300vdc battery string charging

[Madcow]

What are some of the pit falls of using a 300v string
of 12v vrla batteries? Should I sense every battery or
every 48v increment for inbalence? I've heard there are
alot of things to consider befor using a high voltage string.
Any help would be appreciated.

 

[DanEE]

Large strings of 12v batteries that you ask about are common place in large 480VAC three phase UPS systems. e.g. 33 x 12v strings for 396vdc are common.

Given most 12v batteries are typically 3yr batteries (in a few cases 10yr max) the replacement cycle comes around fairly often..

The normal measurements made at installation and 6 month maintenance intervals should catch any batteries beginning to drift out of balance. They tend to stay pretty well grouped voltage wise, unless a defective one somehow snuck in, and that will be evident in the measurements..

While additional measures you mention might be ideal, a practical payoff (in longer battery life) is doubtful.

As they approach end of life, they will start drifting apart, but that should be your indicator that it's time to replace the whole lot.

 

[Madcow]

I read some where that this drift or a shorted cell can cause thermal run away if constant voltage charging is used.
I was planning on thermal sensing the batteries and terminating the charge at 50 degrees C buit thats going to be alot of sensors. Is there a prefered method to charging the high voltage strings? Any saftey issues. Does the case of the batteries have to be isolated from ground?
Yes the app. is a 3 phase ups.
Thanks