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Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Wed, 04 Sep 2019, 19:00
by vk2aaf_hamradio
Hi sparkies!

I'm a retired broadcast engineer dealing with advancing osteoarthritis in both knees after multiple surgeries post getting murdered by a drunk driver while I was on a motorbike. Dear drunks, red means 'stop,' OK? Also a longtime ham radio op with some prototyping & scratchbuilding abilities, so you can hit me with any level of electronic complexity you like.

I have a 24V mobility scooter with a pair of 12V 45AH Pb AGM lumps. They're about 14.5kg each. I carry the moby in the back of my yoot, so I have to lift it, one end at a time, to get it in the bed. As I get older, it gets heavier (checking laws of physics... yep, that's real science). I'm thinking about converting it from Pb to LiFePO4 to reduce the weight.

The 29kg, 45AH pack is supposed to be good for 40km. I will never find out for sure. I only use it to scoot around the shopping mall. At worst, I ride it from Penrith Westfield to the Indian spice shop on the other end of town, about 3km total.

Not sure of the power draw of the motor. Looking for OEM specs on 'Aussie Scooter' brand but not much luck. Maybe 400W? Current draw is beyond the capabilities of my Fluke 89 IV, suspect about 30A on steep ramps etc. Has a 70A PG Drives S-Drive controller. Could 18650s or 38120s source this sort of current? I reckon I only need 10AH to do the range I need.

I don't know enough about how to charge LiFePO4 packs. Lacking docs on the controller so I don't know if I can use a single 24V pack or if it has to be a pair of 12V packs. There's a cable for each battery coming from the controller.

Hoping someone else has done this before so I don't have to reinvent the wheel.

Thanks a bazillion!

-Brian VK2AAF

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Wed, 04 Sep 2019, 20:00
by brendon_m
Surely there's an ebike conversion kit that comes with a lithium battery and charger you can use.
Something like this maybe
https://rover.ebay.com/rover/0/0/0?mpre ... 3696643887

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 05 Sep 2019, 12:26
by T1 Terry
I did one quite a few yrs back, it was one of those fold up units you could take on a plane. All was good until the controller dropped its bundle. The repairer claimed it was because the lithium battery could limit its current ...... a way of getting around that the controller couldn't handle the continuous current because the voltage didn't drop like it does with lead acid batteries. The person I did the battery pack for went back to lead acid batteries.
The take away is the whole thing depends on the quality of the controller, if can handle the full load amps continuously then a lithium conversion will be fine.
Be very careful with the push bike lithium batteries, many are LiPo cells and a very poor quality BMS set up. There have been quite a few reports of these battery packs bursting into flame either while charging or being used on a long uphill climb. The only safe chemistries are LiFeP04 or LTO, they don't burst into flame.

T1 Terry

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 05 Sep 2019, 13:09
by Richo
30A/10Ah=3C continuous and 70A/10Ah is 7C peak.

Using 18650's will be quite a bit of work assembling a pack so I'd probably not recommend it.
Not to mention the voltages don't mach your SLA setup.
A Samsung 18650 "High output" is rated for about 8C.
1.44kg for 8S4P=32 cells $300

LiFePO4 are 3.2V ea and 4 in series gives 12.8V nominal and 14.6V charged.
As long as you existing charger is below 14.6V then this would be ok.
10Ah headway cells should do this as they claim 10C.
2.7kg for 8. $224

LTO's are 2.4V ea and 5 in series gives 12.0V nominal and 13.5V charged.
This may be a bit on the low side for your charger - you need to check what the charge voltage is.
The other option is to go 6 in series for 14.4V nominal and 16.2V charged.
The link below claim 6C/15C.
https://www.aliexpress.com/item/32931057046.html?
3.7kg for 10. $430
4.4kg for 12. $515

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 05 Sep 2019, 14:36
by HuffnPuff
A little outside the box thinking, would it be possible to just buy some smaller lead batteries? You mention you don’t need the range. surely two half capacity batteries would be a lot lighter.

I do get the idea of upgrading though!

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Sat, 07 Sep 2019, 21:00
by Adverse Effects
WheelchairDriver.com

Lithium battery conversion/info

look around there, there is a LOT of good info there

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Mon, 09 Sep 2019, 12:37
by Richo
HuffnPuff wrote:
Thu, 05 Sep 2019, 14:36
would it be possible to just buy some smaller lead batteries?
Yep most SLA can do about 3C.
So for 70A /3C => 23Ah

That'd be ~8kg per battery.

So ~16kg and $210.

The down side is you'd be lucky to get 10Ah at 3C - more like 7.5Ah.
But 3C would be the worst case.

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Mon, 09 Sep 2019, 12:41
by Richo
Adverse Effects wrote:
Sat, 07 Sep 2019, 21:00
there is a LOT of good info there
Wow 173 pages - I think that's longer than any thread here. :o

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Mon, 09 Sep 2019, 19:12
by Adverse Effects
Richo wrote:
Mon, 09 Sep 2019, 12:41
Adverse Effects wrote:
Sat, 07 Sep 2019, 21:00
there is a LOT of good info there
Wow 173 pages - I think that's longer than any thread here. :o
:twisted: :mrgreen: :lol: 8-)

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Wed, 11 Sep 2019, 21:47
by vk2aaf_hamradio
Thanks a bunch to all responders for excellent ideas & links!

Been a bit of a madhouse up here for a few days with expanding the PV array on the house from 7.5 to 10.5kW and adding a 2nd Tesla Powerwall2, rebuilding ham radio antennas murdered by the recent evil windstorms and helping one of the grrrrrlchild's friends move house. Will get back on this in a few days.

Cheers!

-B
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Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 12:26
by Richo
That's the way to do it - 100% covered

Image

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 12:43
by Richo
brendon_m wrote:
Wed, 04 Sep 2019, 20:00
Something like this maybe
https://rover.ebay.com/rover/0/0/0?mpre ... 3696643887
So they are 18650's...
It's title is 24V but the description is 10S5P.
10S would be 36V.
eBay seller wrote:Each cell in every battery goes through a strict testing process for capacity, voltage, and internal resistance.
Well - less so for their add...

The picture of the base plate has 5 prongs on it - presumably for the "balance charging".
So what are they balancing with 5 prongs - each parallel string :?:

Normally you'd balance each battery in series.
If you add the 5 prongs with the + and - power lines that makes it a 6S pack.
So 22.2V nominal and 25V charged.

I guess it would be ok if it really is a 24V pack.
So 2.5kg and $295.

The downside I see is it may not last cycle wise as the other options - but that hasn't been a consideration so far.

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 12:47
by brendon_m
Just to put it out there, all I did was smash "lithium e bike kit" into eBay search and posted the first item as an example. I'm definitely not vouching for it or its quality.

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 12:48
by Richo
Here's a summary:

18650 1.44kg for $300
Bike pack 2.5kg for $295
Headway 2.7kg for $224
LTO-10S 3.7kg for $430
LTO-12S 4.4kg for $515
SLA 16kg for $210

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 12:51
by Richo
brendon_m wrote:
Thu, 12 Sep 2019, 12:47
I'm definitely not vouching for it or its quality.
No I understand - its there for consideration as an option not as a specific "buy this" one.
Realistically I think it is probably the easiest option.

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 12:54
by Richo
My guess is they just copied the add from a 36V unit and forgot to change one number.

Re: Upgrading 24V mobility scooter from PB to LiFePO4 batts

Posted: Thu, 12 Sep 2019, 14:06
by HuffnPuff
An e-bike battery is probably the easiest option. Already packaged up, charger available, plenty of options assuming your current battery box would fit the battery.