18650 pack for 12v with buck

[geo999]

Hi all,

I need your advice for the following situation:
In order to power a camera surveillance system over nigh I've built an 18650 pack in the 7S10P configuration.
from that a DPS3005 (30V max input, 5A max output) buck converter is used to get the voltage down to 12-13v or so.
the DPS3005 needs a minimum of about 13.76v to give 13v output.

for 3.5V min and 4V max per cell we have the following voltages:
 - 7S : 24.5V discharged, 28V charged
 - 6S : 21V    discharged, 24V charged
 - 5S : 17.5V discharged, 20V charged
 - 4S : 14V    discharged, 16V charged - minimum configuration for 13v output of buck.

the question is:
what is the configuration that gives the smallest buck converter loses but still have decent Wh from the pack ?
asking this because the 7S at 28V buck input gives about 5W of loses for 13V @35W of consumption.
Have not yet tested the other configurations, thus the question.

Thank you,
George

[Jwillis]

Best efficiency is when the power supply voltage is closest value  to the load voltage . The 18650 Li ion battery has an optimal voltage of 3.7V  so the best would be 4Series or 14.8V . Each parallel increases Amp hours . An amp hour (Ah) is the amount of current that can be drawn for one hour from 100% charge to 80% charge .  So depending on the mAh of each cell in parallel  added together will give total milliamp hours of the series at 14.8V . Do not mix cells with different mAh ratings . This can led to damage of individual cells .