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Hi Michael,

This could start the battery "religious wars", but perhaps a little
debate would be useful for people who haven't decided yet.

>> a 12v 100ah battery is about the same size, whether lithium or >> lead-acid... the lead-acid just weighs more.
Michael Diehr wrote:
> Disagree.
>
> Lithiums are advertised and rated on their true capacity (e.g. a 100AH
> lithium battery delivers an honest 100AH, at a 1C rate).
>
> Lead Acids are rated on some voodoo number (typically a 1/20th C rate of
> very low current) *and* every time you discharge a lead acid battery below
> 50% (or allow it to sit below 100% charge) you are killing it.

You are falling victim to advertised ratings.

BCI (Battery Council International) has a set of standardized test
methods for batteries. Most lead-acids adhere to these test methods
(though the manufacturers may put their thumb on the scale, by
hand-picking their best battery to test, etc.) Typically, you get a
low-rate (20-hour) amphour capacity, and a high-rate capacity (XX
minutes at 25a or 75a). Peukert's equation lets you predict the capacity
at any other rate.

There are precious few standardized tests for lithiums. Everyone makes
up their own, so they aren't directly comparable.

When someone claims their lithium battery delivers 100ah, that means
they have programmed their BMS to *limit* it to the desired number.
"Full" isn't really full (it's probably around 90%), and "empty" isn't
really empty (it's more like 20%). Your "100ah" lithium is probably
130ah cells with a BMS set for 100ah.

This can also hide the fact that the battery is degrading as it ages. As
it gets old and loses capacity, the BMS eats away at these 10-20% margins.

By the way, you can use a BMS on a lead-acid battery as well. It roughly
doubles their life just as it does for lithiums; by preventing
overcharging and limiting depth of discharge. It's less common because
lead-acids are cheap, and few want to pay extra for a BMS. But lithiums
NEED a BMS; without one, the likely failure modes are disastrous!

>> Safety: Lithium batteries BURN! The failure modes in lithium batteries can
>> easily start a fire; and it's the type of fire that can be very difficult
>> to extinguish. Keep in mind; you are going to use the battery until it fails.
>> And one of a lithium's failure modes is a fire!

> There are multiple Lithium battery chemistries, and this burning issue is
> really not a problem with LFP (Lithium Ferrous Phosphate chemistry, aka
> "LiFePo" or sometimes "Iron Phosphate").
>
>

Read that more closely. LiFePO4 cells are safer than LiCoO2 cells; but
they still burn. It's like comparing gasoline to diesel fuel. They both
burn; one is just easier to light.

For a lithium "house" battery, I agree that you definitely want a LFP
type; *not* a Lithium-Ion, LiCo, or other cheap high-peak "starting"
battery. First, because it's safer. Second, because LFP is more suitable
for long slow discharges than for high engine starting currents.

The big issue is cost. You'll pay 3-4 times more for a lithium battery
that is truly equivalent to a lead-acid. Will it work 3-4 times better,
or last 3-4 times longer? Who knows... do you feel lucky? :-)

> LFP batteries are some of the safest around and (in my opinion) safer
> than Lead Acids by far.

That's your opinion, and you're certainly entitled to it.

But 100 years of using lead-acids, and a billion cars on the road says
they are pretty darn safe. And that is *without* any kind of BMS to
protect them from abuse.

In contrast, it is easy to find examples of spectacular fires caused by
lithium cells. While most of them are LiCoO2, plenty involve LiFePO4 as
well. That is despite almost all of these applications having a BMS. In
fact, a major cause of fires is that the BMS failed to do its job.

>> Lead-acids don't burn. They might leak acid or vent hydrogen; but they
>> won't start a fire except in the most extreme scenarios.
>
> They can explode.? Hydrogen gas is no joke.

"Explode" is an emotional exaggeration. The air space inside a lead-acid
battery is tiny. It can't hold enough hydrogen to do much more than blow
the vent caps or break the case. Very messy if it's a flooded battery!
But not what one would properly call an explosion.

If you put the battery in a larger closed space (like inside your van),
and then seriously overcharge it for many hours, so it fills the entire
space with hydrogen, THEN you can get a serious explosion. But that's
equivalent to dumping a gallon of gasoline on the carpet, then letting
it sit with the windows closed until it evaporates, and THEN setting it
off with a spark or match. Now THAT's an explosion!

