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I am currently building a robotic arm that runs five motors on two 18650 high discharge batteries. The motors run at 6V (the voltage is regulated.) The stall current is 1A each. (The total maximum intermittent current draw is ~5A.)

I am currently using the two 18650 cells in series to get 7.4V and a voltage regulator to supply 6V to the motors. It has been suggested that connecting the cells in parallel and boosting the voltage might be better. I am confused as I am under the impression that boost converting is usually less efficient than buck converting which is what I'm doing currently.

Most conversations I read point out the fact that the choice of series vs parallel+boost is very situation dependent, so I would like to know what reasons would make parallel+boost better for this scenario if any.

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    \$\begingroup\$ 7.4V is nominal voltage. It doesn't remain constant at 7.4V. As the battery discharges the voltage varies from 8.4V to 5.6V. Your 6V buck converter may not be able to fully discharge the battery \$\endgroup\$
    – across
    Commented Oct 22 at 12:37
  • \$\begingroup\$ What sort of voltage regulator topology are you currently using? \$\endgroup\$
    – Andy aka
    Commented Oct 22 at 13:25
  • \$\begingroup\$ Aim for all boost range, i.e. singe cell (1S) or all buck, i.e. 3S. Your average boost without output disconnect won’t turn off your motors unless you have external means for it. \$\endgroup\$
    – winny
    Commented Oct 22 at 13:28

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Good boost converters are more eff. (98%)

The parallel cells prevents early cell death on the slightly weaker cell as weaker cells tend towards over and undercharge. Matched cell voltages is very important, even if the ESR's are slightly mismatched for current sharing.

In series cells, a typical BMS can correct 10% power cell mismatch error by adding more burden to the stronger cells to match the voltages.

For large arrays, both series and parallel are used with BMS to extend battery life.

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  • \$\begingroup\$ Thanks, In your opinion which one would be the better option? I am still leaning towards the series cell config since 100% efficiency is better than 95% efficiency, especially since the gap from 3.7 to 6 is more than the gap from 6 to 7.4 and i feel like less gap in regulation is more optimal. \$\endgroup\$
    – Shakthi Sairam
    Commented Oct 22 at 14:25
  • \$\begingroup\$ Buck converters are less efficient by at least1~2%, some much more, especially as Vin approaches Vout. But your only 100% efficient method is no converter at all, and current limited using PWM by design unless you want the torque and heat rise in the windings. BLDC Motors are generally thermally rated = 10% of stall current and 10x no load current. So you can use 7.4V, just limit the current by design. But this ignores the series cell issues of mismatch runaway, an exponential condition of aging and C stress ratios. If you want high efficiency avoid rapid charge/discharge I^2*ESR losses \$\endgroup\$
    – D.A.S.
    Commented Oct 24 at 1:18

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