SLA %80 Discharge and the Peukert curve/effect

This forum has beat it in to my head not to discharge SLAs more than %80 (and to charge them right away!)

If I were to wear a 12v12ah SLA down to %20 it would be at 2.6v

But according to the peukert effect this 12v12ah battery is only capable of 8ah in an hour.

So what are the applications of these two effects?

Is it 8ah-2.6ah=5.4ah total battery capacity?

or

Is it just flat 8ah?

I'm leaning towards the batter only being 5.4AH. I feel this would explain why everyone raves about how great nickle and lithium is even though they are rated the same AH as the SLA they're compared too.

I don't like the performance of my bike if the SLAs drop down to much. I consider it pretty much spent because it won't move me more than 12mph after 5ah have gone out of the battery. Thats with the same 12v 12ah batteries your talking about.

Not to sound opinionated, but lithium and nickel just plain are better. At least NiCad is ...the jury is sort of out on Nimh's... Lithium suffers far less from Peukert's effect, and tends to deliver near 10ah if it is rated for that amount...regardless of the discharge rate (for the most part). With Nicads you don't get as much capacity for the size, and the discharge rates are somewhat limited.

Anyways I hope that gives you a little more info...

I got 11 brand "new" 12V 12Ah battery ( usually supplied for electric bike)

I fully charged them and made a desulfatation and equalization charge to 14.5v with all 11 battery connected in parallel with an... "intelligent charger"

They have sit for 12 hour and was somewere to 13.2V and fully charged.

I tested every of them at 0.6C ( 7.5A) for 100W power discharge using my CBA II.

I got around 7-8Ah from each and at that C rate that exactly correspond to 80-90% of capacity. normally if they wold hbe in 100% excellent condition, they would give 8-9Ah

The Peukert curve/effect is incredible on SLA. On A123 lithium cell it is like no effect no the capacity eventhou you discharge at low or hig current...

with SLA it's always a percentage of the nominal C/20 capacity you will get. normally at C/20, you would get 12Ah out of a 12Ah battery.. but as you increase the current, you loose capacity

Ex: at 50A, a 12ah battery would only give like 4Ah.. and at 100A it<s like 3Ah

Doc

auraslip said:

This forum has beat it in to my head not to discharge SLAs more than %80

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...and only 50% is better where rated number of cycles for a batt might be 300 based on 80% DOD or 600 cycles if you only discharge to 50%...

(and to charge them right away!)

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My understanding is "right away" means within a few hours... not necessarily immediately... I have (almost) always done immediately w/lead anyway.
Li is more forgiving in this regard... Doesn't care if it is left 1/2 empty, and is usually shipped and stored in this state.

If I were to wear a 12v12ah SLA down to %20 it would be at 2.6v

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Not sure if you typo'ed here. 12V is a nominal value for lead that actually runs from about 14.5V fully charged to 10.5V fully empty. The better way to measure remaining capacity is metering amps consumed to calc remaining, but cheap "gas guages" use voltage to estimate remaining amps...

But according to the peukert effect this 12v12ah battery is only capable of 8ah in an hour.

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...that kinda thing. Truth is, the SLA's folks are using aren't designed for EVs really. The Ah ratings are based on expending batts from full to empty over TWENTY hours. Useful to compare diff. battery sizes, but in absolute terms pretty far removed from ebike reality discharging twenty times this speed...
Watt the Doc said!

So what are the applications of these two effects?
Is it 8ah-2.6ah=5.4ah total battery capacity?
or
Is it just flat 8ah?
I'm leaning towards the batter only being 5.4AH. I feel this would explain why everyone raves about how great nickle and lithium is even though they are rated the same AH as the SLA they're compared too.

Click to expand...

Beyond Peukert... Lead voltage drops more as charge is consumed... Usually translates as lower motor rpms/speed. Lithium chem is "stiffer", delivering almost the same volts `til almost completely empty. And cold temps... w/Lithium a little less affected by cold temps... And a higher energy density for Li translates as a lighter weight pack/vehicle for the same Whs... better acceleration and braking all else equal, or easier at least to lug around the bike or pack when not riding...
Lock

Sorry If i wasn't clear earlier. I was a little tipsy.

A 12v12ah SLA rated a c/20 is good for about 8ah discharged over the course of an hour.

Is this 8ah in an hour a total discharge of the battery? Does it bring the battery down to less than %80 capacity?

If so I need to reevaluate the worth of SLA batteries.

Consider:

Using the battery down to 2.4ah (%80)

8ah @ c/1 - 2.4ah = 5.6 effective amp hours

Using the batter down to 6ah (%50)

8ah @ c/1 - 6ah = 2 effective amp hours.


