The topic of Lithium Battery Management Systems (BMS)) and Charging can become passionate. I have seen some older threads here on the subject but were not really resolved with in depth analysis or conclusion. I have been involved professionally with batteries for about 35 years, built my own Racing Golf Carts (3 now), and semi-retired.
Lithium Batteries are expensive and Fragile unlike their Brethren Lead Acid, NiCd, and any other battery chemistry you can name. Lithium batteries have two sensitive areas. Full Charge and Discharged. If you care about your investment ca$h in Lithium Batteries, you need a Battery Management System or a Strategy in place to protect that investment and get the most out of it.
The fastest and easiest way to destroy a Lithium battery is Over Discharge it into Reverse Polarity. It only takes one careless simple mistake to turn a Lithium battery into a brick or boat anchor. Second is Over Charge, or leaving fully charge for extended periods of time like 24 hours. Over charging is the most common failure mode in Lithium batteries, and is accumulative over time. You want to operate between 20/80% State of Charge (SOC) as best you can or at least 90/10% SOC.
So which method of BMS do you implement and why? Are you a Top Balance or Bottom Balance advocate? You cannot do Middle Balance in the technical and economic sense in which all major Auto Manufactures implement. Auto manufactures like Tesla, Nissan, Toyota, and Chey use Middle Balance or sometimes called Partial State Of Charge (PSOC) or Between the Sheets. They operate their Lithium Batteries between 80/20% State of Charge (SOC). They do this to maximize battery cycle life, and offer the public 8 year warranties. Take this hint and thought with you as you read further.
Manufactures can do many things Joe Plummer cannot possible duplicate or implement. Things like order X00,000 batteries, test, and sort them to 1% tolerance bins. Does not matter if they Bottom, Middle, or Top Balance the batteries because all batteries in a vehicle are matched. If they are say 100 AH rated, each cell is within 1 AH of each other. Lowest would say be 102 AH and largest 103 AH. Of that 100 AH the EV manufactures limit usable capacity to 60 to 80%.
They do not Top Balance by charging the cells to 100% SOC. rather they limit SOC to 80 to 90%. This single act alone enables the manufactures to offer up to 8 year battery warranties. It is quite well know limiting charge to less than 100% can double cycle life of Lithium batteries. That is contrary to lead acid battery mindsets. With Pb it would be unthinkable of not charging your battery to 100% and hold it there until ready to use. Then rush to get it fully recharged again after use. Pb batteries do not like running in a PSOC environment. Lithium thrives in the PSOC environment and we can take advantage of that.
We or most of us cannot duplicate any of that. We order small quantities, off the shelf, and wide variance of capacity. Say you buy 100 AH cells, and they can range from 99 AH at the low end. up to 115 AH at the high end. No matter how you arrange them, you end up with a 99 AH battery. The lowest capacity cell in a series string limits the total string capacity.
So what can we do to mimic what the commercial EV Manufactures do? If we cannot not implement Middle Balance, that leaves us Top Balance or Bottom Balance. Well I guess there is always ignore it.
Let's start with the commercial answer, Top Balance with BMS. As disclosure when I first went into building fast Golf Carts and working with batteries professionally for around 30 years I was a TOP BALANCE advocate. We did not have Lithium when I started. It was an Engineered PRODUCT solution made for sale to automate battery care. I am an Engineer, so go with it. Do it because that is the way we do things, the ole fashion charge until 100% SOC Lead Head crowd.
What is TOP BALANCE? Answer is real simple, charge every cell to MAXIMUM CAPACITY to maximize run time right? Well in the Power Tool, Laptop, tablets, cell phones, or whatever consumer gizmo/gadget you have, the consumer wants the longest run time possible. Cycle life is not critical and will only yield a few years of service before the battery craps out. After a few years, your gizmo is antiquated, and time for upgrade replacement model. That is fine for the disposable market. They traded cycle life, for maximum usable battery capacity. Charging Lithium batteries in excess of around 80 to 90% accelerates aging, cuts cycle life as much as 50%, ending in accelerated capacity loss, and premature failure. Limiting SOC to 80% has proven to double cycle life of a battery. EV and utility storage applications have a different set of priorities due to the large battery investment cost. Replacing batteries every 2 to 4 years is unacceptable period.
