Converting 36v Kuberg kids dirt bike to lithium batteries

[JoelR]

Hi folks,

It's been a few months since I finished my kart conversion project and now I'm onto something else--kids electric dirt bikes.

I found a pair of Kuberg bikes on Craigslist recently and my kids have been having a blast on them. However, the batteries (3 x 12v 15Ah SLA) are tired and while it would be simple to just buy more SLAs, I want to explore an upgrade and would love to get your advice.

In 2016, a guy upgraded a similarly-configured Razor MX500 (link) using a 36v 20Ah 18650-based pack from Luna Cycles. You cannot buy this anymore from Luna, however. There are many options on eBay to buy packs from China and I'm sifting through the options at this time. The best I've found so far is this one: https://www.ebay.com/itm/Panasonic-36V-13-6ah-rechargeable-Li-ion-battery-For-750W-electric-bicycles/273962097611

At $138 shipped from China, I think this will essentially drop right in and is almost as cheap as 3 new SLAs. The Panasonic cells (NCR18650B) are very well known, the pack supports up to 5A charging and supplies enough current to power the 750w motor on the Kuberg. While the 13.6Ah rating is technically lower than the stock 15Ah rating, my understanding is that the actual usable capacity and power of the Lithium Ion batteries is higher in real world use. Oh, and they weigh under 5lbs each!! If the capacity isn't enough, another pack could be added in parallel.

One of my constraints is to keep the existing Kuberg controller (it's 36v, up to 3000 watts). It has built-in controls to limit power and speed which are great for teaching young riders.

What advice and/or thoughts do you have for me?

 

[Functional Artist]

Kool bike

Lithium Batteries have a lot advantages over SLA's (when everything is set up/works properly)
...available power
...less weight
...& faster re-charge time

Keep in mind that there are a lot of variables
...quality of cells & how well are they matched?
...quality of assembly, design, materials & production methods
...for "safe" use they usually require a BMS (battery management system)
(I don't see a BMS included with that battery pack)


IMO SLA's are a lot simpler & almost just as powerful

I have them on many karts & easily get 45 min run time


A nice set of 12V 12AH SLA's from Mighty Max are only (~$70.00)
https://www.ebay.com/itm/Mighty-Max...a=0&pg=2047675&_trksid=p2047675.c100005.m1851

or (same physical size but costs more & has a bit larger capacity)

12V 15AH SLA's (~$100.00)
https://www.ebay.com/itm/Mighty-Max...014770?hash=item2ca42da0f2:g:of8AAOSwE21dCpdm

* You'd be surprised at what a difference a fresh set of batteries can make.

 

[Quinc]

I have had great luck running R/C lipo batteries. Can be had cheap if you watch for sales. So far we are using them in multiple electric motorcycles, gokarts and powerwheels. But read up on the safety concerns and best practices before using them.

 

[Tpdingo]

Kool bike

Lithium Batteries have a lot advantages over SLA's (when everything is set up/works properly)
...available power
...less weight
...& faster re-charge time

Keep in mind that there are a lot of variables
...quality of cells & how well are they matched?
...quality of assembly, design, materials & production methods
...for "safe" use they usually require a BMS (battery management system)
(I don't see a BMS included with that battery pack)


IMO SLA's are a lot simpler & almost just as powerful

I have them on many karts & easily get 45 min run time


A nice set of 12V 12AH SLA's from Mighty Max are only (~$70.00)
https://www.ebay.com/itm/Mighty-Max...a=0&pg=2047675&_trksid=p2047675.c100005.m1851

or (same physical size but costs more & has a bit larger capacity)

12V 15AH SLA's (~$100.00)
https://www.ebay.com/itm/Mighty-Max...014770?hash=item2ca42da0f2:g:of8AAOSwE21dCpdm

* You'd be surprised at what a difference a fresh set of batteries can make.

A properly set up Li-ion pack with a BMS and good charing circuitry will kick the ever living crap out of SLA any day of the week. No doubt SLA is so much easier to use and your absolutely right about the fresh batteries.

BUT!

No, the weight difference is very significant. A 48V big pack for a standard system on SLA would be like (8 * 8lbs) = 48lbs for 48v 30ah

For the same 48V, you could get some 7.4V nissan leaf cells all packaged with a BMS and internal protection circuitry and good cells in a solid container. (7 * 7lbs) = 49lbs for 48v @ 66AH!!

66 AMP HOURS!! Thats over double the riding time, double the distance, double the fun!

All that, and it charges faster, gives more peak power, doesn't go bad every 4 years, is more shelf stable(good for karts that sit over winter), weights less, takes less room. All that for 2x the price. 2x the price, 2x the fun. Seems like a good deal to me .

Don't get me wrong, SLAs still are great for karts, but I mean don't downplay lithium, because they're absolutely fantastic when done right.