Pretty shocking.


dequinox -

With Nicads you don't get as much capacity for the size, and the discharge rates are somewhat limited.

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I thought Nicds were pretty good, a little heavy, but can sustain a high C rate. I hope so, at least because that's what I'm planning on.


Doctorbass:

I tested every of them at 0.6C ( 7.5A) for 100W power discharge using my CBA II.

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I'm confused now. People talk about batteries with a high c rate (c/2, c/3, c/5), but the term c/20 means discharge over 20 hours. When you say 0.6C do you mean a little more than half an hour or do you mean two hours? I'm guessing you mean two hours because that's what most of the manufacturers specs on SLA's have given me.

When i had SLA i only expected 65% of their rated capacity. which was around 7.5AH before the remaining power is very unusable. THe lithium technologies deliver atleast 95% at 5-10C and some cases give 110% of their capacity with very low C rates around 0.5-1C.

On a side note. SLA is a beast that can be tamed. I just had an auxilary flooded car battery in my Scion XB. I took it to the grave and back many times over the past 6 years. Started failing this past year. I have a rather advanced sound system and I would go the the local drive in movie and watch 2-3 movies and not skip an action filled beat. Whenever I got done I always came home and put it on the charger over night.

Since my system is so advanced and power hungry when I switched positions at my job I didnt drive enough miles and along with playing music my car never charged my batteries fully. They constantly sat between 11.8 12,3. This unfortunately is the lower 50% of my battery. It kinda developed a memory. This ultimately killed my battery. They still are able to crank the car fine but i can barely get 20-30 minutes of run time at the movies vs the 5-6 hours I was getting before.
I just replaced it with a AGM Shuriken 100AH battery able to do 2200A peak discharge. Has ultra low internal resisitance. I spent almost 200$ I will take care of her alot better than her predecessor.

On a typical bike, you'd be very lucky to get 20 minuites of full throttle riding out of 3 12 ah slas. So figure how much a battery designed for a 20 hour discharge takes a beating when you discharge it in 20 minuites. Going to half throttle, the peukerts effect is lessened a lot. At that point, say using around 250 watts, the sla's can handle it a lot better. Start to see why the old historic ebikes were so often low wattage before there was even nicads?

The best results I ever had with sla's was on an EV global bike. This bike ran on two 12ah sla's. When I added two more, for 24 ah of 24v, the lead really started to work well. Much much less voltage sag since the peukerts effect was cut in half. I was able to ride more than double the distance with double the battery.

Unfortunately this does not translate well to 36v or 48v bikes. It gets really hard to carry 6 or 8 sla's on a bike, and that many sla's costs too much. Better to buy lithium or something than 6 sla's.

As I said before, if sla's are all you can afford now that's just the reality of the thing. They still make a good starter on a budget, and may allow you to make a few mistakes at the beginning without the cost being so high. Of course they won't take you very far, but maybe you can get enough out of sla's for awhile if you have less than 10 miles to travel.

So a 250w kit would get better range than a 500w kit? That is something to chew on for sure. I wonder what difference in speed it makes...

Hauling ass on that EV gobal was reaching 12 mph, instead of the usual 11. When I put 24 ah of 24v on it, It got all the way up to 13! Who hoo!

Another pretty thrifty motor I've had was a 350 watt fusin gearmotor, very similar to a bafang. With a 15 amp controller, on 36v it could eventually reach 20 mph, but 18 mph was more normal top speed for it. But it was real easy on the battery. Riding a 700 watt motor at the same speed, 18 mph, gave me just about the same range though. So really it makes more sense to have a 500 watt motor and ride half throttle than to ride 250 watts. With the 500 watt you can climb a hill a lot better when you need to. On that 350 watt motor, hills were a slog, and a big enough headwind could nearly stop you.

Yes, it doesn't do so well against headwinds. Had a lot of those here the last few days, and even on high setting it struggles (sounds like a can opener! :lol.

But on it's low setting using an abused 36V 8-9Ah NiMH pack it has gone as far as 11 miles on a trip so far and still not run down past 3 lights on the gauge at rest--under load it is almost always "dead battery" light on at full throttle as I speed up, except in the first few minutes of a ride, and going up to 2 to 4 lights when at speed, which is usually around 14-15MPH. Toward the end of the 11 mile trip it was around 13MPH. Generally this is with me in my next-to-highest gear, 48/18, I think it is, maybe 48/20 (the 48/16 is locked out because of a mounting screw head in the way for my trailer hitch; I need to add a spacing washer on the axle).

But 350W gets me where I need to go, albeit slowly. Even pulling the trailer it only loses a couple MPH but the range is certainly much less. I don't yet know it's max range.