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4S LiFePO4 as a 12V system

Posted: Feb 10 2018 9:31am
by spinningmagnets
I'm just starting this thread as a place to park useful info about 4S LiFePO4 as a 12V battery pack. Depending on the components the pack is powering, the charging system desired, and the LVC needed...using 18650 Li-NCA or Li-NCM can be too low at 3S or too high at 4S, however LiFePO4 has a lower nominal voltage that makes it useful as a lead-acid replacement at 4S for providing 12V. In spite of the recent improvements in 18650-cell Li-Ion systems, some people still would prefer the perceived safety of using the LiFePO4 chemistry.

LiFePO4 is very fire-safe, also
It is known to last thousands of cycles if cared for properly.

The downsides are that since they are not used very much, there are fewer selections available. Headway cells are well-regarded, but the smallest 38120 version is large [5.5inches long, 120mm], so most packs are physically large rectangles.

Also...since the nominal voltage is around 3.2V per cell, it takes more cells to make the same pack voltage compared to the 3.7V Li-Ion selections. For instance, 48V in LiFePO4 is generally considered to be 16S, and using common 18650 Li-Ion cells, it is only 13S...

It would appear that E-motorcycles and EV-cars want the most power-dense cells. 18650-cell packs rarely catch fire, builders are adding individual-cell fuses, and more robust housings...both of which help with a wreck when you crash...

The most passionate information about using LiFePO4 instead of lead-acid are from boat, RV, and aircraft forums. They want the compact power of lithium, along with the long-life, but...they all deeply fear any kind of fire.

Although the factory recommended charge is to 3.65 per cell, they can be over-charged up to 4.2V in order to make certain to get them fully charged (I don't like doing that). The point I want to make is that they can be charged to 4.2V without the electrolyte disassociating and creating gasses, leading to less contact between the collectors, leading to heat (same current flowing through a smaller area of collector) which then causes more off-gassing in a downward death-spiral. This is part of the reason for their perceived safety. If your 3.65V charger goes out of whack 10%, the battery will still be under 4.0V

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 9:31am
by spinningmagnets

"DIY Portable LiFePO4 Power for Ham Radio" (Headway cells in 4S)

2.0V-3.65V max range SOC (it appears there is no usable range under 2.8V per cell) ... 5#p1298285

Starting a V6 car with Headways, 4S / 1P

Load-testing 4S / 1P Headways at constant 80A (sags from 13.3V to 10.5V, at 5 minutes cells got up to 122F / 50C)

The well-known orange end-caps are 40mm square, if you want to calculate the physical size of a pack. The stock bus-connectors are thick copper bar with a nickel coating.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 9:31am
by spinningmagnets
reserved for additional info

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 11:10am
by Alan B
I have used 4S LiFePO4 for 12V systems for many years in Ham Radio and other things. Recovered cells from DeWalt A123 type packs that used to be common, Headway Cells, small 4S packs from Zippy via HobbyKing, quality A123 cell packs from Buddipole, packs from BatterySpace and most recently a larger pack from BioennoPower. I also did one with the 20AH rectangular cells that I got from an electric car place in Sebastopol, don't recall the name.

They all performed well. The little 4S packs from Zippy all swelled a bit, but they never stopped working. I did have some loose A123 cells that eventually self discharged through neglect and were discarded. If they get too low sometimes they can be slowly charged and recovered but not always, and at some voltage they do become damaged and lose capacity.

One benefit for Ham Radio is that these small packs deliver a lot of current, and they do most of their capacity at a voltage above 12.5V. The higher power HF radios (generally 100 watts) prefer voltages well above 12.0V to develop full power and minimize distortion in the final amplifier. Lead batteries are not very good for this, so LiFePO4 are a better fit.

