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Old 03-23-2021, 08:53 AM   #1 (permalink)
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Default LiFe (LFP) batteries for field charging

I started discussion of this on the "what are you working on" thread, but I figured there is enough interest for its own discussion.


Background:
5 years ago, I put together a "battery box" for the 50Ah AGM deep cycle that I use to run chargers at the field. It weighs almost 40lb. It can just barely manage the amps to charge a single 6S at 1C without going into low voltage warning on the charger. And when used at decent amps... at best... you can use 30-40Ah out of it each day without destroying the lifespan.

After 5 years its tired. Its full charge resting voltage has fallen over 1/2 volt, and a capacity test shows its lost at least 30% of its original capacity.




So time for a change. Ive been really interested in, and now that prices took a major dip I took the plunge into something very intriguing - large format LiFePO4 prismatic cells. These are very popular for off grid DIYers, boaters and RV folks so prices and availability are constantly improving. These come in capacities all the way up to 280Ah per cell and there are lots of vendors selling raw cells direct if you are willing to take the risk and wait out the shipping time ordering direct from China.

Around 100ah is the sweet spot. Doing some research I ordered these 90Ah aluminum case cells. A couple months ago a set of 4 was $299 shipped on AliExpress. Shipping took about 2 months. They went by boat to an importer in SanFran who deals with customs clearance, then to me by Fedex.




Remember I mentioned 50Ah (30 usable) of lead was almost 40lb?
Well we are starting at 16lb!!! for 90Ah of LiFe









Some reading while I document the rest of the build out and results:


Large format Lithium battery build resources (written for boats, but the basics apply to us) - I highly reccomend reading this entire article series if you are going to attempt this.
Lithium battery systems | Nordkyn Design

Offgrid forum discussion of building large lithium packs:
https://diysolarforum.com/forums/diy...tery-banks.22/

Off-grid RV webpage, has a lot of good links to components like raw battery cells
https://www.mobile-solarpower.com/ra...eals-page.html


The battery cells I used:
https://www.aliexpress.com/item/3304...2bd5b506a70738

Battery Management System (BMS)
https://overkillsolar.com/product/bms-120a-4s-lifepo4/


Panel meter:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1



Resources for DC wire sizing:
https://www.bluesea.com/support/arti...re_Size_for_a_
https://www.calculator.net/voltage-drop-calculator.html
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Old 03-23-2021, 08:54 AM   #2 (permalink)
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Default Step 1 - Top balancing and initial measurements

The off grid guys dont use special balancing chargers like we do. They typically use solar, or simple CC/CV bench power supplies for charging. To monitor cells they use battery management boards, but those are very limited in balancing capability (typically only a passive resistor netowrk capable of a couple hundred mA of balance current or so..) So the first thing they typically do is a "top balance" to baseline the cells. This is done by wiring them all in parallel and chargeing them to 3.6v until hte current tapers to zero.


I happen to use a PL6 for field charging so I made up a harness and used its LiFe unbalanced mode to do this. Wired in parallel its effectively 360Ah, so at 20A it took over a day to taper down to 2amps or so. A ocuple times the charger timed out and I had to restart.


Overall it took 382Ah in (95Ah/cell) and after resting for 6 hours the cells are at

1 3.345v
2 3.343v
3 3.345v
4 3.345v
(measured on the Fluke)


Note: These cells are shrinkwrapped aluminum boxes. They have bolt terminals with 6MM studs and nuts. No soldering here - everything gets bolted together and they provide nickel plated copper bus bards for interconnects. Hookup is via large gauge wires with crimped ring terminals.
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Old 03-23-2021, 08:54 AM   #3 (permalink)
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Default Building the box

Next up was designing and building a box to hold the battery setup. For my original lead box I used an old toolbox. This time I build a custom enclosure from 1/2 and 1/4 plywood I had leftover from the workshop assembly table project.

