Total efficiency from charger through battery and inverter to mains

[SIdmouthsteve]

I have been looking at my overall efficiency for tariff shifting (charging in the early morning and powering the house from stored charge).

Comparing the power from the mains used to charge my Growatt LifePO4 batteries with the power provided to the house by the batteries shows that for every kWh of charging I get .75 kWh of power. The overall efficiency through the charger, battery and inverter is about 75%.

The 8.5p per kWh that enters my system is actually 11.3p by the time it is used. This is still a very good rate for electricity (Octopus standard daytime charge is 28.5p a kWh).

I am getting about 90% efficiency at each stage. The Growatt SPF5000 inverter is rated at 93% efficiency, the battery charger in the inverter is probably about 90% efficient (I am charging to 90% SOC - efficiency would be better at 80% SOC) and the 4 year old LifePO4 battery stack is probably 95% efficient. 90% sounds good but 0.9*0.9*0.9 is 73% .

Roll on summer! Free electricity from the solar panels again

Happy Xmas.

NOTE: The discussion below suggests we should expect 80-90% efficiency. My efficiency was lower than should have been expected and I will check the metering etc.

PS: The 60W idle power demand from the inverter doesn't help. My figures were based on metered power in against metered power out so the exact efficiency figures for each stage were educated guesswork. Anyone got exact data?

 

[SeaGal]

Sounds a bit low to me.... I measured about 85%. LiFePO4's may only hold 90% of their original charge, but that should affect the end-to-end power loss.

What current are you charging at? Do you have other losses with long cable runs?

 

[AntronX]

Growatt 5000es has bidirectional charger/inverter circuit and should have similar efficiency both ways. I have not tested it myself but 93% * 96.7% (LFP c/d eff.) * 93% = 83.6%

 

[SIdmouthsteve]

If the battery only holds 90% and the inverter has a 90% efficiency this would only give 81% overall efficiency into the house mains. Or is the battery efficiency usually inclusive of inverter losses?

 

[SeaGal]

If the battery only holds 90%

That 90% is capacity though, not efficiency.

e.g. If I put a rock in the fuel tank of my BMW so the tank only holds 90% of what it should, I still get the same MPG, I just can't go so far

 

[SIdmouthsteve]

Growatt 5000es has bidirectional charger/inverter circuit and should have similar efficiency both ways. I have not tested it myself but 93% * 96.7% (LFP c/d eff.) * 93% = 83.6%

I am charging to 90% which probably knocks a couple of % off the overall charging efficiency and I must have done a 1000 cycles on the batteries - another 5%.

 

[AntronX]

That charge efficiency chart looks like it's for lead-acid battery.

 

[SIdmouthsteve]

That 90% is capacity though, not efficiency.

e.g. If I put a rock in the fuel tank of my BMW so the tank only holds 90% of what it should, I still get the same MPG, I just can't go so far

So does the conversion efficiency (95-96%) stay the same throughout the battery's life? I tried a search for this but couldn't get a definitive answer.

 

[SeaGal]

So does the conversion efficiency (95-96%) stay the same throughout the battery's life?

efficiency of the inverter? yes.

 

[SIdmouthsteve]

That 90% is capacity though, not efficiency.

e.g. If I put a rock in the fuel tank of my BMW so the tank only holds 90% of what it should, I still get the same MPG, I just can't go so far

Here was Bing's ChatGPT reply: " it is possible that the conversion efficiency of LiFePO4 batteries may change with age. However, I could not find any specific information on this topic."

 

[SIdmouthsteve]

efficiency of the inverter? yes.

Conversion efficiency of the battery?

 

[SeaGal]

I suspect it is very very small. If you can measure the increase of internal resistance over time, I guess losses within the battery (i.e. creating heat) could be calculated, but my finger-in-the-air guess is that it is insignificant.

 

[RCinFLA]

Typical efficiency numbers in diagram below. Efficiency depends on several factors including total power load. For the typical HF SCC boost converter the efficiency is best when PV panel voltage is near maximum allowed voltage input where the boost converter does not have to do much work to boost up voltage to inverter's internal HV DC bus,

On HF inverters, all power flows through the HV DC bus, with exception of AC input to AC output pass-through.

When charging battery from PV you have the efficiency of SCC boost converter times the battery to HV DC converter efficiency. Battery to HV DC converter has the worse efficiency of all blocks and consumes most of the no-load idle current of the inverter.

For HF AIO inverters, there is many ways for marketeers to spin the numbers so they look better than actually achievable in typical usage.

Worst I have seen is spec sheet claiming 99% PV SCC efficiency, Yes, SCC module efficiency can be 99% when PV array Vmp is near maximum allowed input voltage and power level is about 30% of maximum SCC capability, but in order for user to yield anything from the PV SCC output it has to go through sinewave PWM H-bridge chopper to create AC output power or through battery to HV DC converter to charge battery, both of which have their own additional loss. It's a spec wording game.

 

[SIdmouthsteve]

Conversion efficiency of the battery?

I have just read through some papers on battery efficiency. The key variable is internal resistance. This converts to efficiency according to the load resistance. If the internal resistance is high then if the load resistance is low there will be high losses in the battery. High internal resistance will also affect charging efficiency.

