DC Voltage for Safe Batteries Banks

Risk is relative to the training and ability to focus involved.

At 50 KWH battery capacity I would be pushing for the upper end of the DC working voltage range to reduce the size of wiring.

In the USA we have NFPA 70E which provides guidelines and safety related information on electrical installations and maintenance.

As far as Arc-Flash exposure and personnel electrical contact risks are concerned the cutoff voltage for both AC and DC is 50 volts.

Working on any voltage equal-to or greater-than 50V requires specific personal protective equipment and clothing.

The "Bell Standard" for reliable DC system voltage is 48VDC because any equipment operating at less than 48V suffers severe issues when exposed to minor impedances caused by dirt, oxidization, and moisture.

As already stated the reason for higher electrical system voltages whether AC or DC is the decrease in conductor and equipment physical size.

I recommend you obtain a copy of NFPA 70E and review it thoroughly to help you in your decision especially if the project is in the USA.

If it is in any country other than the USA I recommend you identify what codes, regulations, and laws pertain to the installation and follow them closely.

As others have stated, 48V has been the most used because of PPE, training and responsible personnel for safe operation above that. These systems have grown exponentially in use and higher voltages are now used but only by qualified personnel operating them. 50kwh is going to take up a lot of space, especially if lad-acid is used.

Contact corporations which would/do use these systems. Telephone communication companies are a good place to start but they are very secretive.

Contact manufacturers/suppliers of this equipment. You can get a lot of information during their sales pitch and you can ask them the questions you have. Try:

http://www.staticpower.com/industrial.html

http://www.alpinepowersystems.com/Products-Services/Backup-Power/Backup-Power-Products.aspx

http://www.smartpowersystems.com/

https://powerquality.eaton.com/Products-Services/backup-power-ups/Marine/Default.asp?cx=3

http://www.winkleindustries.com/content/products/electrical/batt-backup.html

http://www.panduit.com/en/home

Why reinvent the wheel when others have done it for you just for the asking?

Good Luck, Old Salt

It's going to take a complete system study to help you decide, you can't just look at voltage and/or current and make your decision. The thought that raising the voltage to lower the current so as to limit the available short circuit current is true, but the amount of energy available doesn't change, 50kW is 50kW whether it's 50Volts times 1,000Amps or 50Amps times 1,000Volts.

From a safety aspect you're going to conclude that physical isolation is the key to minimizing your risk to personnel; then there's a whole host of other concerns such balancing the charge/discharge currents, the physical size of the conductors, managing all those interconnections, protecting the whole thing from a fault somewhere, switching that power level, monitoring/instrumenting the system, venting the hydrogen, etc., etc.

In effect this project is no different than interconnecting a huge field of PV panels and managing the power fed into the grid. You might also want to research what Fredski's buddy Elon is doing with his peak-shavers.

To answer the highest voltage question you might want to base your work on the US Navy Submarine Designs where the peak voltage of a fully charged battery was 385Volts, I'm sure Andy will jump in with some help on how the Royal Navy did it as well.

While I am not sure of the standards involved, I can relay some observations from my own experience, here at work. One of my old programs, used a 30KVA Liebherr UPS. 3phase 120/208V Y output. It used around 192VDC. 16 - 6 cell AGM batteries in series. My current program has a 65KVA MGE UPS system. 480V 3 phase input, 208V three phase out. It uses 216 cells in a single string. 432V nom. The voltages, they chose, may have just been for ease of conversion, rather than any other standards, though. There are ARC flash requirements involved in working with these units.

An option to consider is -48V. One of the systems, that I work on, works at a nominal -48V. That is primarily because the system uses a Bell standard PABX system. -48V systems are a very mature technology. It has been the standard US telephone voltage for decades. Consequently, there is a ton of expertise, data and equipment out there, that is designed for it.

Take a look at battery powered fork lifts.

Their batteries are composed of single-cells,series connected externally to get 48volts.

The cells are huge and heavy,but most economical in terms of maintenance and replacement.

Look at their charging and control and protection schemes for guidance.

They have already done the engineering for these high KWH storage capacity type systems.