The goal was to make the box as compact as possible, leaving room for hte cells, management board, a fuse box and panel with banana, standard 12v and USB hookups. In addition to field charging, I use this as a portable power station for power failures and camping.


The battery end of the box is in the form of a clamp (using 10-24 threaded rods) as some of the manufacturer data sheets for these large cells show longer lifespan when they are maintained in compression. The output panel, top and carry handle are all removable for maintenance.
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Old 03-23-2021, 08:55 AM   #4 (permalink)
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Default Electrical assembly

Next up was assembling the electrical components.


The batteries get mounted in the clamp and wired up with bus bars. I carefully cleaned the aluminum terminals and applied NoAlox before torquing everything to 35in-lb.

There are 2 balancing harnesses you can see. One connects to the 12v/120A battery management board. The other is a custom harness to balance charge from the PL6. 1/4" ring terminals are crimped to the 22ga balance wires.

The negative lead from the battery goes through the BMS (for cutoff protection) and a 100A current shunt feeding the panel meter. The positive lead from the battery connects with 6GA welding cable to a bussman 80A DC circuit breaker and from there to a Blue Sea marine fuse panel. That protects all the outputs.

Outputs:
  • 2x EC5 (10ga wire fused 40A)
  • 2x Bannana jacks (12ga wire fused 30A)
  • 1x 12v socket (14ga fused 15A)
  • aux circuit for a pair of USB outlets and a switchable cooling fan (16ga fused 5A)

All wire is automotive grade or silicone insulated 99.99% pure copper hookup and all terminals are crimped and heat shrinked. The main leads are welding cable. I had to use a hammer crimper for the large terminals on the 6GA wire.

Note these cells could easily manage 100A or more and the BMS is rated for 120. I cant see ever using that so I fused for 80amps allowing me to downsize that main wire slightly.
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Old 03-23-2021, 08:57 AM   #5 (permalink)
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Default Charging and capacity tests

Next up a I did a discharge test. Using teh PL, I discharged to 2.5 vpc (took over a day!) and got 91.2Ah effective capacity. not bad...

After the discharge terminated I let the cells rest for a few hours and measured:
C1 2.61v
C2 3.12v
C3 2.90v
C4 2.88v

Hmm... some imbalance...




Then I hookup up the PL and did a full charge at 30A CC terminating CV 3.6vpc C/10. the charge ran for 4 hours and timed out before it could completely balance. As you can see in the graphs there is one weaker cell... they will hold balance most of the charge, but over 3.5v that one cell runs away.

After the charge finished the cells came to rest at these voltages. Also note the cell IRs the PL measured:

C1 3.353 0.40 mOhm
C2 3.395 0.40 mOhm
C3 3.389 0.50 mOhm
C4 3.358 0.40 mOhm

To double check I also checked temps while charging. The cells warmed about 10 degrees to 80F or so under charge, no problem. I even used a FLIR (had it from chasing down leaks in the walls) to look for hotspots. Looks good.

The cabling gets quite hot however. I make 10GA charge leads and they get to 90+... the 12GA PL input leads (pulling about 35A) get over 100F and the EC5 connector measured 130F at one point. Not sure I believe the claim they are good for 100A+ they would probably melt!




This is why off grid folks do top balancing. Without balancing charge circuits in solar controllers they just push the entire pack to 14.4+ and would have a cell run away and overcharge. I could keep running balance cycles to try and align them, but its only an issue in that last ocuple % and the PL6 takes care of it so I'm not worried. In practice I will define a charge profile with lower cutoffs as I don't ever need that last couple % and its not good to store it full anywat.
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Old 03-23-2021, 08:58 AM   #6 (permalink)
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Default In use..

A few shots of driving chargers. The cool thing is the BMS board comes with a phone app so you can monitor and set protection parameters. You will notice that the imbalance under charging comes right back in line once the surface charge burns off. Also note that when I ramped the current up to 15A the pack overall voltage only dropped by 0.1v or so! A lead deep cycle would already be sagging considerably.




