be lighter in the series case, as it carries half the current.

Series connection often results in simpler wiring and less of it.

The SMPS you have might be able to work up to V, but not 2V, so you couldn't use the series connection without buying new stuff. If it's able to work at both voltages, it might have different efficiency at them.

Parallel batteries need to be balanced in voltage before being connected, but once connected can be charged/discharged as one, they can even be different capacities, as long as they are the same chemistry and voltage. Series batteries need to be balanced in charge state at all times. This is easy to maintain with lead and nickel batteries, as they can accept overcharge to balance their charge state. Lithium needs active charge state balance, as they cannot be safely overcharged. In practice this is not a limitation, as chargers that balance, and BMS ICs are now readily available.

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