>> If you're worried about venting or hydrogen, use a sealed AGM type lead-acid.
>
> True, AGM are better, but I believe these can also vent when seriously abused
> (e.g. if you have a malfunctioning charger)

Yes, I agree!

>> Life: Think about the batteries in your laptop or phone; they are lithium,
>> and most of them get weak and fail within 3-6 years.
>
> different kind of "lithium" - laptop batteries are "Lithium Ion" not LFP.
> Lithium Ion batteries are used because they are energy dense (great for
> lightweight laptops) not because they are safe. They aren't very safe.

Ah, but "very" is a weasel-word; it means whatever the user wants it to
mean. Most arguments boil down to one side saying it's "very large", and
the other saying it's "very small"; when the actual number is "17".

Life does not correlate with chemistry. You can find long-life examples
of every chemistry -- and short-life examples as well. It boils down to
price and quality control.

Tesla used LiCo laptop cells in their earlier EVs, and thousands of them
have lasted 10 years. But they were very carefully chosen cells, and
have a very sophisticated BMS system to protect them.

Laptops and phones use the cheapest, most energy-dense battery they can
get. This also makes them the shortest-life and least-safe types if
anything goes wrong.

> LFP are different - not as energy dense, but much much safer. Also, they
> have some of the best longevity around.? One major brand has a 3000-5000
> cycle estimated life - and at that point, they say the battery still has
> 75% capacity!? They could be lying, of course, but the sorts of stats are
> not unheard of for LFP batteries.

I agree. LFP holds roughly 2 times the energy of lead-acid, while LiCo
and LiPo is more like 3 times. This makes them safer (but I won't use
the weasel-word "much" safer, without supporting data to say "how much"
safer).

> I managed to go through 3 sets of 6V golf cart batteries in 10 years, and
> I'm not a dummy when it comes to this stuff.? Keeping Lead Acids happy is
> not easy.

I'm curious; what kind did you use? I've been using deep-cycle batteries
in my electric car work for 40 years. I generally avoid abusing them,
and so routinely get 7-10 years per set of 6v golf cart batteries. But
it did require proper charging, discharging, and maintenance. The best
types have been Trojan or US Battery; the worst were Walmarts.

Similarly I had a set of Concorde AGMs in my EV for 9 years. No
maintenance, though I did have a BMS. I also have Gates Cyclon
spiral-cell gels that are 20 years old. And I'm still using the
20-year-old Enersys SBS60 UPS batteries.

>> If you want a lithium to last, forget the mass-market "lead-acid replacements"
>> and Chinese ebay specials. Get a used electric vehicle battery -- the EPA
>> *requires* the manufacturers to warranty them for 10 years, so the quality
>> is *has* to be better or warranty costs would be terrible.
>
> This was true a few years ago, but LFP drop-in 12V batteries are quite good
> now.

I wasn't talking about high-end LFP specifically. If you're buying a
Battleborn lithium for $800, you have a right to expect that someone did
their homework, and actually designed it right and tested it.

I was talking about what you get if you go to NAPA or Amazon etc. and
look up a "lithium automotive battery". You'll find lots of Lithium-Ion
types with exaggerated ratings.



And if you buy some Chinese "lithium replacement" battery on ebay,
caveat emptor!

>> Abuse: Most batteries don't die of old age -- they are *murdered* by
>> negligence and abuse.
>
> True, but note: this *only* applies to lead acid batteries.

It applies to *any* battery without a BMS (or with a junky BMS).

Take a naked lithium cell (no BMS), and run it dead. That's likely to be
the end of it; you have destroyed most of its capacity. Or overcharge it
until it gets hot; same problem. And in the latter case, you could even
set it on fire.

With lithiums, you are depending on the BMS to save you.

> Did you know model 3s are now being built with LFP batteries? Almost like
> he knows his stuff ;-)

Yes. Though I credit his engineers; not Musk himself. :-)

Hope this helps,
Lee Hart

--
There is a computer disease that anybody who works with computers knows
about. It's very serious, and interferes completely with your work. The
trouble with computers is that you 'play' with them! (Richard Feynman)
--
Lee Hart, 814 8th Ave N, Sartell MN 56377,

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