LiFePO4 batteries for 12V AUX?

I'll keep you informed, this is interesting for me too - if I am quiet for too long, please try to ping me somehow
I am still more worried about over charging, as there is not a defined float charge voltage for Lithium batteries. At least I could not find any reliable info on that. But LiFe's take over charge a lot better than LiIon's, so that is not a safety hazard. Worst case scenario is that the battery lifetime would be limited.
If it will not be fully charged, with LiFe you'll just end up with lower SOC (which means less usable capacity). In that case a simple work-around would be using AC charger at home. Here in Finland it's pretty common to have engine and interior heaters, and the car already had fixed AC charger for 12V battery when I bought it, so I am ready for that.

I'll do some charging current measurements this weekend, and later I'll add a voltage monitor so I can try to estimate SOC on daily basis.

 

The LiFeYPO4 nominal voltage is 12.8V (3.2V x 4 cells) and the maximum voltage at full charge is 14.6V (3.65V x 4 cells).
If the Prius can't go up to 14.6V, then it shouldn't charge to 100%.
That actually can be a good thing for two reasons:
1. It's safer since this battery doesn't have a protection circuit built-in. The chance of overcharging and starting a fire is less likely.
2. Limiting the state of charge cycles to less than 0% - 100% should increase the life cycles. One seller had this battery listed for 3K Cycles @ 80% depth of discharge. You might end up getting that or even more cycles.

(Winston Battery Specs - Shenzhen Smart Lion Power Battery Limited
More info at vendors: EV Assemble (China), GWL Power / EVPower (EU), and EV Works (AU))

Be careful.

 

I checked today battery voltage with Techstream. 12.7 volts when the car was started without break pedal. When fully on the voltage rose quickly to 14.7 volts.
I do not know the current the battery was discharging at, when 12.7V was measured, so this tells nothing about current SOC :-/

As battery power consumption is pretty small when the car is off, the battery should be fully charged 99% of time. SOC is close to 100 % or even above. Lower SOC would be better for lifetime expectation.

In case there is some temperature compensations, it was around 10 deg C. And found one piece of interesting information: "Dist Batt Cable Disconnect: 1099 km". So now I have had LiFeYPo4 over 1000 km
Is it possible to read the battery temperature or current with Techstream?

 

14.7V (3.675V per cell) seems safe. LiFePO4 can safely tolerate up to 16V (4V * 4 cells). A 4 cell LiFePO4 charger will switch from constant current mode to constant voltage mode at 14.6V (3.65V * 4 cells). At 14.7V, the battery will eventually get to 100%.

The lowest recommended it should go during discharge is 11.0V (2.75V * 4 cells), or more conservatively 11.2V (2.8V * 4 cells). In general, the higher the cut-off voltage during discharge, the more cycles the battery will last. So a cut-off of 11.6V (2.9V * 4 cells) will give more cycles than 11.0V or 11.2V.
LiFePO4 works well at temperatures higher than Li-ion (LiCoO2) can. Cold operation is difficult for all batteries, especially charging which should be done at lower currents in extreme cold. I haven't seen temperature compensation curves for LiFePO4.

Maybe you can borrow a clamp-on type current meter (Fluke 325 or similar).
I don't have Techstream, maybe someone can post about that.

 

A winter update

Summer time here in far-North country went fine. No issues.

Now that it got colder I had one LFP -incident. During the night it was -25 deg C (== -13 deg F, according to Google). In the mornign car would not start - just a lot of lights in dash.. As it was a working day morning, I was in a hurry. Quick measurement from 12-v socket showed below 10 V voltages, when trying to start the car. I decided to connect an extrenal charger. It charged for about 15 s with 2 A current, and then I started the car. Everything worked fine after that.

My scientific method failed here miserably So I am not sure if the root cause was just cold, or did I have an empty battery for some reason.
* If I had measured the idle voltage of the battery I'd have an approximation on the battery level
* I should have measured the actual battery temperature (I had had interrior heater on for 2 hours, but the battery is quite well hidden in the trunk)
* I should measure also the current the car takes in cold mornings - my DIY-electric-car neighbor estimated that a full Winston battery would give only 1 C current (that's 40 A) with 2.5V/cell when down at -20 deg C. I'd guess that the car should take something in range of 10-30A, when starting.
* I should have tried slower startup procedure: press the break, wait 30s (alllow voltage to rise after break booster has taken it's share), then press the start button

If the real issue was with just battery temperature, then the best work around for even more extreme case would be large current consumption for some minutes to heat up the battery. E.g. all lights and heaters on, pump break (get the break booster running) for 2 min, turn all off, wait 30s, and start. I hope I can test this some day

Now we have here only -20 deg C..

