Introduction: Lithium Iron Phosphate (LifePO4) Solar Storage Battery Bank

LifePO4 batteries are fairly new for solar use. My home originally used a 24volt bank of lead acid (see pic on the right). These failed only after a year of use. This was a costly mistake. My replacement bank is LifePO4 25.6volt batteries as in the left photo. These I bought off Amazon for 241usd each.

Each battery has built in BMS for under/over voltage protection, cell balancing and temperature safe guard.
LifePO4 (and any lithium secondary cell) requires 2 stage (constant current followed by constant voltage) charging. In my arrangement, a float voltage was allowed since a load is always drawn by an inverter.

LifePO4 does not explode or catch fire like lithium cobalt (found in cell phones and laptops etc). It does not have toxic chemicals and offers four times the power density at a third of the volume compared to lead acid. For these reasons I felt safe to have such batteries in my Home.

Also the lifePO4 chemistry prefers partial charge and partial discharge. You will have no danger of undercharging these batteries compared to lead acid which will sulphate if undercharged.

If you are serious about solar powering your home, definitely go with lifePO4. Also make your lighting led and use inverter type air conditioners. Be energy efficient and your cost will be much lower.

Step 1: Connections to Each Battery

Since the internal BMS provides short circuit protection, I used a simple single pole single throw toggle switch to provide isolation for maintenance. The current rating of this switch is 20amp and is quite low cost.

For very reduced cost and less materials I soldered the head of a bolt onto one terminal of the switch. This provides a mechanically strong and electrically low resistance connection. To secure the switch to the positive battery pole, just screw in and tighten the nut.

Similarly for the negative battery pole I tightened a nut and bolt. The head of this bolt I soldered a wire directly.

The size wiring is used for each battery is 12awg obtained at a local car audio store. You can of course use a larger cable but the maximum voltage drop when my home draws peak current is 1.2 percent.

Step 2: Metering Per Battery Set.

In order to measure the amp hour capacity of each battery set (I have my batteries in groups of parallel sets), I used a low cost power meter from Amazon. The maximum current through each of these meters in my setup is 10amp as a worst case design load. In practice the current is less than 3amps per meter.

I have parallel sets batteries per meter to allow a low cost metering solution versus a meter per battery. I have saved on cable costs and ancillary hardware (nuts, washers, bolts, lugs).

Generally if the amp hour capacity of a battery bank drops below 60% of the original value then it's time to replace. The energy meters per battery sets will provide such monitoring.

Step 3: Provide a Plug N Play Means to Deliver Power.

My Old lead acid bank was hardwired with large Guage wiring complete with circuit breaker per 24volt string. For my new setup, I wanted an easier way to add/remove batteries from the solar power system.

I used two standard nema-15r outlets each with 6 outlets. Please note that this is not a standard use for such outlets however I put a sign up to warn of the purpose of these outlets. The advantages of this arrangement is that I can safely plug in and out any battery group without affecting the rest of the power system.

The connector on the battery side is a two prong polarized nema-15 plug. I chose the bigger blade as my positive. There is no possibility of the wrong polarity being inserted into the receptacles. Safe, high current, readily available and low cost.

The main cable from the receptacles to the DC loads/solar mppt controller is 2awg that came with an old inverter.

Note that the DC current rating of cables and electrical connectors (not switches or relays) is higher than the AC current rating of the same cable. This is due to no skin effect with DC.

Step 4: Thermal Imaging of Connections.

I took thermal scans of the batteries during night time maximum load. No problems with any of the connections were observed. I also mechanically verified the connections by attempting to rock/rotate each.

Step 5: Having Correct Voltages for Charging.

All lithium technology prefers two stage (constant current followed by constant voltage) charging. My solar charge controller is an outback flexmax 80amp mppt unit. This offers three stage (last stage being float) for lead acids. Below are my settings.

CC: 0.5c per battery or 5amps: total 80amps.
CV: 29.0v
Absorption (CV) timer: 120 minutes
Float: 27.5v

Due to the fact my Inverter is always connected to the Lifepo4 bank, the float voltage would not negatively affect my batteries. If you use Lifepo4 batteries in only standby use then float charging them may become an issue: you will have to use 2-stage (CC followed by CV) charging.

Currently none of the major solar charge controller manufacturers offer profiles for Lifepo4. I have emailed outback power systems and they are not yet willing to make this move. Once you have a programmable charge controller, using the settings I provided will allow you to use the Lifepo4 but please note that there must always be a load attached to the batteries. For example using a Lifepo4 on a desktop ups I will not currently recommend since that float charge in standby service may degrade the Lifepo4.

