LiFeP04 Batteries for Solar & BMS

[karrak]

Bulk / Absorb 13.7, Float 13.1 re-float 12.7v Time limit 30 minutes and ending amps of 20 amps. The EQ setting is set to 14.3 that gets me above 3.55 on the BMS and used to balance if necessary.

Sounds like a sensible setup to me, what is you battery capacity?

Using the EQ setting to do the balance is a great idea.

I tried to bottom balance @ 2.6 on 4 cells. As I recharged them I found that once the voltage gets above 3.460 one cell would climb very fast while the rest were 3.2 -3.3 v.

Yes, that is the problem with bottom balancing, if the cells are balanced at the bottom they won't be balanced at the top. Only way around this which has been stated before is to have a cell monitoring system shut down the Charge Controller when the first cell hits 3.425 or whatever voltage you choose to stop the charging at.

The other problem is that even if you do shut the controller down with some sort of cell monitoring system that at shutoff the weakest cell will be at 3.425 volts and the stronger cell might be at 3.3 volts. That means that every time you fully charge the battery which might be on a daily basis with an off-grid setup that you subject the weakest cell to 0.125 extra voltage. I think it is generally accepted that increase in voltage decreases the life of LFP batteries, 0.125 volts doesn't sound like much but over a ten year plus lifespan that one hopes one gets from these expensive batteries I think it could have an impact. Worst still is that you are subjecting the weakest cell to the extra stress.

Simon

[Sunking]

If you are off-grid you have to keep your battery as close to 100% full as possible during sunny weather as you don't know how long you will have to live off your battery storage when cloudy weather occurs. What voltage would you suggest that you charge an LFP battery to under these circumstances.

That is only true for Lead Acid and , NiCd batteries, does not apply to Lithium. Lithium batteries are made to operate in PSOC (partial state of charge) range. Lead Acid need to be at 100% to slow down sulfation. Operating Lithium batteries at full charge shortens cycle life. That is why no EV manufactures fully charge the batteries. You only need to take lithium up to 90%, and down to 10% yielding 80% usable capacity. Go outside of that and you significantly shorten battery cycle life.

No they don't all have the same capacity, the weakest cell only has 100ah and the strongest cell has 120ah. Using your example, the weakest cell is charged to 95% of its capacity while the strongest cell is only charge to 79% of its capacity. With bottom balancing you only charge the stronger cells up to the same level as the weakest cell's fully charged state, so you loose the extra 20ah of the strongest cell at the top end. With top balancing it is exactly the same but the 20ah is lost at the bottom end.

Simon there is nothing lost. The e120 AH cell you are using as an example that last 20 AH is not accessible. A pack capacity is determined by the weakest cell in the pack. When the weakest cell in a pack reaches 0%, the stronger cells will destroy the weaker cells by polarity reversal. If the weakest cell is 100 AH and strongest is 120 AH you have a 100 AH pack. As soon as the weakest cell reaches 0% you are done leaving that 20 AH in the stronger cell because you cannot access it.

In a Bottom Balanced pack all cells have the exact same capacity at all times. They all reach 0% at the same time thus eliminating the possibility of over discharge. You do not give up capacity, just minimize risk of damage.

Top balancing does not mean that you charge them all to a high voltage >3.6 volts/cell. It means that they are all balanced at the voltage that you do charge them too. You do not run a very high risk of over discharging the cells in an off-grid system as the battery spends the vast majority of its time more than half full and as an emergency backup you have some sort of cell monitoring which will shut down your load or notify you when the weakest cell is flat.

Look I do not care if you Top or Bottom balance. What I can tell you is your controller is not capable of doing either one without modifications. It is much easier to Bottom Balance than to Top Balance. In a Top Balanced system you have to over charge the crap out of the weaker cells to get there. LFP and all Lithium batteries two cycle life killers:

1. Over Charging. In a Top Balanced system you over charge them every time you charge except for one battery, the strongest one. In a Bottom Balanced system you never over charge. You only get close to 100% on one cell only, the weakest. In a 100 AH system only 80 AH are usable. You get 80 AH out of either method. One is just less stressful and extends battery life.

2. Over Discharge is the biggest killer. Top Balanced systems greatly increase your risk of over discharging two fold.

A. It is very possible and 90% of all failures result is the stronger cells driving weaker cells into reverse polarity. Weaker cells reach 0%, but dumb equipment does not know that because the pack voltage still is above Low Voltage Cut-Off and keeps on going because the stronger cells still have capacity and keep the pack voltage above LV Cut-Off . In a Bottom Balanced systems cell voltages are equal at the bottom and Low Voltage Cut off monitoring pack voltages disconnects just before the voltage falls of the cliff, thus protecting the the cells from over discharge.

