[TriLUG] reason not to mix batteries in UPS

[David Burton via TriLUG]

Joe, the LiFePO4 batteries are a different chemistry (only 3.2v per cell),
and should have much better lifespan than the 3.7v per cell Li-ion and LiPo
"lithium" batteries commonly used in things like mobile phones.

However, it doesn't look like this Howell battery
<https://www.amazon.com/dp/B095K38M6C/ref=redir_mobile_desktop?_encoding=UTF8&psc=1&ref=ppx_pop_mob_b_asin_title>
is
a good choice for a UPS.

A key parameter for UPS batteries is the "C rating
<https://www.google.com/search?q=%22C+rating%22+for+batteries>," which is
how fast it can be discharged (amps), divided by the amp-hour rating of the
battery. For a 12v battery to be able to power a 500W UPS, it must be able
to deliver about 50 <https://www.google.com/search?q=500+*+1.2+%2F+12+%3D> amps
continuously, until the battery has run down. For a 7 Ah battery that would
require a C rating of 50 / 7 = 7C.

If a UPS is sized to provide a 10 minute runtime during a power outage
(which is fairly typical for the little UPSs with 7Ah RBC17
<https://www.apc.com/us/en/product/RBC17/apc-replacement-battery-cartridge-vrla-battery-9ah-12vdc-2year-warranty/>
lead-acid
batteries), then the C rating of the battery must be 60 min / 10 min = a
minimum of 6C.

To match the performance of a lead-acid battery, a bare-minimum usable
C-rating for a UPS battery is about 5C, and 10C would be better.

That Howell battery is advertised as supporting a maximum discharge current
of 7 A from a 7 Ah battery, so its C-rating is only 1C.

If you size your UPS large enough to provide a full hour of runtime from
the battery, that would be adequate for your application. 12V × 7A = 84W.
If we optimistically assume 80% conversion efficiency by the inverter,,
that gives us 0.8 × 84W = 67W usable. So if your load is more than that,
your UPS will probably not be able to support it, with that Howell battery.

Dave

On Mon, Jan 9, 2023 at 7:36 AM Joseph Mack NA3T <jmack at trilug.org> wrote:

> On Sun, 8 Jan 2023, David Brain via TriLUG wrote:
>
> > Hi,
> >
> > There are LiFePO4 batteries with the same form factor as SLA batteries -
> > that have a battery management system built in and so im theory could be
> a
> > drop in replacement - e.g (picking at random, there are many)
> >
> >
> https://www.amazon.com/stores/HowellEnergy/page/25E75BEA-EBC2-4EBD-BD0F-646755CECED9?ref_=ast_bln
>
> looks interesting.
>
> > But there's mixed reviews about using them on a UPS - some UPS don't seem
> > to 'like' them and so won't charge or accept them as non faulty.
>
> a regular Pb-acid battery is a two terminal device - you charge through
> the same port as you discharge. All the Li-ion batteries I know about are
> three terminal devices - you charge through one port and you discharge
> through another. The Howell battery is a two terminal device. The list of
> functions of their battery management system seems complete, but I don't
> know how they can do it.
>
> A minor wrinkle is that some of the batteries arrive with 0V output and
> you have to activate them with 18V (12V is not enough). If you had a chain
> of 4 batteries in series and some started discharged and some charged,
> you'd need parallel resistors to balance the charge.
>
> One entry in the review says that you can't put them in series.
>
> Several of the reviews say that they don't work, or die quickly, even when
> used in a device that expects a Li-ion battery.
>
> The specs say a lifetime of upto 10years. I've never had a cell phone
> Li-ion battery last more than 2 years and I take care not to charge it to
> death, take it down to 25% once a month (to reset the meter) and not let
> it get hot.
>
> So even if the Howell batteries work, I'm not expecting them to last
> longer than an SLA.
>
> Joe
>
> > I tried one in a small UPS and so for it's been OK - but it's only been
> in
> > use for a month or so.
> >
> > David.
>
> --
> Joseph Mack NA3T EME(B,D), FM05lw North Carolina
> Don't upgrade a working machine - it won't work any
> better - it could work worse; it may not work at all.
> Homepage http://www.austintek.com/ It's GNU/Linux!
>
>