Max discharge rate/time

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imorton   1 kW

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Max discharge rate/time

Post by imorton » Aug 27 2015 6:46am

Good day, I wonder if anyone can tell me the max CONTINUOUS discharge rate for the 48v 15Ah LifePo4 battery from eBay (sun thing-28)
Here is the specs/details about the battery http://www.ebay.com/itm/48v-15ah-LiFePO ... 1551924820

I hear different theories, some people says to cut the discharge rate in half, others say not more than 1C, etc....

The reason I ask is that I have a 24amp controller and my scooter may occasionally draw 20-24 amps for 2-3 seconds when accelerating, HOWEVER when I climb a longer/steeper hills, the 24 amp draw can last 60-90-120 seconds. Am I causing any harm by drawing 24 amps for that length of time?

The battery is a 15ah LifePo4 pouch battery, so I try to keep the discharge rate to .5C or 1C while cruising, but when climbing hills should I reduce the current draw to 1C (15a) and climb slower, or go full speed but draw 24 amps for 1-2 minutes...?

Thanks for any help, IAN.
Last edited by imorton on Aug 27 2015 8:05am, edited 1 time in total.
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dnmun   100 GW

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Re: Max discharge rate/time

Post by dnmun » Aug 27 2015 6:54am

if the battery is still fully charged it (2C-4C) is not a problem but if the battery is almost discharged try to reduce the C rate of the current draw. especially if it has labored for a long period previous and was already getting hot. high discharge at low state of charge creates a lot more heat than at the top of the charge.

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Re: Max discharge rate/time

Post by KarlJ » Aug 27 2015 7:03am

wow, pretty low spec batt.
24A for a 15H pack should be no sweat.

a similar capacity pack from EM3EV would deliver 40A no problem - 29E packs warm up in both frame packs (11.6AH 48V) and triangle packs ~16AH when pulling 40A
but at 24A they barely rise in temp at all. 20R/25R packs dont even break a sweat at 100A for a 10AH pack! - dual 2000W motors no worries. barely detectable temp rise!!!

Moral of the story.....if cold good, if it gets warm - thats probably OK - if it gets HOT its probably not

Im in Perth WA and summertime - YES i chuck them in the fridge!

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Re: Max discharge rate/time

Post by imorton » Aug 27 2015 8:09am

dnmun wrote:if the battery is still fully charged it (2C-4C) is not a problem but if the battery is almost discharged try to reduce the C rate of the current draw. especially if it has labored for a long period previous and was already getting hot. high discharge at low state of charge creates a lot more heat than at the top of the charge.
So if I am between 100% and 50% SOC, and I climb a hill that requires 24 amps at full speed, If I do this for a few minutes, is it harming/damaging the battery (I presume it is reducing it's life, but to what extent)

Would I do much better for the battery by reducing the hill climbing speed, and draw 15amps versus the max 24amps?

IAN.
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Re: Max discharge rate/time

Post by imorton » Aug 27 2015 8:14am

KarlJ wrote:wow, pretty low spec batt.
24A for a 15H pack should be no sweat.

a similar capacity pack from EM3EV would deliver 40A no problem - 29E packs warm up in both frame packs (11.6AH 48V) and triangle packs ~16AH when pulling 40A
but at 24A they barely rise in temp at all. 20R/25R packs dont even break a sweat at 100A for a 10AH pack! - dual 2000W motors no worries. barely detectable temp rise!!!

Moral of the story.....if cold good, if it gets warm - thats probably OK - if it gets HOT its probably not

Im in Perth WA and summertime - YES i chuck them in the fridge!
Yes, it is not a HIGH SPEC battery, but at the same time, it was $335 usd (delivered) with a 5amp charger. I am sure the EM3EV batteries would cost me between $800 to a $1000 usd when you factor in the charger and shipping.

