Mixing different AH LifePO4

First off and foremost..like alot of people that find their way to these forums I'm sure, I am very new to solar. I am currently replacing my (2) 200ah AMG batteries with (2) 280ah (1) 200ah and (1)100ah LifePO4 batteries. I know I know. Best to have all same same, but it's the hand it's what I have. After some digging here in the forums (wow there is alot of awesome info here) I can see that I can safely run the different ah power level batteries because they are all same chemistry and same 12v running in parallel. Great. My question comes in with the internal BMS of the batteries. With the different power levels I am concerned about battery balancing especially given this is at my cabin on my tree farm an 1hr away, so I am not right there to monitor this while it idling when I am back home running the rat race. Will the internal BMS of each individual battery keep them all in balance. Will it do this both during idle and under load when I am there actively using the system? Or should I invest in an additional external BMS ? If the answer to that is yes, do you have any recommendations which one i should be using? Would it be best to hookup one battery at a time and them them top before adding the next one or can I just drop them all in at once and let the system roll?

The system:
(4) 315watt PV
100amp MPPT charge controller
(2) 280ah (1) 200ah and (1)100ah 12v LifePO4 batteries in parallel
4000watt inverter

CedarRidge said:

Will the internal BMS of each individual battery keep them all in balance.

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Yes.

CedarRidge said:

Will it do this both during idle and under load when I am there actively using the system?

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Yes.

CedarRidge said:

Would it be best to hookup one battery at a time and them them top before adding the next one or can I just drop them all in at once and let the system roll?

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Just make sure they're all at about the same voltage (which should be the case when they're at rest), and hook it all up.

CedarRidge said:

(2) 280ah (1) 200ah and (1)100ah 12v LifePO4 batteries in parallel

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With a 4000 watt inverter, current sharing needs good engineering.

I have run 3x280AH, 2x175AH, 1x100AH in a single bank without issues but with a little tweaking.
The charging profile MUST be moderate and not pushing beyond the Working Voltage Range (3.000-3.400 V per cell) . That said most of use 2.800-3.450 without issue. NB Bulk/Absorb to a max of 3.500VPC but normally to 3.450VPC, then FLOAT at 3.425-3.440.

You MUST respect EndAmps/Tailcurrent. Use the Highest AH Pack for that determination. When EndAmps/Tailcurrent is reached during Bulk/Absorb (Constant Voltage - Constant Current) that changes state to FLOAT (Constant Voltage - Variable Current)
280AH X 0.05 = 14A EndAmps/TailCurrent.

FUSE each Battery Pack. (100AH x 1.25 = 125A Fuse) (280AH X 1.25 = 350A fuse)
*NB* If BMS can only output a max of 200A then 200A X 1.25 =250A is acceptable.
EXTRA SPECIAL ! DO NOT CHEAP OUT ON FUSES/BREAKERS !!!
Ensure the Battery Cables from the common DC Bus to Battery Packs are the SAME Length & Gauge.
Keep Batt Wires close together to prevent EMI/RFI Noise (DC Gets noisy fast)

Battery Cables are NOT All the same ! Use only High Quality Wire such as Southwires Royal Excelene (Welding Cable) with superfine pure copper strands. NEVER EVER USE CCA !!! Copper Clad Aluminium Wires are BAD JUJU and inviting Murphy's Laws to cause you harm.

Here is one of my Simplified Diagrams which may address some questions.

Not that it really matters when mixing cell bank sizes, what’s your system voltage? 12, 24, 48v?

mikefitz said:

With a 4000 watt inverter, current sharing needs good engineering.

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The batteries are hooked up to a bus bar, along with the charge controller and the inverter. I may go down to a 3000watt inverter. The most I have seen on the inverter for wattage being used so far is 2600 and that was literally everything in my cabin running at once to see how much draw there actually would be.

Steve_S said:

I have run 3x280AH, 2x175AH, 1x100AH in a single bank without issues but with a little tweaking.
The charging profile MUST be moderate and not pushing beyond the Working Voltage Range (3.000-3.400 V per cell) . That said most of use 2.800-3.450 without issue. NB Bulk/Absorb to a max of 3.500VPC but normally to 3.450VPC, then FLOAT at 3.425-3.440.

You MUST respect EndAmps/Tailcurrent. Use the Highest AH Pack for that determination. When EndAmps/Tailcurrent is reached during Bulk/Absorb (Constant Voltage - Constant Current) that changes state to FLOAT (Constant Voltage - Variable Current)
280AH X 0.05 = 14A EndAmps/TailCurrent.

