home router battery backup

Jordan Hazen jnh at vt11.net
Tue Jan 18 20:27:37 UTC 2022

On Tue, Jan 18, 2022 at 05:11:57PM +0200, Mark Tinka wrote:
> I don't use the Tesla Powerwall, but Li-Ion is generally the same 
> regardless of who packages it. The difference will be what the OEM 
> decides to set the low-voltage cut-off to on the inverter and/or BMS.

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.

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).

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.

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.

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.

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.

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'm not sure how much the owner can configure a Tesla Powerwall, but 
> with other installations, you can decide when your battery kicks in to 
> run loads, or when it hands back to the grid or generator. This assumes 
> evening time, when solar irradiation is unavailable, of course, as that 
> is generally the preferred source of energy.

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.

> I've heard that Tesla will monitor the weather in your area to 
> "pre-charge" the Powerwall to account for possible power disruptions. 
> While I find that rather invasive, it's a cool feature for folk who 
> "don't want to know". Then again, I also hear that Tesla will limit or 
> withhold support and/or warranty if you do not connect your Powerwall to 
> the Internet for them to "manage". The downside I hear, with that, is 
> that they can remotely adjust SoH (state of health) thresholds to 
> lengthen battery life in order to meet warranty promises. Not sure how 
> true that is, but I've heard it a lot.

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.

> In terms of "reserve" capacity, Li-Ion can go much deeper than Lead 
> Acid. Some inverters are setup to disconnect the battery anywhere 
> between 3% - 20% SoC, depending on the OEM. For LFP chemistries, the BMS 
> will usually turn the pack off at 2.50V, while for NMC, that will be 
> around 2.75V. But different battery OEM's may be more or less aggressive 
> with their BMS's, depending on who you choose to buy from.

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.
> Mark.


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