I have a question about battery banks with a 48v inverter. Let's say i bought 16 of the 210ah lifepo4 and hooked them up with a bms to a 48v inverter. Let's say in 6 months I buy 16 280ah lifepo4 batteries connect to a different bms but still tired to the same inverter. Basically 2 different battery banks completely isolated by their own bms but tied into the same inverter. It's this OK or is this a no no?
Different sized battery banks on a single inverter
- Thread starter Mtwilks25
- Start date
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
A single Battery is just a battery.
Two or more batteries working together is a Battery Bank.
Each Battery Pack must be independently fused.
Multiple Battery Packs can make up a Bank but there are practicalities that come into play as a bank gets larger.
LFP banks can be added to over time, unlike Lead Acid which starts its age cycle right away.
It is best NOT to have too much differential between battery packs. Dor example 200 & 280 is fine generally BUT 100 & 280 there would be issue at both the Top & Bottom End of the Charge State.
Ideally, each battery pack should be configured to be able to handle all the Load/Charge so that any single battery pack can operate as a "Stand Alone" should the other packs disconnect for any reason. This provides Fail-Over Protection.
All BMS's should be identically configured so they so as to not conflict or operate differently than the other packs within the bank.
*BMS Capacity/Capability: Assuming 48V/250A Max Draw & 125A Charge Push capable.
- Battery Packs should be fused to 300A (for surge).
- BMS should be capable of Handling 250A Discharge & 125A Charge
* (DEFER TO BATT SPECS), ie 200AH can only discharge at 200A Max & take 100A Charge) DO NOT Exceed Max Rates.
> FET Based BMS are best DERATED, so a 300A BMS provides safe margin.
> Relay/Contactor Based BMS uses Relays/Contactors to manage the Voltage, these are external and therefore can support assorted amperages & voltage. EV's with Very High Voltages use Relays to handle the volts & amps.
Please see THIS Document, starting Page 17 explaining Paralleling Batteries and the most optimal way. For LFP, we generally suggest & use a "Common DC BUS" with a (-) & (+) BUSBAR with the batteries attached to those with Equal Length Battery Cables from the battery pack to busbar and then equal length cable to Inverter. https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf
TIP: It is strongly recommended to NOT Exceed 250A Drawn from batteries by an Inverter. 48VDC@250A=12,000 Watts which can deliver 100A @ 120V or 50A @ 240V. PLUS Surge Capacity Handling, Low Freq Inverters that is 3X Surge, High Freq Inverter that is 2X Surge.
Hope it helps, Good Luck
Two or more batteries working together is a Battery Bank.
Each Battery Pack must be independently fused.
Multiple Battery Packs can make up a Bank but there are practicalities that come into play as a bank gets larger.
LFP banks can be added to over time, unlike Lead Acid which starts its age cycle right away.
It is best NOT to have too much differential between battery packs. Dor example 200 & 280 is fine generally BUT 100 & 280 there would be issue at both the Top & Bottom End of the Charge State.
Ideally, each battery pack should be configured to be able to handle all the Load/Charge so that any single battery pack can operate as a "Stand Alone" should the other packs disconnect for any reason. This provides Fail-Over Protection.
All BMS's should be identically configured so they so as to not conflict or operate differently than the other packs within the bank.
*BMS Capacity/Capability: Assuming 48V/250A Max Draw & 125A Charge Push capable.
- Battery Packs should be fused to 300A (for surge).
- BMS should be capable of Handling 250A Discharge & 125A Charge
* (DEFER TO BATT SPECS), ie 200AH can only discharge at 200A Max & take 100A Charge) DO NOT Exceed Max Rates.
> FET Based BMS are best DERATED, so a 300A BMS provides safe margin.
> Relay/Contactor Based BMS uses Relays/Contactors to manage the Voltage, these are external and therefore can support assorted amperages & voltage. EV's with Very High Voltages use Relays to handle the volts & amps.
Please see THIS Document, starting Page 17 explaining Paralleling Batteries and the most optimal way. For LFP, we generally suggest & use a "Common DC BUS" with a (-) & (+) BUSBAR with the batteries attached to those with Equal Length Battery Cables from the battery pack to busbar and then equal length cable to Inverter. https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf
TIP: It is strongly recommended to NOT Exceed 250A Drawn from batteries by an Inverter. 48VDC@250A=12,000 Watts which can deliver 100A @ 120V or 50A @ 240V. PLUS Surge Capacity Handling, Low Freq Inverters that is 3X Surge, High Freq Inverter that is 2X Surge.
