Given 80 90 100 % what is the sweet spot?

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Robocog
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Given 80 90 100 % what is the sweet spot?

Post by Robocog » Jan 01, 2018 11:39 pm

With a charger, that can cut off at 80 90 or 100, is 80 worth the short range, traded for longevity? Or is it better to charge to 90, and use your battery, and save the cycles? And, how does this affect the TIME lifecycle of the battery?

GA cells, but all cells should be known.

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dogman dan
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Re: Given 80 90 100 % what is the sweet spot?

Post by dogman dan » Jan 02, 2018 9:30 am

The sweet spot is exactly where it fits your needs. What I mean is this,, if you can't take an hour to finish charging and balancing in the morning before the ride, then 80% charge the night before is not your sweet spot.

If you have plenty of time to get from 80% to full before every ride, then the standard thing to do would be to charge to 80%, then fill up before you ride.

If you need to get going a tad faster, then of course, 90% charge the day before. How much time you need to fill up may not be improved by a big charger, because that last bit is always at much lower amps than the charger puts out when the battery is empty. Its likely to be less than an amp for the last 10%.

Trying to get more time/cycles from your battery by practicing a routine that is a total pain in the ass, is not worth it. If you need to just hop on and ride anytime, then you simply charge to 100% nearly every charge. Charge less occasionally when you know you won't ride tomorrow, but otherwise keep it full and ready to go.

You'll notice I am not advising riding off less than 100% charge. I don't see any benefit to discharging the battery deeper on any cycle. Its time at 100% charge that matters, more than anything. Ride off right away and you have nearly no time full. You are back to 90% in a mile or so.

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Re: Given 80 90 100 % what is the sweet spot?

Post by Robocog » Jan 10, 2018 2:37 pm

That is a keeper reply. thanks. And seems to be what I am doing. But good to have it confirmed.

I also always ride PAS, and 10 amps is max draw. My batts are ran NUDE. NO BMS. So, no full till ready to ride. But I will likely charge to 80%, then top up.

I have checked per cell, even after a few nanosecond shorts of cells assembling, and they are within nothing. To include after 10 charges. I guess I need a 40 amp slow blow fuse added. It has a 30amp switch.

Also, it shifts like nobodies business, so it is usually at 6 or less amps.

14AH batt, 48volts, I always stop at 50% or above discharge. Unless special occasion, and I would charge to 80% immediately.

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Re: Given 80 90 100 % what is the sweet spot?

Post by redilast » Jan 10, 2018 5:44 pm

I agree, it depends on your needs.

The military often charges their long life Li-ion batteries to 3.92v per cell, which is around a 70% SOC providing much longer cycle life, and basically eliminating the CV stage of charging.

NASA in some applications in space has used 20-30% DOD and achieved 10's of thousands of cycles.

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dogman dan
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Re: Given 80 90 100 % what is the sweet spot?

Post by dogman dan » Jan 11, 2018 7:34 am

For most of us, our needs are mostly two things.

Battery small enough to afford, and battery small enough to carry.

You bet, it would be great to charge to 4v, and stop at 20%, but to get very far you'd have to carry a huge, heavy, and more expensive to buy battery. Bear in mind, those nasa and military batteries are not usually running an EV. Their needs are different. Double size battery might mean 5 ounces, not 5 kilos.

But hey, if your ride is 5 miles to work, and can charge there, hell yeah do it.

Back when I commuted, I did 15 miles then charged. Downhill to work, it was usually a 60% discharge there. Coming back, depended on the wind. Often enough in winter, 95% to get back up that hill into the wind. My battery charged to 100% twice a day. That was my needs then, ten years ago. Whatever I could gain charging different was offset by my need to be able to ride in the am, with no fuss. I'd leave the house in the dark so not convenient at all to even top up in the am.

Now retired, I do a lot of 10 mile rides, with a pack that could do 20 miles, or 10 miles full speed (35 mph) I have lots of time to charge before each ride, and the typical deal is a 50% discharge and then storage for a few days at that half charged or less voltage. I charge to 95% or so, 4.15v. I could charge less, but my 50-90-100% charger finally died after many years hard use. If I do discharge deep, I just zap it a few min to bring it up from empty before storing it again.

FWIW, the pack is naked lipo, 18s 15 ah. Its two years old, so getting lower capacity and higher IR.

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Re: Given 80 90 100 % what is the sweet spot?

Post by 999zip999 » Jan 11, 2018 12:11 pm

Do you have sense wires for checking and balancing if needed ? And a cell checker or two ? I don't like a bms but carry 20ah.

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Re: Given 80 90 100 % what is the sweet spot?

Post by Voltron » Jan 11, 2018 12:33 pm

I run several no BMS packs that for sure have had a few groups killed by going too low, so I lean towards the high charged end... plus I work the packs pretty hard and cycle them constantly, so a minor theoretical shortening of the packs life isn't worth worrying about compared to the noticeably hotter acceleration of a nice full charge, for me anyway.

One thing I hadn't seen pointed out... you still want occasional full charges even if you go with the 80% thing, as many BMSs won't start balancing the groups until its hitting full charge.

Edit... never mind... I just reread you're a no BMS guy too.

