Battery Combiner OCPD Necessary?

[PWDickerson]

With L-ion and LFP batteries becoming more popular, I have been thinking about the need to provide fusing for individual battery strings similar to how PV string combiners are fused at a DC combiner box. One manufacturer of a 3.4 kWh, 48V LFP battery states that you can place as many of their batteries in parallel as you need, and that no other OCPD would be required other than the breaker built into each battery unit. I just watched a video where an employee of the product manufacturer stated that many paralleled batteries can be combined together at a DC bus without fusing at the bus, I have talked with tech support at the same company and was told this is generally how it is done in the field.

I know that lots of folks think that paralleling multiple battery strings is a bad idea even for LFP chemistries, but I want to avoid that argument here. I am primarily interested in finding out how others have installed this type of equipment and why you do or do not think individual parallel battery strings should be fused at the DC bus.

I think fusing the strings at the bus should be necessary for the same reason that PV string fusing is required at the DC combiner. A short to ground in a battery cable will have current driven to it by the non-shorted batteries, and the short may not conduct enough current to trip the breakers on board the dozen or so non-shorted batteries in the system. A fuse at the bus side of the shorted cable would trip immediately.

 

[Adamjamma]

when you parallel you are just increasing the storage capacity, not the voltage. Series increases voltage. If a cell dies in a battery, it decreases the storage of that group of batteries that are parallelled. If that group is in series with another group, it will drop the voltage slightly but will be dependant upon the number of cells in the series totals.

So, all you are creating by parallelling batteries is technically the available amperage, not voltage.

Solar panels can be run in series and in parallell but are usually running in series so they can provide higher voltages, then in parallel to raise the amps, and theoretically the voltage can exceed the operating parameters of the equipment.. which is why they get fused often times. But batteries are different.. they only accept so much at a time before the internals act up and they only put out so much at a time before the internals act up, so they in effect have built in limiters. You actually fuse them to keep wire safe on occasion... but I would not be worried about a runaway scenario on batteries as they actually do not generate power like solar systems or generators do.

 

[drcampbell]

I suppose it depends on the specific characteristics. If there are only a few batteries in parallel, they can deliver only double (?) the current the cable can tolerate, and the failure mode is the cable slowly overheating while the batteries become depleted, perhaps not. But if there are many batteries, a very-low-impedance buss and the failure mode is the cable detonating in a spectacular arc-flash event, fuses are definitely called for.

Cable-limiter fuses exist for precisely this application: connecting a cable to a buss that has much more current available than the cable can tolerate.
https://m.littelfuse.com/~/media/el...s/littelfuse_cable_limiter_lfcl_datasheet.pdf

I witnessed an event similar to the one you're contemplating. In the early days of VFDs, (1980s) we assembled a capacitor bank. I don't remember the specific parameters, but it was about 700 VDC (full-wave rectifryer on a 480-VAC supply) and about fifty of the biggest aluminum electrolytic capacitors available. They were about a liter each, the whole assembly was the size of a coffee table and the caps were interconnected by ~5x15 mm copper bussbars. One cap failed, all the other caps dumped their energy into the failed cap, and the result was akin to a bomb going off. After the fact, the failed cap looked like one of Doc Edgerton's high-speed photographs of a rifle bullet passing through an apple, except there was no entry wound.

 

[jaggedben]

Besides breakers, do these battery units have other internal controls? Or are they just unregulated cells? I think it makes a significant difference.

I've only faced this issue with a maximum of 2 high voltage LG RESU batteries. These have internal DC-to DC converters as well as breakers, and I believe the inverter would shut them down if there were a ground fault. I didn't think a fused combiner was necessary and LG confirmed they didn't require one. Nonetheless I sized all wiring for the combined output of both units, following the same principles as for PV panels, and to be on the good side of the general requirements of Article 240.

But my situation doesn't really speak to the issues you might have with many more than two outputs, and/or unregulated cell discharge through a fault.

