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Victron ESS : Multi + MPPT = Battery Life?


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Good day  all,

noob looking for info (probably mostly reflecting my lacking understanding of electricity flow in a circuit – the water in a pipe analogy and google not withstanding).

Short version: in an ESS setup what happens to a battery @100% SOC while the MPPT is providing electricity on the DC side to power loads on the AC side?

Long version:

In a very basic ESS setup connected to grid

  1. intended mainly for backup (i.e. keep battery charged unless grid fails)
  2. but use PV to power loads when (2.1) battery SOC is 100% and  (2.2) PV is available/sufficient
  3. “dumb” LiFEPO4 battery (i.e.internal BMS protection for overcurrent etc. but no CANBUS comms etc.)
  4. no other smart battery disconnect contactor/relay (no BMV etc.)
  5. loads connected on multi AC out
  6. PV + MPPT on DC side of the Multi
  7. Multi + MPPT  connected & controlled by a Rasberry running Venus
  8. no feedback to grid

While the battery is at 100% SOC, what happens to the battery while the MPPT is providing voltage on the DC side of the multi to power loads?

Is the battery constantly “exposed to”/kept at whatever voltage is coming from the MPPT? From my understanding keeping batteries at  ”higher that float” voltage for a prolonged time will shorten the lifespan of the batteries.

Is there some explanation based on potential difference/resistance/magical electron unicorns that will make electricity not take the off-ramp towards the battery but only keep on going towards the multi?  Maybe the ESS limits the voltage coming from the MPPT to Float voltage, with subsequent increased current to maximum available to power loads and technically reduced power available compared to if no battery was used?

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13 minutes ago, introverter said:

Is the battery constantly “exposed to”/kept at whatever voltage is coming from the MPPT? From my understanding keeping batteries at  ”higher that float” voltage for a prolonged time will shorten the lifespan of the batteries.

This is precisely what it does. The water in a pipe (actually, water in a tank is more appropriate) analogy actually works quite well here. When set up in this mode (Keep Batteries Charged), it is like having a water tank with an overflow (the water level correspond with the voltage). The extra simply overflows.

ESS does this by applying a tiny overvoltage, 0.4V to be exact. The solar chargers attempt to push the batteries to 0.4V higher, while the Multi withdraws energy to get it down to the configured charge voltage.

If there is more PV than loads, then the battery voltage rises by 0.4V. This is really not a big issue, and will have just about no effect on the life of your LiFePO4 battery. Temperature variation will have a much larger influence than that tiny bit of voltage.

18 minutes ago, introverter said:

providing voltage

You provide current, not voltage. The MPPT will cut the current when it reaches the voltage it has been instructed to aim for, which is 0.4V higher than the configured charge voltage. In short, don't worry about it: The MPPT will still stop more or less in the same place as it normally does (when running in one of the other modes).

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@plonkster .. thank you. 

10 minutes ago, plonkster said:

ESS does this by applying a tiny overvoltage, 0.4V to be exact. The solar chargers attempt to push the batteries to 0.4V higher, while the Multi withdraws energy to get it down to the configured charge voltage.

I assume when battery SOC is 100% this will be whatever is set as "bulk voltage"   +0.4V.  e.g  bulk set @ 14.4V, the MPPT will try to push to 14.8V as opposed to "Float" (13.5V + 0.4V)?

12 minutes ago, plonkster said:

You provide current, not voltage. 

After barely passing high school science it is this kind of distinction that had me spend my time on the Humanities side of varsity...

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28 minutes ago, introverter said:

I assume when battery SOC is 100% this will be whatever is set as "bulk voltage"   +0.4V.  e.g  bulk set @ 14.4V, the MPPT will try to push to 14.8V as opposed to "Float" (13.5V + 0.4V)?

For accuracy sake, there is also no such thing as a bulk voltage, even though some inverter makers (*cough* voltronic *cough*) use the term. Bulk is the stage where the battery has not yet reached the absorption voltage and the charger is pushing the maximum current in order to get it there. Once it reaches this voltage, it goes into the next charging phase, which is called absorption. So this target voltage is more accurately called the absorption voltage.

Also, in the context of LiFePO4 batteries, there isn't really an absorption phase either in the same sense as with lead acids, so sometimes we speak simply of the charge voltage. Many batteries (eg BYD, Pylontech, etc) don't have two voltages (absorb and float) like a lead acid would, they have only the one.

The state of charge is not determined by the voltage, so the battery might well reach the target charge voltage while still not 100% full. For lead acid batteries this happens very soon. A lead acid battery might easily reach 14.5V at 85% SoC. The really nice thing about the Victron "overvoltage" method is that all the surplus PV can be used for loads (or fed into the grid) the moment the voltage reaches the target, and not only once you read 100% SoC.

For LiFePO4, the battery fills up really quickly once it reaches the target voltage, so generally it will be around 98% SoC by this point.