So how can Joe Plummer use the same Middle Balance techniques used by EV manufactures? You can mimic what they do with Bottom Balance approach. All Bottom Balance means is when the cells are received, you initially Bottom Balance them by connecting all cells in parallel and discharge them until you get a resting voltage somewhere between 2.4 and 2.6 volts for LiFePo4, or 3.2 to 3.3 for LiPo RC Hobby batteries. Then assemble your batteries and charge.
What this does is gives you 2 known reference points. 0 AH capacity and 0% SOC voltage. All batteries all equal in voltage and capacity of ZERO. In a Top Balanced system all we know is 100% SOC voltage reference. That does not tell you capacity. Just that every battery is fully charged up to some amp hour capacity. The packs AH capacity is determined by of the lowest capacity cell in a series string. If you have a 99 AH cell in series with 115 AH cells, you have a 99 AH pack capacity. That is a weak point in Top Balance system we can exploited to our advantage. We don’t want to take any cell to 100%. We want cycle life and concede 80 to 90% is a better long term strategy.
Top Balance systems have two weaknesses we would like to avoid. 1. Is what has already been discussed. Charging to 100% SOC using Pb mentality or stuck in a Lead Acid box mindset that demands 100% SOC. That shortens Lithium cycle life. Very easy to implement and cost you nothing. Just lower charging End Voltage to 80 to 90% SOC voltage. Done. No Bleeder/Monitor boards to buy for each cell are required.
2. This is probable the biggest advantage Bottom Balance has over Top Balance. The fastest way to destroy a lithium battery is to over discharge and have a Reverse Polarity occurrence. If that happens you are done. You have a destroyed cell(s). Bottom Balanced system passively eliminate that threat. It takes no automation or equipment to minimize or eliminate over discharge accidents. If you set your Motor/Controller or Inverter Low Voltage Disconnect (LVD) set point say to 42 volts on a 48 volt system, you have doubled down your protection by adding another layer of protection by using your motor controller or inverter LVD. The LVD will trip 2 volts higher than when the battery voltage collapses when pack voltage is 40 volts. When Bottom Balanced, all cells arrive at 0% capacity at the same time. There is no adjacent cell left in the string that has enough power to force another cell into Reverse Polarity because you removed the possibility of it happening in the first place. Strategy, not more equipment bought you more protection than automation can provide at 0 costs. Nothing to fail because it is Passive Protection designed in.
Top Balance systems are prone, or at least make the conditions right that allows you to over discharge at least 1 if not more than 1 cell to destruction. When Top Balanced, you charge every cell to 100% capacity. But every cell capacity is different at 100% SOC. Say a 48 volt battery 16S 100 AH LFP. 15 cells at say 115 AH and one of 99 AH. You have your LVD set for a conservative 44 volts. Well here is the bad news. At 47.5 volts which is still a strong 20 to 25% pack voltage indication, you now have that 99 AH cell at 0% SOC and being driven into Polarity Reversal by the adjacent stronger batteries thus destroying it without your knowledge. It may take a while to catch on as one of two things tip you off. You notice when you charge, all the sudden the resting voltage after charging dropped 3.4 volts or more. Or the cell catches fire or starts smoking while either being charged or discharged.
Now you can work around that problem in a Top Balanced System with a full Blown BMS that monitors every cell voltage, and capable of operating an external LVC Relay, or send a signal to the motor controller to shut down and disconnect the traction battery. All you gotta do is buy it.
There is another way, requires no additional equipment to buy as that is optional, significantly extend battery cycle life, and eliminates the risk of over discharge. All you gotta do is change your mindset, not your equipment. If you are interesting in Bottom Balance here is a basic over view of how to implement it.
