Replacing lead acid with lithium ion question...

[rg12]

Hey Guys,

I have this guy with a 24v scooter with lead acid batteries that he wants to convert to lithium ion.

From what I understand the fully charged voltage of a lead acid battery is 12.6v and empty is about 10.5v which means that his scooter works at double those numbers (24v).

A 6S lithium ion is fully charged at 25.2v vs the old lead acid which is exactly the same, but the end voltage when most of the lithium ion capacity is used is at about 3.3v per cell which is about 19.8v vs the lead acid battery which is empty at about 21v.

The thing I'm worried about is the scooter's controller which I don't know what is the LVC set to.
I'm afraid that it will cut the power at about 21-22v which can be at around 50% charge for a lithium ion battery.

What do you guys thing?

 

[wesnewell]

Fully charged 12V sla battery is 13.5V, so 27V for 24V system. 7s is li-ion equivalent. Although 6s will also work. For lifepo4, it's an 8s pack. Most 24V controllers built for sla will have an lvc of 21V. A 6s lipo pack is perfect for that lvc keeping an average of 3.5V per cell when it shuts off. You wouldn't want to run a 7s rc lipo pack that low.

 

[Sunder]

Resting voltage might be 13.5v depending on exact type, but you need to expect up to 14.8 as a charging voltage, and higher if the charger can do equalising charges.

A 6S battery would almost certainly get overcharged, and it's not impossible that a 7S would get very marginally overcharged.

Personally, what I would do is:

1. Check the open voltage on the charger. That's likely the termination voltage for a "dumb" charger.
2. If it says smart charger, check carefully for any mention of an equalisation or balance charging phase.

If the termination voltage is under 25.2v or under, go 6S. If it's up to 29.4v go 7s. If it has a smart charger, consider getting a new charger.

As for LVC, easily solved by BMS or LiPo alarm. A 6S might not get the full capacity out of the pack, as the scooter's LVC would cut before the battery was flat.

 

[rg12]

I'm planning on using a new charger anyway.
The old charger says 24v which I don't really believe since no lead acid charger actually charges to 24v.
It will be a Li Ion from Samsung 25R (8C) cells and not LiPo.
3v per cell seems not that low for Li Ion vs 3.5v for a 7s which is a bit high for LVC.
I checked out some discharge curves of lead acid batteries and it shows a drop from 12.6v to 10.5v and read that their charger gives about 13.5v but the resting voltage or something like that (once you disconnect the charger I thing) it settles at 12.6v (I might be wrong).

 

[wesnewell]

If you use 25r cells, then a 7s pack will be about perfect with an average of 3.0V per cell lvc.

 

[rg12]

Thanks alot, I guess I will go for the 7S