question

colj avatar image
colj asked

ESS system overall efficiency AC->Battery->AC round trip

ESS system (Quattro 48/10000 8kW, Cerbo GX, 20kWh batteries (LiFe), 3kw Solar PV).

Hi All,

In testing the system (using an overnight scheduled charge window and then running from battery/solar), I've noticed an increase in my daily average grid usage (from the same month last year). So downloaded the kWh data from the VRM and started looking at the figures between two equivalent states (end of absorption after a full charge). I've been trying to understand the efficiencies at the various stages in my system to explain a large discrepancy in the grid input kWhs from the actually consumed kWhs. Note: The VRM total 'from Grid' in kWh is in line my utility company's actuals (VRM is 5% higher).


These are the numbers between two fully charged battery states over 8 days:


Grid to battery: 88.97

Grid to consumer: 26.97

PV to battery: 17.35

PV to consumer : 32.14

PV to grid: <1

Battery to consumer: 72.8

Battery to grid : 1.46


My understanding is that this is saying (since the battery is in the same state at the start and end of this period) that 88.97+17.35 kWh put into the battery only resulted in 72.8+1.46 kWh coming out. I was expecting some losses but the magnitude of this difference surprised me.


So, I tried to estimate the efficiencies of the individual conversion stages:


Grid AC -> DC : c93% (measured by setting my charge limit to 3kw and noting the DC kW being passed to the battery per the dashboard)

DC -> battery: c90% (I estimate that I'm feeding about 10% more into the battery than I think it needed for top up during the recharge phase)

DC from battery ->AC: less than 85% ( using SoC and min/max cell voltage drops to try to compare what I would it looks like has been drawn from the battery versus what was recorded as being fed to the consumer)


Using these estimated efficiencies, the kWh numbers into the battery and out of the battery would then be quite a good match to what's been measured.


The net is that I appear to 'lose' about 30% of every kW of stored in the battery on a round trip. I would have hoped for 20% or less?


Am I missing something? Have I misinterpreted the numbers or are these within expectations for such a system please?

Colin


battery chargingESS
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6 Answers
Jason - UK avatar image
Jason - UK answered ·

@colj I had a similar situation with high losses and I initially thought it was just the inefficiency of the inverter / charger. Then I replaced the 35mm2 cables with 70mm2 cables. I went from a a 25% overall loss to a 15% overall loss based on a AC grid / PV input to DC battery, then from DC battery to AC consumption. At the same time of replacing the cables I replaced the main fuse holder for a branded one and added another 2 batteries. The reason for the extra 2 batteries was to increase my storage to 21kWh but also to improve the potential discharge rate of my battery system.

On the older set up, my battery connections used to sit at 36*C during discharge but with the new set up, its never event above 26*C so I can only assume my inefficiencies where also to do with the performance of the DC distribution as well as losses from the inverter / charger.

In short, its worth looking at your DC arrangement. The only part of my distribution system that still gets warm is the positive connection from the batteries onto the main fuse when the Multiplus is inverting.

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colj avatar image colj commented ·
Hi Jason - thanks for reviewing the data. Just looked at my system and I have 25mm from each of the four batteries to the busbar then 95mm from there into the Quattro. Are you suggesting upgrade the 25mm cables please? I have the system in an insulated outdoor cabinet attached to the north wall of the house - at this time of year the temperature inside has ranged from 17C to 27C (the high during charging in the middle of the night). I have yet to install a temperature triggered fan to cool the summer highs and warm the winter lows. From the BMS, battery temperatures have ranged from 16C to 25C - i haven't monitored the cable temperatures though.


Colin

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Jason - UK avatar image Jason - UK colj commented ·

@colj I should have mentioned, I still have 25mm between the batteries but 70mm going from the common terminal to the Multiplus. You'll probably find the 25mm is ok but you will also need to look at he maximum and continuous discharge rates of the batteries and then compare that to the minimum discharge rates required by Victron for the Multiplus. PylonTech for example have relatively low discharge rates per battery module because of the connection types. The cable lengths are important. I have 3mtr between the common terminal and the Mulitplus. The greater the cable length on the DC side, the greater the volt drop, the less efficient the system. All of my batteries have 8mm bolts clamping the 70mm cable to each battery terminal so the losses on the battery terminals are minimal. I even have copper components where ever I can and brass bolts due to the improved electrical conductivity. My batteries cells also sit around the same temperatures thought the year as well, 14*C to 26*C.

