Regen/charging/Standard C-rating & voltage questions

[scoot-e]

Couple general questions for all:

1) What is the basic recommended charging speed for, say, Samsung 18650 3ah cells? If they were in a 4p config at 12ah, could one safely charge them with a 3a charger? Is .5C or 1C a standard for best practices charging for decent cycle life? How far could one push it and not see excessive wear, 2c or?

2) Any idea when using regen braking on an e-bike or escooter how much is the general practice for charge through regen, .5C or 1C like wall charging or?...

3) Generally, if one uses regen braking and they fully top off or fully charge their pack and then, say, were to ride down a hill and use the regen braking right away, would the controller and/or bms limit the pack from being overcharged and simply 'shut off' any regen (I've heard from a boosted board eboard user that he experienced this after charging fully and riding down a hill to discover it has no regen power - unlucky for him since there is no mechanical brake or skid ability according to him, but he was able to control it luckily) and thus one would have to rely on a mechanical brake, or could it cause an overcharge fire like we've seen with the hoverboard fiasco in recent months? Just curious others experience with this in general if they have a gearless direct drive motor that uses regen braking to help slow down their e-vehicle without the need for overuse of a mechanical brake

4) For high discharge batteries, like the Samsung 25r or 30a etc that claim 15a or 20a discharge rates, what is best practice or generally accepted continuous amperage and or peak and for how long should one sustain the peak? For example, when e-biking I generally pull about 30a (my max setting on my programmable controller) red-lining it on large hills for maybe a few minutes max and then it levels out on flats or going downhill and rarely needs more than 10-15a to get up to full speed on the flats. I generally try to stay around 2c so on a 15ah pack I try to keep it maxed at 30a for a few minutes at a time mostly and then for most street riding I use about 1c or 15a in general. Any recommendations here? I'd of course like to use the smallest, lightest, cheapest batt pack I can get away with but of course want to use it for the long term and not overdo it or burn them out prematurely. I rarely go above 2c on my packs and again only for hill riding mostly.

5) Lastly, what is considered the standard for 24v, 36v and 48v packs? I have seen 7s for 24v, 10s for 36v but for 48v I've seen anywhere from 12s, 13s to 14s...Any standards here or are they all "correct"?

Thx for your comments!

 

[Hillhater]

A lot of detailed questions specific to certain cells.
Some one may know some of the answers, but beware of generic answers that may not be relavent to your particular cells, The best source for reliable answers is the manufacturers data sheets, which are all available on line and even on this forum if you search.
Here is a starter..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=57291

 

[wesnewell]

1. Depends on the cell, but generally most standard charge rates are ~ 1/2 the mah rating.

2. I have no idea , but it has never hurt my pack. I use rc lipo.

3. If the voltage gets to high the controller will shut off regen, as I guess your friend found out in a very unusual situation. I'd probably prefer it over charge my pack a little, but I do have brakes.

4. The Samsung 25r is an 8C discharge rated cell. I don't consider it anywhere close to a high discharge rated cell. Here's the specs on the 25r. You decide.
http://lygte-info.dk/review/batteries2012/Samsung INR18650-25R 2500mAh (Cyan) UK.html

5. 24V-7s, 36V-10s, 48V-13s, 72V-20s, but you can use whatever you want as long as you know what you're doing.

 

[scoot-e]

Thx and just curious wesnewell, where you get the info you mentioned for #1 (and #5) I believe I have heard its .5C for charge rates as well but not sure if that's common practice or just a quote from a battery spec page somewhere. I know there is what they say to do and then there is what one can safely do long term and they aren't always necessarily the same (or higher or lower)

Re #3 it's not an unusual situation at all. If one charges their pack fully and then unplugs their charger and tries to plug it back in then the bms will limit the charging as the battery is already fully charged and same goes for regen as the Boosted Board owner I met attested to. It makes sense since overcharging cells can damage them or worse but was curious if others had this experience or any thoughts on the matter in addition

And re #4 what cells have you found that are much higher discharge than the Samsung 25R for example? I have seen them rated anywhere from 15a to 20a discharge on various sites where they are for sale which would rate them at 6-8C (although I'd never run them at that rate even for peak rides at short bursts simply because I don't believe they would last very long cycle life wise).
If you are comparing RC lipo pouch batteries from places like Hobbyking, I think they way overrate their C ratings and I would not trust them. However, I haven't seen many 18650 or pouch batts that aren't outrageously expensive or rare to get ahold of that I've heard anyone recommend be used at over 2-3C for long term continuous use...

