DC battery vaults

[joshgarcia]

I have two battery vaults. Both vaults have 60 batteries at around 2 volts per cell. Between them is a manual transfer switch that feeds a DC panel which is obviously the normal load. I want to transfer the switch from one vault to the other but this transfer switch is not a make before break switch. so to avoid dropping out DC voltage and possibly taking the plant offline my question is: would it be safe to wire a wire a a disconnect in parallel with the transfer switch, close the disconnect (joining the two vaults parallel), swap the transfer switch, and open the disconnect? Then power would not be interupted. My DC power from the north vault at the switch is 131.3 VDC and the voltage from the south vault at the switch is 133.5. And polarity is lined up.

Thanks.

 

[broadgage]

IMHO, it would probably be in order to parralel two battery sets of the same nominal voltage, and very similar actual voltage, subject of course to the usuall caveats about proper workmanship and listed materials.

Remember that two batteries in paralel will have twice the available short circuit current.
Therefore a check should be made that the OCPD is listed for this higher fault current.
If the two batteries are some distance apart, then the resistance of the interconnecting cables may limit the fault current to not much more than that of a single battery.

I would advise against the permanent parralel operation of two batteries unless these are of the same age, size, and make, and at the same temperature, since unequal charge and discharge is probable.
However in the short term, to facilitate other work, it should be fine.

Will this involve any "hot" work ? if yes then of course extra care is required, and some would prohibit it.
120/130 volts DC is of course less dangerous than higher voltages but can still kill.
The arc flash danger is generally much worse on DC, and requires carefull consideration with even a small battery.
With a large battery even at "only" 120 volts arc flash can be a substantial hazard.

 

[weressl]

I have two battery vaults. Both vaults have 60 batteries at around 2 volts per cell. Between them is a manual transfer switch that feeds a DC panel which is obviously the normal load. I want to transfer the switch from one vault to the other but this transfer switch is not a make before break switch. so to avoid dropping out DC voltage and possibly taking the plant offline my question is: would it be safe to wire a wire a a disconnect in parallel with the transfer switch, close the disconnect (joining the two vaults parallel), swap the transfer switch, and open the disconnect? Then power would not be interupted. My DC power from the north vault at the switch is 131.3 VDC and the voltage from the south vault at the switch is 133.5. And polarity is lined up.

Thanks.

This could be done by make-before-break rotary snapswitch.

Paralleled battery banks with a single common charger is not uncommon. You would have a DC bus and each battery bank could connect to is with it's own disconnect and diode protector.

 

[LarryFine]

... this transfer switch is not a make before break switch.

... would it be safe to wire a wire a a disconnect in parallel with the transfer switch, close the disconnect (joining the two vaults parallel), swap the transfer switch, and open the disconnect? Then power would not be interupted.

If you mean using a switch to connect the two sources together, it still won't avoid a temporary interruption. If the TS breaks before make, there is a moment when the load terminals are not connected to either source.

If you use the switch to connect one battery bank, let's call it 'source A', to the load around the TS, it will work as long as you always close that switch first and open it last. Is this an automatic or manual transfer switch?

What about two such switches, one for each source? Is there any way a short-term UPS could be added? How about a humongous capacitor bank? Are you sure the loads cannot handle such a short-term interruption?

 

[brian john]

Battery systems are routinely bypassed, one way is to back feed the DC bus utilizing a circuit breaker of fused pull out sized to handle the load and back feed this.

NOT SUGGESTING YOU DO THIS.

But companies that SPECIALIZE in DC power systems have technicians and equipment to perform this task.

 

[benaround]

Call the manf. of the TS and see how much they would charge to change it to M < B ?

 

[topgone]

Short answer is YES. Long answer: Revise your DC system such that you don't have to do temporary wirings when you want to transfer DC loads from other battery banks that you have.
One note of caution though: MAKE SURE THE ELECTRICIANS DOING THESE ARE KNOWLEDGEABLE AND GOOD.