[Nut-upsdev] Megatec and Batteries

[Zeljko Baralic]

Hi to all,

 

I am new on the lists but using nut on megatec for last two and half years.

Looking at archives I saw that there is a lot of people confused about battery voltage and I want to help them to clear out some stuff in their heads.

 

This story is based on my experience with UPS devices from 600VA to 20kVA, from single phase systems to three phase systems, with battery packs from 1  battery to 62 batteries in both technologies interactive (aka stand-by) and on-line (aka double-conversion) but all of them were "speaking" megatec. Anyway this does not necessarily relater only to megatec enabled UPSs this is story for all UPSs and their batteries.

 

On one hand we have UPS manufactures which use their own or OEM boards which are the hart of UPS. These electronics have their battery banks which enables them to work when mains (ups power supply) is out of specified value. Battery bank size varies from ups to ups depending on UPS  technology and back up time. How ups electronics sees battery bank is what you get over megatec driver and this is what I am writing about.

 

Battery in UPS is in 99% of cases electro-chemical product made of Pb, Sulphric Acid, Water and some other chemical stuff to give better performance. It's sealed, non-spill able in order to make it safer for manipulation and what is most important its standardized by its physical and electrical characteristics. Battery is usually made out of three or six cells in one standardized package - this is what you know as one battery. This packed battery has two very important electrical characteristics for your UPS: nominal voltage (V) and capacity (Ah). Usually you will meet 6V (3 cell) and 12 V (6 cell) batteries in UPS. This represents nominal voltage of battery.

In 70% of ups devices usually is used battery of 12V/7Ah in some rare cases 12V/9Ah or 6V/4.5Ah.

Depending on ups power rating in output different number of batteries is used and common values are:

 

UPS Technology                               UPS rated power             number of batteries       total battery voltage

Interactive                          600VA                                   1 (12V)                                  12V

Interactive                          1000VA                                 2 (12V)                                  24V

Interactive                          2000VA                                 4 (12V)                                  48V

On-Line                                1000VA                                 3 (12V)                                  36V

On-Line                                2000VA                                 6 (12V)                                  72V

On-Line                                3000VA                                 8 (12V)                                  96V

On-Line                                5000VA                                 20 (12V)                               240V

 

Since battery is electro-chemical element it doesn't have constantly rated voltage. It's voltage is somehow in relation with its charged capacity and this is usually identified by battery manufacturer but normally you can have following values:

 

per cell

6 cell

charged state

1,60 V

9,60 V

permanently destroyed

1,70 V

10,20 V

Empty

1,80 V

10,80 V

	
1,90 V

11,40 V

bat low start 

2,00 V

12,00 V

> 50% charged

2,10 V

12,60 V

	
2,20 V

13,20 V

	
2,30 V

13,80 V

Fully charged

 

Just for info, charging and discharging of batteries are not linear functions in voltage vs. time.

 

When your UPS tells you battery voltage over megatec driver he tells you actually charged value of its battery bank. This information can be told in two ways: as a voltage sum of whole bank or as average value per battery cell. In first case you may get 38.6V in a second case 2.14V for the same on-line 1kVA UPS. This depend on how manufacturer implemented it.

 

This gets me to megatec driver and how it see battery voltage. I think that this is job for someone to improve megatec driver in order to look at some table to see battery charged value or to create it with parameters to driver.

 

Since I think I am becoming boring if you are interested how to determine charge level I can continue writing... but this involves real time clock inside of driver, some counters... lots of work for someone.

 

All best,

 

Željko Baralić

Pakom Computers doo

SST Service Center Nis

Serbia

 

zeljko.baralic at pakom.com

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