1. Depends on the cell, but generally most standard charge rates are ~ 1/2 the mah rating.

2. I have no idea , but it has never hurt my pack. I use rc lipo.

3. If the voltage gets to high the controller will shut off regen, as I guess your friend found out in a very unusual situation. I'd probably prefer it over charge my pack a little, but I do have brakes.

4. The Samsung 25r is an 8C discharge rated cell. I don't consider it anywhere close to a high discharge rated cell. Here's the specs on the 25r. You decide.
http://lygte-info.dk/review/batteries2012/Samsung INR18650-25R 2500mAh (Cyan) UK.html

5. 24V-7s, 36V-10s, 48V-13s, 72V-20s, but you can use whatever you want as long as you know what you're doing.

 

[scoot-e]

Hillhater, I agree they may be specific questions to some cells but in general have you seen a cell that charging at .5C would cause a problem? (and I don't mean some esoteric cell but rather the generally used decent discharge 18650 or pouch cells)?

And this may be a dumb question but I've seen a lot of people arguing on the forums over charge speed causing potential long term damage to cycle life or cell life in general, but tell me if my math is right: If I have a 2.5ah Samsung 25r cell for example and put 4 in parallel then I have a 10ah pack - if I charge at .5C then I can safely charge at 5a in theory, correct? How do you generally charge your packs (c rating wise), long and slow or generally fast? There's a big difference between charging 5-6 hours vs 2 imho but if it meant I would quickly degrade my pack charging even at .5C then I would prefer the longer and slower route but many new batts seem to allow charging that is relatively fast without noticeable damage to the cells and was curious others takes on this...

A lot of detailed questions specific to certain cells.
Some one may know some of the answers, but beware of generic answers that may not be relavent to your particular cells, The best source for reliable answers is the manufacturers data sheets, which are all available on line and even on this forum if you search.
Here is a starter..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=57291

 

[footloose]

scoot-e,

I asked the same question recently. Wanted to be sure I was correctly figuring c-rate of charge for paralleled cells so I could charge them slow enough to avoid damage.
kdog was kind enough to confirm my assumption that amperage from charger / number of cells paralleled = charge amperage delivered to each cell. Then based on capacity of each cell you can know the actual charge rate for that cell.
His formula: "(charge amps / parallel number) / capacity of cell will give individual cell charge rate in terms of C" .

https://endless-sphere.com/forums/viewtopic.php?f=14&t=76413

 

[wesnewell]

Most of the info I get on 18650 cells is from here.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php
I don't consider anything below ~25C a high discharge cell. I use 10C-20C rated rc lipo. There are some 18650's rated for 10C or more, but since I can put an rc lipo pack together in minutes, why would I want to take hours welding a bunch in 18650's together, and if there's a problem, spend hours more trying to fix it. No thanks.

 

[scoot-e]

Footloose, thx, so to follow your example then:

1) If I were to start with, say, a Samsung 3ah 18650 cell and string 4 in parallel for a 12ah pack total
And were to use a 4a charger
Then I would divide the 4a/4p = 1 and then divide 1 by capacity of cell which is 3ah so 1/3 = .33
So in this case I am charging the cells at .33C (which is below .5C or well within the general standard for charging tolerance).
(In other words, a 12ah pack divided by a 4a charge is a .33C charge rating - or approx 1a charge per 3ah cell and takes me roughly 2hrs to charge fully in my own experience) - Just to confirm is my math correct?