Comments

author
RafaelA63 (author)2016-02-12

Hi thank you for this review.

Common lead acid batteries are not meant to be fully discharged or even halfway discharged.

My question is... Are LiPO4 batteries ok to be discharged more than 50% and still be reliable for the long run?

author
BigEdB (author)RafaelA632016-12-13

Greetings:

I am running a totally off-grid 12VDC system. The ONLY lead acid batteries you should consider are the "so-called" "Deep-Cycle" or "Marine" batteries. I've been using my bat bank for over 2 years now (Careful care and maintenance!)

Good luck. It's worth it!

Mr. B

author
Mjtrinihobby (author)RafaelA632016-02-12

down to 100% according to the manufacturer.

author
charlesd.parker.33 (author)2016-08-11

Take a look at this site, ElectroDacus. It's a solar controller for lithium batteries, I got one and am really happy with it. There are several youtube videos on the use and capabilities.

author
william.swann2 made it! (author)2016-04-02

Great website. I'm doing a project powering a dancing man off of an 8 cell Li battery pack. The cells are 160 a hrs, and I am woindering about some of the settings on the charge controller. Not sure about the Instructables interface.

DancingMan.JPG
author

So the questions are about some voltages, Specifically equalize,boost and float voltages. I know these apply to lead batteries, The charge controller is prompting for these values, I have the overvoltage disconnect set to 27 volts, as I do not want to get into the part of the charge curve where it ramps up steeply at end of charge voltages, I am not using a BMS yet.

author
TylerS52 (author)william.swann22016-04-06

Friend, BMS is critical for LiFePO! I am using a similar setup but with a 6s rc battery, and let me tell you, lipo batteries are very dangerous and difficult to maintain. In your configuration, you are seriously and irreparably damaging your battery and placing yourself and your house at great risk of bodily harm or/and a fire.

If individual cells are charged unequally (the issue that a BMS manages), which happens as the charging and load voltages of the individual cells change independently due to the "daisy-chain" configuration of the battery itself, you may be overcharging a cell. This causes it to explode and start a fire so hot that conventional fire extinguishers will not put it out, not to mention the poisonous chemicals everywhere, or you will draw current from a cell that may not be charged to sufficient voltage, which requires more current (vicious cycle here), further dropping the voltage, which will deplete the battery below the minimum voltage required to ever be charged again. This then puts a great deal more load on the other cells since the load is "expecting" the voltage that it will not be able to produce, so it will try to accommodate this need with more current, leading to a short/more dead cells/fire.

I would test the voltage of the cells independently, and if there are any differences between them I would recommend safely and completely discharging the battery and disposing of it.

Since you're using a fairly large capacity battery these issues may not have been apparent but they are very dire. I have blown and shorted cells before and it is an absolute catastrophe, using only an 8ah battery. Simple voltage controlling is sufficient for lead acid, not lipo.

Check out electrodacus.com and read up on your safety!!! The world needs us DIY guys to make solar/modular power widespread, don't want to hear about an innovator getting hurt. :)

author
william.swann2 (author)TylerS522016-04-06

I'm with you 100 %. And am developing a BMS. Thanks for your precautionary advice. I look forward to perfecting a BMS, because now I am using just a very small portion of the Li battery. I want to stay away from the extremes of the S curve.

author

BMS is not necessary if wired correctly. I have been using mine for several years without one. https://www.youtube.com/watch?v=HPyeQCkcMUE

author
Mjtrinihobby (author)StevenM1422016-07-30

You are a mad man. All series cell Lithium batteries need bms.

author
AlexeyP21 (author)2016-07-29

Hello, I'm very interesting in using a lifepo4 batteries and the most hidden information is reliable of the battery. How long in terms of days will it hold its capacity over 80% of nominal. I have never saw such information in datasheet from seller of the lifepo4, only lifetime in terms of cycling (about 1500 to 3000 cycles 80 DOD). Have you ever test your batteries for capacity and internal resistance?
Great job! Good site!

author
Mjtrinihobby (author)AlexeyP212016-07-30

The capacity is actually greater than the amp hour rating on its label per battery. Any battery with internal bms, you cannot test the internal resistance. In terms of the self discharge rate, these beauties can hold their charge for at least a year.

Thanks for the thumbs up!

author
morsed2 (author)2016-03-24

WRT.

Step 3: Provide a plug n play means to deliver power.

Good idea on the disconnect for the batteries.