 

Gentlemen, I constantly monitor the Prius charging voltages with a Scanguage II attached to the dashboard, and the Prius DC charging system seems to have two distinct states: a 13.6+/-0.1 volt when the 12v battery is assessed to be fully charged and the 12v demands by all other accessories is low, and the “charging” state where the system voltage is 14.6+/-0.1 volt. Thus, I routinely see voltages at 14.7 volts when the 12-volt AGM lead-acid battery is slightly discharged or demands are high, and I have seen spikes to 14.8 volts, although those have always been transient in my experience. So if the limit of the LiFe PO4 battery is strictly 14.6 volts, there might be some negative effects in the long run. Good luck and keep us posted!

 

Summer update

The rest of winter went fine, no problems. Very cold weather shows the increased internal resistance with Prius: You could hear that the break booster was running slower on very cold mornings, but no starting issues other than the one mentioned above.

The Winston LiFePO4 has been in use over a year now, that's about 40 000 km. During coming years it will be only 10-15 tkm per year, as we changed the other car to electric one - it's nicer to drive and cheaper so it is always used if it is available (and range is enough and 7-seater is not needed).

Current and accurate voltage measurements are still pending, I have just been lazy in that respect.

 

Since the battery is smaller, would it be possible to use 2 LiFePO4 batteries in place of the OEM battery?

 

You can parallel connect LiFePo4 -batteries, assuming they are same technology. Ideally they should be on same charge level when parallel connecting - then you should not get any sparks
Just be careful, all batteries of this size are dangerous if shorted.

 

Hi Jarkko, I was going to buy one of these lifepo4 batteries to replace my aux battery. Since its been longer, I was wondering if you have any updates? Any problems so far? I am in South West United states so it rarely freezes here, but it is sometimes 120 degree Fahrenheit . Thanks for being the only person I have seen to try this. I might be number 2!

 

Someone was pinging me to update this (stolen) thread.
My LiFe installation is doing fine - no problems. I installed the battery 4/2018 at 182 000 km. Now the car has about 224 000 km.
This winter has been extraordinary warm here, so no real cold weather testing this year. Last year experience is described above.

 

Thanks Jarkko! I’m going to give one a shot from amazon

 

Short update. The car is still in low usage, now odometer says about 235 000 km.
Last night was quite cold about -22 °C, and I had the same christmas tree light show. This time I stayed calm Just switched car of, press break (and wait for brake booster to finnish), and start - all fine.
So in extreme colds 40 Ah LiFePo4 battery is not sufficient without small extra effort.

In springtime the car was not used for 3 or 4 weeks. No problem there.

 

I finally managed to get some voltage and current measurements with 12V LiFePo4 battery. The measurement system is installed there semi permanently, and all the data is collected to home automation system (Home Assistant).

The measurements are based on a hall effect current sensor, ADS1115 analog to digital converter (for current and voltage), ESP32, BME280 (humidity and temperature). Software is ESPHome, so the measurements are available only when the car is at home Wifi. Hardware could support more data logger features, but I did not want start another software project that I would never finnish . Current measurement accuracy is around 1A, and voltage should go around 10's of mV.

First an idle day picture. Above part are temperatures: trunk temp, dew point, and outdoor. Lower part is the voltage, soon after parking the car after yesterday's drive.


The second picture is a lot more interesting, and more complex too.

From the voltage you can see the time the car was away from home (current is filtered in home assistant, so it draws a straight line when there are no new samples). The low peaks in voltage are basically brake booster running before the car is started. This dip is heavily temperature dependant. In -10 deg C it goes to 11.x Volts. The smaller dip before 2 o'clock is because the battery was warm and very full (I had been driving for 3 hours). Charge voltage does not seem to go too high (I think I have seen it well above 14V, but I have not documented it). When the car is switched off the voltage starts drop slowly, as seen in the above picture.

Current is harder to read. Positive current means draining, and negative charging, i.e. it's current from the battery. Straight line below 0 A means that car is out of our Wifi network (home assistant remembers the last measurement). Brake booster peaks (before the car is started) are 10-20 A. Current into battery seems to peak above 20 A. I do not know how fast the current shrinks, as I tend to leave home soon after starting the car .