FUSE each Battery Pack. (100AH x 1.25 = 125A Fuse) (280AH X 1.25 = 350A fuse)
*NB* If BMS can only output a max of 200A then 200A X 1.25 =250A is acceptable.
EXTRA SPECIAL ! DO NOT CHEAP OUT ON FUSES/BREAKERS !!!
Ensure the Battery Cables from the common DC Bus to Battery Packs are the SAME Length & Gauge.
Keep Batt Wires close together to prevent EMI/RFI Noise (DC Gets noisy fast)

Battery Cables are NOT All the same ! Use only High Quality Wire such as Southwires Royal Excelene (Welding Cable) with superfine pure copper strands. NEVER EVER USE CCA !!! Copper Clad Aluminium Wires are BAD JUJU and inviting Murphy's Laws to cause you harm.

Here is one of my Simplified Diagrams which may address some questions.
View attachment 224727

Click to expand...

I am in fact using copper clad aluminum. What is the problem with it? It's 1/0 gauge wire that seems to work well. The stuff you mentioned is quite expensive and it maybe a bit before I can upgrade to better wire if it's needed. My cables aren't all the same length. Very close though. Nor do I have two sets of bus bars but I like how your diagram is set up. Currently my batteries, inverter, and charger are all on same bus as that is how I saw them in another diagram on a Reddit thread I believe it was. And when I googled it that is also what I found in general. I don't have fuses between the battery and bus but will probably be doing that now at this point after seeing it mentioned by several people in various groups and this forum. What is a smart shunt? I briefly looked it up but am not quite understanding it's actual purpose or why I would need one or should get one. As for charging profile and/or endamps/tail current I have zero control over that. My MPPT controller will only allow me to change one parameter. I can change the voltage to which it will charge to. So with my AMG batteries I would set that to 13.6v for when I wasn't there and 14.8 when I am there actively using the system. That is the only thing the charge controller will let me adjust. Everything else is automatic including what battery type ect.

Best Practices learned over a long period of "doing" is the resource you tapped into by coming here. A few of us are far more involved than it appears and there is a reasonable amount of us Hard Core folks here.

CCA has numerous issues from resistance to amp delivery & assorted risks. We always recommend using ONLY Pure Copper Wire with fine strands for maximum delivery & flexibility. The Southwrire Royal Excelene is pricey but you do get what you pay for. I have in fact just bought a large coil of Kalas Flexwhip Welding Cable (ultra fine copper) to rewire all my packs to common bus as I am finalizing the system. The Kalas wire is also more affordable than Excelene to boot. (I make my own cabling up)
(my supplier is in Canada but you can find it closer to you, likely cheaper too)

A Smartshunt monitors precisely what goes in/out of your battery bank so you know what is really going on and how much power you have available.

Non-programmable Solar Controller ? WTH ??
---------------
Future Reference: Always tell us WHAT YOU HAVE: IE Make & Model of Solar Controller, Inverter/Charger etc... All the bits that matter.
BTW: AGM is nothing like LFP.

Steve_S said:

Best Practices learned over a long period of "doing" is the resource you tapped into by coming here. A few of us are far more involved than it appears and there is a reasonable amount of us Hard Core folks here.

CCA has numerous issues from resistance to amp delivery & assorted risks. We always recommend using ONLY Pure Copper Wire with fine strands for maximum delivery & flexibility. The Southwrire Royal Excelene is pricey but you do get what you pay for. I have in fact just bought a large coil of Kalas Flexwhip Welding Cable (ultra fine copper) to rewire all my packs to common bus as I am finalizing the system. The Kalas wire is also more affordable than Excelene to boot. (I make my own cabling up)

FlexWhip™ Welding Cable - Kalas Wire Most Flexible Welding Cable

FlexWhip™ Welding Cable - ultra flexible welding cable. Available in Gauge 2/0–#2. Insulated with orange EPDM Jacket. Manufactured by Kalas.

www.kalaswire.com

(my supplier is in Canada but you can find it closer to you, likely cheaper too)

A Smartshunt monitors precisely what goes in/out of your battery bank so you know what is really going on and how much power you have available.

Non-programmable Solar Controller ? WTH ??
---------------
Future Reference: Always tell us WHAT YOU HAVE: IE Make & Model of Solar Controller, Inverter/Charger etc... All the bits that matter.
BTW: AGM is nothing like LFP.