When you receive your cells, check the voltage of each cell and make sure all cells are within .1 volts of each other. They should all arrive around 50 to 60% SOC storage voltage. Connect all of them in parallel with your battery bus bars or heavy wiring. Apply a load current to them and discharge them until they come to a resting voltage between 2.4 to 2.6 volts over night for LFP cells. At this point you have your ZERO Capacity and Voltage Reference Point
Assemble the cells for working operation in series. Time for the first charge. You are going to need a charger with adjustable voltage output, and just a simple Float mode charger like the ones they use for EV’s like a PFC 1500, a very versatile charger that can charge any battery chemistry or voltage of 24 volts up to 96 volts at 1500 watts. Charge rate needs to be no less than C/8 and no more than C/2. Initially set the charger to about 75% SOC voltage of the pack voltage. Example for a 16S LFP battery set the voltage to 55.2 volts. Monitor battery voltages while charging. Toward the end of the charge, there is going to be 1 battery with a slightly higher voltage than all the rest. It is the smallest capacity battery and you want to make note of it.
So let the first charge cycle complete and wait at least a couple of hours to rest the batteries. Now measure the voltage of the weakest battery, the one with the highest voltage. You want it to indicate somewhere in between 80 and 90% SOC. If it ends up a little low, raise the charger voltage a bit. If over 90, lower the voltage a bit. Do this a few times until you find the sweet spot. Then periodically check cell voltages one a month for any changes.
Here is the magic. With Bottom Balanced batteries, all cells have the same Amp Hour Capacity. You just mimicked what the big boys do and get the same benefit. When charged all cells have the same capacity, not the same SOC voltage. When discharged all the batteries have the same capacity, but they also have the same SOC voltage of 2.5 volts.
Last thing to do is set the LVD setting on your controller or inverter. Even if they do not allow you to adjust LVD set point defaults are conservative enough because they are likely set up for lead acid default of 1.75 volts per cell. So on a 12 volt system, LVD is set default to 10.5 TO 11.0 volts. Critical cutoff for a 4S LFP is 10 volts so you have a .5 TO 1 volt safety zone. If you can set LVD shoot for as low as you dare. But I suggest 20% SOC or about 2.9 loaded volts per cell so on a 12 volt system 11.6 volts giving you a lot of breathing room. It is like wearing a belt with suspenders and does not cost one red cent. Even if you do not disconnect the batteries, they will protect themselves from being Bottom Balanced and cannot Reverse Polarity an adjacent cell.
Chew on that a while!
Lithium Batteries are expensive and Fragile unlike their Brethren Lead Acid, NiCd, and any other battery chemistry you can name. Lithium batteries have two sensitive areas. Full Charge and Discharged. If you care about your investment ca$h in Lithium Batteries, you need a Battery Management System or a Strategy in place to protect that investment and get the most out of it.
The fastest and easiest way to destroy a Lithium battery is Over Discharge it into Reverse Polarity. It only takes one careless simple mistake to turn a Lithium battery into a brick or boat anchor. Second is Over Charge, or leaving fully charge for extended periods of time like 24 hours. Over charging is the most common failure mode in Lithium batteries, and is accumulative over time. You want to operate between 20/80% State of Charge (SOC) as best you can or at least 90/10% SOC.
So which method of BMS do you implement and why? Are you a Top Balance or Bottom Balance advocate? You cannot do Middle Balance in the technical and economic sense in which all major Auto Manufactures implement. Auto manufactures like Tesla, Nissan, Toyota, and Chey use Middle Balance or sometimes called Partial State Of Charge (PSOC) or Between the Sheets. They operate their Lithium Batteries between 80/20% State of Charge (SOC). They do this to maximize battery cycle life, and offer the public 8 year warranties. Take this hint and thought with you as you read further.
Manufactures can do many things Joe Plummer cannot possible duplicate or implement. Things like order X00,000 batteries, test, and sort them to 1% tolerance bins. Does not matter if they Bottom, Middle, or Top Balance the batteries because all batteries in a vehicle are matched. If they are say 100 AH rated, each cell is within 1 AH of each other. Lowest would say be 102 AH and largest 103 AH. Of that 100 AH the EV manufactures limit usable capacity to 60 to 80%.