Hope it helps, Good Luck
Jbbonnette
New Member
Your post has me concerned about my exisiting project. I have two 16S 280 AH strings, due to room- I decided to make a 3rd string @ 100 AH. What kind of problems would happen in this config? You say issues at top and bottom of charge. Can you elaborate?
Thanks,
James
Nothing too serious. LFP has a very flat voltage curve and batteries in parallel have common voltage. Voltage is not a good indicator of capacity though. The smaller battery will reach voltage knees sooner than the larger one. The batteries will then start transferring energy between them to stay at same voltage. I have 120Ah and 202Ah in parallel and it all works.
Jbbonnette
New Member
Thanks for the info. Question...does the length and gauge of the cable matter? Seems like equal lengths of battery cables matter when you parallel.... But when I have 280ah and 100ah- does my cable need fo be the same length and gauge?
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
It is NOT a Matter of IF they work together, of course they do, but when at the end of the Curve (Charge & Discharge) that is where the size difference becomes an issue. 280AH + 100AH will result in issues. The 280's will attempt to Backcharge the 100AH as it discharges and that has a COST... This is very easy to see as it happens. During charge is less issues because the 100AH will reach designated FULL and platean there and Amps taken will just decrease to <1A taken or even go into storage mode (pending on BMS) at which point the 280 will continue taking everything they can till it/they drecrease Amps Taken and reach full.
The Gotcha which CAN HAPPEN depends on your Charge Setup. You can never exceed 0.5C Rate of the lowest pack in the bank. For 100AH that is 50A Max while the 280AH is 140A Max Charge Rate. Technically you can push as much as 190A for One Hour to charge both up @ 0.5C but such is rarely ever done. Fortunately like Discharge, the Charging is split/shared "Proportionately" not equally relative to battery capacity rating.
Cables, Placement on DC-Busbars do play very important roles as it affects line-lost resistance which is noticeable by Lithium Batteries, unlike Lead which is in essence Brute Force tech. Cable Gauge, Grade & Lengths from BATT Terminals to Common DC Bus should be the same length.
Every crimped lug, terminal, switch etc adds resistance which is Voltage & Amperage drop. These wires should be kept close together to prevent magnetic & electrical noise. Always use High-Quality Fuses & Breakers not only because it is wise to do so, but they are also much better with regards to resistance (which makes heat and that is bad) and line losses.
The charging difference is very easy to see when you are charging. I can watch my bank charge and while the 280AH Packs may be taking 25A +/- ea the 175's will be taking 17A+/- a bit. When discharging you will also see the 100AH will output less Amps while the 280AH will be outputting more.
SOME BMS' have a Sleep / Storage Mode which can be triggered during charge, once the battery pack is fully saturated, or when discharged to Low Volt Disconnect LVD which prevents them from a Full OFF state. In that instance, the BMS wakes up to see if an incoming charge is present, if not, sleep, if yes, open to receive charge, these will cycle this way until a critical point is reached at which time they fully shutoff to protect the battery pack.
*** REMEMBER TO FUSE THE BATTERY TO THE BATTEY ! 100AH Battery should get 100A Fuse and the 280AH a 250A Fuse. Fuses will allow for a short Burst above as will the BMS' and the cells. I have 300A Fuses on the 280's and 200A on the 175AH packs.
A NOTE MANY DO NOT KNOW ABOUT !
We always talk about resistance and how it affects things, The battery Terminals used can be Very Significant for resistance ! Lead Acid Batts use Lead Terminals which are Low Resistance and being "Brute Force" tech is not that significant at that level. No Lithium pack uses Lead Terminals and you cannot even buy them (Controlled substance in MANY MANY Places, (Lead is really Nasty Assed Crap !) ) Almost all Terminals used by us DIY Folk are made of BRASS and are often nickel plated... Brass is not the best of conductors and varies a LOT with the quality of Brass used (it is extremely recycled everywhere, get's whiter each time, as well as more brittle & increased resistance) Then depending on "what" it is plated with can also have effects which are not desirable. Chrome Plating is not equivalent to Nickle or Tin for conductivity. SOME ARE SHOCKED to see as much as 1.0V Drop just across the Battery Terminals and don't get why.... then don't want to hear it when told why.