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Re: Max discharge rate/time

Post by dnmun » Aug 27 2015 8:39am

you have to avoid being drawn into the my battery is better than yours put downs. nothing wrong with your sun-thing guy pack. it will last plenty long

if you try discharging a lithium pack while it is super cold then you end up precipitating out the lithium away from the carbon electrode and that lithium that is disconnected from the electrode is then lost to the cell and your capacity is forever reduced by that amount.

pushing high current for long periods will cause the pack to heat up and these pouch packs do not really have a way to remove heat. but pulling 2C is not much problem until the SOC is reduced so much that the ions are not available to carry the current and that is where the heating increases rapidly.

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Re: Max discharge rate/time

Post by imorton » Aug 27 2015 9:02am

dnmun wrote:you have to avoid being drawn into the my battery is better than yours put downs. nothing wrong with your sun-thing guy pack. it will last plenty long

if you try discharging a lithium pack while it is super cold then you end up precipitating out the lithium away from the carbon electrode and that lithium that is disconnected from the electrode is then lost to the cell and your capacity is forever reduced by that amount.

pushing high current for long periods will cause the pack to heat up and these pouch packs do not really have a way to remove heat. but pulling 2C is not much problem until the SOC is reduced so much that the ions are not available to carry the current and that is where the heating increases rapidly.
Dnmun, thanks for the replies, and Yes, I am not disappointed with my battery purchase. Thanks to all you guys on ES forums, I now understand just a little bit about lithium Batteries, and was able to make a semi-educated decision as to what to buy to replace my SLA's.

The price of the Sun-Thing28 battery was super cheap, LifePo4 & 15ah was the right size and lithium technology for my application, and I would gladly buy another one. If I had a different application and budget, I might get another battery from another source, but for a 24amp controller, the battery fits right in the compartment. I now tend to spend more time doing errands with the scooter, instead of playing with the battery.

Now about the discharge rate, so if I ride up a hill and use WOT for 60 secs (24amps), I am not beating the shit out of a 48v 15Ah LifePo4 "pouch" battery. I can't really check the battery temp as it is under the seat, and protected with foam and plastic bag. I did however check the hub motor on the last ride up that hill, and the motor was still ambient temperature to the touch.

I am a big guy, so these scooters (avatar) have a 2 speed switch, so I can choose "economy mode" and it climbs that hill at 14 km/h and drawing 17amps, OR I can use "high-speed" mode and it draws 24amps while climbing at 22 km/h. I don't know which mode is better for battery health/longevity, economy or high-speed mode?

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Re: Max discharge rate/time

Post by wesnewell » Aug 27 2015 10:57am

You need to realize that cycle life for most lifepo4 batteries is based on a .5C draw, so anything above that cuts into cycle life.17A is ~1.1C, and 24A is ~1.6C, so the lower would be best for longevity. Barring any defects, you should get close to 1000 cycles out of the pack at a 1.1C draw, and I'd think at least 800 at 1.6C. Based on 5 cycles per week, that's at least 3 years at 1.6C, and 4 years at 1.1C. As cheap as it is I wouldn't worry about it too much.
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Re: Max discharge rate/time

Post by imorton » Aug 27 2015 11:49am

Wesnewell, thanks for the reply.

What do you think is the calendar life of a 48v 15Ah LifePo4 battery pack? I don't think I will wear it out by exceeding the cycle life, but what is the average amount of years before a "calendar life" LifePo4 death...?

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Re: Max discharge rate/time

Post by wesnewell » Aug 27 2015 12:55pm

Most lifepo4 is rated at 10 years, but I wouldn't count on more than 6. But a lot will depend on how it's used, stored, and maintained. I use rc lipo only and I'm on my 4th year with it and to be honest I leave it in the garage and dont' care if it's 140F during the summer or 15F during the winter. It's cheap and was I was only planning on 2 years with it anyway. I've got over 12K miles on it now without much loss in capacity.
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Re: Max discharge rate/time

Post by imorton » Aug 27 2015 9:43pm

Wesnewell,

If I get 5 years out of my LifePo4 battery, I will be really pleased... :)

I guess I was worried that running a 15ah LifePo4 pouch battery at 1.6C while running up a long hill for 60-120 seconds might harm the battery.