This only really works if you have a regular routine, but during heavy commuting times I used to use a lamp timer switch with multiple settings, so I would plug in getting home, and it would turn off after a couple of hours at what my normal mid charge was, and then come back on for final charge in the morning. BIG NOTE- I didn't have it set to come on while still asleep! It just saved a morning trip to the garage to physically plug in, and had a smoke detector right over the bike in case I did sleep in a little and it was on for a little unattended.

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dogman dan
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Re: Given 80 90 100 % what is the sweet spot?

Post by dogman dan » Jan 12, 2018 7:58 am

That's a good plan, the timer.

I wanted to get an 80% charge that day I forgot to unplug my charger. Wife woke me up later when the house was on fire.

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Re: Given 80 90 100 % what is the sweet spot?

Post by spinningmagnets » Jan 12, 2018 10:31 am

When looking at the discharge curve of just about any 18650 cell, there is no extra range between 4.1V and 4.2V. The 4.2V number is listed as max charge, but...that is only because it is capable of taking a charge to 4.2V, it doesn't mean its a good idea. Since I found this out, I haven't charged to 4.2V under any conditions, ESPECIALLY when storing over the winter (meaning I make certain I don't even accidentally use 4.2V).

So then...the question becomes, charge to 4.1V (90%) or 4.05V (80%?). I understand the desire to buy the smallest, lightest, and least expensive battery that will "do the job", so then...there is a pressure to charge as high as possible, to get the max miles for the next ride. I've bought four ebike batteries, and so far I've sold one, the oldest and smallest, from back when I wanted to make sure it would fit any bike I'd ever get.

Then, I suddenly found myself getting some overtime at work, and I bought my dream street commuter, a $700 stretch cruiser. Huge triangle space, and comfortable riding position. Right now I have the biggest battery I could find at the time of purchase, 24-Ah. On my longest rides, I never run out of juice, so now?...I charge to 4.0V per cell, and I still never hit LVC...I have no way to prove this protocol will make the pack last twice as long, but...I believe the posted recommendations from Tesla, so that's what I'm doing.

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Re: Given 80 90 100 % what is the sweet spot?

Post by Buk___ » Jan 12, 2018 11:00 am

spinningmagnets wrote:
Jan 12, 2018 10:31 am
... 4.1V (90%) or 4.15V (80%?).
Aren't those percentages the wrong way around, or did you mean 4.05?

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Re: Given 80 90 100 % what is the sweet spot?

Post by Voltron » Jan 12, 2018 1:58 pm

dogman dan wrote:
Jan 12, 2018 7:58 am
That's a good plan, the timer.

I wanted to get an 80% charge that day I forgot to unplug my charger. Wife woke me up later when the house was on fire.
Worrying about accidentally falling asleep after a hard days work with chargers going was the main reason I got a timer. Getting one with multiple on off times for some morning charging was just a bonus.

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spinningmagnets
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Re: Given 80 90 100 % what is the sweet spot?

Post by spinningmagnets » Jan 12, 2018 8:17 pm

Aren't those percentages the wrong way around, or did you mean 4.05?
I blame the beer...thanks!

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Re: Given 80 90 100 % what is the sweet spot?

Post by thunderstorm80 » Jan 20, 2018 10:50 pm

dogman dan wrote:
Jan 02, 2018 9:30 am
You'll notice I am not advising riding off less than 100% charge. I don't see any benefit to discharging the battery deeper on any cycle. Its time at 100% charge that matters, more than anything. Ride off right away and you have nearly no time full. You are back to 90% in a mile or so.
I strongly disagree here.
Although it's true that time spent at 100% can cause a yearly 20%-30% capacity loss if left like this due to aging (at around 25oC temperature), it is ALSO the stress related to undesirable chemical reactions that happens when you are around the 4.2V mark. You can read about many official life cycle tests that charge a battery to 100% and immediately starts to discharge it again or after 5 minutes at most.
So no significant time is spent around the 4.2V mark, and still the specs show a huge difference in the cycle-life count.
Charging to 90% (4.1V@cell) would nearly double your life cycle and so the total energy throughput you can expect through the life of the cell. Down to 80% and you increase the total energy throughput even further, but to a less extent. Going below 80% as your max value gives less and less diminishing returns, but I do tend to believe that charge to 80% (or 4.05V@cell) is the sweet-spot for EV, for economical reasons. (topping up to 90%, and especially to 100% only when you must do so)

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Re: Given 80 90 100 % what is the sweet spot?

Post by DrkAngel » Jan 21, 2018 11:08 am

Getting the smallest - lightest battery might seem attractive ...
10lb battery ($200?) on a 50lb bike!
However ... 10lb battery on 50lb bike w/180lb rider = 240lb @ 1 year usable = $200 per year. ( 0% >> 100% discharge-charge)

Recent documentation demonstrates deep discharging is similarly damaging to cycle life as fully charging!
Optimal at 80% capacity looks to be in the 5% >> 85% to 10% >> 90% range with 400% lifespan.

Compare at 12.5lb battery ($250) on 50lb bike w/180lb rider = 242.5lb @ 4 year usable = $62.50 per year.
  • Extending beyond 400% usable cycles seems pointless due to:
    Age deterioration
    Continued reduction in cost of Lithium cells
    Continued advancements in Lithium technologies
  • Excepting HD cycle applications:
    Multiple cycles per day
    Powerwall
    etc.
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