LiFeP04 Batteries for Solar & BMS

[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]

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]

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]

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.

[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.

[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

[karrak]

You are missing the big picture and still stuck using lead acid battery techniques. Keeping LFP batteries at or near 100% shortens their lives, not maximize as you mistakenly said. That is a lead acid world thing. Keeping LFP at 100% are taking them their daily accelerates their aging. Keeping LFP cell voltages equal are not important at the top as it does not tell you anything about them or has no benefit.

I find this rather amusing as I have never used Pb batteries in my off-grid system, NiCd and LFP yes, Pb no.

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.

Again you logic is flawed and stuck in a Pb world. In a Bottom Balance system every cell has the same capacity, and the system is charged up referencing the weakest cell in the string. In a Top Balanced system the capacity of the pack is determined by the weakest cell. Example in a 100 AH system your AH capacity, cell values are going to range from 100 to 120 AH. The weakest is 100 AH. Bottom Balance them at at any point every cell has the same capacity. You charge them up using the weakest cell. Say 95 AH.

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.

If you Top Balance not only do you stress every cell every time you charge, you run a very high risk of over discharging and destroying the weaker cells. In a Bottom Balanced system you eliminate those risk.

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.

Regular basis? What are you talking about? You do it one time initially at the beginning.

What if the cells age and loose capacity at different rates. If this happens the bottom balance will have to be redone just as it has to with top balancing.

Simon

[Willy T]

Might be worth changing the 'Rebulk Voltage' in the Advanced menu to 13.0 volts to get the FM60 to go back to 'Bulk' charge at a more realistic battery SOC, 12 volts (3.0 volts/cell) is nearly fully discharged for an LFP battery. Could you tell me what your 'Absorbing' and 'Float' voltages are from the Charger Screen and also 'Absorb Time Limits' and 'Absorb End Amps' from the 'Advanced Menu', also what are the 'Equilization' settings.

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.

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. As I watched them the high cell went to 3.785 and the lowest was still at 3.4 v. That let me know that the safest high charge voltage would have to be 3.4 and even that would be to high to be safe. So if I used 3.3v that would make my bulk at 13.2 v and to me that is way to low and giving up to much capacity, so I decided to top balance. I have no idea how temperature effects voltage, I am watching them close until I get a handle on them. I am trusting the BMS, it's better than losing a cell. I have not cycled them yet to see what the lowest safe voltage is yet. It won't be that low, since the lowest it can go is 12.7 before it will recharge off another system.

Real life trumps theoretical BS.

[karrak]

Maybe I haven't found the right combination yet. It's like the system just doesn't want to reset because the voltage is to high and it wants to stay in Float.

Might be worth changing the 'Rebulk Voltage' in the Advanced menu to 13.0 volts to get the FM60 to go back to 'Bulk' charge at a more realistic battery SOC, 12 volts (3.0 volts/cell) is nearly fully discharged for an LFP battery. Could you tell me what your 'Absorbing' and 'Float' voltages are from the Charger Screen and also 'Absorb Time Limits' and 'Absorb End Amps' from the 'Advanced Menu', also what are the 'Equilization' settings.

[Sunking]

Firstly, In off-grid systems you want to charge the battery to as close to 100% when the weather is sunny while also maximising the lifespan of the battery, a tricky compromise. Usually this means fully charging the battery on a daily basis whenever there is sunshine. As has been mentioned in previous posts, bottom balancing a battery will mean that the individual cell voltages will be unequal every time the battery is fully charged which in this situation will mean on a daily or if not daily a very regular basis. I would think that this imbalance could lead to some cells ageing faster than others over the hopefully 10 year plus life of the battery.

You are missing the big picture and still stuck using lead acid battery techniques. Keeping LFP batteries at or near 100% shortens their lives, not maximize as you mistakenly said. That is a lead acid world thing. Keeping LFP at 100% are taking them their daily accelerates their aging. Keeping LFP cell voltages equal are not important at the top as it does not tell you anything about them or has no benefit.

Secondly, off-grid batteries relying on solar power to charge them spend most of their time above 50%SOC as you want to have as much power stored as possible for cloudy weather. It is only very rarely that a battery in a well managed off-grid systems gets to anything near 0% SOC where the protection given by bottom balancing has any impact. If you have cell level monitoring of the battery, imbalance in cell SOC at the bottom end is not an issue.

Again you logic is flawed and stuck in a Pb world. In a Bottom Balance system every cell has the same capacity, and the system is charged up referencing the weakest cell in the string. In a Top Balanced system the capacity of the pack is determined by the weakest cell. Example in a 100 AH system your AH capacity, cell values are going to range from 100 to 120 AH. The weakest is 100 AH. Bottom Balance them at at any point every cell has the same capacity. You charge them up using the weakest cell. Say 95 AH.

If you Top Balance not only do you stress every cell every time you charge, you run a very high risk of over discharging and destroying the weaker cells. In a Bottom Balanced system you eliminate those risk.

Thirdly, doing a bottom balance on a regular basis in an off-grid system is difficult if the battery is your main source of power, especially if you do not have an alternate power supply. I think it also subjects the battery to unnecessary stress.

Regular basis? What are you talking about? You do it one time initially at the beginning.

[karrak]

The CC is a FM60 It's like the system just doesn't want to reset because the voltage is to high and it wants to stay in Float. Sometimes integrated systems aren't all that they are cranked to be. It could be something in the algorithm that is fixed. I said 12.5, it's hard to tell exactly, it could be 12.7. I'll have to watch it again to see. Just getting everything calibrated has been a pain within the voltage range of LiFep04 batteries.

