I too would like an explanation as well as further clarification. In real world application, such as in an RV that is not used full time and spends , say, 50% of the time on shore power, is the loss or degradation significant? For example: if the degradation is say an additional 10% for a system that is rated for 80% of capacity at 10K cycles. Is that really significant? Is it worth scrapping the 3 -12v 100 Ah of BB batteries you bough 3 years ago and cycled 100 times camping during that time period because you wanted to double your capacity at 1/5th the cost by add a Eco-worthy 12V 314Ah? I can understand engineers saying "for best results you need to replace the entire system" but is it cost effective to get maximum efficiency?
I'd be interested in a technical explanation of why different-capacity LiFePO4 batteries which each have their own built-in BMS can't be paralleled, especially if they have essentially the same voltage vs. % SOC profile (same type of internal cells). At a glance, each battery should readily deal with the bus voltage (as a single battery does), and when it reaches full charge or minimum charge, the BMS should happily disconnect it. If the wiring is good, they should even contribute their proportional share of capacity, at least near the end of discharge, where the terminal voltage will allow better current sharing. Is there a physical explanation of any performance detriment that a BMS would allow, simply due to a neighboring battery (or even two or three identical neighboring batteries, together having more capacity) being connected?
The typical response given - from many sources - is that all the batteries (I am assuming All are LiFePO4 with good internal BMS's) should be the same brand/age/capacity,, etc. But this has always confounded me because the only thing any battery sees is voltage. No battery knows about any other battery in it's system - it only sees voltage. If lower than it's own internal voltage (as in a load being applied) then it will discharge some of it's own capacity. If voltage is higher then it Might use that to charge up - depending on what it's own BMS allows. So it appears that with multiple batteries - even if mixed - each will react the same way. Under load there will be a voltage drop which any or all can fulfill be supplying some of their own charge. Under charging each will accept incoming voltage based on it's BMS function. So it appears to me that multiple batteries will be self regulating, as each is simply reacting to the system voltage. While it appears the optimum system have all batteries be identical I don't know why mixed bands and capacities cannot function as well as each is self-regulating.
In My case I built a 500AH system in my CLass A that has worked will for years (as full timers) with very mixed batteries. In 2018 I began my LiFePO4 system with 4 - 300 AH 3.8v Winston cells. Added an external BMS for these as they were older design having no BMS. Worked well but we wanted more capacity so added 1 Battle Born 100 AH in 2019 in order to support an all electric residential refrigerator (best upgrade we ever did!). Then due to the amount of boondocking we were doing, (and adding Starlink) we wanted more capacity so added a LiTime 100AH in 2024. All charged from rooftop solar, internal converter and DC-DC engine converter. Monitored by Victron BMV-712.
So far all have worked well. I periodically isolate each of the 3 batteries to measure voltage and all are virtually identical. Which goes back to my main point. While each battery is different it all comes down to voltage, and they will self-equalize amongst themselves automatically. So while I understand in theory why it might be better to have a system where all batteries are identical, I also understand that each battery has its own regulation and that it all really comes down to system voltage. Which they will all equalize to.
I’m going to get more details from my engineering contacts at Progressive Dynamics, Southwire and WFCO, all of whom make Lithium battery chargers, but don’t sell RV batteries. That way we can get an unbiased engineering answer.
I too would like an explanation as well as further clarification. In real world application, such as in an RV that is not used full time and spends , say, 50% of the time on shore power, is the loss or degradation significant? For example: if the degradation is say an additional 10% for a system that is rated for 80% of capacity at 10K cycles. Is that really significant? Is it worth scrapping the 3 -12v 100 Ah of BB batteries you bough 3 years ago and cycled 100 times camping during that time period because you wanted to double your capacity at 1/5th the cost by add a Eco-worthy 12V 314Ah? I can understand engineers saying "for best results you need to replace the entire system" but is it cost effective to get maximum efficiency?
I'd be interested in a technical explanation of why different-capacity LiFePO4 batteries which each have their own built-in BMS can't be paralleled, especially if they have essentially the same voltage vs. % SOC profile (same type of internal cells). At a glance, each battery should readily deal with the bus voltage (as a single battery does), and when it reaches full charge or minimum charge, the BMS should happily disconnect it. If the wiring is good, they should even contribute their proportional share of capacity, at least near the end of discharge, where the terminal voltage will allow better current sharing. Is there a physical explanation of any performance detriment that a BMS would allow, simply due to a neighboring battery (or even two or three identical neighboring batteries, together having more capacity) being connected?
The typical response given - from many sources - is that all the batteries (I am assuming All are LiFePO4 with good internal BMS's) should be the same brand/age/capacity,, etc. But this has always confounded me because the only thing any battery sees is voltage. No battery knows about any other battery in it's system - it only sees voltage. If lower than it's own internal voltage (as in a load being applied) then it will discharge some of it's own capacity. If voltage is higher then it Might use that to charge up - depending on what it's own BMS allows. So it appears that with multiple batteries - even if mixed - each will react the same way. Under load there will be a voltage drop which any or all can fulfill be supplying some of their own charge. Under charging each will accept incoming voltage based on it's BMS function. So it appears to me that multiple batteries will be self regulating, as each is simply reacting to the system voltage. While it appears the optimum system have all batteries be identical I don't know why mixed bands and capacities cannot function as well as each is self-regulating.
In My case I built a 500AH system in my CLass A that has worked will for years (as full timers) with very mixed batteries. In 2018 I began my LiFePO4 system with 4 - 300 AH 3.8v Winston cells. Added an external BMS for these as they were older design having no BMS. Worked well but we wanted more capacity so added 1 Battle Born 100 AH in 2019 in order to support an all electric residential refrigerator (best upgrade we ever did!). Then due to the amount of boondocking we were doing, (and adding Starlink) we wanted more capacity so added a LiTime 100AH in 2024. All charged from rooftop solar, internal converter and DC-DC engine converter. Monitored by Victron BMV-712.
So far all have worked well. I periodically isolate each of the 3 batteries to measure voltage and all are virtually identical. Which goes back to my main point. While each battery is different it all comes down to voltage, and they will self-equalize amongst themselves automatically. So while I understand in theory why it might be better to have a system where all batteries are identical, I also understand that each battery has its own regulation and that it all really comes down to system voltage. Which they will all equalize to.
Fire away . . .
I’m going to get more details from my engineering contacts at Progressive Dynamics, Southwire and WFCO, all of whom make Lithium battery chargers, but don’t sell RV batteries. That way we can get an unbiased engineering answer.