You can build a bigger and maybe better quality LiFePo4 for a little more money. I just wanted a kit with matched components and didn't care to do the research to figure that all out on my own. I welcome any and all critiques of my methods and the product.
This is the (+) terminal of one cell. The (-) negative terminal is the same with a black shrink tubing. I will refer to the cells (-) and (+) terminals as the battery posts. They are not technically posts but for the purposes of this build I will refer to them as posts. On these cylindrical cells the battery "posts" are lugs with multiple wires running into a single positive and a single negative wire lug. I cut away a small amount of the cell pack's shrink wrap to allow the wires to flex easier.
This is a mock up of my layout of the battery. This is a "4S" (four in series) configuration to add up the 3.2v cells to 12.8v nominal voltage. This is only a mock up at this point to build a container and double check the connections. The copper links are homemade from 1/2" hard copper flattened and drilled. Soft copper would have been easier but this is what I had on hand. I have learned that compressing cylindrical cells is not necessary like prismatic cells. I did no compressing and just built a plywood box to contain them.
The cells come without a full charge and should be top balanced. I won't go into what that is or why as Lauren has described the process very well here...
http://www.tnttt.com/viewtopic.php?f=30&t=74022
I did deviate from Lauren's method by wiring the bank as a 4S including the BMS wiring and charged the bank this way for five hours. This theoretically shortened the top balancing time. Using my Noco 7.2a 12v charger fed three times the watts into the battery per hour compared to when I was top balancing at a lower voltage (more about that later).
To top balance the cells I had to wire up as a 4P configuration (four in parallel). In this configuration the bank starts out at 3.2v, 100 aHr. I purchased a bench power supply to top balance the bank. It is a $50 unit and is the same PS that Will Prose used in his video about top balancing a LiFePo4 battery. I also used a multimeter many times during this process to verify the PS was reading accurately.
https://www.youtube.com/watch?v=x5ABvbbics8&t=237s
The leads from the power supply that are shown are the leads that came with the PS with banana plugs and alligator clips. I made a set of my own using 12awg with ring terminals on each end and hard wired them to the bank. This resulted in less resistance and a more solid connection.
This was the first time I had ever used a bench power supply and I learned a lot by watching YouTube tutorials about their use. The unit I purchased did the job. It has some drawbacks, like only reading to a tenth of a volt, however, for the number of times I'll use a bench power supply it was fine.
Top Balancing
The process I used to top balance is as follows... Leave the leads disconnected (and sparated). Turn both amp and volt dials all the way down on the power supply. Plug in the PS and turn it on. Turn the amp dial all the way up (10a on my PS). Turn the voltage dial to 3.4v. Turn off the PS. Connect the leads to the (-) and (+) posts of the 4P battery. Turn the PS on. This is important to remember... The readout on the PS will show 10a and whatever the current voltage of the battery is, not what you set the dial at. You don't want to adjust the PS while it is on. When the voltage approaches 3.4v the amp reading will drop to near zero. Because I pre-charged the pack (4S) with my Noco 7.2a charger this process took about 50+ hours.
Now repeat the above process, everything disconnected, setting the amperage all the way up and the voltage at 3.6v. Attach the leads and turn on the PS. When the amp reading drops to near zero turn off the PS and let the battery rest for an hour or more. Take a voltage reading. It will have dropped a fraction of a volt or more. Leaving the PS set at 3.6v connect it back up and turn on again. When the amps drop to near zero turn off the PS and let it rest. Take a voltage reading. I repeated this process about four times with the last two charges the PS was set at 3.7v. I was trying to get that last hundredth of a volt to reach a resting voltage of 3.65v. Resting voltage reading ideally should be taken after many hours of rest. An hour or two at the end was close enough for me.
After Top Balancing build the 4S
In this photo I have the cells laid out in their case. The blue straps on the bottom of the box are just scrap straps to help me pull the battery box out of the back of my cabinet.
And in this photo I have the bank BMS taped down to the bank and the bank's negative wire hooked to the B- post of the BMS, the first cell's positive wire connected to the second cell's negative wire, and the second cell's positive to the third cell's negative. That pattern is continued until all that is left is the last cell's positive is connected to a fuse and then to the battery post. The C- post on the BMS has a 10awg wire running to the (-) post.
I believe it is best practice to not plug in the BMS balance wire harness plug until the battery is completely wired. Hooking up the BMS' five wire harness is simply connecting the first (black) wire to the B- post, and in order the next wire to the first cell's (+) post, next wire to the second cell's (+) post and so on until all the wires are on the cells. Again the first wire goes on the BMS which is the negative lead to the BMS. Then each of the rest of that harness goes in order to each cell's positive post.
These next photos are a bit messy, but, nobody sees this but me (and the tnttt world). The white tape is non-conductive and really just keeps the excess wire from flopping around.
I'm about 27% of the way through my capacity test. 446 wh is about 27% of the expected 1200 wh this bank should have. It is going slowly as I don't have an inverter and only low draw 12v appliances. I'll update this thread when I 'm done with the results of the capacity test. Probably tomorrow.
What I learned or would do differntly...
-Any experienced battery builder will notice my battery box posts are a little unorthodox. I went to four local hardware stores and could not find brass 5/16" bolts. To get the job done I flattened more 1/2" copper and wrapped the top of the plywood box with a U shaped "lug" and drilled a 5/16" hole through the box and the lug. Then I used a zinc bolt to tie the battery leads and the load leads to the box. The lugs are tight against the copper on each side. I did a resistance check of the copper versus just a piece of 10awg and there was no difference. Next time I go the the "big city" I'll pick up some brass bolts and eliminate the copper lug wrap thingy.
-After top balancing and then building the 4S bank I hooked up the battery to my teardrop and could not get more than 2.4v to the battery posts. Luckily we have flboy and saltydawg who both have experience with this type of BMS and I was advised to hook up a charger to the battery to wake up the BMS. Apparently the BMS must have a charge of more than the battery voltage to turn on. It had the 14.2v from the battery. It wanted a 14.3v charge in the other direction. This was done with the bench power supply hooked up driving 10 amps at 14.3 volts for less than a minute. When I shut off the power supply everything was humming.
- Without Lauren's encouragement I probably would have replaced my batteries with 100 aHr AGMs. I had actually had two of them in a cart online at $175 each. Thanks Lauren for not giving up on me. This was fun and a little stressful when I couldn't get 14.2 volts out of the BMS. But it is worth the trouble.
- I hope to figure out how to program my Epever Tracer 3210 and MT50 meter to limit charging to 80 or 90 percent. I have no idea how that is done but I'm playing around with the charge controller to figure that out. I'm guessing it has something to do with telling the charge controller to stop charging at a certain voltage. I guess that will be more apparent after figuring out what a full charge voltage and a drained voltage are.
I look forward to hearing if you are able to figure out how to program your Epever devices to limit charging to 80 or 90 percent.