 

@Jarkko This is the first time I've seen this thread. I hope the OP is not offended / upset that his thread was (stolen). I would hope the OP was as impressed with your contributions to this thread as I was was after I read all the post from post #1.
A little about me, I've also been Very impressed with apps originating from .io (I'm a relatively new comer to io), I believe for about a year or two. ie: utox hashcat etc etc etc etc etc....... satoshi .... list goes on and on and on....

I read your post above FIRST and immediately searched Home Assistant, as I already have Hybrid Assistant in use periodically.

I'm in NY US and I too am cognoscenti / concerned with LOW temperature effects on Batteries of all kinds.
Like you I add heat to the car cabin as temps drop LOW. I also add grille block since I'm sure my 17 Prime Plus grille shutters don't close,

I'm impressed by every post you added above, and thank you so much for all the data included.

I have an 06 Civic Hybrid that suffers in the winters and have been hoping to find an economical way to supplement it's small 154 Volt nimh pack for going on 9 years, since November 2012. But since I have the Prius Prime to learn about now, I'm in no rush to make any rash decisions about how to proceed with my still road worthy 06 Honda Hybrid winter problems.

The LiPo reports you've provided above for the 12 volt Prius replacement battery are excellent , especially the recovery / LiPo voltage rebound, during extreme cold ambient temperatures on the 12 volt LiPo pack. I use supplemental heat with low speed warmup to reduce the stress on the old ailing battery in low temperatures.

I'll have to do some more reading from the Home Assistant page, can you recommend a wifi OBDll adapter that works well? Or has the devel included that on the web site somewhere?

Many of us are interested in learning as much as possible about Prius programming and functionality.
thank you again, from the other side of the pond..

 

A slow response on stolen thread, but I have not many on-topic comments now So far think I was still mostly answering the original question: how LFP works as Prius aux battery.

Home Assistant is totally offtopic. It's just about home automation, and in this context it's just for storing the data. For OBD-adapters mini-VCI is the cheap solution for all diagnosis work. But if anyone is interested in implementing something similar to hybrid assistant, then I'd recommend checking out OVMS -project. But this would require own thread - feel free to ping me if you start one

Back to original subject A comment on Robert's and Petrodollars posts. 4 x 3.65 V is a typical voltage for 100% SoC (State of Charge) on typical recommended charge curve. That would be the constant voltage applied until charge current drops to about 0.1 C, and after that the charging should end. If the voltage stays above this for some time on full battery, it definitely has negative effect on LFP battery life. But how big that effect is, and how often the battery is really full?

 

I've been using one in my parents' Prius v for 2 yrs and 5 months now. Bought the S46B24R size off eBay from China. JIS batteries do not have the lips on the bottom for bottom mount but mine did come with them, so it doesn't sit flat. They said they would make changes and I believe that specific vendor now gives the option for "edges". (That's what I called them when I reported the fitment issue. Their English isn't perfect but response time is quick even on weekends IIRC). It has been one of the best investments I've chosen. It weighs a little over 9 lbs compared to 26 lbs for the original.

If I could afford them for the larger batteries in other cars in the household I would buy another in a heartbeat...

Here's my thread on my experience:

I’m in Love, With Our New 12V Battery! | PriusChat

moto g(7) power ?

 

I also want to try, but I heard that its explosive power is still not enough

 

I am thinking about doing this for my 2nd GEn Prius. It seems that you are not using a BMs and just simply charging using the car.

If I wire 4 Lifepo4 prismatic cells in series, should this work? I know I would need to have a case, terminal protection, etc.

Maybe 4 cells like this:
https://www.dhgate.com/product/3-2v-40ah-lithium-ion-cell-3-2v-40ah-lifepo4/517092452.html?f=bm%7cGMC%7cpla%7c911480847%7c43142548502%7c517092452%7cpla-295272882166%7c103006001%7cUS%7cliuzedong3333%7cm%7c2%7c&utm_source=pla&utm_medium=GMC&utm_campaign=liuzedong3333&utm_term=517092452

 

Can confirm lifo4 are usable with current prius setup (gen 2). Also have been using 4 18650s instead of the 12 volt aux for a while now with no problems. I hear it is unsafe but so is living life, at 4.2 volts "fully charged", they never really reach "full" but if 8 volts will start the computer so...no explosions yet....also, BMS is for pusies. And people with money. Who aren't trying to kill their pri.