What's your charge to discharge efficiency?
- Thread starter 1201
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sunshine_eggo
Victron's little biatch
1.0 - (258.3 - 252.7) / 252.7 = 97.8%
Makes sense since I'm operating 50-75% SoC with 23.3kWh 14S NMC & Batrium.
8/10/24 EDIT: DAGNABBIT!
There was this nagging "250-ish kWh, WTF?" in the back of my brain. It finally registered that I had reset my BMV back in June, so this is like maybe 6-7 weeks of use. I had the sense to make a note in VRM...
6/14/2024: BMV-702 history reset - about 5MWh delivered. Max voltage was about 64.4 (temp comp charging on old FLA).
Unfortunately, I didn't record more, and I must not have taken a snapshot. While it would have been blended numbers, it would be interesting to see how FLA influenced the mix.
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ricardocello
Watching and Learning
95.7%
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1201
Net zero
Nice. Thanks for posting. What inverter?
Very nice. Inverter is victron multiplus 5kva iirc?
ricardocello
Watching and Learning
Dual 5kva Quattros
ricardocello
Watching and Learning
This is measuring battery charge/discharge efficiency only, right?
Inverter efficiency (making AC from DC) should be much lower.
Or am I confused?
Inverter efficiency (making AC from DC) should be much lower.
Or am I confused?
1201
Net zero
Lol. I believe you're right.
sunshine_eggo
Victron's little biatch
same.
I'd hate to see that efficiency. My "background" AC power consumption is about 120W, roughly split between inverters. The one time I tried to measure efficiency, it was about 65% - based on measured battery current/voltage and measured AC current/voltage. Fortunately, that includes the "idle" consumption of the inverters.
wattmatters
Solar Wizard
Hybrid LiFePO₄ (15 kWh) + SLA (20 kWh) battery. SLA is kept as reserve capacity for outages.
EASun 8 kW AIO (equivalent to MPP Solar 8048 Max).
Measurement using Victron smart shunt.
LiFePO₄ battery increased from 10 kWh to 15 kWh in May 2023.
TOTAL CHARGE: ------------7,229.4 kWh
TOTAL DISCHARGE: ---------6,959.5 kWh
DISCHARGE TO CHARGE RATIO: --96.3 %
EASun 8 kW AIO (equivalent to MPP Solar 8048 Max).
Measurement using Victron smart shunt.
LiFePO₄ battery increased from 10 kWh to 15 kWh in May 2023.
TOTAL CHARGE: ------------7,229.4 kWh
TOTAL DISCHARGE: ---------6,959.5 kWh
DISCHARGE TO CHARGE RATIO: --96.3 %
timselectric
If I can do it, you can do it.
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95% diy batteries with JK BMS's
My thoughts exactly.
Maybe I'm having a blonde moment, but I don't even understand why this measurement is useful in real life? Maybe it can tell us if we need to have shorter (or fatter) battery cables?
Round-trip efficiency for when I need to charge from Grid at overnight cheap rate in Winter and power the house largely from batteries alone is useful for me to know (which is about 85% in my case, from AC->DC-AC).
wattmatters
Solar Wizard
Not everything we can measure matters. But sometimes it's just curiosity for its own sake.
timselectric
If I can do it, you can do it.
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It's just the efficiency of BMS, batteries/cells and connections.
If any of it is creating heat, that's going to affect the efficiency.
If any of it is creating heat, that's going to affect the efficiency.
ricardocello
Watching and Learning
Would AGM/FLA have a lower efficiency using this measurement?
D
Dont forget about voltaic and couloumbic efficiency of the cell itself.
1201
Net zero
ive read online that agm and fla have much lower efficiencies
timselectric
If I can do it, you can do it.
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timselectric
If I can do it, you can do it.
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I did include cells in the list.
TomC4306
Solar Obsessive
Q-Dog
¯\_(ツ)_/¯
I get 96.7%
wattmatters
Solar Wizard
Yes because they:
i. have a much higher self discharge rate than LiFePO₄,
ii. need to have a float charge applied, which, while a low current, still adds up as it needs many hours, and
iii. there's much greater voltage difference between charging and discharging
On that latter point note there is a difference between coulombic efficiency and energy efficiency.
Most of us are quoting energy in / energy out (which is correct for energy efficiency) but you can also measure coulombic efficiency, i.e. coulombs in / coulombs out. For lead acid coulombic efficiency is generally significantly higher than energy efficiency due to larger voltage difference between charge and discharge than occurs with LiFePO₄.
e.g. with lead acid you might have 85% coulombic efficiency and 70% energy efficiency, although actual numbers will vary depending on depth of discharge and how the batteries are treated. For LiFePO₄ both numbers are going to be in the 90+% range, often high 90s because of the modest cycle depth in many applications.
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