DIY LiFePo4 Solar Battery

[chrisblessing]

In "Mobile Solar Power Made Easy", in Will Prowse' description of how to bottom balance, he suggests that we "...put entire battery on a charger and charge up to 14.4. Check the cell voltages while charging..." May I ask how it's possible to get the voltage that high in the process of bottom balancing my 4s pack? Currently my MPPT charger registers the battery as full at around 13.6v.

While I know that 13.3 is ideal for longevity, I'm trying to get past this bottom balancing stage and want to make sure I get it right.

Thanks to all for any input. I'm loving this little DIY adventure.

 

[Will Prowse]

That was never covered in that book. DIY LiFePO4 packs are covered in my videos and website, not the book.

What type of mppt do you have? Sounds like it does not have a standard charge profile algorithm. And SCC does not know SOC without some form of shunt. Which most SCC do not have. What system is this? I am extremely confused now.

Do not charge to 13.3 because even though the voltage soc charts will tell you that its 90% etc, it will not be 90% soc. need to look at SOC charging curve to notice that you need to charge to 14.1v minimum for full capacity.

Bottom balancing has to do with matching cells at low soc so that when discharged, have the same soc when bms cuts it off. Or do you mean building a pack without a bms?

And I did a community/email update of why the 13.3v recommendation is horrible. Maybe I should post it here on the forum again.

 

[Will Prowse]

Important Update for LiFePO4 Battery Users (scroll to the bottom for a summarized version):

In a couple of my videos, I mentioned charging a LiFePO4 battery to 90% SOC to increase the charge cycle life. I used a SOC chart and charge curves to determine that 13.3 volts for a 4s battery (12v battery), was 90% SOC if charged with CC/CV. Well after doing a week worth of capacity tests, this is wrong.

I have seen a lot of people in the forums argue about the "ideal" charge voltage for this chemistry, which is typically: 3.325/3.45/3.65 volts per cell (most BMS cut off at 3.65v). The problem that I discovered after doing multiple capacity tests, is that LiFePO4 battery voltage will settle a lot after it is done charging. This is not the same with NMC packs (such as the tesla battery).

So you need to charge your LiFePO4 up to 14.0-14.6 if you want 100% capacity (yes, 14.0 volts gave me 100% capacity in my tests). If you charge to 13.3v with CC/CV, you will only have 47% capacity! Not fun.

I was going to do some testing to figure out what absorption voltage would charge to 90% SOC, but then I started thinking about how LiFePO4 can be charged to 4.2v per cell, or 16.8v for a 12v battery (until electrolyte degradation occurs). So charging up to 14.0-14.6 will not cause any noticeable decrease in charge cycle life. The only way it could is if you charge it to 100% and keep the battery in an extremely hot environment. I have a video that covers that as well, with some battery studies as reference.

Also, you can treat these batteries pretty badly and they will still give you 5,000 charge cycles. That is a long time. Calendar aging of your battery may be more of a concern here (especially if batteries are in a hot environment etc).

So I don't see a real need to charge to 90% anymore. These batteries can last for ages and I think the system components around it may break before the battery cells do.

If you wish to actually charge to 90% SOC for your battery, you need a capacity monitor. Once you have charged to 90% SOC, find the voltage while it's charging (before the pack settles) and set your controller.

--------------------------------------------------------
So Long Story Short:

Set your solar charge controller to the LiFePO4 charge setting and do not bother charging to 90% SOC to extend the life of a LiFePO4 pack.

If you want your battery to last a long time, keep it in a cool location. Heat is the enemy! Also, try to size your battery so that the charge rate is less than .4C (which would be 40 amps for a 100ah battery). The larger your bank, the slower each individual battery is charged, and the longer your battery will last.

If you have manual control of your charge profile parameters, it is wise to use Victron's recommendation. This charge profile gives me 100% capacity in all of my tests and none of my cells voltages spike at all:

Absorption: 14.2V
Float: 13.5V
Equalization: Disabled
Temp Compensation: Disabled
Low Temp Cut-Off: 5 degrees Celsius

I also updated my LiFePO4 page with these charge profile parameters: click here

 

[chrisblessing]

That was never covered in that book. DIY LiFePO4 packs are covered in my videos and website, not the book.

What type of mppt do you have? Sounds like it does not have a standard charge profile algorithm. And SCC does not know SOC without some form of shunt. Which most SCC do not have. What system is this? I am extremely confused now.

Do not charge to 13.3 because even though the voltage soc charts will tell you that its 90% etc, it will not be 90% soc. need to look at SOC charging curve to notice that you need to charge to 14.1v minimum for full capacity.

Bottom balancing has to do with matching cells at low soc so that when discharged, have the same soc when bms cuts it off. Or do you mean building a pack without a bms?

And I did a community/email update of why the 13.3v recommendation is horrible. Maybe I should post it here on the forum again.