They do not Top Balance by charging the cells to 100% SOC. rather they limit SOC to 80 to 90%. This single act alone enables the manufactures to offer up to 8 year battery warranties. It is quite well know limiting charge to less than 100% can double cycle life of Lithium batteries. That is contrary to lead acid battery mindsets. With Pb it would be unthinkable of not charging your battery to 100% and hold it there until ready to use. Then rush to get it fully recharged again after use. Pb batteries do not like running in a PSOC environment. Lithium thrives in the PSOC environment and we can take advantage of that.
We or most of us cannot duplicate any of that. We order small quantities, off the shelf, and wide variance of capacity. Say you buy 100 AH cells, and they can range from 99 AH at the low end. up to 115 AH at the high end. No matter how you arrange them, you end up with a 99 AH battery. The lowest capacity cell in a series string limits the total string capacity.
So what can we do to mimic what the commercial EV Manufactures do? If we cannot not implement Middle Balance, that leaves us Top Balance or Bottom Balance. Well I guess there is always ignore it.
Let's start with the commercial answer, Top Balance with BMS. As disclosure when I first went into building fast Golf Carts and working with batteries professionally for around 30 years I was a TOP BALANCE advocate. We did not have Lithium when I started. It was an Engineered PRODUCT solution made for sale to automate battery care. I am an Engineer, so go with it. Do it because that is the way we do things, the ole fashion charge until 100% SOC Lead Head crowd.
What is TOP BALANCE? Answer is real simple, charge every cell to MAXIMUM CAPACITY to maximize run time right? Well in the Power Tool, Laptop, tablets, cell phones, or whatever consumer gizmo/gadget you have, the consumer wants the longest run time possible. Cycle life is not critical and will only yield a few years of service before the battery craps out. After a few years, your gizmo is antiquated, and time for upgrade replacement model. That is fine for the disposable market. They traded cycle life, for maximum usable battery capacity. Charging Lithium batteries in excess of around 80 to 90% accelerates aging, cuts cycle life as much as 50%, ending in accelerated capacity loss, and premature failure. Limiting SOC to 80% has proven to double cycle life of a battery. EV and utility storage applications have a different set of priorities due to the large battery investment cost. Replacing batteries every 2 to 4 years is unacceptable period.
So how can Joe Plummer use the same Middle Balance techniques used by EV manufactures? You can mimic what they do with Bottom Balance approach. All Bottom Balance means is when the cells are received, you initially Bottom Balance them by connecting all cells in parallel and discharge them until you get a resting voltage somewhere between 2.4 and 2.6 volts for LiFePo4, or 3.2 to 3.3 for LiPo RC Hobby batteries. Then assemble your batteries and charge.
What this does is gives you 2 known reference points. 0 AH capacity and 0% SOC voltage. All batteries all equal in voltage and capacity of ZERO. In a Top Balanced system all we know is 100% SOC voltage reference. That does not tell you capacity. Just that every battery is fully charged up to some amp hour capacity. The packs AH capacity is determined by of the lowest capacity cell in a series string. If you have a 99 AH cell in series with 115 AH cells, you have a 99 AH pack capacity. That is a weak point in Top Balance system we can exploited to our advantage. We don’t want to take any cell to 100%. We want cycle life and concede 80 to 90% is a better long term strategy.
Top Balance systems have two weaknesses we would like to avoid. 1. Is what has already been discussed. Charging to 100% SOC using Pb mentality or stuck in a Lead Acid box mindset that demands 100% SOC. That shortens Lithium cycle life. Very easy to implement and cost you nothing. Just lower charging End Voltage to 80 to 90% SOC voltage. Done. No Bleeder/Monitor boards to buy for each cell are required.
2. This is probable the biggest advantage Bottom Balance has over Top Balance. The fastest way to destroy a lithium battery is to over discharge and have a Reverse Polarity occurrence. If that happens you are done. You have a destroyed cell(s). Bottom Balanced system passively eliminate that threat. It takes no automation or equipment to minimize or eliminate over discharge accidents. If you set your Motor/Controller or Inverter Low Voltage Disconnect (LVD) set point say to 42 volts on a 48 volt system, you have doubled down your protection by adding another layer of protection by using your motor controller or inverter LVD. The LVD will trip 2 volts higher than when the battery voltage collapses when pack voltage is 40 volts. When Bottom Balanced, all cells arrive at 0% capacity at the same time. There is no adjacent cell left in the string that has enough power to force another cell into Reverse Polarity because you removed the possibility of it happening in the first place. Strategy, not more equipment bought you more protection than automation can provide at 0 costs. Nothing to fail because it is Passive Protection designed in.
