Which would to be more efficient? In the end I need 14 volts. Am I better off stepping down a higher voltage for stepping up a slightly lower voltage?

I basically have a bunch of 12-volt UPS backups which originally ran off of sealed lead acid batteries, and I have hundreds of 18650s I have acquired to make a large powerwall for backup power.

So my plan is to make a bunch of 18650 battery packs to power each UPS system. I understand I cannot charge the battery packs through the ups. Each battery pack will have a BMS of course.

So would I be better off building 3s battery packs and stepping the voltage up to 14 or building 16 Volt or larger battery packs and stepping the voltage down to 14 volts?


Now keep in mind, I do understand there is better ways of accomplishing this but again I have everything needed to make this happen. (temp control, power monitoring, BMS's) But due to the ongoing crisis and being out of work, I do not have the money to buy a large 3000 or 5000 watt inverter. So for the time being I want to throw this together and in the future can change things up.

 

*** I just came across this forum post and pretty much answers my question. But I'm still curious on your opinions...

Here is what I found:

Boost converters are typically less efficient than Buck converters, but not by much. The fundamental reason has to do with the inductor current flowing directly to ground during the on-time, instead of through the load, like it does on Buck converters. There is an EE-Times article that mentions this: Unscrambling the power losses in switching boost converters

Generally, however, since inductor current flows to ground during the on time, only a fraction (off time to period ratio) flows to the output, as illustrated by the pulsing currents in Figure 2 (this is the reason why boost converters are generally less power efficient than buck converters)

Since the efficiency differences are not significant, you are probably better served deciding on additional criteria instead of regulator efficiency alone, including:

• Charger complexity (single cell is simpler).
• Cost
• Size
• Cells in series will be limited by their weakest cell.
• Cells in parallel tend to charge one another, and there is an efficiency hit due to the chemical process.
• Losses due to higher input current with lower voltages (parallel cells).

 

I doubt it matters much, I would likely buck rather than boost. Something to consider is the current. My UPS units have a pair of 12 Volt SLA batteries for 24 Volts. They are 1500 Watt units. Without even considering any efficiency 1500 watts at 24 Volts is 62 Amps, at 12 Volts that becomes over a 120 Amp current draw under a full load. Just a 1.0 KW true power would be 1,000 / 12 = 83 amps. Drop that down to 500 watts and you are still drawing over 40 amps. That does not consider inverter efficiency.

Ron