SLA vs. LiFePO4 questions

Answer to 1:

Assuming everything else equal, and you took the battery pack down to the same limit of rideability, or to the controller's low-voltage cutoff setting, then the SLA's will charge faster than the LiFePO4's.

This is because you got less energy out of them than the LiFePO4s, so you need to put less energy back in. The standard AH capacity of lead acid batteries is measured at a low C/20 discharge rate - only 2.5 amps for your 50ah pack. But, this doesn't mean you can get 50 AH out of these batteries if you need an average of 40-50 amps to run a full-size scooter - you will actually only get about 30-35 AH of energy out of them. This is because Lead acid battery chemistry suffers a sluggish chemical-kinetics effect called the "Peukerts" effect. All the lithium chemistries exhibit very little Peukerts effect, so you can get the full rated capacity even at high discharge rates.

From the above discussion, the answer to question number 2 should be apparent. You can get more range out of the lithium pack with the same AH rating, because the you can get the full rated energy out of them at high discharge rates. Also, since the voltage declines only a little as a lithium pack is discharges, you are also getting more actual watt-hours (volts*amps*time) of energy for the same "amp-hour" rating.

Of course the weight savings provide more help.

You could say that part of the problem is that the standard ratings for lead acid batteries are "dishonest" and exaggerated for the discharge rates we are interested in. But most good lead acid battery manufacturers do provide data sheets with the energy and run-time you can actually expect to get out of them for a given power (amperage) output.

:?

Gene , I am not the expert but I think If you are using the same charger for the sla battery's on the Lifepo4 battery's It would take the same amount of time because the charger is limiting the RATE OF CHARGE . lifepo4 battery's can be dump charged at a faster rate . 2. lifepo4 bat's give a deeper discharge , and hold their Voltage Longer SO , They will appiere to have more watt hr. the sla bat's will die fast if they are too deeply discharged !!! Someone please correct me if i am wrong ??? Later

OK. It seems as though the discharge and AH rating is very well explained here and I want to thank Gene for posting the topic, and others for providing the great information.

I am struggling with another aspect of LiFePo4's. I have read that some who purchased the relatively low voltage, low AH rated LiFePo4 packs, bought packs with the same voltage and AH ratings as the SLA packs. Some reported cut out when the throttle was applied quickly, and it seemed the battery management system would not allow the required amps "out" of the LiFePo4 batteries to accelerate properly.

So the question is, do you need to purchase a higher rated AH LiFePo4 pack to withdraw the same current that was pulled from the SLA pack? Specifically, in a 72 volt pack, If I have a 300 Amp rated controller (600 Amp peak), will I need 100 AH? 200 AH? A 55 AH SLA battery pack will allow as many amps as the battery is capable of.

According to the info on Cloud EV's page, it is advised that a 12V LiFePo4 pack should not be discharged below 10.8 Volts. is this 10% DOD? Not sure how that compares to the 80% DOD of SLA's.

Thanks in advance.

It depends on what the C-rating of the pack is.
If youre using realy cheap cells such as those from ping ping you will need about 150AH for 300A as theyre only rated to about 2C.
If your using a mid-range pack such as thunderky, you will need 60AH for 300A.
If your using expensive batteries such as K2 or a123, you will only need 20AH to get 300A.

It also depends on how much voltage sag is acceptable. If your willing for each cell to sag to 1.7vpc under the 300A peak load (even if fully charged) you can get away with 40AH cells. The thunderskys sag about 0.2v per C discharge rate.

as for measuring state of charge, use an AH counter, dont base it upon voltage, whether under load or not. The discharge curve of LiFePO4 is too flat, there are too many variables that wil give you an erroneous reading.

hope this was helpful,
regards
Matt

So it sounds like the Cloud EV 100 AH pack would be acceptable?

The bottom line is, when I want to accelerate like leaving the line at the drag strip, will it leave at the 600 amps? (assuming the controller will "ask" for the peak amps.)

The Cloud EV site states the following:"The 100 Amp/Hr Batteries have a C3 Continuous Rating of 300 Amps and a Peak of up to C11 or 1100 amps". "Set your cut off at 10.8VDC per 12VDC pack. That is 2.7VDC per cell, a comfortable margin.

It sounds like these are pretty stout cells.

