Battery Bank – Kelly Flanagan

This is the ninth post of a series of articles documenting and describing our RV electrical upgrade. Our previous post in this series described a simplified model of our battery bank, the wiring of our system, the resistance contributed by each component and associated cables, and the expected system voltage drop. Our goal is to keep our voltage drop to 2.5% or less as recommended in Victron’s Wiring Unlimited. This post describes the outcome, reports actual voltage drop measurements, and compares these with those calculated previously. Finally, we evaluate our final results regarding our initial goals, point out a few things we’d do differently, and conclude. For those interested, this page lists the parts, equipment, and tools we used to build our power system.

Initial Design

This image has an empty alt attribute; its file name is 12-V-Design-v4.png — Figure 1 illustrates the power system proposed in a previous post.

The power system we built, illustrated in Figure 1, was initially proposed in one of our first posts in this series. In our previous post, we introduced the critical path of this system consisting of the circuit from the battery bank to the inverter/charger and back, illustrated in Figure 2.

This image has an empty alt attribute; its file name is Final-Critical-Path-Schematic-1024x714.png — Figure 2 illustrates the components and cables that make up our system’s critical path and associated resistance values.

Figure 2 includes the measured resistance of each cable and component, such as the 400 A class T fuse, battery disconnect switch, shunt, 400 A mega fuse, and the Lynx Distributor busbar system. These resistances are assumed to be worst-case values as we rounded up measurements and the measurement device’s connection to each component was simply the tension applied by the spring-loaded probes. The total measured resistance of components and cables around our system’s critical path is 1.77 mΩ. With this resistance, we concluded that our system should be capable of providing just over 180 A while remaining at or below our 2.5% voltage drop goal.

Final System

After considerable cold winter work, our design came to fruition. As previously described, our power system fits within the left pass-through storage area, illustrated in Figure 3.

Figure 3 illustrates our complete power center providing 800 W of solar, 360 Ah of Lithium-ion batteries, and a 3kVA invert/charger.

Figure 3 illustrates the major components of our system. There are seven major components illustrated from upper left to lower right:

Solar PV disconnect switch
Victron SmartSolar 150/45 solar charger
Victron MultiPlus-II 2x 120V inverter/charger
Victron Cerbo GX monitoring system
Victron SmartShunt
Blue Sea battery switch
Victron Lynx Distributor

Our battery bank is behind and to the right of this location. The solar charger and the SmartShunt attach to the Cerbo GX via VE.Direct cables. The battery voltage monitor (mounted on the battery bank) and the MultiPlus-II connect to the Cerbo GX via two VE.Bus cables.

We used a Victron MK3-USB device to program the inverter/charger for our specific setup and then used the remote console provided by the Cerbo GX to monitor the initial startup. After turning on the battery switch and the solar PV switch, the system immediately started providing inverted power to the RV, and the solar charger started charging the batteries. Next, using a halogen light and my wife’s toaster, we applied some moderate load to the system and took some readings, found in Table 1.

Battery Voltage	Inverter Voltage	Voltage Drop	Current (A)	Resistance mΩ
12.88	12.84	0.04	25	1.6
12.71	12.61	0.10	73	1.4
12.28	12.04	0.24	157	1.5

As expected, as the system current increases, the voltage drop experienced by the inverter/charger increases. We divided the voltage drop by the associated system current to calculate the system resistance. Averaging these three values, we note that the system resistance is 1.5 mΩ which is 0.27 mΩ lower than the measured 1.77 mΩ.

The system experienced a 0.24 V voltage drop with a 157 A load. A voltage drop of 0.24 V is a percentage voltage drop of just 1.9%, well below our limit of 2.5%. With a resistance of only 1.5 mΩ, we should be able to load our system to nearly 215 A without exceeding our 2.5% goal. These 5-10 minute load tests resulted in almost no heat generation by the inverter/charger or any system components. I am looking forward to more extended tests to see how hot things get.

Summary: How Did We Do

Nearly three months ago, we outlined our goals for our new power center in our first post on this subject. We desired the ability to use our microwave, television, and other 120 V AC systems without having to ruin our camping solitude with a generator. In addition, we wanted to minimize the intrusion of our generator while recharging our batteries. We determined that to meet our needs, we needed several items:

400 Ah of lithium-ion batteries
800 Watts of solar power
An inverter that is capable of producing nearly 3000 Watts of 120 V AC power
A battery charger that is capable of consuming our entire generator output to minimize charge time

We have nearly met each of these requirements. Instead of 400 Ah of batteries, we have 360 Ah, and instead of 3000 W of inverter power, we have 3000 VA or 2400 W continuous. We believe each is close enough to call this project a success. Perhaps more importantly, we learned a lot on the journey and had a lot of fun. If our RV needs to be restored to what we had before this project, here is a brief description of the required tasks.

