AGM to LiFePo4 Scooter Upgrade Questions

[YellowBuzzaroundEX]

I own a small travel/boot mobility scooter that easily disassembles to fit in my car trunk/boot. I drive the scooter all over town. Often 3 miles round trip. Occasionally 4-5 miles round trip. I like (and need) that my scooter can fit in my car trunk.

It has two 12V 21AH AGM batteries in series to make a 24V system.

I'm considering converting to LiFePo4 batteries. I could use two 12V 24AH LiFePo4 in series, or two 24V 12AH LiFePo4 in parallel.

(I cannot use one 24V 24AH because the physical layout of my battery box requires two separate batteries to fit the box.)

Would two 24V LiFePo in parallel stay balanced better than two 12V LiFePo in series? Would two 24V be more reliable or longer lasting? What are benefits/implications of these options?

How much more/higher would LiFePo4 voltage be than AGM when comparing at 75% SOC (25% DOD)? i.e. - How much more RPM/speed from LiFePo?

 

[neptronix]

Paralleling or seriesing a pack with a BMS can cause issues. I'd consider maybe getting a 22-20ah lifepo4 and finding one that can fit.

lifepo4 has very close voltage vs AGM, the discharge curve is also flatter. It's a very good swap-in for AGM... non-lifepo4 lithium batteries are a worse match that involves some compromises.

 

[YellowBuzzaroundEX]

Paralleling or seriesing a pack with a BMS can cause issues. I'd consider maybe getting a 22-20ah lifepo4 and finding one that can fit.

I don't understand what you're saying here ^. Are you saying I should use a single 24V battery? A single 24V battery that's large enough would be my preference, but wouldn't fit in my battery box. I have to use 2 batteries.

The 12V LiFePo batteries I'm looking each have a BMS that claims to be compatible for 2 batteries in series.

The 24V I'm looking at each have a BMS that claims to be compatible for 2 batteries in parallel.

lifepo4 has very close voltage vs AGM, the discharge curve is also flatter. It's a very good swap-in for AGM... non-lifepo4 lithium batteries are a worse match that involves some compromises.

I was hoping LiFePo4 had a slightly higher voltage under load.

 

[Hillhater]

Two batteries in parallel will always be in balance with each other.
two batteries in series cannot balance each other…..unless their individual BMSs are connected.
I would not expect you to notice any difference in speed by just changing to LfP batterys

 

[YellowBuzzaroundEX]

I too had heard and thought that 2 batteries in parallel would always be balanced.

However, I watched a Youtube video of a guy who tested this theory. His tests showed that 2 lead batteries in parallel will always balance each other, but 2 LiFePo4 batteries in parallel did NOT balance each other in his test.

I have also found many other sources online that say LiFePo4 in parallel might not stay balanced.

So I don't know if 2 LiFePo4 in parrallel would stay in balance.

Does anyone else want to weigh in on this? Who has actually tested this with LiFePo4 (rather than just assuming)?

 

[wturber]

Two batteries in parallel will always be in balance with each other.
two batteries in series cannot balance each other…..unless their individual BMSs are connected.
I would not expect you to notice any difference in speed by just changing to LfP batterys

This jibes with my experience. I had over five years and nearly 15k miles on eight 36v 10s2P Li Ion "hoverboard" packs wired in parallel. They've stayed in balance all this time and would still be going strong were it not for getting dragged by a trailer for a mile or so. Even with that, the six surviving packs are still in balance. Each pack has its own BMS.

 

[wturber]

The 12V LiFePo batteries I'm looking each have a BMS that claims to be compatible for 2 batteries in series.

The 24V I'm looking at each have a BMS that claims to be compatible for 2 batteries in parallel.

I'd take the manufacturer at their word. I found this article that may explain why people have conflicting experiences.

https://www.master-instruments.com.au/tech-talk-article/3/Explaining%20the%20limits%20of%20LiFePO4%20batteries%20in%20parallel.html

 

[YellowBuzzaroundEX]

OK. The battery manufacturer specs says I can have up to 4 batteries in parallel. I'd only have 2 in parallel. So it's good to go.

It'd be two 24V 12A LiFePo4 in parallel in my mobility scooter. 24V 12Ah lithium battery - LiFePO4 Canada - Free Shipping!

The battery manufacturer also sells a 24V 3A LiFePo4 charger. 24V 3A Lithium Battery Charger (LiFePO₄) - Free Shipping Canada

 

[surgicalfpv]

I have a few mobility scooters, and I upgraded them all from having 2, 12 volt SLA / AGM batteries to 2, 12 volt LiFePo4 batteries.

I always wire them in series, just like they were with the original SLA batteries. So far, I have had no issues whatsoever with balancing. With that being said, I use a LiFePo4 battery charger that is designed and rated for LiFePo4, and that uses the same style XLR connector (3 pin-barrel connector).

Occasionally, I will remove the batteries from the scooter, and charge them individually, to try and keep them balanced. I do this like once a month.

The only downside to the upgrade is that the meter on the mobility scooter is not designed for the discharge curve that LiFePo4 batteries have. So the meter on my dash is not really accurate at all, it just shows that its 100% full all the time, and then eventually, it will go from 100% charge to 0% in a matter of a half-mile. This is to be expected if you are familiar with the discharge curve of LiFePo4 batteries.

A lot to learn, and consider before upgrading. Overall, I think its worth it, but you need to be careful about your batteries dying on you. Keep a charger with you at all times.