Re: home router battery backup

[Mark Tinka <mark@tinka.africa>]

On 1/18/22 22:27, Jordan Hazen wrote:

> Yes, but separate from their absolute low-voltage cutoff meant to
> protect battery cells, these hybrid storage products include a
> user-adjustable reserve setpoint, meant to balance their backup
> role with grid support and peak-shaving.

Yes. In the traditional battery inverter world, it's called "increased 
self-consumption" :-).


> Owners of home-scale PV+battery systems often sign up for
> time-of-use utility tariffs, where per-kWh rates are high during
> peak periods (day and evening), but very low at night.  With a
> large enough system at full charge, such homes can then run
> autonomously for several hours after sunset, covering most or all
> of the peak-demand period, before switching back to grid power, and
> optionally recharging at night when rates and demand are low (if
> local solar production alone isn't enough replenishment).

Indeed.

Obviously, if your goal is to reduce power costs, then this is fine. The 
potential downside is consistent cycling of the battery, which will 
reduce it lifespan.

Then again, batteries are getting cheaper (at least the LFP chemistry), 
so perhaps it might not matter much after you get a solid 5 - 8 years, 
at the very least, out of them.


> Of course, letting the battery discharge all the way to its 0%
> safety limit in its daily demand shifting role, besides prematurely
> aging the cells, would leave the owner without backup were an
> outage to strike at a bad time, in late afternoon or evening.

Agreed.

Between the battery and inverter OEM's, they might not let you get that low.


> With the utility paying for and managing distributed batteries
> itself, though, they'd want to derive the greatest possible return
> on their investment, and so probably tend to set that reserve-limit
> very low.

I know that there are some markets where home owners are given 
incentives to buy batteries that they can use to take some load off the 
grid. Do you know of any markets where this is actually being done by 
the utility, including remote control?


> Some utilities have been experimenting with using these storage
> systems as part of demand-side management / load shedding, where
> during peak periods they can signal for buildings capable of
> operating from battery power to start doing do for a set period of
> time.  With enough aggregate storage available, this could avoid
> having to fire up an expensive-to-operate natural gas peaker plant.

I know the Australians have been fiddling around with remote management 
of HVAC and water heating systems as part of their demand-side 
management program. However, I don't think there is sufficient scale to 
help them reduce production enough so as to continue their use of fossil 
fuels.

Theoretically, it's a great idea. I just don't see it in practice at 
scale, for an entire nation, managed by the utility.


> The battery system owner would receive bill credits, lower rates,
> or other incentives in exchange for allowing this, similar to how
> some data centers and other industrial building will be compensated
> for proactively switching to generator power when asked to during a
> high-demand period.

I've always been a supporter for micro grids... either at the home 
level, or the neighborhood level. It's easier to manage your usage when 
you know how much energy you have for the day.

I don't think renewables at grid scale will work, because the user has 
no visibility of generation capacity and constraints. They'll just keep 
flipping switches.

So if we can put generation responsibility in the hands of the user, 
there is a decent chance that renewables may actually work. Plus, we use 
less land.


> Is anyone aware of data centers yet leveraging battery storage for
> a similar purpose?  It would make zero economic sense with
> traditional lead-acid storage, of course, due to such batteries'
> limited cycle life and intolerance of deep discharge.

I can't imagine any data centre using the battery for pure base load. 
Like they do with UPS's now, I'd expect the battery is really there to 
provide load management between grid loss and activation of the 
generator. You only need the battery for a few seconds to a couple of 
minutes.

So no different from what they are currently doing with Lead Acid 
batteries, but with the benefit of more cycles, deeper discharging, 
higher energy density, less weight and less space, from Li-Ion.


> Uilities are becoming increasingly hostile to solar net-metering,
> where PV system owners are credited for excess energy supplied to
> the grid during peak sun hours, then allowed to "draw this back" at
> parity after sunset, using the grid as a sort of virtual battery.
> They argue that such use incurs uncompensated costs (see "duck
> curve"), and have successfully lobbied for tariff changes in some
> areas to limit or end the practice.  Other locales have never had
> net-metering.  On-site storage can be a good alternative to net
> metering where it's unavailable, and is probably more beneficial to
> grid stability.

Personally, I just avoid all the drama of trying to do grid feed-in with 
our utility. For the effort and pain, the return is not worth it. I'd 
rather dump excess energy into the battery, and throttle back production 
when the battery and house loads are below the PV capacity (thank God 
for automated FSPC - Frequency Shift Power Control)

I heard some utilities in the U.S. were limiting feed-in to about 10% - 
15% of customers, but that some states have been pushing hard to raise 
that to 50%, much to the utility's chagrin, of course.

However, it does make sense, the concerns of the utility. Imagine 1MW of 
PV feed-in coming into and going away from the grid when just a few 
seconds of cloud cover converge over the arrays of multiple customers. 
That kind of swing of generation can be pretty hard to manage, and 
increases the potential for network instability.

On the other hand, nuclear and coal plants do not ramp up quickly... 
they need a couple of hours to spin up and stabilize. So it's easier for 
the utility to keep those generators spinning at steady RPM, even 
without the corresponding load, just so that they don't have to lose 
time ramping up for the evening demand, and potentially having to fall 
back to peaking plants that are more costly. So just because demand 
would be low during the day when consumers are generating their own 
power from PV, does not mean that the utility's costs suddenly come down 
during that period.


> They apparently do this with their vehicles as well.  Claimed
> 0%-100% figures are mapped onto a somewhat narower, but hidden
> true-SoC range at first, which is silently broadened over time to
> compensate for cell aging.  Understandable, but I'm also uneasy at
> such information-hiding.

Me too! And yes, heard the same about the cars as well.

0% SoC isn't actually 0% SoC... more like somewhere between 5% - 10%. I 
really don't like that; rather, set an absolute low voltage cut-off 
limit that indicates true SoC, rather than manipulating SoH over time to 
manage warranty.

I mean, if you are an "aggressive" driver who is consistently pummeling 
the battery over the course of many years, it is likely to die before 
the (is it?) 10-year guarantee. Artificially managing that over the 
period is false, because then it means as the years go by, I am wasting 
more time charging the car, even if all looks like it did when I first 
drove it out the dealership.

My battery OEM considers 100% SoC at 52.4V. However, the true 100% SoC 
for this battery is actually 54V. So within the BMS, they have set it up 
for around 80% SoC true. But at least, they were honest about this with 
me, and since it's a stationery battery, I don't mind, really. It's 
still enough energy to get me through any night.


> LFP (LiFePO4), with its longer cycle life may be the best
> currently- available chemistry for fixed storage, where its lower
> gravimetric & volumetric energy density (vs NMC) doesn't matter so
> much.  NMC has economies of scale going for it, though, along with
> what's likely to be an ever-increasing supply of worn electric
> vehicle packs, replaced after showing reduced range in that role
> but will plenty of life left for other applications.

The Chinese have been experimenting with LFP for vehicles, where NMC has 
been the staple given its higher energy density for the weight it 
carries. I believe Tesla are working with Panasonic to use LFP in their 
trucks (but insist that their standard cars will continue to ship with NMC).

I considered NMC for stationery storage at my house, prior to going with 
LFP. But between being double the cost and somewhat thermally less 
stable, LFP was the way to go.

LFP is a little heavier and is less energy-dense compared to NMC, but to 
be realistic, as Sean Connery asked Catherine Zeta Jones in the movie 
"Entrapment", as they planned the split of their heist:

    "What can you do with 7 billion that you can't do with 4?"

Mark.