Change big Lead-battery in a Victron off-grid system to a big LiFePo battery, some basic questions

DIY
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Hi,

I am using my big lead battery system (24V, 1900Ah) in my off-grid home since nearly 20 years. I think, it is soon time to make the change to lithium. I would like to built a LiFePo battery 8S and 6 strings of 280Ah cells each. My system has a Quattro 8000VA and a Cerbo, 8 kWp solar at DC side (with 6 MPPT 100/50 regulators) and at AC side a fronius that adds another 4 kW solarpower. And there are 3 small wind gennys that add maximum 4 kW directly into the batteries. Surplus power is turned to heat with a AC-Thor. All that is working fine for many years, my experience with this and similar setups lasts for 35 years in between…

In my lead battery system I was used to drive the batteries in summer mostly near 100% (lowest values April to October not under 70%), in winter my offgrid system stressed the batteries minimum down to 50% - if that was (rarely!) the case, I loaded the batteries by a gasoline genny. When using LiFePos, I am planning to drive the batteries in summer between 60 and 80%, in winter more spreaded with a maximum about 90% charge and minimum down to around 30% - this numbers due to a maximum lifetime of the batteries.

Now my questions in preparation to the lithium-project:

  1. Will the charging routines of the MPPT and Quattro can be easily switched to those of such a LiFePo-set?

Especially: will it be easy to tune the planned end-of-charge-points via the cerbo? Or will I have to change the data each time in the Quattro configuration app? Or can I tune the data via the BMS? (at my lead battery system it was simple: I only had to set the “battery-full-voltages” once, as the optimum charge state is allways 100%)

  1. Is it complicated to connect a Daly bms (or more than one, as I will have more than one battery string) to the cerbo and let them work together to control the charging? Are they using the same “language”?

Do you have any further tips and hints what I have to think of? Can anybody with a similar setup will share his experiences?

Iam looking forward to your ideas and experiences!

Martin

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My Battery Setup and Management Approach

Battery communication can be handled through Victron, but after trying it, I found I didn’t like having the battery’s BMS control the entire system. Instead, I use Node-RED for automation and load-shedding control, which gives me more flexibility.

My current battery setup is 36 kWh, and I’m grid-connected with ESS, so any excess energy goes straight to the grid. My daily consumption is quite consistent, and here’s what my charge curve typically looks like:

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I discharge it down to 50%, then allow it to charge through the day until around 4 pm, when it reaches 100% long enough for a top balance. I’m not entirely sure if this routine is optimal for battery longevity, but it seems to work well.

If I wanted, I could have it fully charged by 10 am. The solar system is capable, but I prefer to export in the mornings until a certain state of charge, so it’s not sitting at 100% all day.

My electrician helped reconfigure my MPPT solar strings to make the system more adaptable. The battery can be charged from several combinations with a 450/100 MPPT:

7 Ă— 390 W Ă— 2 (dual string)

7 Ă— 390 W + 4 Ă— 390 W (dual string)

This gives me selectable charge capacities of around 8 kWh, 17 kWh, 25 kWh, and 34 kWh, depending on the configuration.

Rather than running the maximum array all year, I adjust it to match my seasonal energy use.

In winter, for example, we have plenty of sun but no air conditioners running, so I only use about 20% of the battery. In that case, I connect just the 4 × 390 W string to the MPPT. When the other strings aren’t used for charging, they’re routed through grid-tied PV inverters to export power.

If you’re planning to build your own 24 V battery, you can use modular battery boxes that support a 2P8S configuration (16 cells per tower). This setup is more efficient and requires less supporting hardware overall. Just remember, if your ambient temperature drops below the battery’s minimum charge temperature, you’ll need a heating solution.

My battery is now 2 years old with 278 cycles logged. The design life is rated for over 8000 cycles, but realistically, calendar aging will limit its lifespan long before the cycle count ever does.

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You should plan on a sustained 100% charge on your LFP batteries in order to achieve cell balance as Owen said he does in his system. A top balance doesn’t need to be done on every charge cycle but you want to do it fairly often. Generally, a 100% charge doesn’t harm LFP batteries so allowing this each charge cycle is easier to manage.

Charge parameters need to be set in every charging source. There is no central place unless you choose a BMS that controls the charge cycle AND if all devices are capable of external control.

1 Like
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Hello Martin,

The Cerbo does not have any automatic settings for how fully the batteries are charged. Whenever possible, it is always charged to 100%. This topic has been discussed countless times here. The reason for this is that cell balancing only takes place when the charge is very close to 100%. Lithium batteries can be discharged significantly lower than the 30% you are considering; 10% is a practical value. Do you want to stick with a 24V system or switch to 48V?

