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Over-sized lithium bank with small array

JRG

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I'm currently designing a LiFePO4 / Solar set-up for my camper van. With relatively inexpensive DIY Lithium batteries, it seems easy to have a system with a large battery bank compared to the amount of solar that'll fit on a small van roof. Is there any disadvantage to this? For example, I'm leaning towards a 12V 540ah bank with 380W of solar. I realize that the panels will take a long time to charge the bank, but I'd also have a DC-DC charger charging the batteries while driving.

My max load requirements are about 1700Whr (140Ahr) per day, so I really don't need the 540ah bank, but again, DIY cells are fairly cheap...so why not? Am I missing anything?
 
There is supposedly a type of memory effect with LFP, where the voltage rise usually associated with reaching full capacity starts to happen earlier and earlier, if the cells are not fully charged. Whilst the cells have not permanently lost capacity, this effect will fool chargers into thinking that the cells are full because they will reach their high-voltage cutoff point early. Adjusting the charger settings should reverse the effect.

So this would be one possible consequence of having a very large bank in relation to the charging sources. In your case it might depend on how much driving you are going to do.

I suppose you could keep one set of cells offline most of the time and just use them as a backup, but you wouldn't want them to sit fully charged of course, so you'd have to plan ahead and charge them up the day before...
 
Sounds fine to me.
6kWh of battery. about 1700Wh/day PV, 1700 Wh of load. Normally you'd want surplus PV, but you expect to also charge while driving.

I'd say find a way to keep battery bank most of the time at a state of charge (50%?) that maximizes shelf life.
Charge more fully if overcast weather is coming.
Periodically charge more fully so balancing occurs.
 
There is supposedly a type of memory effect with LFP, where the voltage rise usually associated with reaching full capacity starts to happen earlier and earlier, if the cells are not fully charged. Whilst the cells have not permanently lost capacity, this effect will fool chargers into thinking that the cells are full because they will reach their high-voltage cutoff point early. Adjusting the charger settings should reverse the effect.

So this would be one possible consequence of having a very large bank in relation to the charging sources. In your case it might depend on how much driving you are going to do.

Wasn’t aware of memory with these cells. That’s kind of a surprise. Thanks

Sounds fine to me.
6kWh of battery. about 1700Wh/day PV, 1700 Wh of load. Normally you'd want surplus PV, but you expect to also charge while driving.

I'd say find a way to keep battery bank most of the time at a state of charge (50%?) that maximizes shelf life.
Charge more fully if overcast weather is coming.
Periodically charge more fully so balancing occurs.

Yes, my thought was that by not keeping them at a high state of charge that lifespan could be increased.

How periodic would a full charge need to be done?
 
I'm currently designing a LiFePO4 / Solar set-up for my camper van. With relatively inexpensive DIY Lithium batteries, it seems easy to have a system with a large battery bank compared to the amount of solar that'll fit on a small van roof. Is there any disadvantage to this? For example, I'm leaning towards a 12V 540ah bank with 380W of solar. I realize that the panels will take a long time to charge the bank, but I'd also have a DC-DC charger charging the batteries while driving.

My max load requirements are about 1700Whr (140Ahr) per day, so I really don't need the 540ah bank, but again, DIY cells are fairly cheap...so why not? Am I missing anything?
There should be some amount of proportionality between (1) Your consumption (2) your storage capacity (3) your ability to replinish / recharge.

With LFP there is no harm in too low a charge rate, the important consideration is whether you can sustainably replenish what you generally use between periods of charging, and whether you can make up for longer periods of low or no charging.

While ideally you would be able to recharge from empty to full in a day, its not absolutely necessary especially if you have on demand sources (like a generator or an alternator to supplement solar), or if your usage pattern includes reliable grid power every few days or so (long weekend or week long trips for instance).
 
How periodic would a full charge need to be done?
I thought about that after I wrote "periodic".
Just when the BMS shows excessive imbalance. Except, if voltage curve is so flat, how will it know?
I think you'll learn how fast they diverge by bringing to full charge occasionally.

Perhaps the curve has enough slope that you can estimate 50%. If so, charge controller could keep it there.
Then bump setting up to something representing 90% or whatever is good, observe imbalance and operate there long enough to rebalance.
Then return voltage setting to 50%.

Just reasoning on it, haven't used them myself.
 
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There should be some amount of proportionality between (1) Your consumption (2) your storage capacity (3) your ability to replinish / recharge.

With LFP there is no harm in too low a charge rate, the important consideration is whether you can sustainably replenish what you generally use between periods of charging, and whether you can make up for longer periods of low or no charging.

While ideally you would be able to recharge from empty to full in a day, its not absolutely necessary especially if you have on demand sources (like a generator or an alternator to supplement solar), or if your usage pattern includes reliable grid power every few days or so (long weekend or week long trips for instance).

