Battery life paradox

[mars62]

Hi guys, I´m struggling with this idea: if I charge my battery topping it at 80% and draw off just to 30% (as recommended for increase battery life) I need to recharge twice a day. Instead, if I refill to 100% and draw off to BMS cut off (15%?) I only recharge once a day.
So, if trying to protect battery and extend its life, I need to duplicate charging cycles/day, then what would be the benefit?
Am I missing something? A shorter charging cicle accounts less than a longer one?
Battery 7,8 AH-350W geared hub motor - 30km/day commuting use.
Thanks a lot.

 

[Sunder]

It's mainly time in the higher voltages that causes the loss of cycle life, though any wide SOC range will shorten life somewhat. So for example if you charge the battery at 7am while you're having breakfast, discharge it on the ride to work, then charge it again at 4pm just before you leave, it's not all that damaging.

Charging 80-30% (50% of capacity usable), can result in 8-10x the cycle life of 100-15%. So while you are doubling the number of cycles, you are massively increasing the cycle life, so there is a big net benefit.

That all said, batteries do have a shelf life. So no point making your battery last 4000 cycles, if by 3 years (~1000 days), the battery has lost power density, and you need to replace it, because you can't go up steep hills any more.

Find your own balance. Odds are, in 3 years time, there will be some improvements on battery performance anyway, and you might want to upgrade.

 

[mars62]

Cristal clear Sunder, thank you very much for your reply.
Regards

 

[dogman dan]

Charge twice a day, regardless of what you charge to at the top. The less you discharge deep the better.

Your bms will never ever balance the pack, if you don't charge to 100% ever. So do that some.

What makes good sense to me is two chargers. the one at home goes to 100%, and balances the pack as well as the bms can do it, daily. The charger at work can be adjusted to a 90% charge.

 

[spinningmagnets]

I agree with dogman. Two shallow cycles are much better than one deep cycle...by far.

One issue is heat, and the lower the battery is, the higher the resistance. The main issue with long life is time spent resting at the top voltage.

Maybe charge to 80% on a daily basis when the ebike will be resting overnight. I'd charge to 100% once a week (just before a ride) for cell balancing.

Tesla has an 8-year warranty for a battery that is intended to cycle at least 5 times a week (260+ cycles a year, 2,000+ full cycles minimum), so...they are doing something right...

 

[liveforphysics]

Common for modern cells to handle many 10's of thousands of shallow cycles, even when they only handle a thousand full DOD cycles.

 

[MadRhino]

I charge RC lipos 3 times a day at least, sometimes 5. I charge them full, well over sometimes, and I ride right after. That is because I like saving the weight of a big battery, and I also like charging high C rate pretty quick. I never discharge them low, to keep them cool and balanced, and never let them sleep charged above 3.8v per cell. I don’t achieve extraordinary number of cycles, but still do more than their spec life expectancy. When I discard them it is because they begin to heat under fast charging. Yet they are still good for some riders with lower requirements, so most of the time I give them away before they are due for recycling.

 

[dogman dan]

The wisest minds agree, charge every time you can.

 

[quantumcloud509]

nevermind

 

[markz]

Yup I charge up right before I am about to leave, but store it over night somewhere in between so there is just a 20 minute wait to giverrrrrrrrrrrrrrrrrrrrrrrrr

 

[markz]

Question is though, what is a reasonable amount of time sitting at full voltage?

 

[dogman dan]

About as long as it takes to read this, if you want the max cycle life.

But as I often say, make it practical for you to use. A morning ritual where you get up, need 15 min more to finish a charge, but take an hour to get ready to leave might be the best for you. Same kind of thing at the end of your work day. If you need 4 hours to recharge, plug in at noon vs when you get there in the AM. But no need either way to make it a pain in the ass, where you sit there watching a charger finish twice a day. Don't sweat an hour full IMO. Don't make using the thing a PITA.

As said previously, it can depend on the calendar life too. In my case, I rarely reach anything like 1000 cycles in two years. But the cheapo RC packs I often have just start to puff and lose capacity in two years anyway, because of my storage place not being very cool in summer, the nature of the cheap cells, etc.


So for me, it kind of makes little difference. Cells I left full for months at a time lasted two years. Cells I babied last about two years. The only difference is cells I discharge at high rate can fail in months, like a 5 ah pack running a big 40 amps controller.