After being nudged by rando, I’m very happy with the DIY result. The front dinette Fleet can only fit 100 Ah of Battle Born or Victron batteries. The several 200 Ah options, such as LifeBlue are small companies that may not hang around, like Stark Battery. I have a 280 Ah battery I can troubleshoot. If the BMS fails, I can easily replace it. I can identify failed cells and replace them.
Four 280 Ah cells arrived a week ago and the BMS arrived a few days before that. One-day Amazon prime shipping it was not – about 40 times longer. But at 1/3 the price (and, uh, some labor) the waiting was worth it.
In my 2015 Fleet, battery space is quite limited. The maximum width is 8”, so I spent a lot of time working out the dimensions before I built the box. Had I had all of the materials at hand and knew my exact dimensions, it would have taken about 6 hours to build. I used ½” hemlock and aluminum stock from Home Depot. Hemlock is harder than pine, is easy to work and comes in ½” CVG.
One of the most useful tools is a metric-English tape measure. You can use a calculator to convert mm to and from fractional inches. Or you can just look at the tape and read off the conversion at a glance.
The cells are 6 13/16” wide. The box outside dimensions are 7 5/8” x 10” x 12 ¼”. I had about 1/4“ to spare on width, which means that the ¼” threaded rod is as close to the cells and the edge of the end boards as I could get. You could probably go to 12” on height, and length is limited to 21 ¼”. The threaded rods are covered with plastic straws (slit lengthwise) to keep the rod from abrading the cells.
The screws that hold the mounting bars (see picture) to the BMS are metric “PC” screws. There are three common PC screws. These are the smallest diameter, M1 maybe, and come in 5 mm and 10 mm lengths. I had to grind the 10 mm screws down to about 7 mm. The BMS mounting bars are offset from the end board with ¼” sections of 1/8” plastic tubing to allow good air circulation around the BMS. Note that the side of the BMS opposite the side with my mounting bars is a heat sink, so you if you glue or tape the BMS to your box, glue or tape the non heat sink side.
bms.jpg 117.73K
68 downloads
I mounted the BMS on one end of the box and used pass-thru bolts to wire to the battery and to the Anderson connector. This avoided splicing wires.
Bat 1.jpg 107.42K
78 downloads
Before mounting the BMS, I soldered two wires to the battery on-off terminals next to the balancing-wires plug on the BMS so I can disconnect the battery with a switch (available from FWC) in an unused slot in the Fleet lights switch panel. Unlike AGMs, lithium batteries do like to stay topped off. They would much rather sit at 80% to 90% when not in use, so when we get home, I can disconnect the battery from the solar charger while the rig is parked.
Between cells 1 and 2 I inserted a thin sheet of cork with a slit in it so I could slide the BMS temperature sensor into the heart of the battery pack.
Wiring is rather straight forward. Just don’t make a mistake. There’s a lot of energy in the cells.
bat 2.jpg 90.26K
79 downloads
bat 3.jpg 89.03K
78 downloads
Overkill Solar has an excellent how-to guide for the Xiaoxiang BMS: https://raw.githubus...tion_Manual.pdf This guide also has recommended BMS settings, but note that they are in mV and mA while my BMS App wanted Volts and Amps.
Overkill Solar also sells the BMS for about $50 more than Alibaba (but with Amazon one-day prime instead of 25 days). They have excellent customer support and claim (I think it’s real) that no matter how you fry the BMS, you get your money back.
I pulled my AGMs out of the Fleet and switched the Victron MPPT and DC-DC over to LiFePO4 profiles. I also used the MPPT app to turn off solar (enable charging to OFF). I then rewired all wires to the battery to bus bars. The positive post and a small fuse block are behind the heater return plenum. I have an aversion to exposed high current (or voltage) contacts. With the Anderson connector, sliding the battery in and out of the compartment was easy.
Once connected, I checked out the wiring and set my BMS parameters with the Xioaxiang App. You get the iPhone app from the Apple store ($6) or download the Android app from a link on overkillsolar.com site and do a third party install I then re-enabled MPPT charging.
Each lithium cell will arrive with a slightly different charge, but you can only see this when the cell is almost completely charged or discharged, when the voltage differences become obvious. Top balancing is at the charged end and bottom balancing is at the discharged end. I chose top balancing since I plan to operate the battery from about 25% SoC to 98% SoC.
25% is a nod to the original 1938 VW Beetle which did not have a gas gauge. When the engine sputtered, you turned the lever and had another 10 L of gas. So if I find a way to run my battery down to 25% it will shut off. I can then shed load, reset the BMS to 5% SoC, and I’ve got another 60 Ah or so to run the furnaces, fans, lights and refrigerator until I can get sun and/or alternator.
Not having easy access to a suitable bench top DC power supply, I tried a different route for balancing using a $10 active balancing board, the BMS passive balancing and slow charging. It didn’t work, for reasons I now understand. Slow charging was easy – current wildfire smoke greatly attenuated my solar.
So, with alligator clips on a 1 ohm 50 W resistor I bled charge from the highest cells periodically as the battery slowly charged. 50 W got hot. Better with100 W, but 50 W is what I had. This actually did not take that long – well within my attention span. Since I was charging with solar, I had to temporarily set the MPPT to 14.6 V absorption and float to charge the battery close to 14.6 V.
Final.jpg 76.39K
63 downloads
I learned a lot more about the Victron MPPT. It has a float voltage for LiFePO4. Huh? Ahaa! It’s a hysteresis to avoid continually topping off the battery. The MPPT and the DC-DC charger as well charge to the absorption voltage (14.2 V or about 98% SoC) and then go into13.5 V float. At 13.5 V, there’s no charging until the battery drops back to 13.49 V
I also noticed that the Victron MPPT and DC-DC have a low temperature shutoff. There are some significantly lower cost LiFePO4s that don’t have low temperature shutoff that become viable for those who don’t want to build their own or buy Battle Born or Victron.
Update 2021-03-17
The battery has been great. I have almost gotten rid of my battery anxiety.
Except for the Fleet FD (I think), you can use a simpler battery case because the battery compartments are much more accessible. On my Fleet FD, the screw on top makes getting the battery in and out of the battery compartment manageable.
Here are my sources and helpful links:
Alibaba LiFePO4 Cells: https://www.alibaba.....96e2450fqQiVhQ
The 4 cells cost about $330 with air freight at about $250. If you have the patience, I think you can choose sea freight and save a chunk of $.
BMS: https://www.lithiumb...p-battery-pack/
You can also find the same BMS on eBay and Overkill Solar.
Useful WtW Link: https://www.wanderth...dc-dc-chargers/
Also Youtube: https://www.wanderth...dc-dc-chargers/
Will Prowse has a bunch of excellent Youtube videos on Solar and LiFePO4: https://www.youtube....Qq8kmJme-5dnN0Q
Edited by Jack, 17 March 2021 - 06:09 PM.
.
