Typical last mile battery runtime (protecting against power cuts)

Mark Tinka mark at tinka.africa
Sat Feb 4 04:30:36 UTC 2023

On 2/3/23 16:11, Israel G. Lugo wrote:

> Hi folks,
> At $day_job, I have a team of engineers who are oncall for critical 
> services in the United Kingdom. For $reasons, the national power grid 
> is announcing the possibility of rolling power cuts over the coming 
> months. Right now it's "unlikely", but possible. If cuts do happen, 
> it'll be 3+ hours, possibly several times/day.
> I'm looking at the cost/benefit of deploying small UPSes at people's 
> homes, to protect their network access when oncall. Just to power the 
> home router (+ONT if FTTP), and keep a charged laptop. I figure 
> anything smallish should be enough for a few hours.
> Question is, how much battery runtime can I typically expect from 
> ISPs' last mile infra.
> People will have a random mix of DSL, FTTP, DOCSIS. Another 
> alternative is tethering with 4G.
> - For FTTP, I *think* (but am not sure) that the UK mostly uses PON, 
> so guess it would be runtime of OLT and onwards
> - For DSL: runtime of DSLAM cabinet and onwards
> - For CATV: CMTS and onwards, maybe any active equipments in the HFC 
> to the CPE?
> - For 4G: BSS and onwards
> Could anyone with last mile experience help with some ballpark 
> figures? I.e. 15 min vs 8h or 8 days.

Living in South Africa where load shedding is the order of the day since 
the end of last year (and to continue for the next 2 years, at least, if 
not more), I can tell you that until network operators have had to deal 
with this, they are ill-prepared.

For equipment inside major data centres, you will be fine. But for 
equipment in commercial buildings, street cabinets, e.t.c., uptime will 
be directly related to how much faith operators have put into the 
national grid, which translates to some UPS, and whether the building or 
cabinet is serviced by a well maintained generator.

As for the home, a UPS is not a terribly good idea to keep the "wi-fi" 
going... typical UPS's use Lead Acid batteries which are shallow-cycle 
batteries that are not meant to be discharged below 50%. It's the usual 
disclaimer - UPS's are meant to give you time to save your work and 
shutdown, not provide extended backup.

If customers expect the power to be out for 3+ hours at a time, multiple 
times a day, I'd recommend getting a cheap 1.6kW inverter that can power 
a 24V 100Ah Li-Ion battery (either 1x 24V or 2x 12V in series). It's not 
a lot of energy, but if you are powering the CPE + ONU + IP phone, it's 
more than enough (you can easily squeeze 9hrs of continuous run-time on 
a single charge, maybe even more). While these inverters are slow to 
charge Li-Ion batteries (about 10A of charge current), the load is so 
minimal that the battery is being discharged even slower. So it works 
out, even with extended, multiple outages in a 24-hour cycle.

Operators running gear outside of major data centres will want to invest 
in large Li-Ion packs designed for the load and frequency of grid 
outages. Investing in Lead Acid batteries, while cheaper initially, 
will  become operationally expensive, as they don't do well with 
high-cycle counts, even the deep discharge ones. Not to mention, the 
rather poor energy density they possess for the amount of weight they carry.

Depending on the importance of the site, some solar power may be 
necessary, even though the UK is not the most well-lit country in the 
world. For such cases, you can augment the solar plant with a generator, 
mainly to recharge the batteries, rather than to power the load.


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