Any suggestions on setting battery draw down?

[darkskies]

Hi!

I have a LG RESU16 (16kWh) battery on my SolarEdge PV system. Currently, at the end of the day, the battery provides power for my home, until it discharges 20% (80% capacity left). I want to tune the percentage of discharge for the battery, and was wondering if there are rules of thumb, or some guidelines as to how low it should drain, before recharging. Any advice in that area or references to where I can seek some info on how to tune that?

Obviously, when there is inclement weather, I'll want it to be charged fully, and the system does have a "WeatherGuard" capability, which I guess will ensure the battery stays charged prior to storms, but I haven had an opportunity to try that out. Besides, my battery is not integrated yet with the generator (a long standing issue that I'm been trying to get resolved), so when the power goes out, the system immediately switches to generator.

As a result, the battery right now, is acting like an extension of power generation, getting charged in the morning and discharging (a tiny bit) in the evening.

Thoughts appreciated!

[SolTex]

Purely a matter of your preference. I have a Enphase system with two 10T storage modules (20 kWh total). I run a battery profile that sounds similar to yours, except that I allow the batteries to discharge down to 30% SOC each evening. This normally gets us close to or past midnight before the batteries shut down. Like your system, when the NWS sends out a hazardous weather statement, our batteries switch automatically to full backup mode and hold 100% SOC until the alert is canceled.

The way I look at it is this: is it better (financially) for me to use my batteries to power our home in the evenings, or to hold a high SOC in them in case of a grid failure. The way our electric service contract works, it costs me twice as much to import power as I am paid (via credits) to export power. My goal, therefore, is to always minimize the amount of power I import from the grid. I would rather use as much of my daily solar production as possible, rather than send the excess back to the grid. So I choose to draw my batteries down to 30% each day, and take a chance that I won't need them for backup in prolonged grid outages.

Another consideration is what's best for the batteries...cycle them daily or let them sit for long periods at a high level of SOC. I feel it's better to use them. I have mine set to shut off at 15% SOC during a prolonged grid outage. I don't ever want to pull them down to zero - that's not a good idea.

Like I said - it's up to you how you use them.

[darkskies]

Good bits of information there SolTex!

I agree with you (and hence why I want to tune this battery), as only drawing down 20% is leaving a lot on the table. My power from provider is not 2x (yikes!) - from my calculations I have to produce 25-30% more power to recover the cost of what I pay from provider. Although I hear we have one of the highest rates up here in New Hampshire (US).

We only have a few outages a year, and I'm hearing from neighbors that normally they are not too bad (I think they had a few occasions over the years where it was out a week at the longest), so I would like to prioritize for producing vs power outage. I have a generator, but haven't had the chance to compare the cost of powering the house with propane. I suspect it is high though. So, lots of trade-offs.

I think the SolarEdge controls won't allow lower than 10%, I suspect to protect the battery. But I'm wondering about frequently discharging/charging cycles, and whether it is better maintain a charge, do small percentage discharges (say 20-30%), or do deeper discharges (60-80%).

[SolTex]

Don't know about LG, but Enphase battery warranty says " >70% capacity, up to 10 years or 4000 cycles, extendable to 15 years". 4000 cycles is more than one full cycle (100% to 0% to 100% SOC) each day for 10 years. I figure if my batteries really do last that long and still retain 70% of original capacity, then I will have gotten my money worth. Besides, in 10 years battery technology will have progressed so far that my "old" units would be more than ready for a upgrade. Assuming I live that long...

[darkskies]

I dug up warranty info. Looks like 70% for 10 years and aggregate energy throughput of 54 MWh with these conditions:

Capacity measurement condition:

• Ambient temperature : 25 ~30°C(77-86°F)
• Initial battery temperature from BMS : 25 ~30°C(77-86°F)
• Charging/discharging method :
  • Charge: 0.3CP/CV, Cut-off current 0.05C at:
    • RESU16H Prime: 290.5V(BPI*)
  • Discharge: 0.3CP, Cut off voltage at:
    • RESU16H Prime: 210V(BPI*)or cut-off min. cell voltage 3V
• Current and voltage measurements taken at battery side (BPI*)

*BPI :Battery Power Interface

Most makes sense, except for the charge 0.3CP/CV and discharge 0.3CP items. Any clue what that means?

[SolTex]

Most makes sense, except for the charge 0.3CP/CV and discharge 0.3CP items. Any clue what that means?

Sorry, that battery tech stuff is above my pay grade. We just installed our system in March of this year, and I'm still in the steep part of the learning curve. Perhaps a more experienced forum member will clarify.