Click to expand...

Hmm I will have to upgrade my wire. Seems to be working fine for now but I plan on adding panels so I can use the full 4000w of my inverter. A shunt purely monitors battery charge? No other benefit or job? I will look into them. Accurate data would be nice. I attached a picture. When the circle is there is now a cutoff switch. That wire goes to my inverter just out of the picture. The up arrow saying 20amp breaker is on my incoming from PV. The 125amp breaker is between mppt and battery bank. I will post up what mppt I have.

I run a Samlex EVO-4024 Inverter/Charger - 24V/4000W Low Frequency capable of handling 12,000W surge. (Tier-1 product $$$). This runs my home/property including deep well pump & usual gadgets. * I built a hyper efficient passive house and that's a big factor too. I am power frugal but not a miser by any means. Everything I have if as efficient as I could get when purchased, so no legacy appliances. I can run my MIG Welder & Large Air compressor (3HP) without as much as a blink. (Tip: Thanks to the Low Frequency System, a High Frequency system would freak).

At 24V/250A Draw the Inverter can deliver 6000W and my Master Breaker is a 250A between battery bank & inverter. That still allows for a full Surge Demand while providing all the elbow room needed. This Inverter only uses 18W Idle Standby (not powersave) unlike Tier 2 & Value gear.

Case example, with (LED) lights on, Computer & 47" screen, fridge running, usual wall warts & radio going, I am pulling 9A from my battery bank ! THAT'S IT ! IF my heating (radiant in slab) is running as well then I am pulling 14.1A
Kick the MIG and I pull 150A.
Kick the Compressor that adds up to 65A (when reaching cutoff)
Use the big shop vac that pulls 45A.
Deep Well Pump (280' deep) is 120V Soft-start starts at 550W and stages up to 1200W when reaching 54-PSI cutoff.

A Good Inverter/Charger with low overhead operation can serve pretty much any purpose and be frugal when needed & appropriate.
Always ensure the Master Breaker can support the Inverter Capacity (including surge demands).

Solar Arrays + their SCC's are sized to the Battery Bank NOT the Inverter !!
It's about charging the batteries up & replacing that which you used up within the Sun Hours window for your region. But that also must factor in the Low Sun Hour days (like December 21 in the Northern Hemisphere).
I use 2 SCC's for my bank.
1- Midnite Classic 200 - 2080W (4S2P 260W Panels) Array produces 79A Charge (200V input SCC)
1- Midnite Classic 150 - 2370W (2S3P 395W Panels) Array produces 94A Charge (150V input SCC)
* Again these are Tier-1 Product & $$ but this provides more collective charge amperage than a LOT of other SCC's especially those in AIO's (All In Ones). Simply put, smaller arrays that can be tweaked independently while producing max amperage through the day cycle.

Well... I can tell you it does work.

In my RV, originally I had 6 golf cart batteries

Then I upgraded to a pair of 230 amp hour, 200 amp BMS with the iron phosphate batteries

Unfortunately this was still not enough to keep me from having to use generator on a regular basis

So I took 16 of my 304 amp hour lithium iron phosphate batteries out of storage and I built a 4s4p battery and utilize one of my 200 amp BMSs

From there I converted my pair of 230 ah 4s batteries into a 2P4S battery with a 200 amp BMS

I then put the two batteries together in parallel

Most of the time the batteries are in exactly the same voltage... However when I get to a high state of discharge there will be some minor differences... The big difference is that when I am charging as I get up into the higher state of charge the smaller battery will finish early and the closer it gets to charge the less current it will take, and then more current is automatically shifted to the larger battery

This happens because the larger battery has more of a voltage differential

What is kind of interesting to watch is that if I am ever unable to get a full charge, But the smaller battery does have a full charge, I will see current shift from the smaller battery to the larger battery until they equalize

Been doing this for close to 2 years and have had no issues

Charge & Discharge between Battery Packs in Parallel is ALWAYS proportional relative to the battery pack AH.
In my old config with 1x100AH, 2x175AJ, 3x280AH on heavier pulls I would see something like this.
10A, 17A, 17A, 28A, 28A, 28A (General Example) the numbers would float a little +/- 1A or so. This is quite typical & expected with LFP packs like we use.

Steve_S said:

Charge & Discharge between Battery Packs in Parallel is ALWAYS proportional relative to the battery pack AH.
In my old config with 1x100AH, 2x175AJ, 3x280AH on heavier pulls I would see something like this.
10A, 17A, 17A, 28A, 28A, 28A (General Example) the numbers would float a little +/- 1A or so. This is quite typical & expected with LFP packs like we use.

