I want to make a battery pack of 4S2P using 8cells. Is it better to first make 2 sets of 4 cells each (4 cells joined in series to each other) and then join these 2 sets in parallel or first making 4 sets of 2 cells each (2 cells joined in parallel with each other) and then join the 4 sets in series? And Why?
Electrical – Making a battery pack out of 8 cells in 4S2P configuration
powerpower electronics
Related Solutions
First, I second the purchase (from ebay about $15) of a new headlamp that uses 18650 cells.
There are several one chip handful discrete components that would do the trick. HOWEVER, if you place the electronics at your belt (and even if you don't), there will be a significant current which will loose energy on the wires to your lamp. Most headlamps with larger battery packs, usually either place the pack at the back of the head, or up the voltage and buck the voltage at the light head. Unless you are completely rebuilding the head enclosure, that is probably not practical. Since you are wanting to use more capacity to achieve longer running time, I would place all the batteries in series at your belt and use a buck circuit to bring the voltage down to that compatible with your lamp head. If you are ambitious national & linear tech & microchip all have chips to help in this. If you want a nice circuit board and decent circuit low cost, ebay has numerous China vendors which sell small buck circuits to do exactly what you want.
- Your batteries provide a maximum of 216 watts, and likely significantly less in the Real World.
- Simply connecting batteries in parallel is a tricky thing to do without damage or catastrophe. One method of providing a safe method involves using series diodes and/or resistors to "balance" the current and prevent the batteries from charging/discharging each other. Either series diodes or series resistors will further significantly reduce the power from your batter packs.
- A DC-DC converter will further severely limit the amount of power you have available. You probably won't find any DC-DC converters in any serious electric vehicle design because they are just not efficient enough. Match the battery voltage to the motor by selecting the right battery and/or the right motor.
- Grounding is the least of your concerns. Conventionally, you would select the common side of the circuit (typically the negative bus) and call that "ground" and connect it to the chassis. But it doesn't have any great significance connected or floating.
The three main show stoppers in your design are:
- Your proposed batteries don't provide even HALF the amount of power that your motor wants. You need a much better match between battery and motor.
- Simply connecting batteries in parallel is risky and dangerous. NOT RECOMMENDED without taking proper precautions.
- Using DC-DC converter is not efficient enough for anything but a small toy.
Best Answer
It would be better to wire 2 series' of 4 batteries parallel to each other, rather than series wiring 4 parallel pairs together.
This is because you should add shunt/balancing resistors to each battery (or series of batteries) to ensure that every parallel unit "does its share," rather than one battery/series with slightly lower internal resistance burning itself out by performing (sometimes as high as) 90% of the work.
To balance parallel power supplies like this, figure out your anticipated max. amperage load, then size a resistor to cause ~10% voltage drop at that load; then attach a resistor of that value to each lower supply. This will cause a total voltage drop of ~5% in the shunt resistors (so 95% efficiency remaining) at max. load, and less loss at any lower load.
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