geoff-silver avatar image
geoff-silver asked

Looking for Victron SmartSolar settings for LiFePO4 to limit SoC to 90%

tl;dr - I have a Victron SmartSolar MPPT 100/50, a DIY 280Ah 12V LiFePO4 battery, and 450W of solar panels on my camping trailer, which gets used sporadically. My battery cell manufacturer (CATL) recommends keeping the battery SoC between 10% and 90% in order to extend the battery life. I've been fiddling with the Victron settings, but right now I'm finding it difficult to keep the controller from (over)charging the battery. Ideally I want bulk to charge as fast as possible at ~14.4V until I hit ~90% (when the SCC should switch from constant current to constant voltage) and then kick right over to float, but it seems like my SCC wants to charge my battery to 100% (or a bit more) regardless of what settings I use. Looking for advice on how to configure this correctly... more details about my setup below:

My camping trailer gets used one or two weekends per month, and then typically once or twice per summer for 2-3 weeks straight. Between those times it sits unused, and the typical parasitic draw of the BMS, SCC, SOC meter (Simarine Pico), propane leak detector, and radio is <100mA, which results in 1-2A of usage, if that.

When I use the trailer, our typical consumption is between 20A and 50A per day, depending on the weather and location. Sometimes we get full sun the next day and can recharge, but other times we'll go 4-5 days before we get any significant sun. In the latter case I've drained the battery to ~65% SoC before, which means the next day I travel I need to add at least 70A back to get to 90%.

With my older Renogy Rover Elite SCC I found higher voltage settings significantly increase the charging current during bulk mode... to the point that if I set to 13.6V I might get a 1-2A bulk charge rate on a battery with an SoC of ~70% but at 14.4V the SCC was pushing 17A.

With the Victron I have NOT yet had a chance to drain my battery down to ~70% to see the result (planning to do this over Labor Day), but I have noticed that even when the BMS and SOC meter think the battery is 99%+ charged in the morning the Victron SCC will kick over into bulk mode and run that way for 6-8 hours before kicking into float. That makes no sense to me.

My BMS is set up for my CATL battery manufacturer recommendations:
low cut-off: 2.5V/cell (10.0V)
high cut-off: 3.65V/cell (13.6V)
balance: on
balance starts at: 3.4V/cell (13.6V)

Presently my Victron settings are:
absorption: 13.7V <- barely above cell balance voltage)
float: 13.2V <- intentionally below resting state of ~13.3V
equalization: disabled
re-bulk voltage offset: 0.10V
absorption duration: adaptive
max absorption time: 0 hours
tail current: N/A

Note that my LiFePO4 battery's resting state is about 13.3V. My goal with 13.2V float is that the battery does NOT get charged, but the SCC will power any parasitic devices. I think that's working ok.

Also note I've set absorption to 13.7V since that *seems* to affect the bulk->absorption algorithm and I'm trying to prevent the SCC from continually charging a full battery right now, but ideally I want bulk to charge as fast as possible at ~14.4V until I hit ~90% (when the SCC should switch from constant current to constant voltage) and then kick right over to float.

Right now my Victron SCC seems to run in bulk mode pushing 1-2A for 6-8 hours, then goes into float. That seems wrong since I'm starting the day at >99% SoC with 278Ah in a 280ah battery (and TBH I'm not really clear if there's any "damage" being done by trickling current to a full LFP battery). It also seems wrong because at 99% I should be well above the point where the SCC can apply constant current... so in the morning right now (when basically idle) I'd expect the SCC to go right into float mode (or rather to go into bulk for a few seconds, then recognize we're at or above the CC->CV point and switch to absorption for 0 seconds, then go to float).

So as I said in the first paragraph, ideally I'd like a way to bulk charge quickly if (and only if) SoC is below ~90%, then kick over to float when I hit ~90% SoC. Otherwise I just want to float at a low voltage. Any help from those who have done this, or who understand the bulk charging profile better and can explain why this doesn't happen for me (at least when the battery starts off full in the morning)


MPPT ControllersLithium BatterySOC
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k-k avatar image k-k commented ·

[..] high cut-off: 3.65V/cell (13.6V) [..]

seems to be a typo? (->14,6V)

while from 2021: any new results?

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4 Answers
siletzspey avatar image
siletzspey answered ·

I'm curious why the battery manufacturer recommends charging to only 90% SOC, and if the benefits (longivity?) are significant?

Much scientific literature exists on the merits of storing LiFePO4 batteries well below 100%, like 20-50% SOC, especially in hot environments. As such, I usually run my batteries down at the end of each trip.

It's probably challenging to strategically use a lesser top voltage so as to stall the SOC at 90%, and you would run the risk of the BMS not being able to balance the cells. Installing a BMV + Battery Protect to cut off at 90% is doable, but for lack of the BMV being able to see a tailing current, the BMV would loose track of what 100% looks like.

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geoff-silver avatar image geoff-silver commented ·

10-90% SoC is pretty standard for LiFePO4. Cells are rated at ~2500 cycles if you fully charge them, or between 3500 and 4500 cycles (or more) if you only use the middle 80% of the capacity. That feels like a significant increase in longevity to me, though I'm not sure how low C-rate shallow discharges affect that number. (Note the CATL 302Ah cells say "after 4500 cycles, 161Ah remaining pack capacity", so I'm not sure if after 1000 or 1500 cycles that the battery is only really 70 or 80% remaining capacity available).

