Is it okay to oversize the battery bank when using LiFePO4 batteries?

carolinabigfoot

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Sep 8, 2021
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Northwestern NC
I am in the prosses of replacing my dead fla battery bank with lifepo4. The old bank was 24V, 100Ah and powered
by 2 parallel 300W panels (Vmpp=32.5V, Impp=2x9.26A=18.52A). I was looking to replace it with 2 12V 206Ah SOK
batteries in series. (Thinking about adding another solar array later so I can make and store enough energy to run a DC
freezer.) I know that I couldn't do that using la batteries, but from what I read it shouldn't be a problem with lifepo4.
Is there something I missed, something I should be concerned about?
 

Short_Shot

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I am in the prosses of replacing my dead fla battery bank with lifepo4. The old bank was 24V, 100Ah and powered
by 2 parallel 300W panels (Vmpp=32.5V, Impp=2x9.26A=18.52A). I was looking to replace it with 2 12V 206Ah SOK
batteries in series. (Thinking about adding another solar array later so I can make and store enough energy to run a DC
freezer.) I know that I couldn't do that using la batteries, but from what I read it shouldn't be a problem with lifepo4.
Is there something I missed, something I should be concerned about?
No such thing as oversize. Just more run time.
 

cinergi

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Exactly right - because LFP doesn't need to be charged to 100% regularly - it's OK to let the SoC land wherever it lands.
 

Bob B

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I am in the prosses of replacing my dead fla battery bank with lifepo4. The old bank was 24V, 100Ah and powered
by 2 parallel 300W panels (Vmpp=32.5V, Impp=2x9.26A=18.52A). I was looking to replace it with 2 12V 206Ah SOK
batteries in series. (Thinking about adding another solar array later so I can make and store enough energy to run a DC
freezer.) I know that I couldn't do that using la batteries, but from what I read it shouldn't be a problem with lifepo4.
Is there something I missed, something I should be concerned about?
Sounds like you are addressing the only concern I would have .... do you have enough charge capacity .... but the bigger the battery the less you will be stressing it and the longer it should last.
There is no need to keep it fully charge lie FLA.
 

shadowsteve

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In the prior setup if the panels could replace the usage then they'll still replace the usage, assuming you don't add more load. You just won't draw the LFP down as far as the FLA before getting back to full.

I had 448Ah of AGM @ 12v and am in the process of converting to 560Ah of LFP @ 12v and will stay with the 640w of solar for now but I may add another 400w or so. In the AZ winter I would be back to float by noon on a sunny day
 

Bob B

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Sometimes I use more, most times probably less. But we get our share of dark days here in NC.
Exactly .... the larger battery will allow you to get thru more overcast days ... and the extra solar will allow you to recover from those overcast days when you get good sun.
 

HRTKD

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My preference is to create a LiFePO4 battery that is the actual voltage required. In other words, if you need 24v, built a 24v battery. Don't string two 12v batteries in series.

At 206 Ah, you're creating a battery bank that has four times the capacity of your prior battery bank. No problems there. If it doesn't get to 100% state of charge every day, no problem. Lot's of forum members shoot for a maximum state of charge of 90% anyhow.

Too much battery capacity and too much PV is a good problem to have.
 

740GLE

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The other thing to keep in mind is charging LiFePo is a lot more efficient than FLA.

So that same panel array puts more usable watts into the battery, so I’d say try the larger battery with existing array before committing to expanding the array.

That is also assuming your usage stays constant.
 

time2roll

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+1 for a single 24v SOK. Or two 24v in parallel if you need more power.
 

ghostwriter66

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From LA -work in South Texas/New Mexico region
Normally I would never say that anything could be tooo big BUT in this case here is what I would do ... I would rotate your batteries ... if your power-budget only requires 24 batteries and you have 48 ... then throw in some switches and take turns running 24 batteries at a time down to 8% or so and up to 92% or so while the other ones are basically off-line .... I am NOT a big believer at all of taking batteries and just doing a 40-80,40-80,40-80 type thing .. i rather not use some then barely use all .....

Probably NOT a bit of engineering or scientific things I can point you to in the above statement but gut feeling ...
 

Short_Shot

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Normally I would never say that anything could be tooo big BUT in this case here is what I would do ... I would rotate your batteries ... if your power-budget only requires 24 batteries and you have 48 ... then throw in some switches and take turns running 24 batteries at a time down to 8% or so and up to 92% or so while the other ones are basically off-line .... I am NOT a big believer at all of taking batteries and just doing a 40-80,40-80,40-80 type thing .. i rather not use some then barely use all .....

Probably NOT a bit of engineering or scientific things I can point you to in the above statement but gut feeling ...
If you use much less of the capacity on a daily basis they last a lot longer.

Simply running all of them at once and not drawing them down that far is much better.
 

MichaelK

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(Thinking about adding another solar array later so I can make and store enough energy to run a DC
freezer.) I know that I couldn't do that using la batteries, but from what I read it shouldn't be a problem with lifepo4.
Why do you think that? I've powered my standard AC refrigerator 24/7 12 months of the year. Things must be scaled properly to work properly. When they are, it's seamless.
 

carolinabigfoot

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Northwestern NC
I have a small AC fridge as well and it also ran without a problem for almost all the time, but a freezer is a different thing. Here in the mountains of NC we get sometimes longer stretches of cloudy, rainy or snowy days. To unplug the fridge when push comes to shove is not a problem, esp. in the winter when I can throw the food in a cooler and just leave it outside. For a freezer I like to have little bit more wiggle room. I have seen a freezer and its contents go bad before (luckily not mine) and it is not pretty. Before the freezer there will be a generator backup on permanent standby. I hope that I don't have to use this last resort often, hopefully not at all. But better save than sorry.
 
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Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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Rural NE Ontario Canada
I am in the prosses of replacing my dead fla battery bank with lifepo4. The old bank was 24V, 100Ah and powered
by 2 parallel 300W panels (Vmpp=32.5V, Impp=2x9.26A=18.52A). I was looking to replace it with 2 12V 206Ah SOK
batteries in series. (Thinking about adding another solar array later so I can make and store enough energy to run a DC
freezer.) I know that I couldn't do that using la batteries, but from what I read it shouldn't be a problem with lifepo4.
Is there something I missed, something I should be concerned about?
I started with a Lead Acid (8x Rolls Surette S-550's for 24V/428AH gross) based 24V System and migrated to LFP, now with 3X-24V/280 & 2X-175AH for a total 1190AH/30.464kWh.
During summer my system is in float typically by noon.
Winter - especially December is not so good and after 6 days if not charged I have to fire up the Genset.
Charging from the L5:30 (120V/30A) the Samlex Inverter/Charger will pull 120V/22A to charge the batteries at 28V/80A, 1 Hour of Charge Runtime = 10% Pack Capacity (on average due to AC Passthrough affecting it slightly).

NOTE: LFP is 99% Efficient whereas FLA/AGM are only 84-86% efficient (when healthy).
- It is Strongly Suggested to use 24V Battery Packs and AVOID setting 12V Packs in Series ! BMS' do not like that and can fail.
- Pack in parallel divide Charge & Discharge fairly evenly. Each Pack can continue to operate if others cut off.*
* If properly configured to handle Max Load/Charge Capacity of the system, the fault tolerance of having packs capable of operating as "Last Man Standing" increases dependability / reliability. This ALSO reduces Cycles & Stresses on the packs themselves (A very good thing).

SOK 24V/100AH
 
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