BTW / FYI: Many Vendors include busbars with their cells. A Good Majority use CHEAP Chromed Brass Busbars which have a higher resistance while other use proper Nickel Plated Copper Busbars (more costly) with much lower resistance, while some just offer straight Copper Busbars.
* Always use the same type of busbar when assembling a pack, mixing them will skew numbers and affect the BMS (charge/discharge & alancing)
* Try to build packs in a consistent format and replace that with each added pack of same size.
* Do make every effort to use identical BMS's for all packs within a Bank. Makes managing, adjusting & monitoring easier. Also better for Fault Tolerance and Fail Over implementations.
* Check resistance through items like Battery Box Terminals. Common examples shown below.
These are from Morosso but Painless and many others are the same.
Unplated brass has lower resistance as opposed to the Chrome Plated ones.. That's right, Chrome, not Nickel.
The Gotcha which CAN HAPPEN depends on your Charge Setup. You can never exceed 0.5C Rate of the lowest pack in the bank. For 100AH that is 50A Max while the 280AH is 140A Max Charge Rate. Technically you can push as much as 190A for One Hour to charge both up @ 0.5C but such is rarely ever done. Fortunately like Discharge, the Charging is split/shared "Proportionately" not equally relative to battery capacity rating.
Cables, Placement on DC-Busbars do play very important roles as it affects line-lost resistance which is noticeable by Lithium Batteries, unlike Lead which is in essence Brute Force tech. Cable Gauge, Grade & Lengths from BATT Terminals to Common DC Bus should be the same length.
Every crimped lug, terminal, switch etc adds resistance which is Voltage & Amperage drop. These wires should be kept close together to prevent magnetic & electrical noise. Always use High-Quality Fuses & Breakers not only because it is wise to do so, but they are also much better with regards to resistance (which makes heat and that is bad) and line losses.
The charging difference is very easy to see when you are charging. I can watch my bank charge and while the 280AH Packs may be taking 25A +/- ea the 175's will be taking 17A+/- a bit. When discharging you will also see the 100AH will output less Amps while the 280AH will be outputting more.
SOME BMS' have a Sleep / Storage Mode which can be triggered during charge, once the battery pack is fully saturated, or when discharged to Low Volt Disconnect LVD which prevents them from a Full OFF state. In that instance, the BMS wakes up to see if an incoming charge is present, if not, sleep, if yes, open to receive charge, these will cycle this way until a critical point is reached at which time they fully shutoff to protect the battery pack.
*** REMEMBER TO FUSE THE BATTERY TO THE BATTEY ! 100AH Battery should get 100A Fuse and the 280AH a 250A Fuse. Fuses will allow for a short Burst above as will the BMS' and the cells. I have 300A Fuses on the 280's and 200A on the 175AH packs.
A NOTE MANY DO NOT KNOW ABOUT !
We always talk about resistance and how it affects things, The battery Terminals used can be Very Significant for resistance ! Lead Acid Batts use Lead Terminals which are Low Resistance and being "Brute Force" tech is not that significant at that level. No Lithium pack uses Lead Terminals and you cannot even buy them (Controlled substance in MANY MANY Places, (Lead is really Nasty Assed Crap !) ) Almost all Terminals used by us DIY Folk are made of BRASS and are often nickel plated... Brass is not the best of conductors and varies a LOT with the quality of Brass used (it is extremely recycled everywhere, get's whiter each time, as well as more brittle & increased resistance) Then depending on "what" it is plated with can also have effects which are not desirable. Chrome Plating is not equivalent to Nickle or Tin for conductivity. SOME ARE SHOCKED to see as much as 1.0V Drop just across the Battery Terminals and don't get why.... then don't want to hear it when told why.
BTW / FYI: Many Vendors include busbars with their cells. A Good Majority use CHEAP Chromed Brass Busbars which have a higher resistance while other use proper Nickel Plated Copper Busbars (more costly) with much lower resistance, while some just offer straight Copper Busbars.
* Always use the same type of busbar when assembling a pack, mixing them will skew numbers and affect the BMS (charge/discharge & alancing)
* Try to build packs in a consistent format and replace that with each added pack of same size.
* Do make every effort to use identical BMS's for all packs within a Bank. Makes managing, adjusting & monitoring easier. Also better for Fault Tolerance and Fail Over implementations.
* Check resistance through items like Battery Box Terminals. Common examples shown below.
These are from Morosso but Painless and many others are the same.
Unplated brass has lower resistance as opposed to the Chrome Plated ones.. That's right, Chrome, not Nickel.
Similar threads