 

[PWDickerson]

JaggedBen, the battery unit I had in mind is the Simpliphi, Phi 3.4. Each unit has its own BMS and 80A breaker. The manufacturer recommends paralleling multiple units and specifically prohibits series connecting their batteries. They have a 24V model, and a 48V model, and they are meant to be a drop-in replacement for a lead-acid battery bank. The 48V unit has a max short circuit current of 1260A. The Discovery AES is another similar product with a larger capacity and form factor. The Simpliphi and Discovery units are both UL listed. I am sure there are other similar products available as well.

drcampbell, thanks for comment regarding cable limiters. I do not have experience with these. I did some research, and it sounds like this would be a good application for their use. I couldn't find any that were DC rated though. I wonder if they exist. I will keep looking.

 

[drcampbell]

... I couldn't find any (cable limiters) that were DC rated ...

I would expect that a fuse designed, tested, rated & listed for 600 VAC would be quite suitable for use at 48 VDC. Maybe if you contacted an application engineer at the manufacturer, you could get an official letter of approval for 48 VDC. And maybe if you persuade them that they would sell a lot of them for this application, they might go ahead and rate, mark & list them for DC. And with or without the manufacturer's blessing, maybe you could get an approval letter from you AHJ.

Class CC-EV fuses were rated, marked & listed for DC use. If they still exist, they'd be a good option if their DC AIC rating is adequate.
("EV" is an abbreviation for "electric vehicle")

 

[PWDickerson]

Sourcing DC rated fuses is not really the issue. The advantage of the cable limiters is that their form factor could potentially allow the elimination of an expensive DC panelboard. Because they are made to essentially replace cable lugs, they could be used at a low cost copper bus bar and eliminate the need for a bunch of expensive hardware... if they were rated for the application. I agree that they would certainly be suitable for the application, but a picky inspector might not approve their use.

 

[pv_n00b]

Two things come to mind. PV strings are fused mainly to protect the modules from internal faults. One of the UL 1701 tests is to show that a module under a given amount of back feed current will not exceed a specific temperature. The series fuse rating limits the current to less than or equal to the test current. While that will also protect a properly sized string conductor it was not the intended purpose. Batteries do not have the same restrictions as PV modules.

Second is that the conductors inside a battery bank can be considered kind of like tap conductors. A tap conductor can be exposed to fault current above its rating but that is allowed by the tap rules under certain conditions. A battery bank conductor can be exposed to fault current it is not rated for but that is allowed by the code. What the code does do is require protection of the DC conductors from overload when they leave the battery bank. This protects the battery bank conductors from overload but not fault current. Since the conductors inside a battery bank are fairly well protected along with the battery it's considered a reasonable increased risk.

 

[jaggedben]

...

Second is that the conductors inside a battery bank can be considered kind of like tap conductors. A tap conductor can be exposed to fault current above its rating but that is allowed by the tap rules under certain conditions. A battery bank conductor can be exposed to fault current it is not rated for but that is allowed by the code. What the code does do is require protection of the DC conductors from overload when they leave the battery bank. This protects the battery bank conductors from overload but not fault current. Since the conductors inside a battery bank are fairly well protected along with the battery it's considered a reasonable increased risk.

Okay, but what about conductors in raceways connecting two battery units? Last one I did had about 20ft of EMT in a garage.

 

[PWDickerson]

I get that unfused battery conductors are subject to high fault currents that exceed their rating, and I try to design battery systems with fuses located as close to the battery terminals as possible to mitigate the risk. The point of this post is to consider a different risk that I had not really considered prior to designing systems with many parallel 48V batteries. The fact that each of the battery in question has a built in 80A breaker eliminates the risk of a fault in an unfused conductor, but when many such batteries are bussed together, a whole new (to me anyway) issue arises. I spent some time yesterday calculating the short circuit current in a 15-battery system with 20' #6 battery conductors. The 14 non-shorted strings would each supply about 150 amps to the shorted string. The non-shorted batteries would probably take 10-15 seconds for the breakers to trip given a typical response time. Until that happens, you would have over 2100 amps passing through a single #6 wire. That would do a lot of damage.