Top Balance systems are prone, or at least make the conditions right that allows you to over discharge at least 1 if not more than 1 cell to destruction. When Top Balanced, you charge every cell to 100% capacity. But every cell capacity is different at 100% SOC. Say a 48 volt battery 16S 100 AH LFP. 15 cells at say 115 AH and one of 99 AH. You have your LVD set for a conservative 44 volts. Well here is the bad news. At 47.5 volts which is still a strong 20 to 25% pack voltage indication, you now have that 99 AH cell at 0% SOC and being driven into Polarity Reversal by the adjacent stronger batteries thus destroying it without your knowledge. It may take a while to catch on as one of two things tip you off. You notice when you charge, all the sudden the resting voltage after charging dropped 3.4 volts or more. Or the cell catches fire or starts smoking while either being charged or discharged.
Now you can work around that problem in a Top Balanced System with a full Blown BMS that monitors every cell voltage, and capable of operating an external LVC Relay, or send a signal to the motor controller to shut down and disconnect the traction battery. All you gotta do is buy it.
There is another way, requires no additional equipment to buy as that is optional, significantly extend battery cycle life, and eliminates the risk of over discharge. All you gotta do is change your mindset, not your equipment. If you are interesting in Bottom Balance here is a basic over view of how to implement it.
When you receive your cells, check the voltage of each cell and make sure all cells are within .1 volts of each other. They should all arrive around 50 to 60% SOC storage voltage. Connect all of them in parallel with your battery bus bars or heavy wiring. Apply a load current to them and discharge them until they come to a resting voltage between 2.4 to 2.6 volts over night for LFP cells. At this point you have your ZERO Capacity and Voltage Reference Point
Assemble the cells for working operation in series. Time for the first charge. You are going to need a charger with adjustable voltage output, and just a simple Float mode charger like the ones they use for EV’s like a PFC 1500, a very versatile charger that can charge any battery chemistry or voltage of 24 volts up to 96 volts at 1500 watts. Charge rate needs to be no less than C/8 and no more than C/2. Initially set the charger to about 75% SOC voltage of the pack voltage. Example for a 16S LFP battery set the voltage to 55.2 volts. Monitor battery voltages while charging. Toward the end of the charge, there is going to be 1 battery with a slightly higher voltage than all the rest. It is the smallest capacity battery and you want to make note of it.
So let the first charge cycle complete and wait at least a couple of hours to rest the batteries. Now measure the voltage of the weakest battery, the one with the highest voltage. You want it to indicate somewhere in between 80 and 90% SOC. If it ends up a little low, raise the charger voltage a bit. If over 90, lower the voltage a bit. Do this a few times until you find the sweet spot. Then periodically check cell voltages one a month for any changes.
Here is the magic. With Bottom Balanced batteries, all cells have the same Amp Hour Capacity. You just mimicked what the big boys do and get the same benefit. When charged all cells have the same capacity, not the same SOC voltage. When discharged all the batteries have the same capacity, but they also have the same SOC voltage of 2.5 volts.
Last thing to do is set the LVD setting on your controller or inverter. Even if they do not allow you to adjust LVD set point defaults are conservative enough because they are likely set up for lead acid default of 1.75 volts per cell. So on a 12 volt system, LVD is set default to 10.5 TO 11.0 volts. Critical cutoff for a 4S LFP is 10 volts so you have a .5 TO 1 volt safety zone. If you can set LVD shoot for as low as you dare. But I suggest 20% SOC or about 2.9 loaded volts per cell so on a 12 volt system 11.6 volts giving you a lot of breathing room. It is like wearing a belt with suspenders and does not cost one red cent. Even if you do not disconnect the batteries, they will protect themselves from being Bottom Balanced and cannot Reverse Polarity an adjacent cell.
Chew on that a while!
