Is slow Li-ion charging realy harmful?

[cocoon]

Hi guys and girls of ES!

I'm building new battery pack for my new daily commuter. I live in Ljubljana (EU) where city is almost flat. I have chosen 350 w geared motor (i like freewheeling option when i sprint from office to home 8) ). Controller is KT24, 36V, rated current 7 A, max current 15 A. BMS is elcheapo from ebay 10s 36V, 30 A. I want to build bike that is very easy to use so i want to include onboard charger. I ordered 42 V, 2A charger with specs: Input is AC 100-240V ac 50/60Hz , Output is 42V 2A, with LED charging indicator , red light when charging, green light when fully charged. Im stil learning details about battery charging and i read somwhere that slow charging couses formation of the crystals in battery cells and shortens life span. I want to know is this is true?
My battery pack will have 70 x Panasonic NCR18650PF 2900mAh - 10A (LiNiMnCoO2). (i bought them from nkon.nl). Arangment will be 10s7p (36v, 20.3 Ah). I calculated that 2 A onboard charger will need approx. 12 hours to full charge. This is acceptable to me, becouse i will only be doing like 10 km per day. Occasionaly bike will be used for longer tours wiht my GF. I wil be on on normal bike and she on e-bike . I like bike tours that are around 70 km long (4hours riding) so i need battery with at least 20 Ah.

if you have any other suggestion or advice on this battery pack build please feel free to comment.

Thank you!

 

[neptronix]

I've never heard that before and have been charging pretty slowly myself for a long time.. i have a 20ah 36v turnigy battery that i charged at 3A-5A for most of it's life.. it's lived longer than other packs i have.

What lithium batteries really don't like is being discharged close to their maximum amperage.. or being charged too fast. In either situation, you'd notice heat being generated by the cell. Stay away from that.. don't over charge.. don't overdischarge.. and you're cool.

 

[dogman dan]

Is he maybe referring to using a lead charger on li-ion, the trickle charge when its full? That is not good.

The general consensus on charging is this.

What harms batteries most in storage is when they are 100% charged and hot. So whatever strategy you can come up with to avoid that is good. Some leave some at the top, and undercharge. Some charge anything from 50% to 90% each day, then finish in the am. Morning is cool, and you immediately, or fairly soon, start your discharge, so it's cool and short time that you are at 100% charged. Depends on climate,, if it's cool by evening,, the night is not all that long.. It becomes a choice of convenience vs trying for a relatively small increase in lifespan.

You don't want to take a hot battery from discharging, park it in a flaming hot shed, then plug in a charger. Let it cool,, bring in the battery if the parking place is very hot. Charge once the battery is back to room temp, or at any rate no more than body temp if possible. They can tolerate up to 100F fine. I'm saying avoid that 140F metal garage.

Slow charging definitely harms a cell less than fast. However, the typical 5 -8 amps "fast charge" may be plenty slow for a battery of 10 ah size or more.

How you discharge really matters. Whatever the max specified rate is,, you really want avoid doing that, by about half. Deep discharge not exactly harmful, but it does mean you may need more time to get the battery balance charged the next recharge or several to get fully back to balanced.

 

[cocoon]

Is he maybe referring to using a lead charger on li-ion, the trickle charge when its full? That is not good

Source (YT - comment) is referring to BMS balancing.. Cmmenter said That balancing with low currents couses crystal formation.. I didn't find any sources to comfirm that.

Night temperatures here are low also in the summer, they are below 20C My bike will be stored in AC office and at home on balcony with maximmum temperature 40C. I will mostly charge bike at my office or at night.

I have experiences with 10Ah lipo pack. So im not complete newbie :wink: ..

Thank you very much for detailed answers!

 

[Alan B]

Onboard chargers are subject to occasional moisture and condensation as well as severe vibration and shock. They must be designed by the manufacturer for onboard use or they will likely be unreliable and possibly unsafe. There is experience here on ES with failures of this nature.

If the charger is connected to the battery when not charging their leakage current from the battery becomes important.