B Top Balanced system use Vampire Boards aka Bleeder Boards to Top Balance. Bleeder Boards aka Vampire Boards failure mode is Shorted more times than Open. When shorted completely discharges and destroys a cell or cells. Bottom Balanced systems do not have Vampire Boards to fail.

[Sunking]

That means that every time you fully charge the battery which might be on a daily basis with an off-grid setup that you subject the weakest cell to 0.125 extra voltage. I think it is generally accepted that increase in voltage decreases the life of LFP batteries, 0.125 volts doesn't sound like much but over a ten year plus lifespan that one hopes one gets from these expensive batteries I think it could have an impact. Worst still is that you are subjecting the weakest cell to the extra stress.

None of that is accurate. A .125 volt difference is extreme. LFP battery voltage is very linear and flat between 10% Soc (roughly 3.05 vpc under 1C load). and 90% SOC (3.28 vpc under 1C load) From 10 to 90% SOC is only .23 volts difference. A 1% change is only .0023 vpc, so .125 volts is a difference of roughly 50% SOC difference.

Here is a discharge curve of a Calb 180 AH cell that is extremely popular in EV and some solar systems.

[Willy T]

Sounds like a sensible setup to me, what is you battery capacity?

Using the EQ setting to do the balance is a great idea.

Simon

I have 8, 260 amp hr Winston cells, 520 amp hr @ 12 ( 2p4s) . I will eventually use 24v , but use what I had on old equipment to figure them out.

When the voltage jumps, it goes up fast, I was yanking the charger off. All I did was turn my back for a moment.

[Sunking]

When the voltage jumps, it goes up fast, I was yanking the charger off. All I did was turn my back for a moment.

You cannot use 3 or 4 stage algorithms on Lithium batteries especially an EQ setting on a charger made for Pb bateries. That is really funny stuff. Thought you said you knew what you were doing. You should not be giving anyone advice.

[Willy T]

You cannot use 3 or 4 stage algorithms on Lithium batteries especially an EQ setting on a charger made for Pb bateries. That is really funny stuff. Thought you said you knew what you were doing. You should not be giving anyone advice.

Maybe not on your golf cart, but on my system you can. Do you not have any solar experience ? By the way the above was with a power supply. You apparently do not know what EQ is at 14.3v, that runs the voltage up to 3.575 while the boards balance.

[Sunking]

Maybe not on your golf cart, but on my system you can. By the way the above was with a power supply. You apparently do not know what EQ is at 14.3v.

BS Absorb is 14.4. EQ is 15.5 to 16 volts. You are dangerous and should be ignored. Your Winston Cells are to be charged at 3.8 vpc constant voltage until current tapers to C/20 and terminated. I know you do not know what that means but I will give you a hint. 15.2 volts until current tapers to 26 amps. Try reading your specs.

[Willy T]

BS Absorb is 14.4. EQ is 15.5 to 16 volts. You are dangerous and should be ignored. Lithium LFP is charged at 14.4 constant voltage with current limit. You just proved you do not know what you are talking about. Try reading your Winston Specs for a change.

Maybe on your equipment, mine you set the EQ voltage to what you want. 14 -18 volts, you need to get out of the pb box. Dangerous ?? This not a golf cart.

[Sunking]

Dangerous ?? This not a golf cart.

BS no difference in operation. Your batteries are made for EV's. All large format Lithium batteries are made for EV's. Winston's batteries are crappy old Thunder Sky rejects like all the Chi-Com LFP batteries are.

You are dangerous because you do not know what you are talking about or doing. That will get you in trouble as you have already been warned about in the past about unsafe practices. Keep it up and you will be gone. Personally I do not care what you do to your own equipment, but I do care when you tell others how to screw up their batteries.

I can fix ignorance, but I cannot fix stupid.

[Willy T]

BS no difference in operation. Your batteries are made for EV's. They are crappy old Thunder Sky rejects like all the Chi-Com LFP batteries are. You are dangerous because you do not know what you are talking about or doing. That will get you in trouble as you have already been warned about in the past about unsafe practices. Keep it up.

Do you have a draw on your golf cart 24/7 ?? You use it and charge it and it sits until you use it again. A Inverter system is in a constant charge / discharge. it has to be bulked / absorbed and floated to be ready, you can do with your cart as you please.

You keep on adding thing's to your posts, it's hard to keep up.

PM your buddies.I have said nothing but answer your posts and nicely without the name calling. If you don't like the term EQ then so be it, that what it is , equalizing voltages / Balancing cells, every where I use it I said 14.3 volts.

[Sunking]

Do you have a draw on your golf cart 24/7 ?? You use it and charge it and it sits until you use it again. A Inverter system in in a constant charge / discharge. it has to be bulked / absorbed and floated to be ready, you can do with your cart as you please.

