Battery Combining and More

In this webinar, we will discuss a few mounting options, but understand that you also have the flexibility to use the T-channel built into the eFlex battery itself for custom application.  The eFlex was designed around this M6 hammer nut and screw. The hammer nut fits into the two T-slot channels built into the back of each eFlex, to assist mounting to a wall or shelf. If you forget the size of the T-bolt, it can be found in the eFlex installation manual.

The size of the ring terminal for the eFlex can be found in the installation manual and on the specification sheet. The ring terminal size we recommend is 3/8ths.

As far as the cable sizing, each eFlex battery is recommended to be charged at 55A and discharged at 60A. Less frequently, the eFlex can be discharged up to 100A.

eFlex are wired together in parallel, and in parallel, amperage adds up – so for two eFlex in parallel, the recommended discharge rate doubles to 110A with infrequent discharges up to 200A being possible (as well as greater inrush current capability).

During normal system operation, the inverter DC amperage is split between the batteries. For example if two eFlex are on an 8 kW inverter, to determine the DC amperage we divide by the battery DC nominal voltage rating of 51.2V, and so 8000W divided  by 51.2V is 156A.  Divided by two, means that each eFlex will see a maximum continuous amperage is 78A per eFlex. Normally this would be an undersized battery, being out of continuous operating range for the eFlex which could trigger a reduced warranty. So in this configuration, it is important to select a closed loop communication inverter (such as Schneider or SolArk) to keep within the product warranty. Installers may always email techsupport@fortresspower.com for a quick design review.


NEC requires the cable to carry at least 25% more amperage than this (if no other factors call for additional increases in ampacity), and so the cables to each eFlex need to be sized for at least 98A. If combining using a 75C power distribution block, the minimum cable size is #3.

However, it is reasonable to increase the battery cable sizes further, sizing off the battery capability rather than the load. This ensures that if the load ever increases, or if a design mistake occurs by putting to large an inverter on too small a battery, that the conductors are rated appropriately.  The eFlex can deliver 100A per battery, and in the event that one battery is disconnected and the other is working, the battery could deliver 100A. Sizing the cable for 100A using a 75C terminal requires #1 battery cable.

Let’s not get into balance of system material just yet. First, let’s learn a little more about site layout with the eFlex.

First, let’s start with a floor mount. It sounds easy, and it is, but for more than one reason.

The eFlex has a IP65 frame, which means it can take direct water spray, and is sealed against dust and humidity. This does not mean it can be submerged, and the battery terminals need protection from rain as well. But the eFlex can be placed in a dirty, humid, outdoor environment such as a barn or garage and its protective enclosure means you do not need to be as concerned about the durability proofing of the box.

The eFlex can be mounted directly on the floor. Most batteries should not be set on concrete. However, the eFlex has an insulated base that provides protection against conductive wet concrete as well as providing insulation against thermal exchange with the slab. It is a battery for rugged environment.

In extremely cold environments such as Canada it may be necessary to build heated box for the battery. A box may also be built for physical protection in a multi-purpose area. But remember the eFlex is already in a robust case, so the box built around the eFlex can be simple.

Many installers put their eFlex batteries up against the wall. This can be space efficient, but keep in mind that in garage locations, the batteries should not be installed where they can be struck by a car or truck.

The floor mount bracket, included with each eFlex, secures the battery to the wall. This wall mount bracket must be used, because it prevents the battery from accidentally tipping over.

Sometimes it is more desirable to have the eFlex stick out of the wall, which makes the overall layout more compact and reduces battery cable length. But if the eFlex is to stick out from the wall, such as when contained in a box, or even when used outside a box, then how does one use secure the mounting bracket to a wall?

At this point, be creative. A 2×4 could be mounted to the wall, and then solar L-foot plus lag screw left over from the solar array could be used to make the required 90 degree attachment point to land on the floor mount bracket. It only needs to be secure enough to prevent the eFlex from tipping if subject to accidental contact.

When mounting in this orientation, it might make sense to buy angled compression ring terminals. Here is a picture of a ring terminal with a 90 degree bend built into the terminal. This allows the cable to hug the wall in a clean manner, while reducing the cable length.

The only ground mount orientation we do not allow is with the terminals closest to the ground as shown. Be careful – this disallowed orientation also applies when the eFlex is on the wall.

The eFlex Wall mount is another popular option for keeping the eFlex out of the way and protected, as well as reducing the cable sizes as much as possible. The end result is a very professional look. The wall mount kit is an additional accessory, but it is inexpensive and will save money on the battery cable budget. Because the inverter is typically wall mounted, it makes sense to wall mount eFlex batteries.

The eFlex can be mounted on the wall in an upright orientation, but also upside down. It can only mount sideways in one orientation – like the floor mount the orientation with the terminals on the bottom side cannot be used.

When mounting on the wall, the vertical spacing between the wall mount brackets is flexible, only constrained by the distance of the T-slot on the eFlex itself. If the eFlex is mounted on its side, the spacing between wall mount brackets should be the spacing between the eFlex t-channels.

When mounting on a wall, it is easy to transition from the battery into a box. This additional protection is not required by code, but most eFlex installations require multiple eFlex which need to be combined ahead of the inverter. The transition into the box requires a strain relief connector.