Postscript: The Growatt batteries have an internal resistance of 0.1 ohm. 20 amps at 50V are supplied to 1kW load so the power lost to the battery is 40W - ie: 4%. Hence the 96% conversion efficiency that is widely quoted. LifePO4 batteries increase their internal resistance by approx 20% for every 1000 cycles so, as @SeaGal said above, the age of the battery is not highly significant, affecting capacity more than efficiency.

PPS: The power dissipated in the battery is amps squared times internal resistance so if 40 amps are drawn with a 2kW load then 160W are lost to the battery. This is 8% lost to the battery and 9.6% lost after 1000 cycles so the battery conversion efficiency will fall to 90.4% with a medium aged battery and a high load.

Thanks for making me think. My brain hurts now so I will sign off for the day

 

[SIdmouthsteve]

Typical efficiency numbers in diagram below. Efficiency depends on several factors including total power load. For the typical HF SCC boost converter the efficiency is best when PV panel voltage is near maximum allowed voltage input where the boost converter does not have to do much work to boost up voltage to inverter's internal HV DC bus,

On HF inverters, all power flows through the HV DC bus, with exception of AC input to AC output pass-through.

When charging battery from PV you have the efficiency of SCC boost converter times the battery to HV DC converter efficiency. Battery to HV DC converter has the worse efficiency of all blocks and consumes most of the no-load idle current of the inverter.

For HF AIO inverters, there is many ways for marketeers to spin the numbers so they look better than actually achievable in typical usage.

Nice analysis. So the efficiencies for high frequency inverter from utility through to AC output, taking average values:
Batt charge = 0.875
Batt to HV DC = 0.9
DC-AC PWM = 0.97

Multiplied together gives overall 76%. That seems worse than expected and the .96 conversion efficiency of the battery itself is not included. No doubt I have missed something.

The idling power on the inverter, 60-80W, will consume 1.5kWh a day which is 10% of an average 15kWh household consumption. It is beginning to look like an overall efficiency of 70-80% is to be expected for tariff shifting.

Does anyone else have actual measured data of power in versus power out for an overnight battery charged system?

 

[SIdmouthsteve]

I suspect it is very very small. If you can measure the increase of internal resistance over time, I guess losses within the battery (i.e. creating heat) could be calculated, but my finger-in-the-air guess is that it is insignificant.

I did the calculation above.

 

[SeaGal]

Does anyone else have actual measured data of power in versus power out for a overnight battery charged system?

I have measured 85%, as I mentioned This was using my Solis 3.6kW inverter with 30W quiescent draw.

 

[SIdmouthsteve]

I have measured 85%, as I mentioned This was using my Solis 3.6kW inverter with 30W quiescent draw.

I was just looking at the data sheet for the Solis 3.6kW. It looks like a good buy. More efficient than the Growatt.

 

[SeaGal]

Postscript: The Growatt batteries have an internal resistance of 0.1 ohm. 20 amps at 50V are supplied to 1kW load so the power lost to the battery is 40W - ie: 4%. Hence the 96% conversion efficiency that is widely quoted. LifePO4 batteries increase their internal resistance by approx 20% for every 1000 cycles so, as @SeaGal said above, the age of the battery is not highly significant, affecting capacity more than efficiency.

PPS: The power dissipated in the battery is amps squared times internal resistance so if 40 amps are drawn with a 2kW load then 160W are lost to the battery. This is 8% lost to the battery and 9.6% lost after 1000 cycles so the battery conversion efficiency will fall to 90.4% with a medium aged battery and a high load.

I do not believe the 0.1 ohm value is correct. How did you measure that?

My battery pack will drop only less than 1.2V with a 100A load, which works out about 12 mOhm. That includes the resistance of busbars and wires to the BMS. So, if all the connections had zero resistance, that is well under 0.1mOhm per cell.

If your battery had an internal resistance of 0.1 ohm it would drop 10V at 100A.

 

[RCinFLA]

Internal resistance of LFP battery is not its dominate loss. Overpotential voltage slump during discharge and voltage bump up during charging is the dominate loss at moderate cell current. Overpotential (also called polarization voltage) is the overhead power consumed to drive the migration of lithium ions within cell.

For example, a relatively new 280 AH EVE cell, internal ohmic loss is about 0.2 milliohms.

At 56 amps of discharge current there will be 0.63 watt loss due to 0.2 milliiohm ohmic resistance plus about 2.2 watts loss for overpotential slump, for a total cell loss of about 2.8 watts.

Aging and cooler temperatures increases this loss.

LFP is fairly equal in overpotential between discharging or charging current. At 56 amps of discharge and charging, round trip power efficiency is about 96%. (AH round trip efficiency is about 99% on a Columb counter. Difference between power and AH efficiencies is the terminal voltage slump under current.) This is overall cell capacity summation. If you stick to upper 50% of cell SoC the efficiency will be better due to slightly higher cell terminal voltage.

For 6' pair of 1/0 battery cables and cell bus bars at 56 amps, there is 4 watts of loss for cables and 8 watts loss for typical 15 total copper core, nickel plated bus bars and their terminal surface contact resistance (0.05 milliohm for each cell terminal to bus bar contact, 0.07 milliohm each bus bar for 0.17 milliohms total for each bus bar). This is for a 16-cell series stack 48v system.



Actual measured cell at 40 amp charge and discharge rates.