You do not Bulk, Absord, or Float any Lithium battery. That is a Pb algorithm aka 3-Stage. You charge Lithium at constant current until they reach target voltage and terminate the charge. Makes no difference what application they are used for. Try doing research and learn something. I got a 35 year head start on you with research and experience.

[Willy T]

You do not Bulk, Absord, or Float any Lithium battery. That is a Pb algorithm aka 3-Stage. You charge Lithium at constant current until they reach target voltage and terminate the charge. Makes no difference what application they are used for. Try doing research and learn something. I got a 35 year head start on you with research and experience.

Thank You for the Information, you charge yours the way you want and I'll charge mine the way I want.

[karrak]

That is only true for Lead Acid and , NiCd batteries, does not apply to Lithium. Lithium batteries are made to operate in PSOC (partial state of charge) range. Lead Acid need to be at 100% to slow down sulfation. Operating Lithium batteries at full charge shortens cycle life. That is why no EV manufactures fully charge the batteries. You only need to take lithium up to 90%, and down to 10% yielding 80% usable capacity. Go outside of that and you significantly shorten battery cycle life.

It is actually not true for NiCd batteries, you do not have to keep them at 100%, one of the big advantages of NiCd over Pb. I think we have a misunderstanding about what I mean by fully charged, what I mean is whatever level of SOC or maximum cell voltage that one defines as fully charged. In my case fully charged is a maximum cell voltage of 3.45 volts at a charge rate of C/20 this equates to a SOC of ~90%-95%. In this post http://www.solarpaneltalk.com/showth...l=1#post140805 you state an average full charge voltage of 3.45 volts, so at least we agree on something.

Simon there is nothing lost. The e120 AH cell you are using as an example that last 20 AH is not accessible. A pack capacity is determined by the weakest cell in the pack. When the weakest cell in a pack reaches 0%, the stronger cells will destroy the weaker cells by polarity reversal. If the weakest cell is 100 AH and strongest is 120 AH you have a 100 AH pack. As soon as the weakest cell reaches 0% you are done leaving that 20 AH in the stronger cell because you cannot access it.

I agree that the extra 20ah in the strongest cell is not accessible in both a Top and Bottom balanced pack. The stronger cell will not destroy the weaker cell if you monitor and act on individual cell voltages. In off-grid situations where the battery is usually above 50% charged you are rarely if ever going to get near a situation where the weakest cell is near 0%.

In a Bottom Balanced pack all cells have the exact same capacity at all times. They all reach 0% at the same time thus eliminating the possibility of over discharge. You do not give up capacity, just minimize risk of damage.

This is only true if the bottom balance doesn't drift, i.e. the cells all age and loose capacity at the same rate. I can only see this happen if all the cells are manufactured exactly the same, and are all subjected to identical operating conditions. Me thinks this is an unlikely scenario.

Look I do not care if you Top or Bottom balance. What I can tell you is your controller is not capable of doing either one without modifications. It is much easier to Bottom Balance than to Top Balance. In a Top Balanced system you have to over charge the crap out of the weaker cells to get there. LFP and all Lithium batteries two cycle life killers:

I designed and built my MPPT controller so it is capable of whatever I designed it to do and whatever I program it to do. If you mean commercial charge controllers, as long as they have settable End-Bulk, Float, Absorb, Re-Connect, and Equalize settings which the majority of goods ones have you can set them up to work with LFP batteries just as Willy T has. If you top balance it is simply a matter of making the End-Bulk voltage = wanted fully charged cell voltage x number of cells. Set Absorb time to something small and if you have an end Absorb amps setting, set this to whatever current you want to terminate the charge, say C/20. Set Reconnect to something like 2.5-2.75volts x number of cells depending on what SOC you want to start charging again at. If you have balancing boards to balance your cells set equalize voltage to whatever the balance boards will balance at (I wouldn't use balance boards that balance at a higher voltage than 3.55 volts) x number of cells and set the time between equalize cycles to be say around a month. With bottom balancing unless you like living dangerously as per SKs post http://www.solarpaneltalk.com/showth...l=1#post141179
you have to stop the charger charging when the weakest cell hits the full voltage you choose. This is far more complex.

1. Over Charging. In a Top Balanced system you over charge them every time you charge except for one battery, the strongest one. In a Bottom Balanced system you never over charge. You only get close to 100% on one cell only, the weakest. In a 100 AH system only 80 AH are usable. You get 80 AH out of either method. One is just less stressful and extends battery life.

If one top balances the battery before you start using it which you should do, all the cells reach the same fully charged voltage at the same time so all the cells are put under the same stress. With bottom balancing the weakest cell will be first to reach the fully charged voltage and may go higher than this if you use SK's averaging scheme outlined in the post http://www.solarpaneltalk.com/showth...l=1#post141179
so it will be put under more stress than the rest of the cells.

