LiFePo4 battery bank.

littleharbor2

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

Dave Angelini

Down on the baja? What is it like these days? Lot's of fish tacos and pacifico I hope!

  If the BMS is off? The spec for the LFP I use is, 

Self Discharge (25°C / 77°F)

< 3% per month (Battery Off)

littleharbor2

Dave Angelini said:

Down on the baja? What is it like these days? Lot's of fish tacos and pacifico I hope!

  If the BMS is off? The spec for the LFP I use is, 

Self Discharge (25°C / 77°F)

< 3% per month (Battery Off)

Hey Dave, 
 It's been hot and sticky long enough. Time for a new season.. Oddly Pacificos are hard to find in our area. Damn Covid 19 affected beer production right away and it has taken a while to recover.
I'm waiting on the BMS. At the moment There's nothing connected to the batteries. They're just parallel wired together. I will keep an eye on the voltage.
Thanks!

Dave Angelini

Damn that is bad news on Pacifico.  Plenty in our small town.    When we were there,  the empties were refilled and so you saved them. The cost was almost free if you hauled the empty bottles in the boat or RV.

Fall is almost here in the southern Sierra. We did well this summer. Some really bad smokey hot ones of late. 

Horsefly

littleharbor2 said:

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

Hey Mike - The self-discharge rate of LiFePO4 goes up as the SoC goes up. At 3.269V yours are probably not much over 50% SoC, so the self discharge will be pretty negligible.  You know about top balancing them? You'll want to do that before you hook them up as a 8s 24V pack. What BMS are you getting?

littleharbor2

Horsefly said:

littleharbor2 said:

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

Hey Mike - The self-discharge rate of LiFePO4 goes up as the SoC goes up. At 3.269V yours are probably not much over 50% SoC, so the self discharge will be pretty negligible.  You know about top balancing them? You'll want to do that before you hook them up as a 8s 24V pack. What BMS are you getting?

Haven't decided on the BMS as of yet. So now that I have the batteries balanced  am I to understand I need to charge them up to  near fully charged and balance them again before commissioning them?

littleharbor2

Dave Angelini said:

Damn that is bad news on Pacifico.  Plenty in our small town.    When we were there,  the empties were refilled and so you saved them. The cost was almost free if you hauled the empty bottles in the boat or RV.

Fall is almost here in the southern Sierra. We did well this summer. Some really bad smokey hot ones of late. 

Funny how there's more Mexican beer in the USA than here in Mex. Cheaper too. The days of dirt cheap Coronas are long gone. Funny thing, You can buy Coors Light and Bud Light cheaper than domestic beer down here. Oh and gas is about equal to So.Cal, pricewise.

Horsefly

littleharbor2 said:

Horsefly said:

littleharbor2 said:

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

Hey Mike - The self-discharge rate of LiFePO4 goes up as the SoC goes up. At 3.269V yours are probably not much over 50% SoC, so the self discharge will be pretty negligible.  You know about top balancing them? You'll want to do that before you hook them up as a 8s 24V pack. What BMS are you getting?

Haven't decided on the BMS as of yet. So now that I have the batteries balanced  am I to understand I need to charge them up to  near fully charged and balance them again before commissioning them?

Yep, that's basically it. In the middle of the charge/discharge curve (where your cells are) the SoC can actually vary considerably. That's because the LFP curve is so flat between the 95% SoC and 10% SoC. So to make sure your cells start out mostly balanced, you need to first top-balance or bottom-balance them. In a solar application the cells will spend more time near the top than near the bottom, so top-balancing is better. So you keep them tied parallel, hook up a charger set to 3.65V (need a good multimeter to make sure), then leave it alone. Depending on how many amps your charger can put out, it could take days for the cells to get up to 3.65V. Then you leave it charging until the tail current drops down to nearly nothing. Then they all are at the same state of charge at the top. With any luck you won't have to worry about them getting too far out of balance after that.

littleharbor2

Horsefly said:

littleharbor2 said:

Horsefly said:

littleharbor2 said:

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

Hey Mike - The self-discharge rate of LiFePO4 goes up as the SoC goes up. At 3.269V yours are probably not much over 50% SoC, so the self discharge will be pretty negligible.  You know about top balancing them? You'll want to do that before you hook them up as a 8s 24V pack. What BMS are you getting?

