Clarification on the 20% Rule

I charge my battery to 80% when I’m commuting. All good.

After the return journey the battery usually ends up around 40% at rest.

During the ride home the battery indicator is often in the red, below 20%, as I ride.

So i am asking if i am harming the battery by breaking the 20% rule or not?

What kind of battery are we talking about?

Regular shark pack. 48 volt 14ah

Taking any chemistry pack to below 20% - resting, is doing some damage. What is the % indication after an hours rest at home?

like i said, about 40%

sounds like it's the resting percentage that is important

So it's a lithium battery. Important to know these things since all batteries are different.

Long ago, NASA found out that damage happens when a cell is fully charged and remains fully charged for long periods of time.
I have never seen any research papers or experience from anyone on this forum showing that extended times at lower SOC are harmful.

Now if you fully discharge a pack and let it sit there for months, then you've done some damage, because it would have self discharged itself below it's low voltage cutoff. ( this happens extremely slowly )

I'm sure you're fine. Lithium is OK with a 100% charge and discharge. but for extending cycle life, it's best to charge it to the lowest voltage you can get away with.

I got about 600 cycles out of a lipo pack designed for 300 cycles by cutting 10% off the top and 10% off the bottom. If i had a BMS on that pack, i'd cut 20% off the top and 0% off the bottom.

Yeah having it sag to under 20% under load isn't the same as it resting at 20%. But if I were you I'd give it at least a bit of a charge when you get home, to get it closer to 50% before letting it sit overnight.

yeah, one key to my RC Lipo's long lifespan was keeping it at ~50% charged when i wasn't using it.

Lithium batteries are stored at 50% after being produced for a reason

Yeah generally running a cell in the lower 2/3rds of it's voltage range is better long term than running it in the upper 2/3rds.

ahh, so it is also much better to charge it to 50% for overnight storage?

i usually charge it to 80% ready for the next morning

Charge it to the voltage you need - if that's 95% then so be it. Just make sure you only charge it to that voltage immediately before you use it. If you only need 1/3rd of your pack's total capacity to get somewhere, you might as well charge from 33% to 66% as needed. But for a long trip you can't avoid the high SOC.

**Storage** sitting at lower SoC is good for longevity.

But regularly **cycling** to low SoC guarantees **much** shorter lifetimes.

Every chemistry I've seen, just different sloped curves.

Avg cycling DoD vs predicted cycles is a standard spec chart. Lab conditions, wildly optimistic compared to IRL.

But when you need to save weight. . .

Some recent literature:
View attachment 271 Cell SOC and longevity.pdf

tl;dr

High SOC = short life
High temp = short life
High SOC and high temp = very short life
High SOC, high temp and high discharge rates = even shorter life.

pollywaffle said:

ahh, so it is also much better to charge it to 50% for overnight storage?

i usually charge it to 80% ready for the next morning

Click to expand...

80% is fine. Most of the benefit comes from avoiding the top 15%. Below 80% you're into rapidly diminishing returns. 50% is ideal for longer term storage. Vacations, winter hibernation, that kind of thing.

No need for getting out of bed in the middle of the night to last-minute charge before work. Remember the battery is meant to work for you, not the other way around :wink:

pollywaffle said:

I charge my battery to 80% when I’m commuting. All good.

After the return journey the battery usually ends up around 40% at rest.

During the ride home the battery indicator is often in the red, below 20%, as I ride.

So i am asking if i am harming the battery by breaking the 20% rule or not?

Click to expand...

What exact instrumentation is giving you these SoC % estimates?

If based only on voltage GIGO. . .

Better to actually report the voltage itself, since their "translation" will always be wildly inaccurate.

Except at 0% (ideally set above 3.0Vpc) and 100% after charging to say 4.2Vpc

So, say you charge TO (CC only no CV) 4.1Vpc, don't let the battery sit there but as close as possible to heading out on a trip.

that will give much better longevity than 4.2V especially with CV time and letting it sit there overnight

but will only cost maybe 4-6% off "vendor spec" 100% SoC, minimal sacrifice of range.

And now you won't get voltage sagging as low under load toward the end of your trip, **and** your average DoD will be much shallower, big boost for cycle longevity.