We did a couple of things right and a few we’d do differently with the new knowledge we possess:

We can’t properly express how great the copper bar approach to connecting the battery disconnect switch and the SmartShunt to the Victron Lynx Distributor is. Using a short segment of 4/0 wire and a lug at each end results in a rather long connection. The copper bar approach saves space, looks clean, and in our cramped environment made our layout possible. You could save ten bucks if you want to make your own, but we saved ourselves the cutting, drilling, and the likely mistakes and bought a pair.
I wouldn’t have initially skimped on our torque wrench purchase. Our fitst purchase had a torqu range of 10-100 ft-lbs and barely registered when being used at the low end. We ended up twisting a bolt head on a battery lug clean off. This was dangerous and could have resulted in a bolt being unretrievable from an expensive battery. Fortunately, just enough bolt was left to enable its removal with a pair of vicegrips. We love our second torque wrench, the Park Tool TW-6.2.
We would definitely use boat/marine wire instead of the 6/3 Romex that we installed. Our RV, like most, is full of Romex making us comfortable that this was a reasonable choice. In addition, 6/3 Romex contains stranded conductors, but not like ultra flexible boat/marine wire such as Ancor’s Triplex Cable.
We would have used lugs suggested by Victron Energy. The 4/0 sized lugs we used are great, but don’t fit very well within the Victron Lynx Distributor. I suspect, but have no evidence that the lugs they suggest would fit much better.

We’re done, it looks clean and neat, and above all else, it works!

This is the third post of an ongoing series of articles documenting and describing our significant RV electrical upgrade. In our previous post, we discussed a potential need for an alternative design and presented its schematic. We also discussed a 3D model of our finished product, and we shared our results. In this post, we describe our battery bank design, discuss where we intend to place it, and the details of making space for it.

Battery Bank Design

Our first article of this series discussed our requirement for a 400 Ah battery bank to meet our electrical needs. However, our 400 Ah requirement assumed a 12 V system. We have since determined that we’d like to implement a 24 V system that reduces our Ah requirement. To enhance clarity, we should discuss our needs in terms of Watt-hours (Wh). A 12 V system capable of delivering 400 Ah is a 4,800 Wh system. A 4,800 Wh 24 V system delivers 200 Ah. We need a battery bank design for both a 12 V and a 24 V system until we determine whether or not Victron will produce a 24 V version of their MultiPlus-II 2x 120V inverter/charger.

Whether we implement a 24 V or 12 V system, we intend to use four Lion Energy UT 1200 lithium-ion batteries to implement our battery bank. We would use their newer UT 1300 batteries, but we already own two of the UT 1200s and found two used UT 1200s for less than half the price of new units. The UT 1200s are rated at 1,152 Wh, while the UT 1300s are rated at 1,344 Wh. So while our goal was 4,800 Wh, we’ll settle for the 4,608 delivered by our four UT 1200s.

24 Volt Battery Bank

We will connect two UT 1200 batteries in series to create a 24 V set. This set can deliver 2,304 Wh, which is well below our goal. Two of these sets can be connected in parallel to yield our desired 24 V battery bank, providing 4,608 Wh. Figure 1 illustrates this configuration. This implementation also adds redundancy in case a battery fails while in the field.

Figure 1, This is a 24 V series-parallel battery bank configuration built using four 12 V Lion Energy UT 1200 batteries. The fuse holder on the left of the figure will hold a 300 A Class T fuse.

Because wire resistance is nonnegligible in these types of systems, it is essential that the circuit length from the positive terminal of any battery, through the load, and back to the negative terminal of the same battery be equal for all batteries in the bank. In practice, this means that the connecting wires between batteries should be of equal length, and the positive and negative connections to the load should be from opposite ends of the battery bank. For example, Figure 1 shows that the positive connection comes from the upper bank, while the negative connection comes from the lower bank.

According to a comprehensive document published by Victron Energy called Wiring Unlimited, an alternative configuration includes a connection between the midpoints of both sets. This configuration is illustrated in Figure 2.

Figure 2, This is a 24 V series-parallel battery bank configuration built using four 12 V Lion Energy UT 1200 batteries and includes a midpoint connection. The fuse holder on the left of the figure will hold a 300 A Class T fuse.

According to the Wiring Unlimited document, this approach requires midpoint monitoring to detect deviations in voltage between the upper and lower batteries. In addition, when deviations are detected and reported, they must be acted on to deal with potential issues. Batteries in the battery bank may be damaged if reported problems are not resolved. Monitoring the midpoint has the advantage of detecting these issues that might otherwise go unnoticed. The Victron SmartShunt, which we intend to use in our system, can monitor midpoint voltages and raise alarms when problems arise.