I am just curious why many retail ebikes and escooters sell chargers that are in the 1a-2a range for the most part with similar size packs (8ah to 12ah) if for any other reason than cost? If .5C or less is plenty slow enough to maintain a long cycle life without undue degradation it seems that charging at .25C or even less isn't necessarily that much more beneficial. I'm plenty happy charging my 10ah or 12ah packs at 3a or 4a in a couple hours, works for me...

scoot-e,

I asked the same question recently. Wanted to be sure I was correctly figuring c-rate of charge for paralleled cells so I could charge them slow enough to avoid damage.
kdog was kind enough to confirm my assumption that amperage from charger / number of cells paralleled = charge amperage delivered to each cell. Then based on capacity of each cell you can know the actual charge rate for that cell.
His formula: "(charge amps / parallel number) / capacity of cell will give individual cell charge rate in terms of C" .

https://endless-sphere.com/forums/viewtopic.php?f=14&t=76413

 

[scoot-e]

Interesting wesnewell
What amount of amp discharge are you using either continuous or peak and for how long peak? Like I mentioned, for hill climbing I find 30a a good amount minimum to climb most hills for a few minutes at a time. Curious your experience if you've measured with a watt meter ever or if you know what you are pulling off hand?

But in practical experience I have used, for example, Hobbyking Turnigy 20c rated 4s 5ah cells (3 in series for 12s and then 2 in parallel to get 12s 10ah pack total) and even though they claim to be rated at "20C" which would mean each 4s 5ah cell can be discharged at 20 x 5 = 100a it is nowhere near that before they "pop"/die...I've killed many single packs (luckily they simply die rather than puff and explode) out of the entire pack when running at only 30a (3C) discharge which is well under their 100a (20c) rating that you are quoting and I'm not running them at a full 30a for more than a few seconds burst mainly so that's only 3C on a 10ah pack...not anywhere near 20C, although my 30C or higher generally can withstand the higher discharge but are clearly exaggerating their true C ratings either way...Not sure I've read that most of the 18650 cells are actually ok with running much higher than 2C for continuous riding (not a few seconds of burst but general hill riding over several minutes daily) but that's why I'm asking others as well to comment here vs just reading off the charts I've seen posted...

The pouch lipos are also easy to puncture, puff/swell or catch on fire compared to the 18650's which come in a cylindrical metal harder metal casing and lastly they don't have a small gas relief hole like the 18650's have but in general if you are familiar and careful with handling the pouch batts they are a bit cheaper and easier to assemble or disassemble as it were, I agree...

Most of the info I get on 18650 cells is from here.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php
I don't consider anything below ~25C a high discharge cell. I use 10C-20C rated rc lipo. There are some 18650's rated for 10C or more, but since I can put an rc lipo pack together in minutes, why would I want to take hours welding a bunch in 18650's together, and if there's a problem, spend hours more trying to fix it. No thanks.

 

[Hillhater]

.?? I dont know what you are doing to "pop" those lipos running 30A, but i have several 5s, 5ahr, 20c hk lipos ( configured 10 s , 2p = 38v, 10ahr ). running 30amps , often for mins at a time, and those packs are now over 5yrs old.....with no pops !

 

[wesnewell]

The pack I've been running now for almost 4 years is a 24s2p pack made from Turnigy 20C 5ah 4s hardcase packs. My controller is limited to 40A, so I've probably pulled no more than 35A burst. 10A will get me ~1000W on a full charge. The pack is rated for 200A continuous and 300A burst. I've got ~15K miles on the pack now.

 

[scoot-e]

Forgive my questioning, but how did you know they are running at 30a peaks? Did you test with a standalone amp meter or some other reliable source? Just curious...
Good question, but I did it several times and it could be a bad bms (from BatterySupports in China btw) and it wasn't all the packs just one or two here or there and only a cell or two out of the single packs too and after a month of rides or so but I may have been pushing 35a but nothing higher and they would just die on me and have to be replaced, but when not pushing them too hard they have lasted many many months of great long rides without problem so I really don't know what is causing it...I assumed the C rating was incorrect and maybe I should bump up to 30C or higher or try a new bms...was charging with a 4a charger as well (again, running a 10ah pack) which is another reason I asked the original questions above to try and trouble shoot...

.?? I dont know what you are doing to "pop" those lipos running 30A, but i have several 5s, 5ahr, 20c hk lipos ( configured 10 s , 2p = 38v, 10ahr ). running 30amps , often for mins at a time, and those packs are now over 5yrs old.....with no pops !