When networked, the Victron MPPT are controlled by the Cerbo, which receives its values from the BMS.

I operate an off-grid system for friends in Germany with currently 2x MPPT RS 48/200, Mp2 8k, 40KW Pylontech, Cerbo and a diesel generator.

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Hi Owen, Kevin and Dirk,

thanks to your detailed answers, this brought some new aspects…

I learned (no: I remembered…) that 100% at LiFePo is not a problem, instead: it is necessary like the equalization on lead acid batteries. I had been irrtated by the fact that for my e-car it is supposed to stay between 20 and 80%… But clearly: LiFePos need balancing. So it seems I have to control the charging maximums by hand, not via a percentage but via the voltage. And I have to look whether I should change the values on my Quattro (which is awkward) or better in the BMS (but I do not yet know how this works).
@Owen: I would be glad to have so many energy sources that I could sell it so often . My winter is the hard season…But it was my aim to stay on my off-grid-island…Unfortunately I did not get along with Node-Red to arrange my automation by myself. How did you configure your battery cells/strings (Voltage, Strings, cell-Ah)?
@Dirk: Good to hear that you think I can drive the batteries down to 10%. Though I will stay on the safe side and choose a nearly similar total capacity compared to my lead acids: about 40 kWh Li instead of 48 kWh Lead. I already expected the question why I have a 24V system at such big loads/currents. The answer has to do with the history of my system: I started in 1993 with 400Wp solar and 1 kWp wind with 8 pieces of 24V/105Ah “solar batteries”. Over time these units where replaced, first the bad batteries, later the solar modules that became cheaper, then a second wind genny, a bigger inverter (and so I landed at Victron…), and so on. In about 2010 I should have made a cut and upgrade to 48 V, but I missed the right moment, as now all my setup (at least Inverter, MPPTs and wind gennys) only work at 24V.
So I have to handle up to 300A current, using wires up to 105mm². When my Quattro dies, maybe that could be a moment of change, but the MPPTs and moreon: wind gennys are not easy to be replaced…Dirk, as I am living in Germany, too it would be nice if we could contact directly, is it possible?

Thanks again to you all,

Martin

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When I had the Victron system installed, I already had two grid-tied PV inverters, each 5 kW on separate phases, with 6.5 kW of solar per inverter.

The electrician reconfigured the system so that the east and west arrays were connected to the Victron 450/100 MPPT. He installed DC breakers to isolate each array, which allows me to easily switch solar between the MPPT and the grid-tied inverters.

If I want to increase or decrease the solar input to the battery, I can simply turn off the DC breaker, unplug the MC4 connectors, and either plug that string into the MPPT or reconnect it to the PV inverter.

He also added an extra 4 panels dedicated to covering our base house load when we’re not home. That way, the battery charges more gradually using a smaller solar input, instead of being hit with the full array output.

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Hi Martin,

Gerne können wir uns weiter zu dem Thema austauschen. Der Sinn des Forums ist aber auch, dass andere mitlesen , mitlernen und mitdiskutieren können, daher gibt es keine PN Funktion hier :wink: Bei mir klappt das am Besten in Deutsch. Entweder schwenken wir hier, oder du machst einen neues Thema auf….. Und hast du einen Plan der aktuellen Anlage den du hier posten magst?

GruĂź

Dirk

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I’d like to add a few thoughts here. First of all, why are you replacing the entire battery system? A friend of mine runs both in parallel and it works very, very well. The key is to set up the system so that the voltages run in parallel. Personally, I would leave the old lead system running as long as it’s still functional and slowly start building up a lithium system in parallel. The second point is the BMS-Cerbo communication. I myself run 7 Lifepo4 blocks with a capacity of around 100 kWh without any communication with the Cerbo. Victron can regulate the charging currents just as well. Third topic…cell balancing!!! In my experience, cells don’t drift very far apart at low charging rates. I’m talking charging rates of 0.1 to a maximum of 0.2 C. I also use a completely different balancing strategy with my system. I balance across the entire voltage range, but I have a slightly higher voltage difference. The result is that the mid-voltage range isn’t balanced anyway, but the cells are balanced in both the lower and upper voltage ranges. This isn’t a problem for a battery system that runs at low charging rates and isn’t run completely full and empty every day. The next effect is that, with 7 blocks, I don’t have serious consequences if one block is disconnected from the BMS. These are just a few thoughts on this thread.