A note on the BMS. It hooks up in the negative lead and is basically a solid state circuit breaker with cell sensing and temperature monitoring. It will monitor charge and discharge currents and allows you to define safety cutoffs:
  • Pack hi/low voltage cutoff
  • Individual cell high/low voltage cutoff
  • Hi/low temp cutoffs (separately for charging and discharging)
  • Charging and discharging current cutoff


You can also enable passive cell balancing, but I turned that off as the PL6 is much better.



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Old 03-23-2021, 08:59 AM   #7 (permalink)
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Default Final thoughts

So whats the bottom line?

My old system:
  • 38 lb
  • lead acid
  • 12 volt nominal, often drops to 11.x v under high load
  • 50Ah nominal capacity, in reality only 50% is usable for a good cycle life
  • Capacity drops with amp draw.. If you pull 30 amps it only provides 30Ah nominal (15 usable)
  • Lifespan is 300-500 cycles, 3-5 years per the datasheet.

New system:
  • 24.5 lb
  • LiFePO4
  • 12.8v nominal, 13v+ in practice even under high load
  • 90Ah nominal capacity, full 90Ah usable
  • Minimal capacity loss with amp draw. You can pull 90A and the pack barely sags and you still get all 90Ah
  • Lifespan... 2000 cycles if you use 100%. 4000+ if you limit to 80%. Calendar life is potentially decades.


I fly mostly 3S 3000s, 6S 1500s and 3700s. Only the old system I could charge the 3700s maybe 3 times before killing the AGM. The new system calculates at least 17 charges on a fresh pack.




All in I spent $300 for the cells, $120 for the BMS, probably $600 all up when you add the components and box (much of which I already had on hand). To get the same *usable* capacity with lead acid would require at least a 150Ah AGM... which would weigh over 100lb and cost about $250 for the battery or about $400 all up.


{Edit: And that's for a turnkey solution. You could build a bare minimum pack with just basic protections for 300-400 and then you are really competitive with a $250 lead battery]



So an extra $200 cuts my carry weight by 75% and at least doubles the lifespan I will get out of this. If you calculated the lifecycle cost per cycle/year its actually significantly cheaper than lead in the long run. Unless you have the power requirements that a Honda inverter makes sense, I think this solution is a no brainer for us. Especially compared to what we spend on everything else in this hobby.



Closing thoughts: While I highly recommend this, please don't attempt it if you are inexperienced working with large batteries, heavy gauge wire and crimped connections. While these LiFe cells are in many ways potentially safer than Lead Acid they still can cause real damage/injury if you make a mistake. Direct shorted these cells can push thousands of amps and weld your watch to your arm, etc. Not fun! Please research, work safely and do not cheap out on tools, wire or components.
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Old 03-23-2021, 10:30 AM   #8 (permalink)
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Nice setup. Just getting into LiFePO4 cells myself.

Would suggest using cell dividers for any ajacent cells in series. Something more substantial than just 2 layers of shrink wrap. With the CALB cells I bought, the entire aluminum shell is connected to the ground. A dead short would be dramatic to say the least.


Happy flying.
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Old 03-23-2021, 10:41 AM   #9 (permalink)
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Quote:
Originally Posted by kavic5150 View Post
Nice setup. Just getting into LiFePO4 cells myself.

Would suggest using cell dividers for any ajacent cells in series. Something more substantial than just 2 layers of shrink wrap. With the CALB cells I bought, the entire aluminum shell is connected to the ground. A dead short would be dramatic to say the least.


Happy flying.

Excellent point... I was concerned about this, but I peeled the plastic cover off one and did a continuity check... both terminals are isolated from the case by plastic and using the DMM I get no continuity from either to the case.

But you have got me to thinking, I may take it apart and put a piece of thin plastic in between just in case.... I already took a lot of precautions to make sure nothing conductive could come near them or the cables, heat shrinking over the clamp rods, etc.