One solution is to fully pot the electronics, this increases the weight. It should be done by the manufacturer since it can affect circuit operation in a number of ways that need to be considered in the design, and it must be tested for water ingress during manufacture.

I know of one ebike charger that is designed and specified for onboard use (ebikes.ca Satiator). The 48V Satiator indicates water resistance, however the 72V unit states for indoor use only, as do most products in the market place. The Satiator specifies very low output leakage back into the charger. I've not seen a spec for that on other chargers.

Best of luck on your project,

 

[cocoon]

I will reconsider about using onboard charger.. The slow ones are cheap so i can have one at office and one at home. With this setup i will have an option to use fast charger if neaded and avoid possible problems with onboard charger.

Now ill just have to wait for 18650 battery holders from china (most difficult part of build ).

 

[dogman dan]

Hmm,, I'm certainly no battery engineer, but I never heard of the slow charging causing crystals. But if the bms balancing includes overcharging as part of the routine,, that could be a problem,, and it is slow,,, waiting for bms balancing.

Most bms,, or RC balancing chargers, they work by charging full as possible, till one or more cell groups is at full voltage. Then the fullest, sometimes even overcharged cells get discharged some by the bms. Then the charger may stop, or it may restart and refill the weak cell again. So if the charger restarts the lowest capacity cell, the one growing crystals fastest already, gets full charged repeatedly till the larger capacity cells finally fill. If it doesn't start, then the weak cell gets fully charged, while the rest get a break, undercharging a bit. So either way, crystals are going to grow more on the cell getting topped off at the higher voltage, till a discharge is started.

This I would call an effect of how a bms works, combined with a weak cell. Not really a caused by slower rate of charge thing.

IMO, if you do have a really shitty cell in there, It sure as hell won't like faster charging better than slow. But most likely your cells are ok enough, to make using a faster 5 amps charger no problem.

RE the onboard. cheap chargers just aren't made to be used onboard. The movement from road vibrations can cause shorts, or solder breaks, etc. But a good solidly made onboard charger simply needs to be disconnected from the battery to avoid any issues with trickle discharging the pack through the charger.

When I commuted,, I just had a fast charger at work, and the slow one at home. Sometimes, once in a while, I needed to leave work early. So I wanted a full charge of 500wh or so as quick as I could get it.

 

[cocoon]

Define slow?

I'm planning to charge 10s7p battery with 2A charger.

Do you know any EU source for faster chargers?

Panasonic cells i have are now uniform charged. I tested all 70 cells and voltage is 3.50v

 

[Alan B]

I also a faster charger at work and an overnight slower charger at home, for the same reasons dogman cited - one might need to leave work early for some reason, and one generally has a longer time window at home. In addition the other reason to charge faster is so that charging can be completed while you are nearby to monitor it and improve the safety.

What is actually "fast" or "slow" for the battery really depends on the size of the battery and its design. While we tend to think of lithium battery types as being similar, in reality they vary quite a bit depending on the details of the chemistry and the form factor.

If we look to a commercial system for comparative guidance, for the Nissan Leaf "quick" charging is done at 2C and replaces 80% of the energy in 30 minutes. This is 125 amps into a 65 amp hour pack, or 2C. Anything significantly less would be considered "normal" charging, and "trickle" charging is around 3 amps, or 0.05C.

 

[cocoon]

This could be my "fast" charger:
http://m.ebay.co.uk/itm/Charger-5A-for-Li-ion-Batteries-36-37V-Charging-voltage-optional-quiet-/322344313314?var=&hash=item4b0d37e1e2%3Am%3Am4Dmwd65BPsALIS-8TNNVfA&_trkparms=pageci%253Af3527ab7-d1c5-11e6-84ab-74dbd1e03e7e%257Cparentrq%253A64dc5c211590a60cd587a0aaffcb29b1%257Ciid%253A13

 

[Voltron]

The generic Battery University type articles warn about low charge rate... but all the scientific one where they're actually analyzing the batteries all link high charge rate and low temperatures as a combo you don't want, as it leads to metallic lithium plating. Lowering charge rate in low temperatures was cited as reducing the problem.