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colj avatar image colj Jason - UK commented ·

Thanks again Jason. My 95mm cables are certainly the short side of 3M so similar length to yours. Batteries are Leoch - 48-100TCAN and nothing jumps out as being a mismatch. Can't say how the installer connected the cables but I would be surprised if there's any problem there. I will ask though.

Just seen a comment in a question regarding Leoch batteries that

"It is stated quite clearly 100A for 1 but in Parellel it is 20A max. "

If this is the case could that be a problem since I could well be trying to draw more than 20A?

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paulcupine avatar image
paulcupine answered ·

25mm2 is quite small... unless your 20kWh batteries are 10 x 1 kWh units

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nickdb avatar image nickdb ♦♦ commented ·
For each of four 5kWh batteries to the busbar? Doesn't sound unreasonable for 10kVA.
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johnone avatar image
johnone answered ·

I'm seeing 26% losses. System is HQ2226 MP-II 48/5000/70-50 in garage with temp av of 7C, 4x 3.5kWh pylontech in the house (other side of wall from MP-II, thus <2m cable) with battery temp range 23-28C, 70mm busbar to MP cabling, thermal camera shows battery to busbar 25mm cables at 35C (when charging from grid). Numbers from VRM over last 3 months:

Grid to battery: 677
PV to battery: 102
Battery to consumer: 573
Thus 573/779 = 0.7355

So that's 206kWh gone 'missing', around 2.2kWh per day - definitely not all being lost from heat in the cables/connectors, but some is. With the front cover off, a thermal camera has shown the big MP-II transformer at 50C when doing overnight battery charging. Maybe it's all lost in heat produced in various parts of the system? Wonder how this compares with the transformerless inverters?

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Al avatar image
Al answered ·

Round trip efficiency will vary throughout the year, depending on how the batteries are being charged and discharged, I'm not sure how ESS may make a difference though.


I started a thread which has a few people sharing theirs, mine was around 85% then, but is currently 78% as expected for older lead acid in winter.


https://community.victronenergy.com/questions/112182/whats-your-battery-round-trip-efficiency.html


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johnone avatar image
johnone answered ·

Hi @Al, are there two different things at play here? I'm interested in system efficiency which might relate more to Multiplus-II. Whereas, battery efficiency is another aspect of overall efficiency and would typically include a distinction between lithium and lead acid batteries? And for most people installing domestic ESS systems, lead acid isn't a factor - i.e. it's about newer lithium technologies?

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osullivan avatar image
osullivan answered ·


MultiPlus-II 48/3000/35-32 GX+ Pylontech 5000 + Pylontech 2000 here.


My first month of ESS usage is over and i can report a Round-Trip efficiency of 74%

My setup is placed in my garage at relatively low temperatures right now (10-18°C)

Let´s see how this performs over the next year. I have it in my monitoring and can report more in-depth data soon.


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sharpener avatar image sharpener commented ·

You are doing well then. Over a full year I have only managed 69.5%, see this thread.

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Al avatar image Al commented ·

We're off grid with a diesel generator backup, so not ESS, but since I posted in the thread above the efficiencies of our flooded Lead (Pb) batteries this time last year, and we now have 26kWh of LFP, here are our updated efficiencies of both the batteries themselves and the system efficiencies:

Last 90 days on LFP:

Solar = 657kWh

Generator = 302kWh

Consumption = 758kWh

758kWh / 959kWh = 79% System Round Trip Efficiency

With Pb over the same 90 day period last year:

579kWh / 891kWh = 65% System Round Trip Efficiency


LFP:

SmartShunt 100% to 100% SOC - Discharged: 687kWh Charged: 717kWh = 96% Battery Efficiency

Pb: (Old Bank)

SmartShunt - Discharged: 4,453kWh - Charged: 5,684kWh = 78% Battery Efficiency


So a 14% System efficiency gain from Pb to LFP, or 18% battery only efficiency gain of LFP from Pb. 14% of last years 90 day 891kWh generation = 125kWh lost due to Pb inefficiency, or 14% less diesel needed with LFP!

Obviously there are many variables and we don't have an ESS setup, but it's still an interesting comparison :)

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