[Bob Moffit]

Hi all,
I have a 10KWh Solar Edge battery. It was set up with a draw down to 30% each day and I did this for several months. However, I started tracking the energy in and out, and noticed that the losses with this is ~15%. For example, if it is fully charged and draws down to 30% (which should be 7 KW of capacity used) I only get about 5.8 KW of power out. I expected to have some losses, (the battery is rated with a 94.5% peak round trip efficiency). I've analyzed the data over very long periods (months) and also looked at 5 minute intervals for a given day. I also turned off the grid power after the sun went down one day and just ran battery until sun-up to eliminate as many variables as possible. The result is always closer to 15% loss than 5% loss.

Has anyone else seen similar performance or have any ideas on why I am seeing this?
Thanks

[bcroe]

All batteries lose some energy in a charge-discharge cycle, the
voltage is higher charging than discharging, even if the same
number of electrons pass. The higher the power level, the more
energy will be lost in the resistance of the battery elements.
95% efficiency sounds too good, to me. Bruce Roe

[SolTex]

Bob,

Based on my experience with my Enphase storage modules, I would say your Solar Edge battery is pretty efficient! I have two 10T modules for a nominal 20 kWh storage capacity. I looked at the past week and my battery performance is very consistent. Our batteries make the round trip from 30% to 100% and back to 30% every 24 hours (assuming a decently sunny day). As you can see from the representative screen plot below, the recharge power required is 1.25 times the discharge output. So I only get back 75% of what we put into our batteries. On my battery spec sheet, Enphase claims Round Trip Efficiency of 89% which i would say is a bit optimistic. Perhaps in a lab...but not in the real world.

I don't really worry about these numbers, because our system produces (over the average year) a lot more energy than we can ever use. If we waste some of it due to the way our batteries work, who cares? I'm giving it away "free" to our utility provider because we can't use all the credits we generate, so IMO its nothing to lose sleep over.

Screen Shot 2022-12-04 at 5.11.24 PM.jpg

[MattSl]

Unless there is a storm coming, I set mine to self-consumption and 0% (actuality is 5%). I'm more focused on achieving ROI, and maximizing the 12 year warranty on my batteries. Mine typically lasts through the night, unless there was a crappy solar day and they didn't charge past 50 or 60% (or higher, in case it's a big laundry day). I've got about 53kwh of storage, and a pretty heavy electric usage household as everything is electric in the house, and 5 people - including 3 teenagers (hot water woes anyone?). Jan 8th was a big laundry night, mass battery discharge that evening.

sshot-344.png

Roof Mount - 20.4kw: 51 Tesla T400H, TBG2, 4xPW2+

[darkskies]

Very nice! On average, what is your daily consumption? Is it around 30kWh?

I only have 16kWh battery (though could expand to 32kWh in future). My big issue is getting my installer to enable the ability for me to set the reserve for the battery. Currently it is fixed at 80%, so I only get about 3-4 kWh from the battery each day .

[MattSl]

My average is around 40% of 53kwh, so around 21kwh-ish. Of course it varies, and I only have about 5 months of statistics, but I'm sure it will be higher in the summer with AC running far longer. It's been a strange winter, where I've only run the heat pump consistently for 1 week. Dehumidifiers - run pretty constantly.

On average, my house consumes about 66kwh per day, so if there was no sun, and I didn't adjust usage, I'd deplete the batteries in a day.

[SolTex]

On average, what is your daily consumption? Is it around 30kWh?

Our average daily consumption over a one year period is about 60 kWh per day.

[soby]

Very nice! On average, what is your daily consumption? Is it around 30kWh?

I only have 16kWh battery (though could expand to 32kWh in future). My big issue is getting my installer to enable the ability for me to set the reserve for the battery. Currently it is fixed at 80%, so I only get about 3-4 kWh from the battery each day .

You may want to review this regarding NH net metering: https://www.solarreviews.com/blog/ne...e-net-metering

Depending on how your utility compensates you for solar power fed to the grid, you have very little incentive to charge/discharge your battery daily. South of your border in MA, we have full 1:1 net metering so the utility acts like a battery for our solar. I only charge/discharge the battery when there is an incentivized demand response event.

[darkskies]

You may want to review this regarding NH net metering: https://www.solarreviews.com/blog/ne...e-net-metering

Depending on how your utility compensates you for solar power fed to the grid, you have very little incentive to charge/discharge your battery daily. South of your border in MA, we have full 1:1 net metering so the utility acts like a battery for our solar. I only charge/discharge the battery when there is an incentivized demand response event.

Yes, very familiar with net metering in NH. My previous house had 1:1, but they changed the rules many years ago and you no longer get 1:1. Over the past year, I've found that I need to produce (on average) 23% more power to break even (not counting RECs, which in NH are tiny anyway).

Because of the disparity, it is a big incentive to discharge the battery as much as possible - not to export to the grid, but to self consume, as NH has very high rates (last month $0.3217/kW).

Right now, I only get about 3 kWH from the battery (out of 16 possible).