BUT

It is dangerous, very dangerous, yes I did read your warnings. But you only need a John Doe to walk in and connect a battery to an 110v extension cord, and BOOOOM!

Also: The pins of the plugs are uninsulated, and if dropped onto the battery that are connected to, or onto a metal surface, guess what, BOOOM!

Sorry to be negative, great idea, simple to implement, and cheap. I like cheap!!! I am going to implement a similar scheme using RCA connectors, so who am I to criticize?

BUT

I have a suggestion, for $2.00 per connection you could use Anderson Power Poles. I use these connectors for bi-directional 12 volt connections for my Ham Radio equipment. They are genderless (I think that's the word) simple, and fully insulated, and finally, not reversible (within limits). Link below.

http://www.powerwerx.com/anderson-powerpoles/power...

author
Mjtrinihobby (author)morsed22016-03-25

Great suggestions. I just wanted the plug and play feature and I'm the only person who is anywhere near these batteries.

author
sspence (author)2016-03-18

At 60 Hz in copper, the skin depth is about 8.5 mm, so skin effect is not an issue at the low frequencies found in house wiring. DC has greater arc quenching requirements, so switches and breakers rated for ac have to be derated for DC.

author
Mjtrinihobby (author)sspence2016-03-18

I will debate the skin effect with u but the breakers you are correct.

author
FahdA3 (author)2016-01-27

Hi there! Your guide is really inspiring for people like me who are new to the world of solar home systems.

Quick question: I have read on some sites that LiFePO4 batteries are not ideal for solar home systems because they are not deep cycle like conventional batteries. Your article clearly shows this not to be the case. Could there be different types of LiFePO4 batteries (maybe some designed for vehicles, others for backup power etc)?

author
Mjtrinihobby (author)FahdA32016-01-27

LiFePO4 chemistry is perfectly suited for deep cycle and engine start applications. To date all my LiFePO4 batteries are performing admirably. My wish is that the prices will drop in the near future. There is only one type of LiFePO4 unlike lead acid that has a few variants (Gel, AGM, Flooded, various plate alloys etc). I hope my response was helpful to you.

author
tiggeroush2 (author)2015-12-29

Nice setup, the way battery prices are going the lead battery will to costly to buy. Nice to know LifePO4 can be used.

author
Mjtrinihobby (author)tiggeroush22015-12-29

Best solar battery in my opinion.

author
Fabv2 (author)2015-12-16

Hello Mjtrinihobby, thank you for a great instructable! I have a couple of questions, how much does your battery bank powers in your house? What kind of solar panels do you suggest using? Thank you!

author
Mjtrinihobby (author)Fabv22015-12-16

Those batteries power everything including one of my air conditioners for one night period. The panels I use are monocrystalline for better efficiency.

author
Fabv2 (author)Mjtrinihobby2015-12-17

Thanks!!! There is a great feeling and excitement when you do things that make a change...

author
FredericC2 (author)2015-08-06

I am attempting to switch my yurt solar from lead-acid to LiFePO4. In doing so, I bumped into the same issue with chargers. Now, I use RC chargers/balancers for my LiPo batteries and some of them now have a profile for LiFePO4. These chargers/balancers require a 12V DC input and need to have a wire running to every single cell. Do you think that one could be used with a solar panel?

Here is an example of such chargers:

http://media.hyperion.hk/dn/eos/

Thank you for a clear and informative instructable.

Frederic

author
Mjtrinihobby (author)FredericC22015-08-07

No. Definitely not. Those chargers are meant to be powered from a stable dc supply. They are not solar charge controllers. Please don't use them like how you suggested.

author
FredericC2 (author)Mjtrinihobby2015-08-19

Thank you for the reply. I will continue following your instructable.

Frederic

author
Mjtrinihobby (author)FredericC22015-08-19

No problem and thanks for the appreciation. Solar charge controllers are purpose based battery chargers. Trying to outfit a household powered charger for solar use is a really bad idea. Once you are dealing with significant solar power, its better to buy a reputable solar charge controller to avoid the "OH CRAP!" moments.

author
samuel vermeulen (author)2015-06-17

tp.pa.12 hello

questen? can you plais send me a pictuer from te battery plais

thank you very much.

email samuelghoost@gmail.com

author
alwayshungry (author)2015-05-30

Bob

author
tp.pa.12 (author)2015-05-21

Morning, I just used a Photoshop type program to edit your picture, it took awhile, but I figured it was better to do that, than try to put it all in words. You still can insert switches with each wire for you have your disconnect, or wire connectors that come apart, but then of course you want quality connectors for you don't add resistance.

