Large Off-Grid- 192 x 2V cells in series

lazza · April 2018

HI

I'd like to ask the forum a technical question about having so many 2V FLA batteries in series.

There are new solutions for PV for large scale users farms, say 60 or 100kW, by existing UPS companies like Riello. They consist of an all-integrated battery charger, PV grid-tie inverter etc.

It sounds like a great option compared to conventional large-scale off-grid at 48V such as 6 or 9 off-grid inverters along with some grid-tie inverters controlled by frequency shift. These conventional systems I find get very clumsy once you go over 45kW.

However, these new systems use high voltage DC. i.e. the 2V FLA batteries are in one single series of 192 cells! This has the obvious advantage of maintaining low currents, but cries out to me as being problematic in terms of charge equalization between cells.

What are the forum's thoughts on these solutions? Are so cells in series an issue? Any direct experience with such systems?

Thanks
Lazza

mike95490 · April 2018

Because of the voltage and danger in a nearly 400V DC system, I'm not going in the building. Even dripping water into the batteries could be deadly.

Cell balancing, 24 cells or 200 cells, the same principal applies

BB. · April 2018

My concerns with large numbers of series batteries (any chemistry) is that (pretty much) all rechargeable batteries have failure modes that happen (shorted/open cells, over charging, over heating, over discharging, reverse charging, etc.).

When you have a 12 volt (lead acid) battery with 2 volt cells--One dead cell (shorted, zero charge, etc.0, and you are looking at 10.x volts and the obvious inverter/DC loads failure.

Put 192 cells in series... The difference between 384 volts and 382 volts--Nothing... The battery bank and loads/chargers will operate "normally" until something happens... Cell melts down, catches fire, explodes, etc...

For a bank that large--At the very least I would like to see a voltage monitor for ever 6 cells (12 volts) or so... If any of the 12 volt "blocks" (aka batteries) "goes" out of balance (unequal voltage within a few 1/10s of volts), you have an immediate alarm to alert that there is maintenance required before something goes bad.

But I do not see many (if any) systems like this for Lead Acid battery systems (unlike the many you see for various Li Ion battery banks).

-Bill

lazza · April 2018

Thanks for the replies. Am I right in assuming that Li Ion doesnt have the same issues because it already has the BMS integrated?

westbranch · April 2018

Bill, probably something like these Liebert UPS's....625 -1200 KVA
.https://www.vertivco.com/globalassets/products/critical-power/uninterruptible-power-supplies-ups/liebert-exls1_00.pdf

BB. · April 2018

BMS for Li Ion batteries (many high energy density chemistries) pretty much require some version to reduce the very real risk of fire and explosions if you overcharge or take cells dead (does it approach 100% chance of dangerous failure of rules are not followed? I do not know).

Lead acid batteries can fail with fire or explosions, but it is less common. Usually the cells just stop working (open or shorted).

Also for home applications (off grid systems), generally we only have 6 to 24 large AH lead acid cells in series, vs the 192 small AHcells or so you see with electric cars and other Li Ion high density banks.

Flooded cell lead acid cells can be overcharged safely. They simply gas a bit of hydrogen and oxygen.

There are other Li Ion chemistries like LiFePO4 that are a bit less energy dense and have less spectacular failure modes. This are good options for off grid systems where weight and size of batteries is not a huge issue. Plus many folks run these cells without BMS. Just monitor cell voltages with a hand held Meter. And a hard cutoff for high or low bank voltage (to prevent cell damage and loses).

Bill

Large Off-Grid- 192 x 2V cells in series

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