 

[dogman dan]

That's what I thought,, fast and hot,, worst case.

But 7p,, that's what 15 ah 20? I don't see a problem with 5 amps charge for that. .3c or less. A 10 ah pack you might like to limit to 3 amps. Again,, below .5c rate for the charge.

When I said I charged fast at work,, it was a 5 amps charger, on 15 ah. So really,, not so fast. But faster than the 2 amps charger I had at home. Those batteries lasted for years,, no way I cooked them with 5 amps charger.

 

[Voltron]

No, fast and cold was the worst case. The low temperatures slowed down how fast the ions could migrate, causing the plating, so reducing charge rate in freezing temperatures helped.

 

[wineboyrider]

I charge my 12s 16ah Multistar packs at 10.6 amps so far they don't even get warm when charging or discharging....

 

[dogman dan]

Boy,, I read shitty.

Oh,, frozen. You definitely damage a battery by charging it frozen. I thought you meant above freezing cold.

Seems like I definitely heard charging a too hot battery,, like way hotter than ambient when it's summer, is bad too. Anything that makes a cell really hot is always bad.

So on both extremes of hot or cold,, don't charge IMO.

 

[BikePilot]

I usually charge my 6s4p pack at 6 amps which is just under 0.5C (Tesla cells). it doesn't get warm at all and I think could be charged faster, I just have no need of that. I use SMC lipos in my RC truck and they specifically instruct that charging must be done between 1 and 2C. I charge two 2s 7200mah packs in parallel at 20amps most times. They don't get warm at all at that rate.

 

[Overclocker]

i had a similar thread: https://endless-sphere.com/forums/viewtopic.php?f=14&t=83043

 

[KarlJ]

I run a 16.5AH samsung 29E pack (em3EV triangle)
i discharge it at upto 40A albeit for short periods
I charge to 4.1V overnight from a DIY powerwall which is 3kWh of laptop cells (otherwise new but only good for about 1C)
its 7S 20P + BMS 20A x 3 bricks in parallel charged by solar.
DC-DC converter running at about 1A charge rate.
I'll plug in the satiator about 20mins before i get on the bike and it will charge at a couple of amps to 4.2 V /cell or 58.8V

PAck will still be cool and bike performance a little lackluster and i'll run it not so hard until 2km in where it will be quite snappy and warmed up somewhat.

by the time i get to work 35km and 40min later - cool morning pack is still warmer than i'd like
onto laptop power supply + same type of DC-DC as on the DIY powerwall. run that at about 80W so fully charged by about lunchtime - always connect when i arrive as forgetting is downright shit to pedal the bike home with 20% remaining capacity!

Home is pretty hot as always keen for a beer and always late leaving work = icepack on the battery cool her down from the 60C the pack is at.

I think this is too hot and have another 30Q 14S2P pack coming (independant BMS) to hopefully lower the C rate reduce the DOD and reduce the temp rise and add to its longevity.

KArl

 

[liveforphysics]

Charging slow is OK. Charging hot is OK (provided it's under say 55degC)

Charging when very cold, especially at a high rate is destructive to the anode.

Faster charging tests sometimes end up looking better because the slow charge tests take so long to get results (only getting say 3-4cycles a day), and calendar decay period of staying warm for so many months/years before the test completes vs a faster charge profile that completes in much less time with the cell sitting at temperature. Thermal kinetics and the related decay mechanisms are ultimately a battery against entropy.

 

[dogman dan]

Interesting point about the time the test takes affecting the results.

Also interesting that charging in 55c ambient is ok. I consider my south facing garage too hot in summer, when it reaches 120 f/50c. That's only on the very hottest days though, when it's 110 outside anyway.

Above 100f/40c ish,, I like to get my batteries into someplace cooler if not in use. Am I wrong to bother if it's only 40c? I kind of use body temp as my rule,, if it's hotter than body temp, I like to bring them inside for storage, or place in my old fridge in backyard at least, which in summer stays below body temp.