2. Over Discharge is the biggest killer. Top Balanced systems greatly increase your risk of over discharging two fold.

A. It is very possible and 90% of all failures result is the stronger cells driving weaker cells into reverse polarity. Weaker cells reach 0%, but dumb equipment does not know that because the pack voltage still is above Low Voltage Cut-Off and keeps on going because the stronger cells still have capacity and keep the pack voltage above LV Cut-Off . In a Bottom Balanced systems cell voltages are equal at the bottom and Low Voltage Cut off monitoring pack voltages disconnects just before the voltage falls of the cliff, thus protecting the the cells from over discharge.

Where are your stats to back this up? Maybe true for EVs where you have total control over the charging process and not so much control over the discharge process, compared to off-grid and solar powered systems where you don't have as much control over the charging process as the discharging process.

B Top Balanced system use Vampire Boards aka Bleeder Boards to Top Balance. Bleeder Boards aka Vampire Boards failure mode is Shorted more times than Open. When shorted completely discharges and destroys a cell or cells. Bottom Balanced systems do not have Vampire Boards to fail.

Great use of emotive language to describe Balancing boards. Good quality well designed electronics doing simple jobs like cell balancing should be very, very reliable. I would think there are far more disasters caused by human stuffups than things like transistors on balance boards failing short circuit. That sort of fault would be caught if you have some sort of cell monitoring system in place.

Simon

[Sunking]

Simon EV manufactures DO NOT TOP BALANCE. They run their batteries between 10 and 90% SOC. They do not allow a user to fully charge or discharge the batteries. They do that to extend battery cycle life, and offer decent warranty term without extreme risk. . Chevy Volt is the most conservative running their batteries between 20 and 80%.

Commercial EV manufactures use Middle Balance. They can do one thing you and I cannot possible do. They buy their cells in quantity of hundreds of thousands. They then test each cell for capacity and performance. They segregate each cell into bins with like matching cells. The tolerance in each bin is extremely tight and balanced. They build their battery packs with matched cells. You and I cannot do that. They Mid Balance the cells before assemble at 50 to 60% SOC. Each cell have the exact same capacity at any SOC voltage. They are precision balanced from the start.

DIY solar and EV builders cannot do that, but they can mimic that with Bottom Balance. I never said that a DIY EV or Solar System does not use a form of BMS, when in fact they do. The smarter users use a battery monitor with control to monitor and control charging and discharging. In fact they use two Fail Safes like I do.

On the Charge side I monitor every cell voltage, charge at C/2, and terminate when th every first cell reaches 3.5 volts. Any Cell. The second Fail Safe on the charging side is I set the charge voltage to 56 volts on my 16S battery. That would be 3.5 volts per cell. If I topped balance would have to be at a minimum of 57.6 volts and use Vampire boards to bleed off weaker cells.

On the discharge side Fail Safe 1 is I use Battery Monitor to disconnect the pack if ANY CELL dips to 2.9 volts for more than 15 second, or immediately disconnect if ANY CELL touches 2.5 volts. Second line of defense is to use the Motor Controller Low Voltage Disconnect if the pack voltage touches 46.5 volts. Across 16 cells that is 2.9 volts per cell or 10% SOC. Well above the panic 2.25 volts or point of no return cliff.

Lastly for a gas gauge I use a calibrated Coulomb Counter. It measure Amp Hours In, and Amp Hours Out. I know at any point in time what the AH capacity is in the batteries. WhenI see 10 to 20 AH remaining on my 100 AH battery, I go home and recharge.

I learned this the hard way. Like you I thought Top Balance was the way to go. I then researched the heck out of Lithium battery management and ran across Bottom Balanced. At about the same time I learned how the EV manufactures manage their batteries with Middle Balance. DIY cannot do Middle Balance and do Distributive Active Balancing lie commercial EV manufactures do. They can only use Top Balance, or mimic what EV manufactures do using Bottom Balance. I use the safer and least expensive method with 2 fail safes. I do not have to sacrifice any capacity, eliminate risk, and maximize cycle life.

[PNjunction]

Willy - I see what's happening here. You run the higher voltages to activate the top-balance bleeders. Ok, but just consider that you don't need to do that every cycle!

Once you are satisfied that you are top balanced, drop your charger voltage lower so that you do not actually activate the bleeders. I'm not familiar with the charger you are using, but if you can, there is no need to go beyond 14.0 / 14.1v for a 4S system after you are satisfied that you are in a state of somewhat sane balance. And of course don't drop those winston's down too far. If you feel the need to balance again, run the higher voltage / bleedoff routine, and once satisfied, drop it again for normal use.

As long as you are willing to monitor things at least occasionally, you should be good *for your purposes*. I can understand.