I'm not actually sure who makes these cells, they come from a reseller. Pretty sure they are not CALB.. might be some lesser name like LittoKala or Shensen Basen. Not even sure if they are truly "grade A factory fresh" but for the way we use them I'm not sure that matters so much? I picked them because of the price and the fact they are a lot smaller and lighter than a lot of the CALBs.
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Old 03-23-2021, 10:55 AM   #10 (permalink)
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I thought CALB was that style of battery? Oops. Anyways the cells I bought look like the cells you bought, except for the dimensions. Mine are wider than they are tall. And the blue covering on mine is just heat shrink. The entire shell is aluminum.

Makes me cringe every time I see a youtuber build a pack with these without any sort of cell dividers.
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Old 03-23-2021, 11:54 AM   #11 (permalink)
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CALB is a manufacturer... China Aviation Lithium Battery. Their most commonly seen cells are actually plastic case:





A fellow named Will Prowse seems to be the guru among the solar DIYers and has a page that shows various form vactors and vendors. A lot of those YouTube videos are probably his channel...


I agree that there are a lot of people out there taking a lot of risks in videos... sometimes they have websites full of safety warnings but then in videos they are obviously ignoring their own advice.


The other thing I see is many people on the DIY solar forums diving into this with obviously little to no battery knowledge and are learning the hard way everything we already know from 10+ years working with A123s. I cringe when I see them hook up a battery to a 10amp CC/CV bench power supply and just set the voltage to 3.6 and walk away... but that is what many of them do...


Frankly I think the engineers at FMA could teach these folks a lot (speaking of I had a great email exchange with Tim & Jamie Marks about PL6 settings as I did this) ... and probably could give a lot of great info to the companies making solar chargers but they appear to all be figuring it out as they go. Maybe reading the early white papers NASA put out when they did testing of the first generation A123 26650 cells for the space program. But as far as I can tell most of them are just experimenting and adapting existing gear. A lot of the solar stuff is the same hardware they used with lead acid, just with different voltage thresholds. Even some of the "high end" brands like Victron still appear to use simple time based absorption (CV) charge profiles... per cell charging simply isn't a thing in that world yet
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Old 03-23-2021, 08:35 PM   #12 (permalink)
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Would I need to buy the management electronics or could I charge the cells individually using my icharger?
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Old 03-23-2021, 10:53 PM   #13 (permalink)
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@jharkin - Great job on your project! Thanks for sharing.
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Old 03-24-2021, 07:55 AM   #14 (permalink)
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Would I need to buy the management electronics or could I charge the cells individually using my icharger?

The bottom line, is for our use you dont. Just make a balancing harness and use the A123/LiFe mode on the charger. (a 4S JST-XH balance pigtail with some 6mm/22ga ring terminals crimped on will do nicely) Then when you power chargers at the field make sure to enable low source battery cutoff in the charger settings.... anything over 11v or so *should* be safe enough to protect you from a single weak cell. Id be shocked if you even see much less than 13v in practice unless you try charging 12S5000s at lots of amps...


A simple build could be a 4S pack in a plastic toolbox with a single class T fuse for protection and some welding cables wired to the output. You should be able to do that for well under $400 and then the price is very competitive to lead acid.


The BMS is just extra protection the solar guys use as their equipment doesn't have per cell monitoring,etc. I used one and all the other stuff mostly as an interesting science project, not because it was really necessary. Basically I made a hembrew Goal Zero power station for a fraction of the price. I do get some possibly useful stuff from the BMS like usage logging and thermal/cell protection but its not necessary if you are careful. And Ive seen some horror stories about cheap BMS causing more harm then good so if you use one use a good one. The unit I posed above seems decent and is supported by a USA based company.
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Old 03-24-2021, 08:05 AM   #15 (permalink)
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Some other thoughts...


I did the math on these cells and I get a raw energy density of 155Wh/Kg! If you look at the spec that is on par with high end hobby LiPos like Pulse/Hyperion/Gens.... Really impressive. Much higher than A123 can cells and it tells me that both small format LiFes and LiPos (Lithium Cobalt Oxide) probably have some significant room to improve in energy.