You do have a nice set-up though, your imagination is the limit on what you can do with it if you wish, and the worst part, the capitol.

Tp

author
Mjtrinihobby (author)tp.pa.122015-05-21

i know alot of car dudes have profile pics of lovely chics. this is the first time I'm seeing an instructables member have a pic like that. good one!

author
tp.pa.12 (author)2015-05-20

Here is a picture showing the best way to hook a battery bank in parallel for equal charging & discharging.

Best Way To Run Parallel.jpg
author
dw96790 (author)2015-05-03

Hi, My name is Tinkerer yeah big surprise. I own the site Gajetest.com. I had all but given up hope on working with the LiFePO4 batteries for I feel they are the best for renewable energy systems. I have a couple of questions that you seem to understand and address. I just want to be sure. I have found in the Asian market farther advanced power charger/controllers that are adjustable. I have been told in which your informative article debunks that LiFePO4 48 volt system can only be made in a factory, and if one cell goes bad the whole system shuts down.

Now if I am using say 30 3.2 60ah LiFePO4 and want just want around 48 volts to run my inverter, is this possible? I want the (ah) as they make the Kilowatt hours I need for backup systems. Or do I have to install a voltage control to lower the D.C. volts?

I have been told you cannot tie together the LiFePO4 48 battery packs to make more power like you could with lead/acid. This I was told was; because, of the BMS controller issue. You cannot use more than one BMS at a time. Yet on this site I have seen every battery having a BMS system. I have an advanced inverter that I can adjust the amperage going to the batteries in charge cycle. Since usable suntime at best is 4 to 5 hours a day even with my tracking system. I want to use as much amps as possible to charge the LiFePO4 battery pack. I deal in many sustainable generators and want the rapid charge ability of the LiFePO4 .

The batteries I have been looking at can handle 100/150 amps charging. This will enable me to make the most use of the Solar, Wind, Hydro, or Biofuel generators I have found. I have seen the battery wiring diagrams, and have a choice of a multitude of BMS controllers. Or should I just rely on my adjustable charge controller to power the pack? Of course it has float, MPPT, battery overcharge, dumping all the bells and whistles that is why I chose it over all other inverters. I know this is long and thank you for taking time to read it your information and pictures has set my company back on path. To reach me use Tinkerer@Gajetest.com my site Gajetest.com is a site where I promote all kinds of renewable /sustainable enrgy systems.

Thank you

Tinkerer

author
Mjtrinihobby (author)dw967902015-05-05

Also, a side note on charge controllers. The major manufacturers "Morningstar, Outback, Midnite) currently do not have profiles for LifePO4 charging. The Lead acid profiles will suffice once properly applied. The emerging LIFEPO4 charge controllers are a nice addition to the choices on the market but I personally will wait until reliability issues are worked out on those units before I replace my current Outback and its spare.

author
Mjtrinihobby (author)dw967902015-05-03

Hi. For any Lifepo4 battery bank I recommend a bms per battery string. This provides under/over voltage protection plus cell balancing. My outback mppt fm80 can handle up to 60v battery bank so I theoretically can recharge a 64v Lifepo4 bank by adjusting the settings on the fm80. Yes you can safely have strings in parallel (each with its own bms). My batteries charge safely and thermal scans have not shown any appreciable temperature rise (highest being 7c above ambient). With Lifepo4 or any lithium you must use two stage charging. That is, constant current followed by constant voltage. Avoid float charging without a DC load attached! Private message me for any further information and I will try my best to help.

author
lusch (author)2015-04-15

I am amazed by your competence and curiosity in tinkering and solution finding ! I also want to complement your environmental awareness ! Great Instructables, throughout!
Greetings from Germany!

author
Mjtrinihobby (author)lusch2015-04-15

Wow thank you for that very positive comment! I have always been into electronics since i was 6 years old and as an adult I just love to improve systems or make new stuff that fits my automation or energy needs. You can view my other instructables if you wish. My aim is to try to give other hobbyists information to help them as they have helped me in the past.