ALSO::

Second note for both FA and the OP, look into LiFePO4, its like Lithium-Ion, but with a bit less capacity, and a lot less affinity to catch on fire or blow up. LiFePO4 is a very bulletproof technology, especially good for high discharges very quickly like in a go kart and are very hard like SLA to overcharge and what not. Also seems to be a bit cheaper.

 

[Functional Artist]

A properly set up Li-ion pack with a BMS and good charing circuitry will kick the ever living crap out of SLA any day of the week. No doubt SLA is so much easier to use and your absolutely right about the fresh batteries.

BUT!

No, the weight difference is very significant. A 48V big pack for a standard system on SLA would be like (8 * 8lbs) = 48lbs for 48v 30ah

For the same 48V, you could get some 7.4V nissan leaf cells all packaged with a BMS and internal protection circuitry and good cells in a solid container. (7 * 7lbs) = 49lbs for 48v @ 66AH!!

66 AMP HOURS!! Thats over double the riding time, double the distance, double the fun!

All that, and it charges faster, gives more peak power, doesn't go bad every 4 years, is more shelf stable(good for karts that sit over winter), weights less, takes less room. All that for 2x the price. 2x the price, 2x the fun. Seems like a good deal to me .

Don't get me wrong, SLAs still are great for karts, but I mean don't downplay lithium, because they're absolutely fantastic when done right.

ALSO::

Second note for both FA and the OP, look into LiFePO4, its like Lithium-Ion, but with a bit less capacity, and a lot less affinity to catch on fire or blow up. LiFePO4 is a very bulletproof technology, especially good for high discharges very quickly like in a go kart and are very hard like SLA to overcharge and what not. Also seems to be a bit cheaper.

I've got one on (ElMoto) my motorcycle conversion

I've been lookin into these 60V ~50AH Lithium Battery packs. (~$500.00)

https://www.ebay.com/itm/223290104145

Module removed from 2017 Chrysler Pacifica Hybrid.
Battery made by LG Chem. NMC chemistry.
Battery pack is made of 16 pouch cells welded together.

Module size

Length: 15 1/4 inches
Width: 8 1/8 inches
Height: 6 1/4 inches
Weight: 40 lb

Cell Voltage spec
Maximum Voltage: 4.2V
Nominal Voltage: 3.7V
Minimum Voltage: 3V

Module Voltage spec
Maximum Voltage: 67.2V
Nominal Voltage: 59.2V
Minimum Voltage: 48V

Yup, that's a 60V ~50AH Lithium Battery Pack that only weighs ~40lbs.

But,
As, I've said, I can get ~45 min. of run time out of a "good" set of 12V 15AH SLA's
...that only cost ~$100.00 & weighs ~50 lbs

Where as, this "well built" Lithium Battery Pack should provide a lot more "run time"
...but, costs ~4-5 times more & still weighs close to 50lbs.

But, here is "the thing" in my experience 45 min. is plenty of time "on a kart" (for us old'er fellas anyways)
…& more importantly these electric motors, controllers & even (sometimes) the wiring/terminals get warm or even hot.

So, IMO 45 min. seems to be a good "stopping point"
...to re-charge the battery pack
…& to let 'er kool down

 

[JoelR]

Thank you for the replies, guys. I had notifications off or somehow missed them so my apologies for my late response.

The battery I referenced arrived from China and I hooked it up for some testing. So far, it's working great and the low weight is very, very noticeable on a bike this light. I love how it's dwarfed by the SLAs in the picture and can tuck into the lower tray, eliminating both weight and lowering the center of gravity.





Here are the original Kuberg battery cables. Nice quality silicone insulation! I don't love spade-type connectors in general but these are decent examples.


After converting the battery to use an XT60, I made this adapter and stole a boot off an alligator clip to cover the positive side.


I'm a little puzzled on how to proceed with charging it. The Kuberg uses a 2.1mm barrel jack that wires into the motor controller. I haven't opened the controller box to see where these 3 wires go. The Chinese lithium pack also came with a 2.1mm female socket to charge it. However, I don't know which route to take:

Route 1: Ignore (tape off) Kuberg 2.1mm jack and use the provided female socket on the battery pack. This is what I've done so far and it's fine but seems a little tacky to have the unused jack taped over.


Route 2 (PREFERRED): Ignore battery pack's 2.1mm jack and plug lithium ion pack's 2.1mm charge plug into the Kuberg's 2.1 jack. I haven't tried this route and am afraid of causing harm to the controller and/or improperly charging the battery by bypassing its provided charge circuit. If this route is viable, I love the simplicity and how it uses the bike's features.

Route 3: Cut the Kuberg 2.1mm jack wiring and splice onto the charge wires of the pack. If I do this, I'm curious if I should keep the inline diode + resister on the positive cable. I learned that the controller assumes it's being charged if this cable isn't present so I'll at least need to defeat this feature to take this route.