Thanks!

One potential downside is that this set up may put extra wear and tear on the alternator, and reduced MPGs from the load, since the alternator will be trying to fully charge it (unless Hedges idea of partially charging would work).
 
An alternator wouldn't know how to manage a lithium battery. It also can't handle the high current lithium batteries accept when alternator tries to put out 14+ volts.

You did say "I'd also have a DC-DC charger charging the batteries while driving."
So long as you select a DC-DC charger, or charge controller with both PV and battery (alternator) inputs, it will limit current to something your alternator can handle. You just want adjustable voltage on Lithium side, if that is how you can switch between 50% charge vs. 90% or so.

Without occasional balancing, running at 50% would mean the cells would eventually diverge until one hit < 10% and BMS disconnected, or one hit > 90% and BMS started to balance. Until that happened, degradation could be happening faster for some cells than others. Because balancing is gradual, if you let them diverge far then you couldn't immediately make use of full battery capacity. You'd have to bump into/dwell on the limits where imbalance was detected and rebalancing occurred.
 
Thanks!

One potential downside is that this set up may put extra wear and tear on the alternator, and reduced MPGs from the load, since the alternator will be trying to fully charge it (unless Hedges idea of partially charging would work).
I've never seen the effect on MPG quantified or even ballparked (this is something I would like to learn more about). I have seen Dacian (the maker of the electrodacus SBMS) say that Alternator charging is at a minimum $1 per kWh, I'm not sure how he came up with that figure.

In either case, because alternator charging is not renewable, not sustainable, not free, and not the cleanest option, I would not use it as the primary charge source unless I had to. A switch could be added to make it 'on demand'.

Another note, DC-DC chargers limit current. Usually to a somewhat low value (Victron's has models from 10 to 30 amps I think, other brands go up to 40 or 60). So at the most, your alternator could replenish 10% capacity or so per hour of driving, and likely closer to 5%

There may be combination solar charge controller / B2B chargers that let you define priority (i.e. preference solar, fall back to (or supplement with) alternator charging. I'm not up to speed on that. There are only a few of these combo devices available and the most popular one (Renogy DCC50S) only works with '12V' solar panels. CTEK and/or Redarc and/or Kisae are a few brands to look into if you want to find a combo device that may have that functionality.
 
Thanks to both of you. This gives me some things to think about!
 
What is keeping you from putting more solar up on the roof?

I know little about vans in particular. I had a Chevy for a while. Did the back up in Avocado Green shag carpet, ripped out the 8 track and installed an awesome new cassette deck. It's been a while.

From what I remember of mine, I am sure I could stuff 900-1200 watts on the roof.

Something to consider as far as charging from your alternator. Automotive alternators are not made for continuous use at full output. They REALLY hate high, continuous loads at low RPM. I saw somebody let the Magic Smoke out out of a 10si that way. I had a spare can (MS) on the shelf but as hard as we tried we couldn't refill it.... :) :)

I would not charge LFP's from my vehicles charging system unless I put in place some sort of current limiting like the Victron mentioned earlier.
 
There is supposedly a type of memory effect with LFP, where the voltage rise usually associated with reaching full capacity starts to happen earlier and earlier, if the cells are not fully charged. Whilst the cells have not permanently lost capacity, this effect will fool chargers into thinking that the cells are full because they will reach their high-voltage cutoff point early. Adjusting the charger settings should reverse the effect.

So this would be one possible consequence of having a very large bank in relation to the charging sources. In your case it might depend on how much driving you are going to do.

I suppose you could keep one set of cells offline most of the time and just use them as a backup, but you wouldn't want them to sit fully charged of course, so you'd have to plan ahead and charge them up the day before...

This is in conflict with both claims of manufacturers and actual PRACTICE of EV manufacturers where they are NEVER truly charged to full.

Please cite your source.
 
What is keeping you from putting more solar up on the roof?

I know little about vans in particular. I had a Chevy for a while. Did the back up in Avocado Green shag carpet, ripped out the 8 track and installed an awesome new cassette deck. It's been a while.

From what I remember of mine, I am sure I could stuff 900-1200 watts on the roof.

Something to consider as far as charging from your alternator. Automotive alternators are not made for continuous use at full output. They REALLY hate high, continuous loads at low RPM. I saw somebody let the Magic Smoke out out of a 10si that way. I had a spare can (MS) on the shelf but as hard as we tried we couldn't refill it.... :) :)

I would not charge LFP's from my vehicles charging system unless I put in place some sort of current limiting like the Victron mentioned earlier.

I’ve got an available roof area of about 64” x 62” for panels. Because of this I seem to be limited to 2 panels. The ones that I’m looking at are two of the 190W Grape Solar panels. I’m open to others though if it’ll get me more wattage.
 