Haven't decided on the BMS as of yet. So now that I have the batteries balanced  am I to understand I need to charge them up to  near fully charged and balance them again before commissioning them?

Yep, that's basically it. In the middle of the charge/discharge curve (where your cells are) the SoC can actually vary considerably. That's because the LFP curve is so flat between the 95% SoC and 10% SoC. So to make sure your cells start out mostly balanced, you need to first top-balance or bottom-balance them. In a solar application the cells will spend more time near the top than near the bottom, so top-balancing is better. So you keep them tied parallel, hook up a charger set to 3.65V (need a good multimeter to make sure), then leave it alone. Depending on how many amps your charger can put out, it could take days for the cells to get up to 3.65V. Then you leave it charging until the tail current drops down to nearly nothing. Then they all are at the same state of charge at the top. With any luck you won't have to worry about them getting too far out of balance after that.

Could I wire them up in series and charge to 29 volts (29.2 being max charge) then take them off and parallel wire them again letting them sit and balance? If not where do you get a 3.65 volt charger? I have individual 18650 cell chargers but that doesn't sound right.
 Please forgive my ignorance. This is completely new to me.

Horsefly

littleharbor2 said:

Could I wire them up in series and charge to 29 volts (29.2 being max charge) then take them off and parallel wire them again letting them sit and balance? If not where do you get a 3.65 volt charger? I have individual 18650 cell chargers but that doesn't sound right.
 Please forgive my ignorance. This is completely new to me.

You don't want to charge the pack in series to 29V mainly because - without a BMS - it is virtually certain that one or more cells will get past 3.65V (maybe well past) while others are still down at 3.4V or 3.5V.  Funny thing is, the probability is high because the cells haven't been top balanced.

There are a ton of 0-30V / 0-10A bench power supplies on Amazon, all made in China. These work fairly well, but at only 10A it takes a long time to charge a parallel pack of 8 x 280Ah cells. www.circuitspecialists.com has a little higher quality supplies, some that go to 20 or 30A. I actually have 3 of the cheap supplies and 1 of the circuit specialist's supplies. If I was near Baja I'd be happy to loan one to you for however long you need it.

Horsefly

littleharbor2 said:

Haven't decided on the BMS as of yet.

I can give you some recommendations on a BMS if you like. There are lots of choices. There are some pretty expensive ones, but for most of us here there are some acceptable ones that are relatively inexpensive.  PM me if you want some links.

BB.

Horsefly, you are welcome to post links here too.

More information is always good.

Bill

Horsefly

BB. said:

Horsefly, you are welcome to post links here too.

More information is always good.

Bill

Thanks Bill. I'll post some names, and maybe a couple of links.

Some general BMS concepts:

1. The purpose of the BMS is to protect the LiFePO4 cells. All should have a LVD (Low Voltage Disconnect) where if any one cell gets to 2.5V, the BMS shuts off discharge from the battery. Similarly, a BMS should have a HVD (High Voltage Disconnect) that would shut off charging if any one cell gets up to 3.65V. Most also detect short circuits and will cut off the battery. A few have temperature probes, and allow charging to be cut off if the cells get below a certain temperature (LiFePO4 cells can be ruined if charged below 32°F), or discharging below or above a certain temperature.
2. Most BMSs these days seem to use MOSFETs to connect or cut off the battery from the rest of the system. Some still use relays / contactors. There tends to be a religious argument about whether MOSFETs or contactors are better. Since MOSFETs are directional, they have the advantage of being able to cut off charging or discharging or both on a single connection to the battery pack. A few BMSs allow you to disable the MOSFET control and drive an external contactor. 
3. There are "dumb" BMSs and "smart" BMSs. The basic difference is that a dumb BMS has some hard-coded thresholds on all the settings, and it just - hopefully - does what a BMS is supposed to do. A "smart" BMS has at a minimum a Bluetooth interface and a phone app, where the user can see the status (voltage) of each cell. Better smart BMSs will also allow you to modify the settings, such as the LVD and HVD voltages, temperature cutoffs, etc.  A good BMS should also have a Coulomb counter built in, so you can see the SoC of the battery. Finally, some of the smart BMSs allow your to calibrate the voltage readings, since an accurate voltage reading by the BMS is pretty critical (this requires a high quality multimeter).    I personally think there it is a really bad idea to use a dumb BMS, especially for anyone doing DIY. It's useful to see on your phone how the cells behave during charge and discharge cycles, and it helps to see where the best bulk/absorption voltage is for your SCC, and where your LVCO on the inverter should be. 