Battery cables are commonly located within the same room as the inverter, often less than 5 feet away from the inverter. They are not required to be in conduit – although an inspector may want to see additional physical protection based on the use of the room. Keeping exposed cables short and tidy on a wall will make your inspector happy.

Strain relief takes tension out of the cable which might otherwise stress the battery terminals, resulting in a poor terminal connection or even worse, the cable pulling out of the crimp or terminal creating a safety hazard. When cables transition into a box, a strain relief connector is used to suppress tension in the cable.

Another strain relief strategy is to use fine stranded cable. Fine stranded cable has an additional advantage of having a smaller bending radius than regular stranded cable which allows it to bend inside a box with ease.

The use of 3/8ths compression ring terminal, a strain relief connector, and a durable fine stranded cable is a great way to transition from the eFlex wall mount into a gutter before transitioning to the inverter.

We will talk more about battery combining strategy at the end, but for now let’s look at eFlex layout with two eFlex on the wall and a 48V residential battery inverter. This configuration can easily fit on the back wall of a garage, and only requires a few feet of battery cable. It also looks good and is well protected. In areas prone to flooding, wall mounting the system above flood level will dramatically improve the value of the system to the user.

The last eFlex mounting option is shelf mounting or rack mounting. It’s called both shelf and rack mounting, because the eFlex was designed to fit onto a server rack – but rather than attach directly to the rack, it rests on a shelf that is attached to the rack. The server rack bracket is then used to secure the battery to the shelf.

Server racks are not cheap, but can open up some interesting applications. An open server rack may nearly organize a large number of eFlex in a compact, cost-effective manner. An enclosed server rack costs more, but is a good application to protect the0 eFlex against accidental contact in a multi-purpose room.

A server rack may be small, containing a single eFlex and a small rack mounted inverter to protect a refrigerator, internet equipment, and some LED lights while tucked out of the way on a wall in a kitchen. Larger server racks are used in commercial installations inside shipping containers supplying power to large buildings.

It is not necessary to use a server rack to shelf mount an eFlex, although the eFlex should still be secured to the shelf in some manner and the shelf must be rated for the load.

Let’s end the discussion by talking about some balance of system material for battery combining. Almost all eFlex applications will require at least 2 eFlex, and so combining eFlex together is a necessary task.

Daisy chaining eFlex power cables is not recommended. The result is uneven cable lengths between the battery and the inverter – these differences can result in shorter battery life or even trigger the battery management system shutdown. So the last step in eFlex design is how to combine the eFlex in a cost-effective manner.

There are multiple ways to do this, but a metal gutter fits well between the eFlex and the inverter, and if it is large enough, such as an 8” x 8” square tube gutter, all the material necessary for battery combining can be installed inside.

This device is called a power distribution block. It is UL-listed, commonly rated for 75C. To find the right sized power distribution block, one must identify the size of the inverter cable, DC amperage, and the size of the battery cable. It must also be listed for fine stranded wire.

Most power distribution blocks require ferrules with fine stranded wire.  Ferrules are larger than the wire sizes which they contain – a ferrule for #1 wire has a diameter about equal to 3/0 wire.  After additional research, a power distribution block rated for fine stranded wires is found, and it is a simple matter of seeing if the inverter cable and battery cables are appropriately sized. Alternately, a power distribution block with studs for ring terminals could be used.



Let’s digress for a moment to discuss battery cable. There is a UL listing for “battery cable” but it is not available in wide supply. The product literature of the power distribution block mentions DLO cable, which is a fine stranded wire which is similarly robust like battery cable, is more widely available, and is also UL-listed.

Note that that finely stranded wire has a larger diameter than traditional stranded wire, and it can flare out when the jacket is stripped off. So when using a #1 AWG cable with a power distribution block, it is important to find one with a 2/0 terminal so that the finely stranded wire can land on the terminal port with ease.

We also need to determine the cable size for the inverter. This popular battery inverter mentions a maximum inverter charging capability of 185A, as well as a 9kW rating. 9kW divided by a nominal voltage is 51.2V is 175 A. Using the greater of the two, with an additional 25% safety factor capacity, results in a 218A necessary cable rating. This results in 3/0 cable using 75C terminals, although 4/0 is often used due to better availability.

At any rate, the power distribution block is rated for 500 kcmil wire and so will accept a 4/0 cable readily.

The end result is that we need a short run of 4/0 DLO cable, this power distribution block, an 8” metal gutter, another run of 2/0 DLO cable, some strain relief connectors, and some compression ring terminals. The overall balance of system cost is kept to a minimum while providing a quality, code compliant, and fine looking installation.

As a final note, there are power distribution blocks available with studs rather than terminal ports. In general, the studs are rated for 90C, allowing amperage calculations to be performed at the higher temperature rating. This can reduce the required cable size.

That is all the time we have for today. Remember a copy of this presentation is available in the dealer resource section of our website, and installers, distributors, and designers may always email techsupport@fortresspower.com with project questions at any point in the project.


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Posted July 23, 2020 by John Cromer in category "Intro to Batteries

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