Other questions:
Cloud EV keeps refering to a 12 Volt pack. 3.2V(4)= 12.8 volts. Is this similar to a 12V SLA being charged to 13.5 Volts?
Can you charge a 12.8 volt pack with a 12 volt battery charger?
For 72 volts, do I go with 22 cells for 70.4 volts? or 24 cells (6-12.8 volt packs) for 76.8 volts?
Can we still use (6) B&D 2/4/6 amp chargers like you would with (6) 12V SLA's?

I appologize for all the questions.
Thanks again.

One thing I would like to throw into the mix is, how long will the batteries last?

If you have a low Ah pack and pull lots of current out of it over a short time, will it last as long as a higher Ah pack that you pull less current (per cell) out of it over the same amount of time? Sure the lower Ah pack may be made better and allow you to pull more current out of it, but do you want too?

I think long term testing is the only way to tell if you are getting a better deal or not. $$$$ for a lower Ah pack or $$ for a higher Ah pack.

Deron.

Some reported cut out when the throttle was applied quickly, and it seemed the battery management system would not allow the required amps "out" of the LiFePo4 batteries to accelerate properly.

The BMS's with current limiters are not designed with EV's in mind. They are not needed because the controller already limits the amps, and instead seem to be mostly for over zealous warranty protection.

An improved BMS is being developed by Bob Mckee on the endless sphere forum. But for now it is limited to an 8A maximum charging rate. Check it out here:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=3345

Some reported cut out when the throttle was applied quickly, and it seemed the battery management system would not allow the required amps "out" of the LiFePo4 batteries to accelerate properly.

The BMS's with current limiters are not designed with EV's in mind. They are not needed because the controller already limits the amps, and instead seem to be mostly for over zealous warranty protection.

This is not strictly true. Just like SLA batteries LiFe batteries come in many different versions. "power" versions offer high discharge rates (say up to 10C) and are good for EVs. "energy" versions offer only low discharge rates (say 2C) and are more suited to storage scenarios. If your batteries are large enough and you're controller amperage limiter is low enough than even "energy" cells would work (say you had 40Ah cells and a 50A max draw then you'd be fine because that's a 1.2C max. draw).

Bottom line, at the moment one still needs to look at the spec sheet for the cells you're considering.

deronmoped,
The batteries will last longer if discharged slower, it is the same for all brands.
The K2 cells are rated to 2000 cycles at 3-5C discharge, or 800 cycles at 15-20C discharge.

The lower C rated packs are usually cheaper from an energy stored point of view. However, from a power output peak point of view it a little more complicated. case in point:
K2 cells are slightly more expensive per kw, but more than double per kwh compared to thundersky.
Ping Ping batteries are about 3/4 the price of thundersky (actually about half for scooter sized packs) per kwh, but are the same price per kw.

so it very much depends on what your weight, power and energy requirements are.

also it depends on how many cycles you expect to put on the pack in 5 years, since this is the suggested shelf life of lithiums.

Ive found with designing/modifying an EV, you spend an extraordinary amount of time with your calculator out.

Racermike39,
For best performance, set the current limit to where the max power point will be (its not likely to be 11C). To work this out you need to know the internal resistance of the cells.
The max power point will be where the voltage sags to just above half its floating voltage.
Just make sure the current limit is below the max suggested C rating.

For instance,
the max power point on the 1.125AH k2 cell is 30C, however since the manufacturer suggests now more than 20C peak, you would set the controller to the current that corresponds to 20C.
The max power point on thunderky cells is 8C, which since it is below the impulse rating of 10C, you would set it to 8C.

You can charge a lithium pack with a SLA charger (i do) as long as it has a BMS to interupt charging if something bad happens. One thing you must note is the CC/CV change over voltage. This voltage should be 3.9vpc or there abouts, so for a 4 cells block that will be 15.6v, which is much higher than most SLA chargers. The way i got around this was to charge 15cell blocks with a 48v SLA charger. the change over voltage is 59v, which is 3.93vpc.

for 72v, use 24 cells, although finding a suitable charger could be a bit tricky.
Theres an ebay seller called e-crazyman who has been making custom 2 stage chargers for people here in Australia (hes based in taiwan).
Ive also be buying chargers of kipoint, but im not sure if they do anything custom, i haven't needed a custom one yet, as the ones they do make happened to fit my design.

As for BMS, if cloud EV doesnt make or sell one, cant remember if they do or not, try www.ev-power.com.au . they make the BMS i use.

Matt