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I'm faced with a problem and looking for advice.

I'm planning on making a 5V battery pack from 18650 batteries (6 of them). I have no idea witch connection is better for this specific purpose.

I came up with these options: 3S2P; 2S3P; 6S. I'd like to get a BMS for charging/discharging. I will be using DC-DC converter, either step-up or step-down, depending on the battery pack voltage. Primary target is capacity, then everything else.

Any advice?

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    \$\begingroup\$ How any of these configurations is giving you 5V ? \$\endgroup\$ – Eugene Sh. May 7 '18 at 18:12
  • \$\begingroup\$ They're not, sorry forgot to mention I'll be using a DC-DC converter. \$\endgroup\$ – pauliucxz May 7 '18 at 19:40
  • \$\begingroup\$ You can’t design any power source and less you have specifications for min max voltage and min max power and efficiency but I would lean towards 2S3P step down for high current since 2S balances are common if using protected cells other wise add 10A PTC’s or micro fuses depending on use. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 7 '18 at 20:22
  • \$\begingroup\$ Best advice: Buy a battery pack from a reputable source. Avoid no name batteries and chargers. See also: batteryuniversity.com/learn/article/… \$\endgroup\$ – Misunderstood May 10 '18 at 1:53
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You can use even 1S battery if you use DC-DC upconverter like NCP1402SN50T1G and you'll get 5V from it. Just make sure that your converter of choiсe can handle the current. Or you can use a linear low dropout voltage regulator like MC33269 to get 5V from 2S battery (or more series batteries) but I wouldn't recommend using more than 2S battery since the conversion efficiency will reduce. When choosing a linear regulator, make sure it accepts voltages down to 7.2V since minimum voltage a single li-ion battery can give is 3.6V and (again) that it meets your current consumption requirements. And don't forget the heatsink if you're planning to power something like RPi.

UPD

Now I see, you need maximum battery life. In that case, you'll need maximum conversion efficiency. Maximum efficiency can be reached with properly chosen DC-DC converter, so your question narrows down to whether use a step up or step down converter (1S or 2S battery respectively). In general, step-down converters are more efficient than step-up but would you give up the ease of 1S charging and handling for a few percents of efficiency? LTC3441 or TPS63002 converters, for example, can reach 92-93% efficiency in step-up mode if your device draws 300-500 mA, it's a very good performance.

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    \$\begingroup\$ Good point on 1S battery. When I was looking for compact rechargeable 7V power I started researching all those "BMS with balancing" modules. Then I realized that using 1 cell with DC-DC will be much simpler and safer option. Not necessarily a good fit for the OP, of course, depending on current/capacity requirements. \$\endgroup\$ – Maple May 7 '18 at 19:05
  • \$\begingroup\$ From advices given, I'm leaning towards 2S3P, or should I just use 2S1P? Size and portability isn't necessary. \$\endgroup\$ – pauliucxz May 7 '18 at 20:25
  • \$\begingroup\$ @pauliucxz what is your maximum current? How much time do you need your device to work? \$\endgroup\$ – Archimedes May 7 '18 at 20:26
  • \$\begingroup\$ Maximum and constant current of 2A, I need for as long as possible \$\endgroup\$ – pauliucxz May 7 '18 at 20:27
  • \$\begingroup\$ @pauliucxz I personally don't really see the point of using a 2S battery at this current. If you need maximum time then just go with maximum capacity you can afford. Regarding 1P or 3P, it doesn't really matter. Using 3 cells is better in terms of reliability and in some cases more efficient enclosure space usage. And if those are 18650 cells, they are all same size, so you'll get more working time for 3 cells obviously. \$\endgroup\$ – Archimedes May 7 '18 at 20:32

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