Any benefit to storage at body temp vs ten or twenty degrees F higher? I generally wait for a battery to return to body temp before charging,, generally don't discharge at a rate that gets them above body temp anyway. Generally don't recharge the same day anyhow, now that I don't commute.

Obviously on a simple vehicle like a bike, you pretty much have ambient temp battery in use. I like it warm for discharge, perky battery at 40c.

 

[ScooterMan101]

55 Degrees C , you are talking about the ambient air temperature ?

At what Degree Celsius / and under, is it still safe for the battery pack itself while charging ?
( 18650 Li-ion , and , Hobby Lipo's )
am I right to guess that a higher charge rate amperage than 0.5 c , is fine as long as one has a smart charger ( RC Lipo Charger , not a cheep bulk charger ) and keeps the pack temperature within a certain range ? ( or would 5 amp and higher charge rates just for hobby lipo's that have a much higher C rating than 18650 Li-ion ) ?

Reading through Battery Charge Threads/Posts, there are two different thoughts

1) you should charge a battery with an amperage that is one half ( 0.5 ) of its total cell amp hour capacity. ( 6 cell lipo pack that is a 5800 mAh , so 2.9 amps ?

2) you should charge a battery at 25% of its total mAh x c rating , so a 25c rated lipo pack that is 5800 mAh could be charged at around 37.5 amps ?

Charging slow is OK. Charging hot is OK (provided it's under say 55degC)

Charging when very cold, especially at a high rate is destructive to the anode.

 

[danielrlee]

Charging when very cold, especially at a high rate is destructive to the anode.

Just for clarity, how cold is 'very cold'?

 

[spamseonameremoved291]

Liveforphysics didn't say or mean ambient... 55°C ~=130°F. If that is your ambient temp, please seek shelter immediately. :lol:

Consult the datasheet for your particular battery, but many will state near 60°C-75°C upper operating limit. Charging fast (fast=relative to the specific battery) can add a substantial amount of heat and should be avoided without active cooling.
Some modern cells can easily reach 50-70°C while discharging at moderately high rates (some as low as 3C) without active cooling.

What liveforphysics shared is that time spent at high temperature affects test results more drastically for slower discharge than high discharge (since the battery spends more time at high temp during cycle testing). This extrapolates to storage temperatures. Basically heat is molecular friction which degrades battery performance over time (see entropy). To illustrate, imagine a pot of water. If kept at room temperature, it would evaporate slowly, perhaps over weeks. If heat is added the process would accelerate. If frozen, the process would nearly cease (ignoring sublimation). The water in this illustration is your battery capacity.

Rule of thumb: If it's too hot or too cold for you, your batteries are affected as well. This is 10°-45°C imo, but -10<->0°C is too cold, generally speaking.

 

[dogman dan]

Yeah, I think he meant charging without letting the pack cool down from the discharge.

But I've seen 104 F ambient outside plenty. I saw it daily for a lifetime in an outdoor microclimate. Park your vehicle on the south side of a building,, 140F easy right up against the wall. I've definitely seen 120 ambient in my garage, if I have the door open very long. There is a damn good reason I put white covers on my bike seats. Yeah, you take shelter, but maybe your bike or car can't.

So I was asking about ambient. I don't charge batteries in that garage in summer anymore. I was just wondering if that kind of temps was worth the fretting about, if you have no choice in a situation. You commute, one plug to use, its on the south side of a building.

Obviously you have no choice,, I just wondered if he had any thoughts about just how bad it might really be to do that, say, half the cycles of a battery. Most of the day,, not full, but on charge and plenty damn hot.

I no longer keep my safer chemistry batteries in the garage in summer, I bring them inside where it's 80 F or so, and keep em in my fireplace. I start doing this when it gets above body temp in there. But could I draw that line a bit warmer with no particular worry? Not 55 c, but maybe 40c? Again,, nothing to do about it if you need to use the vehicle. Its 105/40c out there, that's your battery temp at minimum. It's spec operating range is like you said,, take shelter.