People want a fast calculator to help on their custom 18650 battery design, however, since things are complicated with different voltage and capacity of each cell, we think people designing the battery packs should know some basics of lithium 18650 battery design.
18650 Battery packs achieve the desired operating voltage（ie: Total Battery Pack Voltage） by connecting several 18650 cells in series( S in short ); each 18650 cell adds its voltage. Parallel( P in short) connection attains higher capacity by adding up the total ampere-hour (Ah).
to help you further understand how it works, see below explanation:
#1 For the series connection, batteries with same(almost) voltage and capacity are connected to raise the voltage of final battery packs. The positive terminal of the first battery is connected to the negative terminal of a second battery and so on until the desired voltage is reached. The final voltage is the amount of all battery voltages added together while the final capacity(Ah) remains unchanged.
#2 While for parallel connection, batteries with same(almost) voltages and capacities are connected together to increase the capacity of the overall battery pack. The positive terminals of all batteries are connected together, or to a common conductor, and all negative terminals are connected in the same kinds. The final voltage remains unchanged whilst the capacity of the assembly is the sum of all individual cells together for such parallel design.
let’s use Samsung 18650 3.7V 2.6Ah( 2600mAh) for example.
What is the most important factor to consider when designing a 18650 battery pack?
We think the first important factor to consider battery size constraint, size limitation will decide which battery cell to choose from, a 18650 cell is a standard battery cell with 18(C)*65(H)mm in size, people will need to choose lithium polymer battery cell if size limitation of thickness less then 18mm like 10mm.
Is there any way we can calculate a battery pack cost on myself?
Yes and No:
For the Yes part, for battery packs that draw working current less than 5A (like power banks), you can calculate the cost by about 1.5 USD/2.6Ah Chinese 18650 battery cell, 2USD/2.6Ah Korea cells. plus 20%-30% cost (PCM and assembly cost )
For the No part, for battery packs that is quite thin(less than 18mm) or require high discharge current, like ebike or EV, will first need to confirm which cell is the best fit and then check what BMS is required for the battery packs. All are not clear and will result in a big difference in pricing.