I’ve got an available roof area of about 64” x 62” for panels. Because of this I seem to be limited to 2 panels. The ones that I’m looking at are two of the 190W Grape Solar panels. I’m open to others though if it’ll get me more wattage.

I am guessing that you may already have "stuff" on the roof that you have to work around. The panels I am installing today are 72"x39" and are 300 watts. I was envisioning those on a blank roof thinking that you could fit 3-4 on the roof of a van.
 
JRG, in my van remodel I changed from having 210AH of 12v AGM to a planned 560AH of 12v lifepo4. I wasn't able to get my battery heater built to install my batteries outside quickly enough ,so I took my first trip with just one 280 installed inside and left my 210AGMs outside, and installed switches to turn both banks off or run them in parallel. I have 300w of solar and a Renogy 50A DC-DC converter. In my first three weeks on the road with it, in mostly cloudy PNW weather the lifepo bank never got below 80% and that was three days without any driving. I only switched on the AGMs when I ran the espresso machine and the water heater in the morning. I was concerned about having too little solar as well, but not so much any more. The combo of solar and alternator keeps it charged pretty well.
 
JRG, in my van remodel I changed from having 210AH of 12v AGM to a planned 560AH of 12v lifepo4. I wasn't able to get my battery heater built to install my batteries outside quickly enough ,so I took my first trip with just one 280 installed inside and left my 210AGMs outside, and installed switches to turn both banks off or run them in parallel. I have 300w of solar and a Renogy 50A DC-DC converter. In my first three weeks on the road with it, in mostly cloudy PNW weather the lifepo bank never got below 80% and that was three days without any driving. I only switched on the AGMs when I ran the espresso machine and the water heater in the morning. I was concerned about having too little solar as well, but not so much any more. The combo of solar and alternator keeps it charged pretty well.

Are you still planning to put in the full 560Ah of lithium?
 
I'm currently designing a LiFePO4 / Solar set-up for my camper van. With relatively inexpensive DIY Lithium batteries, it seems easy to have a system with a large battery bank compared to the amount of solar that'll fit on a small van roof. Is there any disadvantage to this? For example, I'm leaning towards a 12V 540ah bank with 380W of solar. I realize that the panels will take a long time to charge the bank, but I'd also have a DC-DC charger charging the batteries while driving.

My max load requirements are about 1700Whr (140Ahr) per day, so I really don't need the 540ah bank, but again, DIY cells are fairly cheap...so why not? Am I missing anything?

I would say go for it!

If you ever pay for a campsite with power you could run a charger for 24 hours and really get your money’s worth ?
 
Are you still planning to put in the full 560Ah of lithium?
I'm on the fence here. For the first three years in the van I had 105AH of useable capacity. Living 3-4 nights per week in it through the winter I rarely needed more. But...there is something to be said for reserves! I also didn't have a water heater or espresso machine initially. Funny how those two little things made me spend a bunch more on electrics. If I can convince myself that I can keep the lifepos functioning well and heated down in my outside battery box at 0F I'm sure I'll add the second one.
 
This is in conflict with both claims of manufacturers and actual PRACTICE of EV manufacturers where they are NEVER truly charged to full.

Please cite your source.


"Memory effects in LiFePO4 cells were discovered and studied by Sasaki et al. [6] and the results published in Nature Materials in 2013. The authors illustrated that, under specific circumstances, the prior cycling history of a cell alters the voltage curve during charging by causing the voltage to increase faster and earlier than expected."
 

"Memory effects in LiFePO4 cells were discovered and studied by Sasaki et al. [6] and the results published in Nature Materials in 2013. The authors illustrated that, under specific circumstances, the prior cycling history of a cell alters the voltage curve during charging by causing the voltage to increase faster and earlier than expected."

That's not a source. That's yet another citation.

Here's the source:


The testing in which those cells were evaluated included charges to 4.4V, which brings any conclusions into question.

Claiming a "memory effect" is misleading based on the general public's understanding. The "memory" effect as re-defined by that paper is minimal and has negligible implication to this thread or any other application related to the focus of this forum. It doesn't affect capacity. Nor does it affect SoC computations in any meaningful way.

Here's a more accurate presentation of the data:

"Short cycled" (not fully charging between partial or full discharges) LFP cells will experience slight alterations in voltage response which can slightly change prior SoC to voltage correlations. Test data indicate this phenomenon is localized in the charge/discharge curves and occurs almost immediately. There is no evidence this condition accelerates or changes over time. "New" cell voltage response can be obtained by a single charge to full. Once a cell has been short cycled and a SoC to voltage response range is established, there is no data to suggest that this behavior will change over time unless the cells are charged to full thus resetting the original relationship.

Note that I didn't use "memory effect" anywhere in there because there is no memory effect in these cells based on any definition outside of that research paper.

Please do not continue to spread misinformation.
 
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