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Old 08-29-2021, 12:56 AM   #1
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DIY 12V Prismatic Lithium Battery

Hey Folks,


Meant to post this a while back but......life is busy. So my dead 4D battery prompted me to take a look at lithium batteries. I ended up running into this site and went down the rabbit hole:


http://diysolarforum.com


After a lot of reading and such I decided to build up my own LIFEPO4 cell and ended up replacing my 4D agm with a smaller 31M battery to act as a backup for the starter. The lithium will be the main house load battery, but I can switch it over to the House AGM in case I drain it.



Many folks are familiar with the smaller LIFEPO4 cylindrical cells. With the EV car market like it is, there are tons of different larger amp hour "prismatic" cells out there. Most, if not all, originate in china. They can be bought on ALI, Ebay, Amazon, etc.



I chose smallish 120AH prismatic cells. I wanted a small form factor and lighter weight. I bought Cells from a company named NINTHCIT. They boast a USA warehouse, although I am not sure if that was true. I bought through Amazon, figured that gave me a safety margin with returns etc. I paid a bit more, but ordering direct from China can be dicey if your cells are not good grade or arrived damaged. There are less than reputable suppliers.


My cells came well matched (Voltage) and looked to be in great shape. They came with studs, nuts and bus bars. First thing you do with new cells that are in good balance is to TOP Balance them. You hook them up in Parallel and charge them up to 3.6 to 3.65 volts. You need a reasonable bench power supply to do this and it takes a long time if you are working with a 5 amp supply like I was. These took the better part of a week. There are very detailed instructions on how to do this on diysolar. I reviewed my findings on these cells in this thread:


https://diysolarforum.com/threads/ni...-review.21603/


I will say here that I most likely did not save any money over a packaged Li battery, but I learned a ton and that is worth it to me. I bought a decent Victron smart charger and a bench power supply, so that put me over. But I now have a true 120AH cell that will run a long long time.


Second part of the build is making a box to add compression to the cells. Some cells like a bit of compression, or to not be able to expand during charging. An overcharged Lithium cell will blow out of it's housing and is done and could be a fire risk. Many folks have made the mistake of upping the voltage over 3.65 on a Top Balance and wrecked their cells. I kept mine under 3.6 and quit. Weird thing about LI is it will sit in the 3.4-3.5 range for days while charging, then suddenly just take off to 3.6. Remember, these are the individual cell voltages. They are in parallel when being top balanced charged.


I made my box out of plywood and used a floating front to compress the cells will threaded rod. It worked great. My cells came a little bulged on the big flat sides. A little compression and cycling took that bulge out. Once in the housing it is time to add a BMS.


Most critical part of an LIFEPO4 setup is the Batter Management System. This is the small unit that tracks cell voltages, system voltages, charging and discharging amps, and should also have a thermocouple to shut down charging below freezing and perhaps at high temps. Good ones have bluettooth and custom programming and monitoring through an app. Mine is DALY, but there are other better ones out there. A highly rated one is the overkill solar. Mine is only rated to 100 AMPS which means 100A of charging or discharging. I have mine custom set bit lower. Another feature is to be able to shut down charging and discharging at certain voltages. You have to also look at the specifications on the cells. They will give charging and discharging numbers like 1C, that means 1 capacity, or for a 120AH cell, 120 AMP max discharge or charge. Some are 0.5C, which would mean half the amp hour capacity in amps charging etc.



Use caution when buying a BMS, it can get confusing. they go by the number of cells in series. In my case, I need a 4S for 12V. The BMS has individual sensor leads that go in order onto the terminals. The BMS then can read each cell's voltage as well as charging and discharging currents. This way you can also see if one or more of your cells is lagging or leading. It is important to have matched, balanced cells that do not lag or lead each other, this reduces capacity as the BMS will shut down charging or discharging depending on those leading and lagging cell voltages.
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Old 08-29-2021, 12:58 AM   #2
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Enough text, here are some pics:


A shot of the battery in it's plywood case. You can see the individual cells. I was topping it off a bit here with the bench supply






A side shot showing the threaded rods for compression. I built the back, bottom, and sides as one piece with the front floating to compress the cells. I added a plexi shield that I bent with a heatgun and promptly cracked screwing down because I am impatient and stupid sometimes.



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Old 08-29-2021, 01:04 AM   #3
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Here's a pic of the business end. You see a nice AGM fuse and block, the BMS, and a battery monitor with shunt. This battery monitor is a little goofy, but it gets the job done. I think it is off about 10AH over the 120AH count.






Here is a screenshot of the IOS App for the BMS. You will see the capacity meter, the total volts, and the individual cell voltages as well as the delta between the cells. My cells have not exceeded more than 25mV in delta and are often no more than 10mV. That is pretty great. Not shown is the settings screen where you can really dial in the cell charge and discharge protection amps and minimum and maximum volts for charging and discharging.


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Old 08-29-2021, 01:24 AM   #4
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So the whole unit weights about 30 lbs and 13x9x8. I used it on a road trip with no charging and it ran my dometic fridge for 5 days straight in hot weather and still had 18AH left.