I had a quick look at the FM60 manual. It is not very clear to me (pretty poor manual!) but I think the FM60 should go back to Bulk charge after what they call 'Sleeping' then 'Zzzzz...' then 'Snoozing'. 'Sleeping' starts when the panel voltage is 2 volts below the battery voltage, then after 3 hours of 'Sleeping' it should go to 'Zzzz...' then when the panel voltage gets 2 volts above the battery it should go to 'wakeup' then I assume on to bulk.

I would check that it is going through this cycle each night. I couldn't work out if the battery voltage needs to go below the 'Rebulk Voltage' at night before the FM60 will go back to 'Bulk' mode when it wakes up, maybe someone else can help here.

Simon

[Willy T]

My MPPT charge controller is homemade so I can program it however I like. I have programmed it to start a new bulk charge at the start of each new day or if the battery voltage goes below 3.25 volts/cell (13 volts for a twelve volt system). What is the make and model of your charge controller?

Simon

The CC is a FM60 and a FNdc all connected to a GFX 1312. I have tried a lot of settings and with and without the FNdc controlling the total system and it still seems to happen. Maybe I haven't found the right combination yet. It's like the system just doesn't want to reset because the voltage is to high and it wants to stay in Float. Sometimes integrated systems aren't all that they are cranked to be. It could be something in the algorithm that is fixed. I said 12.5, it's hard to tell exactly, it could be 12.7. I'll have to watch it again to see. Just getting everything calibrated has been a pain within the voltage range of LiFep04 batteries.

[karrak]

Do you have any issues with your CC triggering a full Bulk, Absorb, Float cycle until the voltage drops below 12.5. Sometimes my CC will not automatically re-trigger and it will limit the number of output amps and stops at the float setting unless the voltage drops below 12.5. I can set a bulk cycle, but that requires manual intervention.

My MPPT charge controller is homemade so I can program it however I like. I have programmed it to start a new bulk charge at the start of each new day or if the battery voltage goes below 3.25 volts/cell (13 volts for a twelve volt system). What is the make and model of your charge controller?

Simon

[Willy T]

From my research and experience with an LFP battery that has been part of my off-grid power system for over two years now I am not so sure that bottom balancing LFP cells in off-grid systems and probably most power systems that rely on solar energy as their primary source of power is as good or as practicable as top balancing.

Firstly, In off-grid systems you want to charge the battery to as close to 100% when the weather is sunny while also maximising the lifespan of the battery, a tricky compromise. Usually this means fully charging the battery on a daily basis whenever there is sunshine. As has been mentioned in previous posts, bottom balancing a battery will mean that the individual cell voltages will be unequal every time the battery is fully charged which in this situation will mean on a daily or if not daily a very regular basis. I would think that this imbalance could lead to some cells ageing faster than others over the hopefully 10 year plus life of the battery.

Secondly, off-grid batteries relying on solar power to charge them spend most of their time above 50%SOC as you want to have as much power stored as possible for cloudy weather. It is only very rarely that a battery in a well managed off-grid systems gets to anything near 0% SOC where the protection given by bottom balancing has any impact. If you have cell level monitoring of the battery, imbalance in cell SOC at the bottom end is not an issue.

Thirdly, doing a bottom balance on a regular basis in an off-grid system is difficult if the battery is your main source of power, especially if you do not have an alternate power supply. I think it also subjects the battery to unnecessary stress.

I agree that for EV use bottom balancing cells has many advantages but the charge/discharge profiles, expectations and requirements for an EV battery and for a battery in an off-grid system are very different.

Simon

Finally someone with a off grid system and not someone testing batteries on a bench or a golf cart. Be careful, the last off grid person got banned for talking about their system and being called a lot of names.

Do you have any issues with your CC triggering a full Bulk, Absorb, Float cycle until the voltage drops below 12.5. Sometimes my CC will not automatically re-trigger and it will limit the number of output amps and stops at the float setting unless the voltage drops below 12.5. I can set a bulk cycle, but that requires manual intervention.

[karrak]

From my research and experience with an LFP battery that has been part of my off-grid power system for over two years now I am not so sure that bottom balancing LFP cells in off-grid systems and probably most power systems that rely on solar energy as their primary source of power is as good or as practicable as top balancing.

Firstly, In off-grid systems you want to charge the battery to as close to 100% when the weather is sunny while also maximising the lifespan of the battery, a tricky compromise. Usually this means fully charging the battery on a daily basis whenever there is sunshine. As has been mentioned in previous posts, bottom balancing a battery will mean that the individual cell voltages will be unequal every time the battery is fully charged which in this situation will mean on a daily or if not daily a very regular basis. I would think that this imbalance could lead to some cells ageing faster than others over the hopefully 10 year plus life of the battery.

Secondly, off-grid batteries relying on solar power to charge them spend most of their time above 50%SOC as you want to have as much power stored as possible for cloudy weather. It is only very rarely that a battery in a well managed off-grid systems gets to anything near 0% SOC where the protection given by bottom balancing has any impact. If you have cell level monitoring of the battery, imbalance in cell SOC at the bottom end is not an issue.

Thirdly, doing a bottom balance on a regular basis in an off-grid system is difficult if the battery is your main source of power, especially if you do not have an alternate power supply. I think it also subjects the battery to unnecessary stress.

I agree that for EV use bottom balancing cells has many advantages but the charge/discharge profiles, expectations and requirements for an EV battery and for a battery in an off-grid system are very different.

Simon