 

[scoot-e]

I certainly don't trust the "200a or 300a" quote from HobbyKing (I used the same 20c 5ah 4s hardcase packs) but it sounds like they've held up for you for a long time so I don't know why I've had so many problems. And you aren't even using any bms with the packs? And if not, how are you monitoring voltage out of curiosity?

The pack I've been running now for almost 4 years is a 24s2p pack made from Turnigy 20C 5ah 4s hardcase packs. My controller is limited to 40A, so I've probably pulled no more than 35A burst. 10A will get me ~1000W on a full charge. The pack is rated for 200A continuous and 300A burst. I've got ~15K miles on the pack now.

 

[wesnewell]

While the rating for my 24s2p pack is 200A continuous, I've never pulled more than 40A from it, and that would just be for milliseconds, and probably no more than a minute at 30A. I pull less than 5A most of the time. I only monitor the cells when I charge. I never run the pack below 10% soc, and most times not below 20% soc. I've set my LVC to ~88V, roughly 3.66V per cell. While riding I only use this voltmeter to monitor pack voltage.
http://www.ebay.com/itm/RED-DC-Voltmeter-15-120V-Waterproof-Dust-proof-Shockproof-Panel-Meter-DC-24V-36V-/321405804645
For the last few months I've been bulk charging with a 5A 100.8V charger. I've had a couple of 24s bms's for ~ 6 months but have been too lazy to install them. Before bulk charging I split the pack and charged as 12s 20ah with an rc charger.
I've had a couple of packs bulge over time. The worst one actually broke the plastic case. I broke it down, poked a hole in each pouch to release the gas and then resealed it. I use it for my light and to start my portable generator. It was never used on the bike.

 

[999zip999]

I run heavy 24s lifepo4 A123 20 ah pouches. I charge as two 12s @ 16amps each pack. I just hooked up regen and get back 8-10 percent. It has the most benefit as braking. Heavy rig with a muxus 3,000. Being very robust cells is the key for long life. Samsung 25r is a lighter cell, but less cycle life and not as flat a discharge curve. Plus I also have mega clamp style torque arms. A must.

 

[footloose]

Footloose, thx, so to follow your example then:

1) If I were to start with, say, a Samsung 3ah 18650 cell and string 4 in parallel for a 12ah pack total
And were to use a 4a charger
Then I would divide the 4a/4p = 1 and then divide 1 by capacity of cell which is 3ah so 1/3 = .33
So in this case I am charging the cells at .33C (which is below .5C or well within the general standard for charging tolerance).
(In other words, a 12ah pack divided by a 4a charge is a .33C charge rating - or approx 1a charge per 3ah cell and takes me roughly 2hrs to charge fully in my own experience) - Just to confirm is my math correct?

I am just curious why many retail ebikes and escooters sell chargers that are in the 1a-2a range for the most part with similar size packs (8ah to 12ah) if for any other reason than cost? If .5C or less is plenty slow enough to maintain a long cycle life without undue degradation it seems that charging at .25C or even less isn't necessarily that much more beneficial. I'm plenty happy charging my 10ah or 12ah packs at 3a or 4a in a couple hours, works for me...

Yes, per my understanding your math is correct.

Re why so many vendors sell relatively low amp chargers with their batteries. I think it is an economic issue. Over-building a system raises cost, without necessarily yielding significant performance benefits. First, the charger itself costs less. Second, it makes sense that they may use a relatively less robust (expensive) BMS in those batteries as compared with high-performance packs... and the BMS might not support a high charge rate. Third, a lot of relatively lower performance battery configurations seem to use barrel connectors for charging, and barrel connectors have their own relatively low amp limits. A couple of well-respected vendors here offer low amp chargers with their bottle / dolphin / shark packs. My assumption is that they've run the numbers and find that the price / performance / reliability equation works out well. I'm convinced enough that I've ordered a dolphin battery and 2A charger to try on my spouse's bike. If it takes a bit longer to charge, no big deal.

I charge my existing battery packs primarily with a 5 amp charger, with a resulting resulting charge rate of .42C for the larger pack, .84C for the smaller pack. .84 may be a bit higher than theoretically ideal but seems ok, the battery doesn't warm up noticeably.