12 Volt Battery Bank

Alternatively, four UT 1200 batteries can be connected in parallel to yield a 12 V battery bank, delivering the same 4,608 Wh. This configuration also adds more redundancy in case a battery fails while in the field. The obvious configuration to accomplish this is shown in Figure 3.

Figure 3, This is a 12 V parallel battery bank configuration built using four 12 V Lion Energy UT 1200 batteries. The fuse holder on the left of the figure will hold a 400 A Class T fuse.

Notice in Figure 3 that the positive connection comes from the upper left battery in the bank, and the negative connection comes from the upper right or the last battery in the bank.

In a 12 V system, the currents are double those of a 24 V system. These higher currents demand a 400 A Class T fuse and wire with double the cross-sectional area of wire used in a similar 24 V system.

A cleaner wiring layout that still respects the principle of keeping circuit lengths equivalent is illustrated in Figure 4.

Figure 4, This is a 12 V parallel battery bank configuration built using four 12 V Lion Energy UT 1200 batteries. This layout uses the halfway approach of keeping circuit lengths equivalent. The fuse holder on the left of the figure will hold a 400 A Class T fuse.

In the hopes of reducing system currents, voltage drops, and associated ripple, we desire to create a 24 V system. However, if this is impossible due to the lack of availability of an appropriate inverter/charger, the 12 V layout, illustrated in Figure 4, will be used.

Battery Bank Placement

Our RV is a 2016 Outdoor RV Blackstone 240 RKSB. The floorplan of this model is illustrated in Figure 5. Our new Power Center will be located in the left-hand side of the pass-through storage area that is accessed by an exterior door located in the upper right-hand corner of Figure 5. Batteries should be located as close as possible to their associated invert/charger to reduce voltage drops that may occur over long cables. To keep the cables short but still leave storage areas available for their intended purpose, our battery bank will be located under the foot of the bed.

Figure 5, This is the layout of our RV, a 2016 ORV 240 RKSB.

In our last post, we determined that space between the drawers was available for our battery bank. This open space was found during our modeling efforts and is illustrated in Figure 6.

Figure 6, The space between the drawers is sufficient for four Lion Energy UT 1200 batteries.

The UT 1200 batteries measure 10.2″ long, 8.8″ tall, and 6.6″ wide. We started demolition to determine the best way to orient these batteries within the available space. We knew from our previous inspections that two cross beams were holding up the false floor of the storage area above the drawers. We removed the drawers and, using a jigsaw, removed the portion of the false floor between the beams. This roughed-out hole and the exposed space are shown in Figure 7.

Figure 7, This is the rough cut opening that exposed the space we intend to use for our battery bank.

While the drawers are only 17″ from front to back, the glides they’re on are 18″ long. The distance between the backs of one set of glides and the other is insufficient for two side-by-side batteries to sit between them. Unfortunately, 17″ drawer glides are unavailable, and the next size down is 16″.

The lack of space necessitated converting one of our drawers to 16″ glides and adjusting the drawer to match. After removing the glide supports and drawer glides, we constructed a new and more robust support structure shown in Figure 8.

Figure 8, This is the drawer space with a new support structure and drawer glides in place.

We acquired and installed new 16″ drawer glides. In addition, we disassembled the associated drawer, cut 1″ off the sides and bottom, and reassembled it better than new.

The remaining space between the backs of both drawers is now more than sufficient to hold two UT 1200’s side by side. The distance from right to left in Figure 8 is also more than enough to hold two UT 1200s longwise. There is also enough space for the fuse holder that needs to be located near the battery bank.

Figure 9 shows the battery bank space with two UT 1200s in place between the installed cleats.

To assist in holding the batteries firmly in place, we added two cleats to the floor spaced to allow the batteries to sit side by side with little to no room to spare. The cleats and batteries can be seen in Figure 9. Also, notice in Figure 9 that the roughed-out access hole has been cleaned up using a round-over bit in our router, some chisel work, and a bit of sanding.

To restore the utility of the storage space used to provide access to the batteries, we drilled finger holes in the remaining false floor. We also added another false bottom over the top of what can be seen in Figure 9. This can be seen in Figures 10 and 11.

Figure 10, Finger holes were added to the existing false floor to facilitate the removal of the new false floor.

Figure 11 shows that the new false floor restores the storage area while protecting the hidden battery bank.

The End Result

In the spirit of making incremental progress and leaving the trailer functional for camping, we’ve reached the end of this sub-project. We successfully created an excellent space for our battery bank that will work for either a 24 V or a 12 V system. The only negative impact of this sub-project was one drawer is now an inch shorter. In future posts, we’ll install the batteries, wire them together, create a small shelf for the fuse holder, and wire the battery bank to the battery disconnect switch and the SmartShunt.

Tag: Battery Bank

RV Power Center: Final Results