Other lithium chemistries with 4.2V peak and 3.7V nominal outputs do not work well with this 12 volt equipment. 4S peak voltage is 16.8V which is more than most of this equipment is rated for, and 3S is too low, delivering 11V nominal for much of the charge cycle. A few radios are optimized for these lower voltages, but most equipment performs poorly compared to the nominal 13.6 or 13.8V rated nominal voltage.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 11:26am
by spinningmagnets
Awesome info, thanks!

Charging and LVC methods

I have a 100W 14V solar panel for emergencies, and 12V car battery chargers / alternators often actually put out a variable voltage around 14V. The 4S LiFePO4 at a fully-charged 3.65V would be 14.6V.


If I drain to only 3.0V per cell for long life (and also for safety, so it has a margin to stay away from too low of a voltage),

How do I add a reliable 12.0V LVC?

Digital voltage readout with adjustable low voltage alarm, $5 ... 6442!US!-1



Can I use a dumb 12V car battery charger (13.8V output), and add some clever and cheap device to reliably limit any accidental overcharge?

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 12:43pm
by e-beach
Sounded like you pounded your Headways! :lol:

I will be moving into the 12000 mile range on my 12s 1p 15a Headway pack sometime in late March or early April. They do however sagggggggg on the hills now. But that gives me an excuse to actually peddle.

I was thinking about a "hybrid" Li-ion/Li-Polymer Battery Pack and LiFePO4 electronics set-up myself.

I was thinking, for longevity putting together a 18650 pack with a LiFePO4 charger and bms. That way all cells would be charged to 3.65v and balanced at that voltage. That would be 14.6v in a 4s pack. The fact that the cells would never charge to 4.2v would (theoretically) contribute to longevity. A 12v cutoff or alarm would be helpful. One wouldn't get maximum watt hours out of this setup, but it could last for a long time.

Edit: BTW I have used lead chargers 13.8v more or less on my LiFePO4 pack on many occasions. Doesn't charge to full voltage, and the bms won't balance, but it did give me watts when I needed them. Just make sure you don't over-amp the cells with a big charger.


Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 1:23pm
by spinningmagnets
Thanks! There are several amp-output versions of the Headway cells ("HP" version shown above, 20C, 8Ah, 160A), the continuous 80A test for five minutes was the highest amp version. He dead-shorted the weakest cell at the end and got over 300A for a few seconds.

The factory advertising claimed 160A continuous, but they got to 122F / 50C at a true 80A continuous by the end of a five minute test. The temperature is what I use, so I would say these are a true 80A continuous cell (this is why he probably chose 80A for the video test, after the initial testing beforehand). That is truly awesome performance, is not a 160A Cont. cell...

Like most cells, you can get higher amps or higher capacity (in the same format and form factor), these are the smallest Headways at 38120 (38mm dia, and 120mm long / 5.5-inches). The standard blue-shell Headways are 10-Ah, at a claimed 100A Cont. but...based on the video, I'd guess they're probably closer to an actual 50A (temporary PEAK amps for most lithium cells I've studied is typically double the actual continuous capability, based on heat)

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 6:41pm
by Alan B
Batteryspace has some BMS's for these.

BioennoPower has built-in BMS's on their packs.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 10 2018 9:23pm
by e-beach
This company claims to adjust the bms to suit certain needs. ... 138V1.html


Re: 4S LiFePO4 as a 12V system

Posted: Feb 11 2018 8:06am
by spinningmagnets
Thank you e-beach. That is a very good and useful link.

On a side note, Harbor Freight Tools / HFT (yes...I know)...they have water-resistant Pelican case clones now under the Apache brand name. I hate when China clones something, but...Pelican cases are already made in China so...whatever. Apache brand is roughly 2/5ths the price of Pelican.

