LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[john61ct]

Originally Posted by Steve_C

I thought that would be 3p4s ?

3 cells in Parallel and then in a series of 4 ?

That would be the best design as long as the bank is intended to remain a single unit.

Originally Posted by newhaul

3 separate 1p4s 100ah then 3s for 300ah total. 3 separate 12 volt batteries .

Actually, no.

Yes each 4S string is a 12V 100Ah battery.

But putting these three 4S strings **in series** (3S as you wrote) gives you a 36V bank at 100Ah.

If you want to stay at 12V, you parallel the strings to get 300Ah.

Are you planning to break apart then rejoin each of these strings to function independently, as say mobile powerpacks, power a dinghy or tunes on a picnic?

That would be the only justification afaic for serialing first.

[john61ct]

Excellent choice on the cells themselves though.

[newhaul]

Originally Posted by john61ct

That would be the best design as long as the bank is intended to remain a single unit.


Actually, no.

Yes each 4S string is a 12V 100Ah battery.

But putting these three 4S strings **in series** (3S as you wrote) gives you a 36V bank at 100Ah.

If you want to stay at 12V, you parallel the strings to get 300Ah.

Are you planning to break apart then rejoin each of these strings to function independently, as say mobile powerpacks, power a dinghy or tunes on a picnic?

That would be the only justification afaic for serialing first.

correct it should have been 1p4sx3p.
And no not for mobile pack usage i already have the Lfp 75ah suitcase .
I am a retired navy hull tech ( one of my specialties was damage control) I like the redundancy . If there is an issue with a cell I can quickly isolate to protect the rest and still have 200ah with simply throwing a switch.

[Steve_C]

Originally Posted by newhaul

correct it should have been 1p4sx3p.
And no not for mobile pack usage i already have the Lfp 75ah suitcase .
I am a retired navy hull tech ( one of my specialties was damage control) I like the redundancy . If there is an issue with a cell I can quickly isolate to protect the rest and still have 200ah with simply throwing a switch.

So really its just 4s3p right? You can't really do 1p (1 in parallel)

or just 4s then connected 3p by switches???

remember that most of us get confused in this thread REALLY easily

[Q Xopa]

Originally Posted by newhaul

correct it should have been 1p4sx3p.
And no not for mobile pack usage i already have the Lfp 75ah suitcase .
I am a retired navy hull tech ( one of my specialties was damage control) I like the redundancy . If there is an issue with a cell I can quickly isolate to protect the rest and still have 200ah with simply throwing a switch.

Yes I like this approach too. Im still considering exactly which LFP setup I will go with. But I was thinking of 2 individual strings on seperate On/Off batt switches. It is expensive BMS wise but I like the idea. Yes Im also a redundancy fan. More than one Alternators, Inverters, fuel tanks, fuel filters, bilge pumps, Mppts etc. Why not Batteries?
Obviously more upfront cost and more maintenance and things to break. But I'm leaning heavily this way.

[john61ct]

Originally Posted by Steve_C

So really its just 4s3p right? You can't really do 1p (1 in parallel)

or just 4s then connected 3p by switches???

remember that most of us get confused in this thread REALLY easily [emoji3]

Well you can of course **do** 1P. but the notation is redundant, usually left out.

3 strings in parallel isn't too bad, but going to a higher number can def lead to balancing problems. There are higher-level balancing gadgets to help with those, but violates KISS reliability.

If you want to isolate a 4S string with switches, then the BMS on each string needs to be independent of the others, in effect designing as if the picnic scenario.

Personally I think a single Reserve/starter 12V for contingencies, then a normal 3P4S layout makes more sense.

Just one BMS, easy to expand if needed without the balancing problems.

If a cell goes bad (very unlikely), use Reserve/starter while you reconfigure to 2P4S 200Ah, keep the same BMS and settings, just watch C-rates until fixed back to 300Ah.

Just my 2¢, obviously your rig do what you like.

[anthem00]

Originally Posted by Q Xopa

Yes I like this approach too. Im still considering exactly which LFP setup I will go with. But I was thinking of 2 individual strings on seperate On/Off batt switches. It is expensive BMS wise but I like the idea. Yes Im also a redundancy fan. More than one Alternators, Inverters, fuel tanks, fuel filters, bilge pumps, Mppts etc. Why not Batteries?
Obviously more upfront cost and more maintenance and things to break. But I'm leaning heavily this way.

It sounds redundant and simple. And Genasun way earlier in this thread - actually advocated having two banks in parallel for supposed redundancy. Everyone seems to espouse against it with balancing between banks issues, etc (theoretically they should balance themselves out between battery banks like a normal lead-acid bank as well). I'm not sure why there is so much/many against it. But i've been playing with this setup. It does complicate things a bit - namely how you are going to handle BMS unit and what not. Namely :

Are you going to run two BMS units ? It essentially requires this, unless you're BMS units run only on the cell level. I think the Orion can handle this. The Rec BMS cannot and specifically advise against using their product and will even tell you to use a competitors product rather than two BMS units for whatever reason. But if the BMS runs for LV/HV protection only at the cell level - then it doesnt matter.

Are you treating the banks completely differently ? As in two separate supply busses, and two load busses. Though you can one way isolate the load bus for them to be on both. Because when you do a cut off, the guess is - that you wont want to cut off both banks if its only one bank having an issue. Load bank - you want a shared load bank, but then isolate the load bank when you need to.

Charging - generally the bank will be considered in parallel most of the time. So its essentially connected, charged and managed as one.

Anyhow, something I've been planning to do to double the amount of AH that I currently have.