Thanks for the quick reply Will. My reference was this website. The charger in question is an Eco-Worthy 40amp MPPT charge controller. Other than the ability to select the battery type it doesn't appear to be programmable. And, to be clear, I wasn't really aiming for 3.3v or 90%. I was just trying to get it fully charged. That just appears to be as high as it would charge. I'm a complete newbie so just figuring this out. Tomorrow promises a full day of sun so I'll see what happens and report back.

 

[Will Prowse]

Oh ok no problem. I would get a different charge controller. The reviews on that thing and lack of functions would scare me away. And lifepo4 needs 14.1v to charge up to.

 

[Will Prowse]

Just read about the controller and it says you can program it. What is your current settings?

 

[b.james]

Its pretty well established Will that LFP should only be brought up to 3.65V on the initial charge to create the SEI layer. After that stay under it to avoid laying down more which will choke the cell over time . There is a video on here that explains it well, called "Whats inside a lithium Ion cell" or something like that. I think it was Pensylvania Uni ?.That's the heading put up about 10 days ago or so. LFP,LiFePo4 is classed as a Lithium Ion cell for those purposes.

Lithium-ion battery, How does it work?

This is a well-made video I found on an engineering channel on YouTube I follow and while looking at new up and coming battery innovations I ran across this little jewel. This is for beginners and very detailed while explaining in simple terms using excellent animations. I hope some of you find...

diysolarforum.com

So on a 12V pack thats 3.65V x 4= 14.6V max
Running day to day I stay at no more than 3.5V but no manufacturer tells you that that I have seen. 0.100 Volts is about 10% charge by my rule of thumb.
So on a 12V pack thats 3.5V x 4= 14V.

You know ,what you take from that is what you want, but I'm finding the difficulty is with setting the silly lead acid orientated chargers to just stop there ,without equalisation or float or bulk crap ,just turn off. I'm just gonna make a switch to turn the PV off when that 14V is reached.

14V accurate zener and a latching relay should do it.Some of the old Silicon chip magazines had controllers you build and they did a better job than todays lead acid crap. I've got about 3 stuck around here somewhere . Just have to get the voltage right.

 

[chrisblessing]

Apologies for the delay. The charger in question is from Eco-Worthy. The model is ECO-MPPT40. I see no means of programming it other than manual switches for battery type. The manual, limited though it is, also makes no reference to programming any aspect of the device.

 

[bobstay]

I was going to do some testing to figure out what absorption voltage would charge to 90% SOC, but then I started thinking about how LiFePO4 can be charged to 4.2v per cell, or 16.8v for a 12v battery (until electrolyte degradation occurs). So charging up to 14.0-14.6 will not cause any noticeable decrease in charge cycle life. The only way it could is if you charge it to 100% and keep the battery in an extremely hot environment. I have a video that covers that as well, with some battery studies as reference.

I'd still be interested in knowing the absorption voltage for 90% SOC to be honest. I'm the sort of person who wants to be quite conservative with cell degradation, and I haven't yet found enough testing/research to be confident that regular charging to 100% SOC won't reduce cycle life. Or, put another way, I'd like to extend cycle life, if possible, by being gentler than necessary with my cells.

 

[CheezWiz]

Thanks for the quick reply Will. My reference was this website. The charger in question is an Eco-Worthy 40amp MPPT charge controller. Other than the ability to select the battery type it doesn't appear to be programmable. And, to be clear, I wasn't really aiming for 3.3v or 90%. I was just trying to get it fully charged. That just appears to be as high as it would charge. I'm a complete newbie so just figuring this out. Tomorrow promises a full day of sun so I'll see what happens and report back.

I notice this in the user manual for that charger: "Only connect LiFePO4 batteries with integrated BMS! "
Assuming this is indeed that charger: https://www.eco-worthy.com/download... MPPT Solar Charge Controller User Manual.pdf

 

[robaroni]

Will,
I did some prelim tests on my LiFePo4's. What I found is that charging to 3.5 volts gives you 100% charge. Charging to 3.65 doesn't give you anymore than that.
Once you hit ~ 3.55 to 3.6 volts the battery isn't as stable as it is at lower charging voltages so if you have batteries in a series bank you start to get a run away condition and the balance between the cells is harder to maintain.

 

[chrisblessing]

Will,
I did some prelim tests on my LiFePo4's. What I found is that charging to 3.5 volts gives you 100% charge. Charging to 3.65 doesn't give you anymore than that.
Once you hit ~ 3.55 to 3.6 volts the battery isn't as stable as it is at lower charging voltages so if you have batteries in a series bank you start to get a run away condition and the balance between the cells is harder to maintain.

Thanks robaroni. I purchased a 2nd charge controller (Rich Solar 20a) and the results are the same. The Rich Solar controller is still in MPPT mode with the cells at around 3.3-3.4v but there hasn't been any movement beyond that. The Eco-Worthy 40a, on another 4s, is holding at 3.3v and hasn't moved for some time.