If you are into building your own packs head over to diysolarforum and read up.



The prismatic cells I used were 120AH. The make everything from 50AH to 300AH in these and lot's of solar folks use them.


When I get this permanently mounted in the van I will be using a Victron 15AMP DC to DC charger. It is a slick unit with bluetooth and is very customizable.


One thing I am doing is not fully charging or discharging it. My BMS and chargers are set to 14.1V max instead of 14.4 and the BMS will cut out at 12 Volts and not 10. This is tested and recommended by the DIY Solar crew to greatly extend your battery charge cycles.


Thought this might be fun for anyone looking to get into Lithium and save a couple bucks.
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Old 08-29-2021, 09:46 AM   #5
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Great write up Tim!! Thank you!

By setting your low voltage cut off to 12.0vdc I am wondering how Li reacts to a heavy draw such as a microwave? I can see the voltage drop into the mid to high 11's on our 350ah AGM bank after about 30 seconds of running. If in full sun the large solar panel helps keep it above 12.0vdc.

5 days running the Dometic and drawing ~100amps implies about 0.9A/hr usage - that is super efficient.

I have been looking at the SOK batteries with interest. Their 100 and 200 AH batteries seem very attractively priced. Other than for the deep learning experience is there an "off the shelf" battery you would be drawn to?
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Old 08-29-2021, 11:37 AM   #6
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is there an "off the shelf" battery you would be drawn to?


Battleborn is well respected and many others sell less expensive...Maybe second string would be Renogy....A lot of mfrs are jumping in...there are several threads on this adaptation in the forum..
Remember you cannot just drop in a Li technology battery...you will at least need to address charging and voltage control with, for instance a DCDC converter...You also need to look at the battery jump start from house and charging both house and starter batts built into many Sportsmobiles.
Fun project, but not plug and play...
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Old 08-29-2021, 11:57 AM   #7
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I have 16, 3.2V 280ah batteries on the way. That will make 1,120ah of lithium power. I will also be using a DALY BMS, and have them heated also for cold temps.
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Old 08-29-2021, 12:36 PM   #8
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Quote:
Originally Posted by 225 View Post
I have 16, 3.2V 280ah batteries on the way. That will make 1,120ah of lithium power. I will also be using a DALY BMS, and have them heated also for cold temps.

Would like to see your setup when you install it. My DALY BMS is working pretty great. I know some had trouble with them. Mine seems just fine and the Bluetooth range is excellent. I guess if you hook them to a computer you can also define Depth of Charge curves for more accurate capacity readings. I am just using the cheap Amazon shunt and meter.
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Old 08-29-2021, 12:40 PM   #9
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Quote:
Originally Posted by 1der View Post
Great write up Tim!! Thank you!

By setting your low voltage cut off to 12.0vdc I am wondering how Li reacts to a heavy draw such as a microwave? I can see the voltage drop into the mid to high 11's on our 350ah AGM bank after about 30 seconds of running. If in full sun the large solar panel helps keep it above 12.0vdc.

5 days running the Dometic and drawing ~100amps implies about 0.9A/hr usage - that is super efficient.

I have been looking at the SOK batteries with interest. Their 100 and 200 AH batteries seem very attractively priced. Other than for the deep learning experience is there an "off the shelf" battery you would be drawn to?



AGM and LI are pretty different. I am not sure if AGM sufferes a voltage sag during heavy discharge. I don't think that is the case with LI. They have very different SOC profiles. LI is very flat in comparison. An LIFEPO4 at 12V is actually at 9% SOC. So my settings to to stop discharge there are to protect the bottom end and get more cycles.


on the SOK, if you look at their website and go to the 200AH model, they have a video with Will Prowse. Will is the guy owns diysolarforum and writes many of the how to's. I think he reviews that SOK. If he says it is decent, I would take his word for it.


Many off brands have entered the market. Without cracking the case and looking inside it would be real hard to know what you are getting and what the BMS is etc. I like the cases SOK uses. Watch that review.
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Old 08-29-2021, 12:49 PM   #10
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When I get my LI mounted, this is the charger I will be using to charge it from the alternator. My LI will be completely separated from the AGM's and the alternator. I will also be isolating my loads so I can keep the batteries separated. The starter and house agms can charge from the alternator with the normal ACR setup.


https://www.victronenergy.com/upload...50-400W-EN.pdf


I have the 18 Amp version, they make a 30 as well. It has a voltage sensor to tell when the alternator is on and it will begin charging.



I really like the victron smart products. You can do custom charge profiles for everything. I will use 14.1 for bulk and absorb and 13.5 for float and storage. Lithiums will settle into that 13.5 range even when charged to 14.4 volts.



YOU NEVER WANT TO CHARGE THEM HIGHER THAN 3.65V per cell or 14.4 for a 12V setup.


I also got one of these battery chargers. Another fantastic product with custom settings and tells you how much charge you applied etc.


https://www.victronenergy.com/upload...20V_EN_web.pdf




It was these two items that kind of put me over the top $$ wise, but I will have a much better system when done.
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