 

[pv_n00b]

Okay, but what about conductors in raceways connecting two battery units? Last one I did had about 20ft of EMT in a garage.

I would consider that two separate battery banks connected together, and those connecting conductors leave their respective battery banks and need to be protected at the point they leave each bank. The unprotected conductor allowance only applies inside a battery bank enclosure. I can't have a site with 20 different interconnected battery bank locations around the campus and say it's really one battery bank.

 

[pv_n00b]

I get that unfused battery conductors are subject to high fault currents that exceed their rating, and I try to design battery systems with fuses located as close to the battery terminals as possible to mitigate the risk. The point of this post is to consider a different risk that I had not really considered prior to designing systems with many parallel 48V batteries. The fact that each of the battery in question has a built in 80A breaker eliminates the risk of a fault in an unfused conductor, but when many such batteries are bussed together, a whole new (to me anyway) issue arises. I spent some time yesterday calculating the short circuit current in a 15-battery system with 20' #6 battery conductors. The 14 non-shorted strings would each supply about 150 amps to the shorted string. The non-shorted batteries would probably take 10-15 seconds for the breakers to trip given a typical response time. Until that happens, you would have over 2100 amps passing through a single #6 wire. That would do a lot of damage.

The 150A output for a shorted string seems very low since batteries are usually great sources of fault current. I looked over the battery data sheet and I did not find a fault current rating for this battery. Usually, there is enough fault current to trip the CB faster than that.

 

[PWDickerson]

The 150A output for a shorted string seems very low since batteries are usually great sources of fault current. I looked over the battery data sheet and I did not find a fault current rating for this battery. Usually, there is enough fault current to trip the CB faster than that.

The battery side of the shorted string conductor will experience up to 1260 amps, which is the battery's short circuit current rating (in the manual, but not in the spec sheet), and will immediately trip the battery's breaker. However, the bus side of the shorted string conductor will receive current through the bus from the rest of the batteries in the system, and the short circuit current will be evenly divided between them. If the string conductor is long enough and only sized for the 30-40 or so amps that it will carry, it might limit the short circuit current within it to a value that would not immediately trip the breakers in the other batteries.

 

[pv_n00b]

The battery side of the shorted string conductor will experience up to 1260 amps, which is the battery's short circuit current rating (in the manual, but not in the spec sheet), and will immediately trip the battery's breaker. However, the bus side of the shorted string conductor will receive current through the bus from the rest of the batteries in the system, and the short circuit current will be evenly divided between them. If the string conductor is long enough and only sized for the 30-40 or so amps that it will carry, it might limit the short circuit current within it to a value that would not immediately trip the breakers in the other batteries.

At 160A the 80A breaker will trip in a few seconds unless they are some strange slo-blow CB. But it comes back to the same reason, protected conductors inside battery banks are considered relatively safe from faulting. There is nothing stopping you from putting in more fuses if you want, just like there is nothing stopping someone from putting in fuses when doing a feeder tap. The NEC just says that you don't have to. If it really bugs you that the conductors are unprotected then fuse away.

 

[PWDickerson]

View attachment Fused Battery Combiner1.pdf

Here is a simple schematic showing the issue. The fuse on the battery end of the shorted conductor will blow eliminating current from the battery to the short. The other 23 batteries that aren't shown will supply current to the short through the bus. The fuse at the bus will protect the "bus side" of the shorted conductor. If there were no fuses at the positive bus, the other 23 battery breakers could supply 1840A (23 x 80) to the conductor without ever blowing a fuse. The resistance in the wire would limit the short circuit current to approximately this amount

In this type of installation, all the battery conductors need to be the same length to keep the current the same in each battery circuit, and so the battery conductors tend to be longer than one might expect. The are typically routed through gutters/conduits, and are not all within a single enclosure.

As I said in my original post, I don't understand why fusing wouldn't be required in this case when it is required in a PV DC combiner panel in order to protect a shorted string from current supplied by the other strings connected to the bus. Seems like the same thing.