Swapping out SLA Batteries for Lithium Iron Phosphate

[evest]

So I got a new bike it's a 72v Emmo Knight Sport. I came with 6 12v 20 AH SLA batteries and they wanted $ 1300 for the upgrade to Lithium 30 AH. So I wanted to build my own battery pack using the K2 26650 3800mAH cells. I am going to be making a pack that will be 24s 8p so it will be 76.8v 30.4 AH
Each cell specs are:
Brand: K2
Model: LFP26650E
Capacity: 3800mah
Nominal Voltage: 3.2
Fully Charged Voltage: 3.65 (recommended), 4.1 (maximum)
Fully Discharged Voltage: 2.5 (recommended), 2.0 (maximum)
Continuous Discharge (A): 12
Pulse 30 Seconds Discharge (A): 28
Charge Current (A): 3.8
So my question is the Continuous Discharge is 12 amp since I'm running 8 in Parallel that would be a Continuous discharge of 96 amps. The BMS is rated for 100 amps. The controller is what is in Question if I would need to replace it or not?

 

[Gionnirocket]

Those are maximum battery specifications... I can't imagine you drawing that much even drag racing up hill with a 32kph max speed.
That said... What are the specs for the controller?
How does the battery you are building compare to the factory li_ion battery?

 

[evest]

The factory li_ion is made up with 18650 cells that are 3.7v per cell. The Pack that they offer is 72volt with a 30ah. It looks like the factory controller is that of 30ah it's hard to read unless I take a bunch of panels off the bike..

 

[evest]

I was hoping that I could just make the battery and do the swap with no other mods but a new charger for the Lithium Battery as they require a different charger.. If I did have to change the controller I would think that if may only have to be like a 40ah controller as I don't think that the motor would allow for much more then that.. But am I correct with the BMS of 100 amps though?

 

[evest]

It will do 50 all day but that's not what I'm going for. I want a battery that will last longer and go farther between charges.. That is my goal that and take away about 60 pounds or so from removing the SLA batteries..

 

[Gionnirocket]

I was hoping that I could just make the battery and do the swap with no other mods but a new charger for the Lithium Battery as they require a different charger.. If I did have to change the controller I would think that if may only have to be like a 40ah controller as I don't think that the motor would allow for much more then that.. But am I correct with the BMS of 100 amps though?

In theory the battery will never be put under any more load than the controller allows. But sizing it to the batteries potential is not a bad idea and is probably best practice for reliability and safety.
One thing you can do is to monitor your current riding loads and see how close and how often you come to the controllers limits. Fyi.. the controller is rated by amps not amp/hour.
Curious... How much do you think this is as far as savings $
If it were I, I probably would’ve gone with the factory battery. It'll be warranted, come with the correct controller (though probably the same)and BMS.... but most importantly it'll be packaged correctly for vibration and fit in place perfectly.

 

[evest]

yes so i took the panels off to get a better look at the controller and it is in deed a 38 amp if figured it would be 40amps or less. I know the controller goes by amp and not amp hours. My concern was that the battery pack itself is 96 amps max output. What I was thinking is yes the controller would only allow it's max amps so really it would only pull half of the battery max so in the end would also last longer no? Or am I missing something? As far as the price difference yes it is half the price. So the $650 savings is worth it to me. I have made smaller packs before and they work awesome. Thanks for all the help and input.

 

[Gionnirocket]

yes so i took the panels off to get a better look at the controller and it is in deed a 38 amp if figured it would be 40amps or less. I know the controller goes by amp and not amp hours. My concern was that the battery pack itself is 96 amps max output. What I was thinking is yes the controller would only allow it's max amps so really it would only pull half of the battery max so in the end would also last longer no? Or am I missing something? As far as the price difference yes it is half the price. So the $650 savings is worth it to me. I have made smaller packs before and they work awesome. Thanks for all the help and input.

Ok... As long as you know what you are doing on the build.
Last longer than what?
You need to compare ah rating between the SLA, Factory LI ion and your build

So yes the li ion both have higher ah ratings.
The 96 amp is what it can max deliver continuously, not how long it can deliver it, that is the ah

EDIT: so your build can deliver 30 amps for an hour as compared to the sla of 20a in one hour.
So if you typically only pull 10a...it would last 3 hours as opposed to 2 for the sla.
If you pulled a continuous 96a it would only last about 18 minutes..

 

[evest]

the new pack is 30.4 ah as the old is 20ah So yes I will get more out of the new pack that way. The old pack can deliver a max w/h of 1,440 but really can only discharge to 50% with the new one it can deliver a max w/h of 2,188.8 and can discharge to 70% But I was also looking at it as it would be putting a lot less of a strain on the battery.

 

[evest]

I am thinking of it like the new battery pack can be charged 5-10 thousand times as the SLA is only good for about 3-4 hundred charges. And if only used lightly maybe will get even more charges from them. I would think that it would be easier to charge a half drained battery then a fully drained battery. I could be wrong but that my logic of thinking how it will work out. I was just looking for some input from anyone who may have done this already. I know lots of people do this you can get a lot of info on YouTube but I take that with e grain of salt. Some real hacks on there and wouldn't trust them but with that said I know there is lots out there that know what is what. Like I said I have built smaller packs and had no issues.

 

[Gionnirocket]

I am thinking of it like the new battery pack can be charged 5-10 thousand times as the SLA is only good for about 3-4 hundred charges. And if only used lightly maybe will get even more charges from them. I would think that it would be easier to charge a half drained battery then a fully drained battery. I could be wrong but that my logic of thinking how it will work out. I was just looking for some input from anyone who may have done this already. I know lots of people do this you can get a lot of info on YouTube but I take that with e grain of salt. Some real hacks on there and wouldn't trust them but with that said I know there is lots out there that know what is what. Like I said I have built smaller packs and had no issues.

Yes, not draining it fully is good practice.... As is not charging it fully, especially if not used right away. I try to think of it as li ion are happiest at near nominal voltage.
And yes, be cautious following youtube as many times there is no follow up to a video... So how is it 6mo - a year later?
If you really are cost and earth conscious... One thing you can do is to continue to research and use the life out of your sla. It's paid for already and if it provides the range you need... You're good to go.

 

[evest]

Yes I have plans on using the SLA but in a different set-up right now they're set -up as 72 volt @ 20ah new set-up will be 12volts @ 120ah for a different project. So they won't go to waste, yeah part of the problem is the bike will last about 3 trips back and forth to my work then it starts slowing down. As with the new set-up I am hoping to get the week out of them before having to charge them again.