I think the primary reason for the recommendation is there's very little voltage difference in the middle of the charging curve, but a tremendous change at either tail, so think of it as minimal "wear" in the center where voltage doesn't change much but significant "wear" at the edges.

This is the discharge curve from the 271Ah CATL cell, and a second chart which is viewed as the generic LiFePO4 SoC chart.



The big problem with using a lesser voltage is that the majority of the capacity available is within a few 0.1th's of a volt. Because of that it's very hard to stop at 90%... you either end up stopping at 50-60% or you hit 99% when you step up the next 0.1V. The only place you really end up playing in when trying to shut off based on voltage is above 99%

In theory the charging profile is dumping full current at (or below) whatever the absorption voltage is set to when in bulk mode (13.8V or less in my case). When the battery hits around 90% the charging profile should switch to constant voltage. At that point the number of amps drops as the voltage is held constant (13.8V in my above case). Then the adaptive absorption and float algorithms should kick in.

FWIW I have a 10A variable DC bench charger at home which does operate this way. I can set it to 9A and 14.4V and it will feed 9A continuously until the battery gets around the 90% mark, at which point the voltage creeps up to the 14.4V limit and the amperage drops. I don't know if the bulk charging algorithm that Victron uses is flawed, if my C rate is just too low for it to work as expected (17A/280A is barely C/20 and that is my peak... more often I'm generating just a few amps), or if there's some other configuration going onI don't quite understand.

Incidentally I have a SmartSense attached to my battery as my SCC is about 15' from it (via 8 AWG wires)

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klim8skeptic avatar image klim8skeptic ♦ geoff-silver commented ·

Cell voltages @ Soc are different when you compare a discharge plot vs. a charge plot.




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k-k avatar image k-k klim8skeptic ♦ commented ·

is CATL using yttrium in their cells?

AFAIK only Winston (the yellow ones) is doing that, so that curves won't fit to CATL

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Nev avatar image
Nev answered ·

We are fulltime traveller's here in NZ and for the first time in many years we have settled down for a winter break on mains power.

Our now 7 year old 4 cell 300Ah Sinopoly LiFePO4 has both started the Canter truck based motorhome and powered our house behind. Solar and alternator battery charging have been our only methods.

I have for the first time connected the Victron 30A mains charger and have had about 6 weeks to play and tweak charging levels.

Our numbers untouched for the last 3 weeks or so:

Victron 75/50 solar controller bulk and absorption - 13.25V.

Victron 30A battery charger in power supply mode - 13.25V

Multiple loads here all connected to the battery. TV, notebooks, tablets, 255l compressor fridge etc.

The Victron Smartshunt and Juntek battery monitors both match and maintain a constant 89% SOC. A weekly or so synchronisation is necessary to maintain any meter drift accuracy.

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geoff-silver avatar image geoff-silver commented ·

How much capacity do you use each day? How much solar power do you have? When your charger goes into bulk mode, is it really bulk charging (lot of Amps) or does it charge really slowly when absorption voltage is set to 13.25V?

If absorption voltage setting doesn't affect the number of amps being applied during bulk charge then I will have to try this

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Nev avatar image Nev commented ·

When travelling that is very hard to determine with solar/alternator delivering the energy most days. Overnight we have no charge sources and typically draw in Ah about 100.

24V Solar panels. 3x 270W paralleled.

Those numbers in my post are only while sitting here, not travelling and once the battery was initially charged to 100% SOC.

The battery is essentially in "float" mode and doing nothing. The mains charger is delivering our energy requirements. When we occasionally draw in excess of 30A for example making toast (c60A) the deficit is gradually restored by the mains charger maintaining 13.25V.

Our usual travel numbers are very different.

Victron MPPT controller.

Bulk 14.10V - Absorption 5 minutes (the battery is always @ 100% SOC) - "Float" 13.40V.

I still occasionally experiment by tweaking the numbers. Because I can.

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geoff-silver avatar image geoff-silver Nev commented ·
Ah ok thanks. At first I thought you'd solved this. But if the charger is floating at 13.25V and maintaining 90% I'm not sure that setting an MPPT controller to do the same will necessarily provide the same result. That said I will still try it when I have a chance.
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devodave avatar image
devodave answered ·

I use a SmartShunt to broadcast accurate battery voltage to my MPPTs. (I have 400A/h of LFP and four solar panels with a SmartSolar MPPT each to avoid shading issues.) Using a SmartShunt gives much more accurate voltage/current/SOC indication as the shunt is closely tied to the batteries and the voltage sense is a separate wire instead of using the wire carrying the charge voltage/current.

It has only bein in place for a few days but I see all four of the MPPT's go into float (13.5V) at the same instant when it happens.

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geoff-silver avatar image geoff-silver commented ·
I do have a Victron Smart Battery Sense attached at my battery, so the Victron Smart Solar MPPT *should* be getting the correct battery voltage regardless of wire length or gauge.
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sowilo avatar image
sowilo answered ·

I also want to extend the life of my investment. (24V Smart LiFePO4)

Is this correct: What can be meassured is Voltage and Current. SOC is just a roughly computed and not very accurate value.

So even if 90% SOC is a good limit - it cant be used for control purpose.

What can be used is the voltage that the smart batteries are meassuring very correct.
And that should be < 3.55 V per cell (= 14.2/28.4 in a balanced LiFePO4)

So my question is:
-- Is this reasoning correct?
-- Is cell voltage of < 3.55 V a good value for long life

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