[Q Xopa]

Originally Posted by john61ct

Well you can of course **do** 1P. but the notation is redundant, usually left out.

3 strings in parallel isn't too bad, but going to a higher number can def lead to balancing problems. There are higher-level balancing gadgets to help with those, but violates KISS reliability.

If you want to isolate a 4S string with switches, then the BMS on each string needs to be independent of the others, in effect designing as if the picnic scenario.

Personally I think a single Reserve/starter 12V for contingencies, then a normal 3P4S layout makes more sense.

Just one BMS, easy to expand if needed without the balancing problems.

If a cell goes bad (very unlikely), use Reserve/starter while you reconfigure to 2P4S 200Ah, keep the same BMS and settings, just watch C-rates until fixed back to 300Ah.

Just my 2¢, obviously your rig do what you like.

Yes I agree with what youre saying.
However with my 24v house/ windlass and 12v start arrangement its not so simple. I would like 24v starter motors then just use some converters for some 12v devices. But havent been able to track them down.

[Chotu]

After reading through this, I think I can sum up the thread in one sentence.

"This is why we can't have nice things."

[newhaul]

Originally Posted by john61ct

Well you can of course **do** 1P. but the notation is redundant, usually left out.

3 strings in parallel isn't too bad, but going to a higher number can def lead to balancing problems. There are higher-level balancing gadgets to help with those, but violates KISS reliability.

If you want to isolate a 4S string with switches, then the BMS on each string needs to be independent of the others, in effect designing as if the picnic scenario.

Personally I think a single Reserve/starter 12V for contingencies, then a normal 3P4S layout makes more sense.

Just one BMS, easy to expand if needed without the balancing problems.

If a cell goes bad (very unlikely), use Reserve/starter while you reconfigure to 2P4S 200Ah, keep the same BMS and settings, just watch C-rates until fixed back to 300Ah.

Just my 2¢, obviously your rig do what you like.

boat manufacturers do it every day with lead based batteries . Why not Lfp .
As to doing a reconfigure while running off a start battery ? Anyone that has ever tried to do serous work while offshore and on a timeline would prefer to have an easy to carryon in till in better conditions . ( nothing breaks in calm weather. )

[Q Xopa]

Originally Posted by anthem00

It sounds redundant and simple. And Genasun way earlier in this thread - actually advocated having two banks in parallel for supposed redundancy. Everyone seems to espouse against it with balancing between banks issues, etc (theoretically they should balance themselves out between battery banks like a normal lead-acid bank as well). I'm not sure why there is so much/many against it. But i've been playing with this setup. It does complicate things a bit - namely how you are going to handle BMS unit and what not. Namely :

Are you going to run two BMS units ? It essentially requires this, unless you're BMS units run only on the cell level. I think the Orion can handle this. The Rec BMS cannot and specifically advise against using their product and will even tell you to use a competitors product rather than two BMS units for whatever reason. But if the BMS runs for LV/HV protection only at the cell level - then it doesnt matter.

Are you treating the banks completely differently ? As in two separate supply busses, and two load busses. Though you can one way isolate the load bus for them to be on both. Because when you do a cut off, the guess is - that you wont want to cut off both banks if its only one bank having an issue. Load bank - you want a shared load bank, but then isolate the load bank when you need to.

Charging - generally the bank will be considered in parallel most of the time. So its essentially connected, charged and managed as one.

Anyhow, something I've been planning to do to double the amount of AH that I currently have.

I definately dont have all the answers. Which is why Im here.

As I said still considering options.

I realise there would have to be seperate BMSs for each string. Thats why I was saying it would be expensive with BMSs.

[nebster]

Originally Posted by john61ct

3 strings in parallel isn't too bad, but going to a higher number can def lead to balancing problems. There are higher-level balancing gadgets to help with those, but violates KISS reliability.

In my direct experience, this is false. Please cite your reference or experience, or at least explain why magical imbalance begins at 4p and is not an issue at 3p.

[john61ct]

Just like the 50% discharge guideline for lead, it's not a magic B&W line, as you get past two strings the potential issues increase in incremental greyscale.

The context on this issue was in the past, with lead banks, being installed for customers that wanted good longevity and didn't want to worry about monitoring details.

But with so many balancing devices now readily available, if that design path is important to you go for it, keep an eye on your "sub-packs" ideally with regular capacity testing, I'm sure you'll be able to head off any problems.

[john61ct]

Originally Posted by Q Xopa

Yes I agree with what youre saying.
However with my 24v house/ windlass and 12v start arrangement its not so simple. I would like 24v starter motors then just use some converters for some 12v devices. But havent been able to track them down.

Common as chips, but good stuff not as cheap.

Seems to me you should go 24V throughout.

Little low-watt for specific PoL circuits are IMO better than multiple backbones.

eBay is a great source for industrial quality NOS but recently discontinued, get the datasheets off Mouser or Digikey.

Seen no-reserve auctions of pallet loads of say TDK-Lambda converters end up going for under a dollar each, sellers just clearing the warehouse eat most of the shipping too.

[john61ct]

Originally Posted by anthem00

having two banks in parallel for supposed redundancy. Everyone seems to espouse against it with balancing between banks issues

Just a pair of strings should be no issue at all.

Nor is a large number of **paralleled cells** down at the bottom level.

So say you have 8P4S for a 12V bank.

Connecting a midpoint tap from a BMV-712, you will be alerted to any voltage imbalance while in use.

Reconfigure to 4P4S using the higher-SoH cells, and then as you have time, see where the problem lies with the weaker half.

Once identified, if one cell actually failed (very unlikely) put it back to 3P4S and continue on.