I can see that my battery appears healthy as it's resting voltage is @ 53.33v, and while drawing .5C it drops to @ 51v. If I am drawing 1C it drops to 50v, and I notice that when I climb that long steep hill it drops to @48-49v while drawing 1.6C

I repeatedly drained the battery recently to 85% DoD a few times, and the resting voltage seems normal. I wished I could have drained it right down, but I don't want to push the scooter home... :(

I have seen that the charge cycle seems normal, and the charger tends to balance the battery very quickly (cycling between red/green).

IAN.
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Re: Max discharge rate/time

Post by vicens » Aug 28 2015 1:36am

Can you tell me battery lead-acid before? How many cycles you get to the battery?

What is the autonomy with the old battery lead-acid and the lithium?

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Re: Max discharge rate/time

Post by imorton » Aug 28 2015 1:59am

The scooter had four 12v x 12ah lead batteries. They were giving me @ 6-7ah of useable capacity, so the 15ah LifePo4 gives me @ double the capacity.

I actually sold my lead batteries to another guy who owned a scooter, and used the $100 towards the new battery pack. Needless to say, I am very pleased with the LifePo4 battery, it's simple to use, doubles my range, 1/2 the weight, and the 5amp charger makes for quick "on the road" recharges.

Plus I will get @ 750-1000 cycles OR 5 years use out of it, compared to @ 200-300 cycles with the SLA's

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Re: Max discharge rate/time

Post by dogman dan » Aug 28 2015 6:37am

My experience with similar cells in pingbatteries has been that 1.5c is a comfortable rate for them. 4 years is more typical, but cycles is so dependent on how deep you discharge, so that number is almost meaningless.

So your 15 ah pack can handle just about 22 amps just fine, with not too much sag, not too much heating of the battery.

However, you will find that your typical wot cruise speed will very likely take less amps than that. 15-18 amps more likely on the flat, without headwind.

It can do 3c, but that will make it sag, and likely some cells will under deliver capacity, and the battery will get out of balance real bad. So if you size your controller so the max spike is 2c, that will work better. 30 amps, but really a 25 amps max would be best.

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Re: Max discharge rate/time

Post by imorton » Aug 28 2015 7:02am

Dogman Dan,

I agree with what you say and I find that the scooter draws @ 400 watts (8amps) at 20km/h, and it draws @ 800watts (16amps) at WOT throttle of 31km/h. What is amazing that a 50% increase in speed requires double (or 100% increase) the power.

I have been babying the discharge rate, so I guess what I was curious about, how long can it run at 24amps? 1min, 2min, 5min, 15min, 30min, 60min, continuously...?

I hope to treat the battery so that it lasts 4-5 years, and at least 500 cycles.

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Re: Max discharge rate/time

Post by dogman dan » Aug 28 2015 7:07am

I've run at 15 amps for as long as an hour many many times, with the exact same size battery. That one lasted me 2.5 years. But it swelled up on one end eventually. BTW, that pack flew over my head while I was riding, and landed on one end after flying about 20 feet. I was riding off road, and the battery box broke on a huge bump.

So IMO, that was what caused the early death of that one.

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Re: Max discharge rate/time

Post by imorton » Aug 28 2015 7:20am

Dogman Dan,

Lol, I am not a electrical engineer, but having a battery fly 20ft and breaking the battery box could very likely cause some swelling... :)

IAN.

PS: so having the battery discharge at a constant 1C, and the occasional 1.6C for those hills will not likely cause any premature death. And even if it did, I would order another one right away to replace it.
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Re: Max discharge rate/time

Post by dogman dan » Aug 29 2015 6:34am

No, you are good to pull 1.5 continuous, till the pack is empty.

FWIW, my pack ran normally for 2 years after that broken battery box thing. It was in an additional plastic box, so it had some protection when it flew and landed. Dirt, so not quite as hard a landing.

But there is a line you can cross with pings. They really don't like more than 2c. A few 2c spikes ok, but not 2c for long durations, or 2c spikes over and over and over like dirt riding does. I had heard the ping pouches were rated 3c, but cut the c rate in half gets you 1.5c, which I found a ping can take for the entire continuous discharge.

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