As for brands of BMS (this is pretty dangerous for me to stick my neck out like this):

1. For those that are really serious about it, there are some il primo BMS companies: Batrium (made in Australia, I think), Orion (rooted mostly in electric vehicles), and REC.  These are quality units, but if you are building a DIY battery from LiFePO4 cells you probably will be paying more for the BMS than you did for the cells. 
2. There are quite a few Chinese BMS companies. The one that gets the most play on Alibaba and AliExpress is Daly. They are distinctive because of their very slick-looking bright red paint and cool looking heat sinks and fans. Unfortunately, it seems they have traded some quality control and design detail for those pretty looks, as I think I've seen more complaints about Daly than most others.
3. I've seen some positive comments about Ant BMS, QUCC, and Heltec BMS, but not enough that I can say they much one way or the other about them.
4. Chargery is another Chinese company that started out for EVs but has broadened their application and it seems there are a number of DIY LiFePO4 battery builders that are using them.
5. There are some down in the weeds BMSs that get very little play but I have heard of them. TaoBMS seems to have lots of functionality. There are some others I can't think of at the moment.
6. JBD (also Chinese, and on AliExpress) is my current choice, and from what I've seen most people are pretty happy with them. I have 5 of them right now. They are smart BMSs, have temperature-triggered cutoffs, and can be calibrated. They are entirely MOSFET-based, so if you don't like MOSFETs you should not look at JBD.  Perhaps best of all, there are at least two companies in the U.S. that are reselling JBD and provide support and calibration of the BMS before shipping. Best of all (if you are in the U.S.) the shipping gets to you in a few days rather than the 30-60 days from China. The two companies are https://overkillsolar.com/ and https://www.currentconnected.com/. They both charge quite a bit more than JBD on AliExpress, but for some it is worth it.
7. There are a ton that I'm forgetting, and probably even more that I don't know anything about.

For most of the BMS brands that I've mentioned, you should be able to just ask the Google. For the Chinese brands, you can search for them by name on AliExpress.

Whew!

RCinFLA

littleharbor2 said:

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

If just passively paralleled they won't be perfectly balanced, but that was not your question.

At 3.269v rested open circuit, if DVM is accurate, the cells are in ballpark of 30% state of charge.  They will have between 1.5% to 2.5% cell capacity per month of cell self discharge depending on ambient temp.  So you have a while.

If you only passively paralleled to balance, some cells may be greater or less than 30% state of charge.  It takes 10-20 mV of overpotential before there is any small amount of cell current.   While parallel connected, they will all read the same voltage, but if you separate them, some will change as much as 10-20 mV above or below 3.269v, after any surface capacitance charge bleeds off which could take a couple of days.  That could mean some cells are as low as 20% state of charge and some as high as 40% state of charge.

littleharbor2

Horsefly said:

BB. said:

Horsefly, you are welcome to post links here too.

More information is always good.

Bill

Thanks Bill. I'll post some names, and maybe a couple of links.

Some general BMS concepts:

1. The purpose of the BMS is to protect the LiFePO4 cells. All should have a LVD (Low Voltage Disconnect) where if any one cell gets to 2.5V, the BMS shuts off discharge from the battery. Similarly, a BMS should have a HVD (High Voltage Disconnect) that would shut off charging if any one cell gets up to 3.65V. Most also detect short circuits and will cut off the battery. A few have temperature probes, and allow charging to be cut off if the cells get below a certain temperature (LiFePO4 cells can be ruined if charged below 32°F), or discharging below or above a certain temperature.
2. Most BMSs these days seem to use MOSFETs to connect or cut off the battery from the rest of the system. Some still use relays / contactors. There tends to be a religious argument about whether MOSFETs or contactors are better. Since MOSFETs are directional, they have the advantage of being able to cut off charging or discharging or both on a single connection to the battery pack. A few BMSs allow you to disable the MOSFET control and drive an external contactor. 
3. There are "dumb" BMSs and "smart" BMSs. The basic difference is that a dumb BMS has some hard-coded thresholds on all the settings, and it just - hopefully - does what a BMS is supposed to do. A "smart" BMS has at a minimum a Bluetooth interface and a phone app, where the user can see the status (voltage) of each cell. Better smart BMSs will also allow you to modify the settings, such as the LVD and HVD voltages, temperature cutoffs, etc.  A good BMS should also have a Coulomb counter built in, so you can see the SoC of the battery. Finally, some of the smart BMSs allow your to calibrate the voltage readings, since an accurate voltage reading by the BMS is pretty critical (this requires a high quality multimeter).    I personally think there it is a really bad idea to use a dumb BMS, especially for anyone doing DIY. It's useful to see on your phone how the cells behave during charge and discharge cycles, and it helps to see where the best bulk/absorption voltage is for your SCC, and where your LVCO on the inverter should be. 