Also, you don have to use the cells I did. You can get these things in sizes as small as 1-20Ah (cylinders and prismatics) all the way up to 280-300Ah. Or parallel/series them for even more.



Small form factor cells are available at Batteryspace:
https://www.batteryspace.com/LiFePO4...Batteries.aspx


For large cells you can either try your luck at Alibaba/AliExpress (cheapest) or if you want really high quality cells from a trusted USA supplier I've been told this is a good source:
https://www.electriccarpartscompany....smatic-lifepo4




If you find this intriguing but don't want to take the risk there are a number of outfits that make "drop in" lead acid replacements. Basically the cells and a BMS inside a box that looks like a traditional deep cycle marine battery. Expensive but fool proof. Battle Born, Victron Energy and Lithionics are some top brand names Ive seen.. . but you will spend 2-3x what I did for the same capacity.
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Old 03-24-2021, 08:18 AM   #16 (permalink)
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But you have got me to thinking, I may take it apart and put a piece of thin plastic in between just in case.... I already took a lot of precautions to make sure nothing conductive could come near them or the cables, heat shrinking over the clamp rods, etc.

You really got me thinking Kavic. I tested a cell again. Continuity test from both terminals to the case shows nothing, but a leakage test shows about 2.7v on the case and 11uA (micro amps) of leakage current from the negative terminal.


Digging around on the solar forum I found a number of discussions on this. Apparently the consensus is that certain manufacturers tie one terminal to the case, but most don't, yet there is still some leakage current as the elecytolyte inside is in direct contact with the outer shell. Seems to be some disagreement whether the blue plastic wrap is sufficient protection. The site admin claims it is, and is what the manufacturer intends... but other point to some battery spec sheets calling for insulators.


I will continue to research...
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Old 03-24-2021, 12:41 PM   #17 (permalink)
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The question I have is does your charger have enough balancing current to keep those gigantic 100a cells in line?

I've been using one of these to balance my 3s 10ah lifepo4 pack.

https://www.ebay.com/itm/NEW-2S-4S-8...r/313224828973

Thats the newer version with a jst-xh connector already on the board. The one I bought is the older style and its good up to 17s.

If you end up getting one of those, make sure to never disconnect any of the balance leads in the middle. Will Prowse did that with the 1.2a version and fried his. Not surprisingly he gave the things a bad review and questioned whether they actively balance.

I also have the 1.2a version and tested it twice. They do in fact transfer current from the high cells to cells at a lower state of charge. The 1.2a one is just too slow for anything over 2,000mah IMHO.
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Last edited by kavic5150; 03-24-2021 at 02:24 PM..
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Old 03-24-2021, 02:16 PM   #18 (permalink)
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Interesting project guys, I can see building one in teh near future. I mostly fly my gaui x3 and nitros. So my needs aren't huge. But i do have a logo 600 on 12s. Maybe I can get some smaller cells that can charge the logo a couple times, and 5x for the gaui.

I tired a mobile charging setup from my car direct before, but chargers don't like when the alternator kicks on and off. This is a viable solution.
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Old 03-24-2021, 02:20 PM   #19 (permalink)
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The Powerlab does up to 1A balancing current.

Yes, it struggles... but the BMS boards the solar guys are using don't even have that... I can't find a spec but I believe its less than a couple hundred mA, just a passive bleed resistor.


The thing is, the imbalance only shows up in the last couple % of charge. Easy solution is just to terminate charging at 3.5vpc and set a low CV timeout. I'd never miss a lost 1Ah or so of capacity in practice.
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Last edited by jharkin; 03-25-2021 at 10:44 AM..
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Old 03-24-2021, 03:09 PM   #20 (permalink)
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And what is the reason you stopped at 12v? Why not make it 24v to make it easier on the chargers?
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