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Hi Sarowe,

thanks also for your aspects on my plan! Using lead acid batteries and LiFePos really parrallel seem to me very very daring. Though you are right, it would be a pity to give a still running battery bank of perhaps 15 to 20 kWh to the scrap. But I don’t think that according to the different voltages at the ending of charge these two blocks would stay in harmony… I could use switches, but had to readjust the charging parameters each switching time.

The topic of the charging rates is surely also important: As my solar panels provide up to nearly 400A at 24V, the old lead acid batteries rise up to over 29 or more Volt even when they are rather empty. Then my AC-Thor jumps in and dissipates the power before the batteries are regulary full loaded. So I think I can exploit the solar power much better by using LiFePos that allow higher charging currents. But of course, the lower the C rate the gentlier the process. When we speak about the balancing I think there is much to learn for me, still… But I also think, some facts are generally not completely clear up to now, for example, in which charging state the balancing should be done with the best and gentliest success. So I’ll wait till I know which parameters can be set at the BMS and mess around with them…

@Dirk: dann ist es wohl am besten, ich lasse unsere direkte Kommunikation erst einmal in diesen Thread integriert. Ăśberhaupt: bin ich hier auf dem internationalen Forum, auf dem manche Threads dann einfach auch auf nicht-englisch laufen oder gibt es ein getrenntes .de-Forum?

Einen schlichten Übersichtsplan von meinem Setup habe ich nicht, nur reichlich detaillierte Verschaltungspläne, um da selbst den Überblick zu behalten. Aber ich werde mal was zeichnen und hier posten - ist ja vielleicht für alle ganz interessant. Gibt es eine einfache Software zur Gestaltung solcher Pläne?

Martin

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Hallo Martin,

es gibt ein deutschsprachiges Unterforum

Zeichne doch einfach mal einen Plan mit der Hand und fotografiere ihn zum hochladen ab. Es gibt KiCad, aber meistens bin ich zu faul dafür und zeichnen ebenfalls mit der Hand. Der Übersichtsplan wäre interessant um zu schauen was schon da ist und was fehlt. Mit ein wenig Glück ist der Umstieg relativ einfach

Was du an den LiFePo lieben wirst ist, dass sie sehr hohe Ladeströme abkönnen und diese auch bis fast bis 100% SOC aufnehmen können.

Hattest du schon mit einem Akkuhersteller geliebaugelt?

GruĂź

Dirk

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Dann können wir das auch in Deutsch machen. Also erstmal die Verwendung von lifepo4 und Bleiakkus halte ich nicht für gewagt, es gibt sogar Leute die die Behauptung aufstellen sie würden sich sogar von den Eigenschaften gut ergänzen aber das beruht weniger auf eigenen Erfahrungen als Berichten von Bekannten. Deshalb will ich es auch nicht weiter qualifizieren. Ich wollte es nur erwähnt haben.
Warum mich das Thema etwas triggert ist das ich mich zur Zeit auch etwas mit dem Thema unterschiedliche Zellchemien beschäftige. Dabei geht es weniger um Blei in Kombination mit Lifepo4 sondern Lifepo4 und Zellen auf Natrium Basis. Der wahrscheinlich nächsten Generation an Speichern. Wir sind ein kleiner Kreis an Heimspeicherbetreibern die eine Art Stammtisch betreiben und in letzter Zeit häufig das Thema diskutiert haben. Ein Ansatz beim Betreiben unterschiedlicher Zellchemien ist es die einzelnen “Chemien” einzeln nach Spannung vom Gesamtsystem zu trennen. Es gibt von Victron sogar ein Produkt dafür, frag mich jetzt spontan aber nicht nach den Namen. Der Grundgedanke dabei ist die unterschiedlichen Akkutypen in der Überschneidungen Spannungszone parallel zu betreiben, fällt die Spannungsgrenze unter das Niveau eines der Akkutypen trennt man es automatisch vom System und schaltet es automatisch wieder dazu wenn das System soweit gestiegen ist, das es wieder passt. Im Umkehrschluss für den anderen Teil des Systems wenn die Spannung zu sehr steigt. Die Ladeparameter der Lade/Entladekomponenten würde man über die gesamte Range laufen lassen. Für mich persönlich wäre das ein Ansatz um dein Bleissystem weiterhin zu nutzen bis es völlig am Ende ist. Mal sehen wann die ersten wirklich Marktreifen Natriumsystem erhältlich sind, ich denke ich werde es mal testen. Bitte nicht falsch verstehen, meine Beiträge versuchen immer Lösungsansätze zu finden und nicht den einzig gangbaren Weg zu benennen

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Hi Dirk & Sarowe,

I opened a new thread in the german community, where I also added a sceme of my setup:

, so we can discuss the themes in German (hope, others will follow in spite of this fact…)

Martin