As for brands of BMS (this is pretty dangerous for me to stick my neck out like this):

1. For those that are really serious about it, there are some il primo BMS companies: Batrium (made in Australia, I think), Orion (rooted mostly in electric vehicles), and REC.  These are quality units, but if you are building a DIY battery from LiFePO4 cells you probably will be paying more for the BMS than you did for the cells. 
2. There are quite a few Chinese BMS companies. The one that gets the most play on Alibaba and AliExpress is Daly. They are distinctive because of their very slick-looking bright red paint and cool looking heat sinks and fans. Unfortunately, it seems they have traded some quality control and design detail for those pretty looks, as I think I've seen more complaints about Daly than most others.
3. I've seen some positive comments about Ant BMS, QUCC, and Heltec BMS, but not enough that I can say they much one way or the other about them.
4. Chargery is another Chinese company that started out for EVs but has broadened their application and it seems there are a number of DIY LiFePO4 battery builders that are using them.
5. There are some down in the weeds BMSs that get very little play but I have heard of them. TaoBMS seems to have lots of functionality. There are some others I can't think of at the moment.
6. JBD (also Chinese, and on AliExpress) is my current choice, and from what I've seen most people are pretty happy with them. I have 5 of them right now. They are smart BMSs, have temperature-triggered cutoffs, and can be calibrated. They are entirely MOSFET-based, so if you don't like MOSFETs you should not look at JBD.  Perhaps best of all, there are at least two companies in the U.S. that are reselling JBD and provide support and calibration of the BMS before shipping. Best of all (if you are in the U.S.) the shipping gets to you in a few days rather than the 30-60 days from China. The two companies are https://overkillsolar.com/ and https://www.currentconnected.com/. They both charge quite a bit more than JBD on AliExpress, but for some it is worth it.
7. There are a ton that I'm forgetting, and probably even more that I don't know anything about.

For most of the BMS brands that I've mentioned, you should be able to just ask the Google. For the Chinese brands, you can search for them by name on AliExpress.

Whew!

Horsefly said:

Wow, now that's a lot of good info coming from experience. Again, thanks so much for your help

littleharbor2

RCinFLA said:

littleharbor2 said:

I currently have a  bank of 8 280 ah. batteries wired in parallel and sitting perfectly balanced at 3.269 volts. My question is can I leave them sitting, wired in parallel for an indefinite length of time while I assemble the rest of the components needed to put them into service as a 8s,24 volt bank. I'm in no big hurry to put them into service as my current AGM bank is somewhere shy of 3 years old and I figure I have at least 2 more years left on them.

If just passively paralleled they won't be perfectly balanced, but that was not your question.

At 3.269v rested open circuit, if DVM is accurate, the cells are in ballpark of 30% state of charge.  They will have between 1.5% to 2.5% cell capacity per month of cell self discharge depending on ambient temp.  So you have a while.

If you only passively paralleled to balance, some cells may be greater or less than 30% state of charge.  It takes 10-20 mV of overpotential before there is any small amount of cell current.   While parallel connected, they will all read the same voltage, but if you separate them, some will change as much as 10-20 mV above or below 3.269v, after any surface capacitance charge bleeds off which could take a couple of days.  That could mean some cells are as low as 20% state of charge and some as high as 40% state of charge.

Fluke DVM I trust it. The batts varied a bit before being passively balanced and a couple were farther off than the majority. Now they are all sitting still parallel wired and have been for a couple weeks now. I will disassemble the pack and watch the voltage.