I hope I can assume that you're right - 3.3 to 3.4 means 100% charge.

Thanks.

 

[chrisblessing]

I notice this in the user manual for that charger: "Only connect LiFePO4 batteries with integrated BMS! "
Assuming this is indeed that charger: https://www.eco-worthy.com/download/manual/ECO-WORTHY 20-40A MPPT Solar Charge Controller User Manual.pdf

Yes. I read that as well - after I purchased it . That said, with the Daly 60 amp BMS I'm not certain what the difference would be between my 4s and a "drop in". And in my exchanges with Eco-Worthy, which included photos of my setup, the difference never came up.

Thanks though for looking into.

 

[CheezWiz]

Yes. I read that as well - after I purchased it . That said, with the Daly 60 amp BMS I'm not certain what the difference would be between my 4s and a "drop in". And in my exchanges with Eco-Worthy, which included photos of my setup, the difference never came up.

Thanks though for looking into.

So you have a BMS on your battery then? Since you were trying to self-balance, I assumed you did not. Typically this whole top balance vs bottom balance is moot if you have a BMS as it is always balancing. Or were you trying to achieve your goal with the BMS removed?

 

[b.james]

Typically this whole top balance vs bottom balance is moot if you have a BMS as it is always balancing

Hey ?? Most BMS's have a small balancer circuit built in to handle down in the milliamps range . Current continues to flow through the whole battery until one cell reaches 3.65V and then the whole lot shuts down.

Say you have 8 amps flowing through the battery. That's 8 amps flowing through every cell with say one getting high and feeding an amp to ground through a resistor. Its still going up at 7 amps as opposed to its neighbours which are getting 8 amps.

Generally I have found the balancer to be inconsequenial (or moot if you like) and a top or bottom balance must be performed with all the cells in parallel if you want them to stay under the cut off voltage of 3.65V which is damaging anyway if you get to that again.

I find a bottom balance is the safest. I don't know whats moot about that?

 

[TCgreg]

Oh ok no problem. I would get a different charge controller. The reviews on that thing and lack of functions would scare me away. And lifepo4 needs 14.1v to charge up to.

You can charge LiFePO4 to any voltage. I only charge to 13.6v.(3.4v/cell) Anything past that produces very little added capacity and requires a reliable top balancer.

 

[CheezWiz]

Hey ?? Most BMS's have a small balancer circuit built in to handle down in the milliamps range . Current continues to flow through the whole battery until one cell reaches 3.65V and then the whole lot shuts down.

Say you have 8 amps flowing through the battery. That's 8 amps flowing through every cell with say one getting high and feeding an amp to ground through a resistor. Its still going up at 7 amps as opposed to its neighbours which are getting 8 amps.

Generally I have found the balancer to be inconsequenial (or moot if you like) and a top or bottom balance must be performed with all the cells in parallel if you want them to stay under the cut off voltage of 3.65V which is damaging anyway if you get to that again.

I find a bottom balance is the safest. I don't know whats moot about that?

So how often do you find it necessary to rebalance a system to get reliable operation?

 

[TCgreg]

So how often do you find it necessary to rebalance a system to get reliable operation?

My bottom balanced cells have stayed tight for 2 years (since I built it) No need to rebalance as of yet. 13.65v is about where the cells stat to drift. At 13.6v max charging voltage no issues with imbalanced cells at either end. Staying between 3v/cell and 3.4v cell leaves quite a bit of room for something to slip a little but so far they have been rock solid. Our system only sees moderate to low charging and discharging rates for the size of the bank though so hammering them harder might make a difference. Our 2800 watt inverter is no match for 600ah bank capable of more than 1C

 

[robaroni]

My bottom balanced cells have stayed tight for 2 years (since I built it) No need to rebalance as of yet. 13.65v is about where the cells stat to drift. At 13.6v max charging voltage no issues with imbalanced cells at either end. Staying between 3v/cell and 3.4v cell leaves quite a bit of room for something to slip a little but so far they have been rock solid. Our system only sees moderate to low charging and discharging rates for the size of the bank though so hammering them harder might make a difference. Our 2800 watt inverter is no match for 600ah bank capable of more than 1C

That's exactly right, if you look at the curves you can see where the voltage rise is almost exponential, that's because there's very little more charge they can take that will make any difference and the internal resistance drops causing balance problems because at the knee of the rise battery resistance differences are greater.

I'd say your 3.4 volts per cell is pretty close to the right place to be for most manufacturers cells. My charge controller is set to 54.4 volts for 16 cells or 3.4 volts per cell bank.

 

[Dave1]

This discussion is making me feel better about the fact that my MPP Solar 12V PIP is automatically cutting off charging of my DIY LiFiPO4 pack at 13.6V (it then settles to 13.4V). It also helps that I did a capacity test and it came out at the rated capacity. I have the bulk charge voltage set at 3.65.