FLA to LFP battery upgrade timeline adverse effects (If any)

EvilChuck

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Jan 25, 2021
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7
Considering I intend to change my FLA batteries (At least some of them) by LFP in the next few months (Let’s say 8x280Ah in a 12V system), what would be the effects/consequences of adding the same number of batteries (Same Ah), 1 year later to complete the migration to LFP. Obviously, doubling the capacity (Ah) of the 12V system is a gimme. My question is more in terms of; will batteries used for 1 year « match » the new ones? Or the discrepancies due to usage, specs (IR), batch differences would inder the whole system significantly? I´m assuming (Bad thing to do, I know...) I would find the same brand/model/manufacturer... which is less than certain as I noticed some very well known batteries becoming out of stock while new ones (capacity or otherwise) are appearing.

What are the risks of going for a 2-step project instead of a complete migration? Budget is key here, hence the timeline. The question is really pertaining to batteries as the rest of the infrastructure (BMS, Shunt, active balancer, etc...) would be planned to accomodate the entirety of the project in the first stage.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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Oct 29, 2019
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Rural NE Ontario Canada
Unlike FLA, LFP does not have similar degradation issues. You can certainly add another pack in Parallel a year later with little issue. If you build with 280AH cells then it is best to stick to the same size or very close so that the battery packs will be as close to each other as possible as that makes things simpler. If packs are too far apart in AH capacity, the BMS' on the smaller capacity packs will do Hi & Lo Volt cutoffs before the larger packs in the bank. This can be handled but must be planned for and worked around.

I moved from big Rolls Surette FLA to LFP which is now my Primary Power Bank while keeping the FLA as a secondary bank. These are SWITCHED so it's LFP or FLA.

For 12V you need 4 LFP cells. It is more prudent for fault tolerance & load sharing to make 2x 4S (12V) battery packs and set them in parallel.
 

EvilChuck

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Jan 25, 2021
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Thank you Steve_S.
As mentionned, I was looking into buying a single BMS for the entirety of the project (Daly SmartBMS 500A common port) in the first « step » (8 batteries 280Ah for a 12,8V system -> 4 groups of [2 * 3,2V cells paralleled] in series for 560Ah). THEN, a year later, simply add 8 new cells (After balancing, equalizing, etc...) thus making 4 groups of [4 * 3,2V cells paralleled] for a 12,8V @ 1120Ah.

As you indicated, that is not the way you’d go and I can see why your approach would be much simpler/secure at the time the second set of batteries would be added with its own BMS. My concerns (Or at least one of them) would the « load sharing » (I’m losely using the term as I’m a telecom guy) between the 2 packs created over time. I will upload some schematics to illustrate what I was planning. Nothing is written in stone yet...

The idea is to provide at least 500A @12V for those hungry loads (110VAC @ 11A for the submersible water pump, air conditioning during a couple of weeks during, LP dryer, washer, etc...). The rest (Present at 1st + future a year later) would be my reserve for those long winter days (Short in terms of sun presence but long in terms of moral...) and/or cloudy ones over the course of a year.

At the end of all this, I would love not to use my generator (e.g. no gasoline -> less pollution and expenses) to charge the system all year long or at least a very few times.
 
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HRTKD

Boondocker
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Apr 24, 2020
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Somewhere South of Denver
Just make two separate batteries. Parallel them at the common bus bar. No big deal. Yes, there will be some differences in resistance, but will it be enough to worry about? I would have no problem adding a third or fourth battery to my system a year after the original install.

I started out on the 2p4s route, but was convinced by many members on this forum to split that out to two separate batteries. In the end, I think that worked out much better than a single battery.
 

EvilChuck

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Jan 25, 2021
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I‘m leaning towards a 2-pack config now. Twice the number of BMS (Of smaller size/price) but 100% functionnality over time.

As mentionned, here are some amateur schematics of my project... Fourth year offgrid, time to go LFP!
 

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EvilChuck

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Jan 25, 2021
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I guess if I keep my wiring the same lenght for each packs (Among themselves and connecting to other devices -> chargers, PV controllers, etc...) it would minimize the effect of the pack being different ages/usage.

Thanks for the comment HRTKD.
 
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