 

[scoot-e]

You're probably right, it's just a cost/numbers situation but in comparing prices it doesn't seem like more than a few bucks for to go from a 2a charger to a 3a or 4a and to charge in 2-3hrs vs 5-6hrs is a huge difference in my opinion if one uses it to commute or wants to recharge, say, while at a Starbucks having a coffee for an hr or 2 and I'd bet most would pay $20-40 more for a fast charge as long as it didn't diminish the cycle life appreciably...


[/quote]

Yes, per my understanding your math is correct.

Re why so many vendors sell relatively low amp chargers with their batteries. I think it is an economic issue. Over-building a system raises cost, without necessarily yielding significant performance benefits. First, the charger itself costs less. Second, it makes sense that they may use a relatively less robust (expensive) BMS in those batteries as compared with high-performance packs... and the BMS might not support a high charge rate. Third, a lot of relatively lower performance battery configurations seem to use barrel connectors for charging, and barrel connectors have their own relatively low amp limits. A couple of well-respected vendors here offer low amp chargers with their bottle / dolphin / shark packs. My assumption is that they've run the numbers and find that the price / performance / reliability equation works out well. I'm convinced enough that I've ordered a dolphin battery and 2A charger to try on my spouse's bike. If it takes a bit longer to charge, no big deal.

I charge my existing battery packs primarily with a 5 amp charger, with a resulting resulting charge rate of .42C for the larger pack, .84C for the smaller pack. .84 may be a bit higher than theoretically ideal but seems ok, the battery doesn't warm up noticeably.[/quote]

 

[scoot-e]

Sure, but what's the cost of A123 cells (if one can even find them easily) and the weight of lifepo4 vs lipo/li-ion packs and my scoot doesn't have the room for 20ah pouches regardless but I get your point...

I run heavy 24s lifepo4 A123 20 ah pouches. I charge as two 12s @ 16amps each pack. I just hooked up regen and get back 8-10 percent. It has the most benefit as braking. Heavy rig with a muxus 3,000. Being very robust cells is the key for long life. Samsung 25r is a lighter cell, but less cycle life and not as flat a discharge curve. Plus I also have mega clamp style torque arms. A must.

 

[scoot-e]

I see, gotcha
So you are using your controller's discharge setting and LVC (both programmable I assume?) to limit the levels.

A voltmeter is cheap and handy for sure

Wouldn't "poking a hole to let the gas out" make the thing burst into flames? Not sure one can "poke a hole" in a pouch lipo while it still retains energy...

While the rating for my 24s2p pack is 200A continuous, I've never pulled more than 40A from it, and that would just be for milliseconds, and probably no more than a minute at 30A. I pull less than 5A most of the time. I only monitor the cells when I charge. I never run the pack below 10% soc, and most times not below 20% soc. I've set my LVC to ~88V, roughly 3.66V per cell. While riding I only use this voltmeter to monitor pack voltage.
http://www.ebay.com/itm/RED-DC-Voltmeter-15-120V-Waterproof-Dust-proof-Shockproof-Panel-Meter-DC-24V-36V-/321405804645
For the last few months I've been bulk charging with a 5A 100.8V charger. I've had a couple of 24s bms's for ~ 6 months but have been too lazy to install them. Before bulk charging I split the pack and charged as 12s 20ah with an rc charger.
I've had a couple of packs bulge over time. The worst one actually broke the plastic case. I broke it down, poked a hole in each pouch to release the gas and then resealed it. I use it for my light and to start my portable generator. It was never used on the bike.

 

[wesnewell]

No, my controller isn't programmable. I can change both the max amperage and the LVC via hardware mods. I haven't changed the max amps from 40A, just the LVC from 60V to ~88V. Here is the controllers I use.
https://endless-sphere.com/forums/viewtopic.php?f=2&t=29469
No, poking a hole in the pouch won't do anything unless you short it out with the plates inside. It might burst into flames then, But I was very careful. I used a straight pin, but it would be better to use something non conductive to be safer. IIRC they were at ~3.8V when I did mine. That was quite a while back and they haven't puffed again. And I didn't notice any capacity loss either.