Symmetra PX Battery Bus voltage, how?

[ojfsdi]

I noticed in the web interface of our symmetra px, battery voltage is listed as  214vDC.

Nominal Battery Voltage:	192.2 VDC
Actual Battery Bus Voltage:	214.2 VDC

But each battery back has 8x12v sla batteries.

8*12=96v right?  So how do they come up with 192 or 214v?

96*2=192.  So does this UPS use two battery modules in series to get 192v?  It so, should I keep the A &B Columns even, and c&d columns even to keep it running smoothly?  OR does the UPS monitor what bus needs a boost in voltage the most and use relays to change the DC bus its ok.

[al_buffington]

Hi,

The 214v is the float voltage level. The nominal voltage is 192vdc. As you have surmised the  batteries are connected in series to make 192vdc.  The "AB"  batteries make up the positive string and the CD batteries make up the negative. You can find a detailed description of the Symmetra PX batteries here: https://www.apc.com/us/en/download/document/SPD_AKAR-88TTJW_EN/

[ojfsdi]

Well.  I wish I had known that!  Sure does explain why cartridges that did not have a neighbor were always cold, even if they had been warm somewhere else previously.

It's a real shame that the web interface and front panel don't communicate that at all.  If a C module "fails" then its paired D module should be listed 'unavailable' or something, not merely "ok".

Interesting design choice.  I suppose necessary for their igbt approach.  but I would have at least labeled them A1, A2, B1, B2.  Or +a+b  -a -b.

It sounds like shelves L1-L4 are one unit, and L5-L8 (in an 80kw system) are another, though it's probably of little consequence if they're paralleled together.

Are the bus bars fully rated?  in other words, if I had 1c, 1d, 2c, 2d, 3a, 3b, 4a, 4b all vacant, would the remaining cartridges on the right side of L1 and L2 balance out the left side of L3 and L4 without damaging the bus bar  between L2 and L3?

[al_buffington · Accepted solution]

Hi,

A few additional comments based on your reply

 If a C module "fails" then its paired D module should be listed 'unavailable' or something, not merely "ok".

It actually will do this but it depends on the scenario. There are four ways in which the PX 10-160 will declare a battery as being in a fault condition.

1.  A "wear out" condition. The UPS monitors the midpoint voltage of each battery pair on a shelf (AB / CD) and will declare the pair as being failed if the appropriate criteria is met.  This condition can only be detected when the UPS is running in battery operation.
2.  The UPS monitors the discharge current from each battery pair. If the measured value is below a certain level the UPS will declare the pair as being failed. Each battery module is individually fused. The log files stored within the Intelligence Modules will record this as a blown fuse alarm (though technically we are not monitoring the fuse itself).
3. Overtemperature. Each battery module contains a temperature sensor. If the measured temperature  of any battery module exceeds 140F (60C) then that individual battery will be declared as failed. This is the only scenario where you could see a "B" battery reporting as OK while its pair in the "A" position is reporting as failed. 
4. Mixed battery. Schneider Electric sources the batteries for the PX from multiple vendors. Batteries must be installed in like pairs (A must match B and C must match D). There is a sticker on the front of the battery which will identify the type. Mixing batteries within a pair would not allow for proper fault monitoring on items 1 and 2 above hence the fault declaration. 

I would have at least labeled them A1, A2, B1, B2.

The batteries are labeled in this manner. Here is a screen capture from a Symmetra PX40 NMC. This is also available through  the display in a slightly different format.

It sounds like shelves L1-L4 are one unit, and L5-L8 (in an 80kw system) are another, 

Yes, but not from an electrical perspective.  This is only how the monitoring works. The battery monitoring board present in the UPS can only monitor a maximum of 4 shelves. You'll find one of these boards in each Symmetra PX 40K and then 2 in each external battery cabinet. 

Are the bus bars fully rated?  in other words, if I had 1c, 1d, 2c, 2d, 3a, 3b, 4a, 4b all vacant, would the remaining cartridges on the right side of L1 and L2 balance out the left side of L3 and L4 without damaging the bus bar  between L2 and L3?

The bus bars are rated to handle the full current capacity of the system. By reducing the number of batteries you would be reducing your runtime. The load (while on battery) or the number of power modules (while online - charging current) is what would dictate the current draw. It would not be standard to install the batteries in incomplete shelves. 

[ojfsdi]

Interesting.  Every fault I've seen has been a single battery module.  And they were all, obviously thermal (overheat) related.

Once I've gotten some of the older cells out and new ones in their place I'm going to to a runtime cal, and then schedule a weekly test to suss out any worn out pairs.