Three sizes, the middle size interests me for a 12V / 4S-Headway suitcase battery pack. If you remove all the foam inside (it is full to the brim with "pick and pull" foam squares) the interior dimensions are approximately 12 X 9 face when you open it like a laptop computer...and 5 inches tall (removable one inch thick foam in the lid). This size is $30, and they have one size smaller and one size larger. This 2800 model is popular with owners of large pistols.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 12 2018 8:02am
by Alan B
I use one of the cheaper 11 inch Harbor Freight cases for enclosing my 4S2P 32AH Headway pack made from the larger 16AH 40mm cells.

The same type of case also fit four 20AH rectangular cells as well as the compression boards those required.

These cases are not water sealed or as thick or tough but they are adequate for most uses.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 13 2018 12:44am
by spinningmagnets
Alan, if you were buying Headway cells today, would you have one or two retailer recommendations?

Re: 4S LiFePO4 as a 12V system

Posted: Feb 13 2018 10:44am
by Alan B
Sorry, no up to date data here. I bought those in 2010 when I started and was planning to use them on my first ebike. Bad idea.

Today I buy packs from BioennoPower or BatterySpace, and they're not Headway cells.

Talk to Matt at Empowered Cycles. He built many ebikes using Headways, which he no longer uses, but he would have a more recent idea about suppliers.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 18 2018 3:08pm
by EbikeAus
Huge fan of the LiFePO4 chemistry here!

I have a largish off grid LiFe solar system (160 ah @ 48 volt).

I also have a few spare 60 ah cells. I've been playing with one 12 volt pack recently doing some basic tests on different bms.
I currently have the Chargery Power BMS8 connected.

I've been charging this pack using an old 19v laptop charger via an ISDT Q6 Plus charger.

Re: 4S LiFePO4 as a 12V system

Posted: Feb 18 2018 9:42pm
by spinningmagnets
Thanks for posting EbikeAus. Lately I have been thinking about putting together a hybrid system. Originally, I thought I would get four 20-Ah high-current flat foil packs from EIG ($220). Now I am leaning towards four large LiFePO4 15-Ah cylindrical cells, and a capacitor bank to help with burst amps. It would provide 200A burst current for a few seconds. Other than the boring #1 job of charging cell phones, flashlights, and laptops in a short power outage, it would also be...

2. Car starter boost pack, portable...

3. Power supply for my spot welder, so I don't have to rely on a 130C LiPo pack (200A burst required). Uses a common 12V car battery charger...

It appears as though for the capacitor bank, the minimum to make a couple of start attempts on a 4-cylinder engine is...six of the Maxwell 2.7V units, or five of the Amperics 3V units. Both are about the same performance and price, but the Amperics is one cap smaller, and they come with overly-thick copper buses for low resistance...

edit: the Maxwells have a much lower ESR compared to the Amperics. So...I just bought six of the 2.7V's, 3.2-mOhm...The Amperics have 7-mOhms ESR, which is fine for starting a car engine, but not for powering the kWeld...

edit: It appears this system will not work for the spot-welder, but I am still building it. The six Maxwell SC's showed up, but...I was about to pull the trigger on the Headway cells, and I stumbled across an unexpected offering of rare LTO cells. I bought six to play with now. However, I plan to build a second case that will definitely use LiFePO4. The resulting cases will be tasked with:

1) Starting a car engine that has a dead battery

2) Back-up 12V power for cell phones, flashlights, and laptops.

The cases will be chargeable by a car's 13.8V charging system, a homes 120V AC wall socket, or...a 14V solar panel.

Re: 4S LiFePO4 as a 12V system

Posted: Apr 19 2018 6:01am
by marcexec
Just wanted to add my experience from a while ago (38120P) cells:
- drop-in without BMS (which are obviously much cheaper these days).

Re: 4S LiFePO4 as a 12V system

Posted: Apr 22 2018 7:09pm
by parabellum
Here are some very affordable 18650 A123 cells, 30A discharge, 75A for 10 seconds. Also Vruzend V2 kit is already out 20A rated but sure can take short bursts much higher. I am seriously considering to assemble starter battery to replace my dying LA pig. ... =9&t=93396