I learn from my failures and I love to share my successes/lessons. Stay tuned for more instructables i plan to publish soon (eg. wind turbine installation at home!).

author
dasimpson1981 (author)2015-03-20

i have a cc cv unit and i have set the unit to the float voltage of the bettery bank in my case 4.2 volt i find that it does the 2 stage by simply once it hits the 4.2 volts it can not allow current as the forwed path for electrons are gone with the input voltage been equal to the battery voltage and seems to work well all be it never really get over 1 amp do to clouds but it will take upto 4 amp so a 40 whatt or so

author

Hi,

The batteries you mentioned are lithium ion yes? at cv setting of 4.2V this indicates Li-Ion. The batteries i use are LiFePO4 which are extremely safe. I dont want to discourage you but those Li-Ion batteries you are using are dangerous if overcharged or charged/discharged too fast; they will catch fire. 4.05V is a safer voltage for them. Also, lithium batteries cannot accept float charging like lead acid. I always have a load on my batteries so they never actually float even though the OutBack FM80 does enter the "float" stage.

Please my fellow hobbyist, be careful if you are using Li-Ion on a solar setup for home/heavy power (not portable power packs for charging cell phones or mobile electronic devices). They are many good resources on the web (eg.

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

) that can provide information to you.

All the best!

author

i tested with a single cell first the i have 12v 4 amp solar panel arrangement they go to the adjustable cc and cv unit is set to 4.2 volt and 4 amps this is dropped down i put a 600mah cell on and left it i outside in a metal box and what i found was the closer the battery got to the 4.2 volkt of supplied voltage the less current was drawn once the battery reached the 4.2 volt they was no current flow at all and left it like this for a good couple of week and nothing changed it just stayed like that so it would run content current untill the 4.2 volt was reached then stayed in the constent voltage range i am now running a 1s70p pack that charges my phone every night and runs a couple of usk led bulb looking things so all seem good but thanks for your concern if you would like to see what i have done please take a look here https://www.youtube.com/watch?v=tUQ_ws7plos

author

Very nice setup. Please keep me informed on how the system behaves in the future. Congrats and all the best! You definitely have my interest here.

author

i also find the battery pack never quite reaches full charge do to the battery pack been 140ah and the solar panels only putting in max 4 amp but from what i see the avarage current supplied to the bank is 1 amp

author
slamdstorm (author)2015-03-14

Excellent system and explanation. I'm considering installing solar energy as soon as my lack of knowledge and resources is remedied. I'm a beginner with just a basic understanding of the technology but this has helped a great deal. What type, size and quantity solar panels are you using? Have you considered posting a full wiring diagram for a beginners reference to go along with this? If not would you recommend some online references. Thanks for your time.

author
DacianT (author)2015-02-26

Hi Mjhobby,

Glad to see another user of LiFePO4 for energy storage. Not sure you seen my Solar BMS That may have made things simpler for you.

I needed to design that for my OffGrid house since nothing is available. For long battery life is better to carge LiFePO4 with just CC (constant current) and not use the last part constant voltage that will not gain much extra capacity and will just degrade the battery since it dose not like to stay at that top voltage.

Is not great to have this batteries with build in protection since if that works correctly will cut your power when any cell gets to 3.6V or whatever they set.

I can guess they used that 3.95V IC made for LiCoO2 and that will degrade the batteries much faster than necessary.

Anyway contact me I will love to have a conversion with you. You can easyly find me if you search for Solar BMS

author
Mjtrinihobby (author)DacianT2015-02-27

Ah! I'm very happy to meet another brave soul who has experience with these lifepo4 solar batteries. I think I may be the only one in the Caribbean using them in this fashion. My mppt charge controller can't do constant current alone. It's hard coded for nasty lead acid. My god.
I agree with your assessment. My implementation is based on cost and available materials at the time. We must converse much more my fellow hobbyist!

author
DacianT (author)Mjtrinihobby2015-02-27

I will be glad to do that (converse more). I have almost two years of OffGrid on my LiFePO4 battery and it works great. In spring I will do a capacity check in exactly the same way I documented on my youtube video so I will find out how much they degraded. I do not expect much at most 2 or 3% but I will see. They were used quite hard you can see from a 7 day energy production / consumption graph I just made at the beginning of the month just scroll down to bottom on my webpage electrodacus.com there is also my email address there.

author
Simulation314 (author)2015-01-07

Hi Mjhobby, this is a nice system, I never listen about this Lifepo4 batteries, could you tell me how long is the average lifetime of this kind of batteries?, just anoher ask /question: ¿Usted solo usa estas baterias para alimentar su casa?, ¿es suficiente la potencia que le entrega el sistema?

author

Lo siento amigo. No comprendo mucho del espanol. Puedes escribir en ingles?

Yes the lifepo4 battery bank provides all power to my home. They are the safest and these I use have built in BMS protection.

author

Lifespan is 2000 cycles at 100 depth of discharge. For me approximately 8 years using my software management setpoints.

author
skepticaljay (author)2014-11-22

wonderful write-up. good job.

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