Click to expand...

I originally had my wires cut to length so they were as short as possible. I am now understanding why it's better they all be the same length so they are all same resistance for charge and discharging. I rewired my battery bank so all wires are the same length and put them on one bus that is connected to another bus that has the mppt and inverter attached like your diagram. At this point I just removed the AGM from the equation and am using the four lifepo4 batteries.

You'll probably start noticing some differences, if you can use BMS Apps to monitor the packs you'll see more.

Steve_S said:

You'll probably start noticing some differences, if you can use BMS Apps to monitor the packs you'll see more.

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I am going to have to get some more panels before I get a Bms first. With the new battery capacity it's taking much much longer to charge LOL

Hahahaha Indeed !. I started my solar adventure with Rolls Surette S-550 6V Heavy Leads ! 4s2p for 24V and that only gave me 220AH I think ? 2KW solar array with 79A Charge capability & okie dokie.. but life changes & tinkerers tinker and of course MANY of us would benefit from a 12-Step Program !

Steve_S said:

Hahahaha Indeed !. I started my solar adventure with Rolls Surette S-550 6V Heavy Leads ! 4s2p for 24V and that only gave me 220AH I think ? 2KW solar array with 79A Charge capability & okie dokie.. but life changes & tinkerers tinker and of course MANY of us would benefit from a 12-Step Program !

Click to expand...

I will probably pick up 4 more of the same 315 watt panels I have up now. That will bump me to 2560watts. I was planning on adding a wind generator but more panels might have to come first. I fore see a 12 step in my future lol. I am constantly thinking about various aspects of the system and how I can improve it or increase safety to prevent failure god forbid fire.

I have finally & fortunately arrived to my final & complete build. I'm in the process of completing the majority of it now with the powerhouse reorganization etc. Then one final thing will be reworking my ground mount arrays for better winter generation, summer is never an issue but winter... oi.

CedarRidge said:

I will probably pick up 4 more of the same 315 watt panels I have up now. That will bump me to 2560watts. I was planning on adding a wind generator but more panels might have to come first. I fore see a 12 step in my future lol. I am constantly thinking about various aspects of the system and how I can improve it or increase safety to prevent failure god forbid fire.

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Be very wary about small wind generators. Rarely do they meet their (usually greatly overstated) marketing claims. Pay close attention to minimum average wind requirements.

CedarRidge said:

Hmm I will have to upgrade my wire. Seems to be working fine for now but I plan on adding panels so I can use the full 4000w of my inverter. A shunt purely monitors battery charge? No other benefit or job? I will look into them. Accurate data would be nice. I attached a picture. When the circle is there is now a cutoff switch. That wire goes to my inverter just out of the picture. The up arrow saying 20amp breaker is on my incoming from PV. The 125amp breaker is between mppt and battery bank. I will post up what mppt I have.

Click to expand...

Is that 125A breaker a DC rated breaker?

Also, I see your MPPT charge controller is laying on it's back. Does the manual say that's okay? Usually they need to be on a vertical surface for the heat to vent properly. If it's not made to be on it's back, it can overheat the components inside and shorten it's life.

It seems the only way a small wind generator makes sense is if you receive it for free. Even then, it might be more trouble than it's worth.

I started with a 35ah AGM and a 20w panel, lol

740GLE said:

I started with a 35ah AGM and a 20w panel, lol

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Look at you now... Hahaha

Still modest in grand schemes but more than meets our needs.

340w pv
3k MPII
100/15 SCC
24v 230ah (soon to me 510ah)

I would like to hit 800-1200w PV then cones the mini split.

sollap said:

Is that 125A breaker a DC rated breaker?

Also, I see your MPPT charge controller is laying on it's back. Does the manual say that's okay? Usually they need to be on a vertical surface for the heat to vent properly. If it's not made to be on it's back, it can overheat the components inside and shorten it's life.

Click to expand...

Yes the breaker is a DC rated breaker. As for the controller they do recommend mounting vertical Long story short... It's gonna have to be in there for the time being. I'm trying to find a solution to why I had to do that in the mean time I have been watching the temp to see if it is getting warm. I have a 4" fan goung in there as well.

cs1234 said:

It seems the only way a small wind generator makes sense is if you receive it for free. Even then, it might be more trouble than it's worth.

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That's what I am afraid of.