The 0.4V offset is for a 48V inverter. It uses 0.1V for each 12V increment. On a 12V system, the offset would be only 0.1V. The voltage is also added to whatever the voltage used for the current charge phase, so if you are in absorption phase (14.5V), it will aim for 14.6V, and if you are in float (13.5V), it will aim for 13.6V. As I said, really nothing to worry about.

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1 hour ago, plonkster said:

For accuracy sake, there is also no such thing as a bulk voltage, even though some inverter makers (*cough* voltronic *cough*) use the term.

..."bulk" is also proudly displayed on the inverter icon of my *cough* raspberry/Venus *cough* when the multi is chugging along to get the battery to absorption before the next loadshedding kicks in 😉 (after doing my homework on how to hopefully not turn a lithium battery into a small fission device through overcharging AND having to use VE Configure to set voltages etc, since the battery is less intelligent than others and not officially supported, it is difficult to get away from the terms)

 

59 minutes ago, plonkster said:

....As I said, really nothing to worry about.

Thank you, I accept that.

My pedantic voltage questions are because I prefer to try and understand/know how something works but it also leads into the next question which is around calculating potential realistic AC load that can be run when using a specific PV array size + MPPT spec.

Knowing that ESS is unlikely to turn my battery from a "10 year wonder" into a 6 month "has been", removes a potential con from the pro/con list. All of this should help me decide whether spending money on an MPPT and solar panel/s will add real value to what I have (despite the allure of ESS just seeming like an awesome setup)

To test my understanding of battery charging and the working of ESS: 

12v multi with max charge current set at 20A, absorption voltage set as 14.4V,  and Float as 13.5V. MPPT 100/30 .After an early evening loadshed the battery was happily charged from the grid during the night to 14.4V, followed by 1 hour absorption until the multi indicates "Float". The next day when the sun is sufficient, ESS will potentially have available (13.6V x "up to max amps MPPT+panel+conditions allow") to power a load on the AC side?  e.g. if there is a 200W load on AC side, ESS will provide roughly 17.3A @ 13.6V from the MPPT to power the AC load and because the battery is in any case "full" with a very low self discharge rate, the battery will not draw/"accept" the 17.3 A. As it is a 100/30 MPPT there is a theoretical "surplus" of 12.7A or the ability to power an additional 135W on the AC side... .... show your calculations: (200W/13.6V)/0.85 = 17.3A

If however there was another loadshed early morning, and the multi started charging the battery just as conditons were ideal for the PV side, then:

ESS will push (14.5V x 20A) from MPPT to charge the battery and at the same time if PV allows have 14.5V x 20A to power a load on the AC side?

 

 

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8 hours ago, introverter said:

bulk" is also proudly displayed on the inverter

That's the charge phase, but fair enough, we do have some lead acid terminology hard-wired into the thing 🙂 I do apologise for being a tad pedantic.

8 hours ago, introverter said:

since the battery is less intelligent than others and not officially supported

The Smart Lithiums are technically also "less intelligent". The VE.Bus BMS that is used with them is not nearly as "smart" as the batteries with a CAN bms. I say this so you know that this is not some weird edge-case that is never done. It is actually more common than you might think. The one thing these BMSes do have is dry contacts that can signal when you should stop charging, or stop discharging. You can wire those to the auxiliary contacts on the Multi, and then you can install the two-signal BMS assistant. This is just as safe as the "smart" options.

LiFePO4 is also unlikely to go up in flames. It's the safest of the lithium chemistries. Of course they do still destroy themselves, which you don't want, but the BMS should always be the last defense: There should be a big old contactor that opens up to protect the battery.

Regarding precise calculations of surplus power, that's something you can't really do, precisely because conditions differ so much. It is rare to see more than 90% of your PV modules' capacity, and the Multi is anything from 80% to 92% efficient (depending on power level), while the MPPT is also somewhere between 90% and 95%. So its largely academic anyway. Even the voltage isn't a given: It will hover between 13.5V and 13.6V (for example) depending on which component (solar charger or inverter) is presently winning the tug of war. Cause that's what they are doing. But the effect of 0.1V times a current value that is highly dependent on conditions, temperature, etc... you'll just make yourself crazy trying to calculate it 🙂

 

Edited by plonkster
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13 hours ago, plonkster said:

 I do apologise for being a tad pedantic.

some times I drive past a very small plattelandse primary school (total of two buildings, 3 cars in the staff parking area, the shade of the big blue gums being more attractive to the children than the multi-coloured klim-en-klouter play area..) on the entrance gate from the tar road a sign that says "kom in om te leer" ... think a similar slogan is apt for a forum like this and the knowledge that people like yourself freely share...👍

13 hours ago, plonkster said:

you'll just make yourself crazy trying to calculate it 🙂

 

..that boat has already left the harbour..

any case, as per original topic/question, answer is that ESS is unlikely to on any practical level reduce battery lifespan.

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