Battery charger for 4 x lithium batteries connected in series

Coasta007

100 µW
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Jun 17, 2021
Messages
7
Hi all.

Just after some battery charger advice. I upgraded the batteries in my son's scooter from SLA to lithium. The scooter runs at 48v so I'm about to replace the SLA with Lithium batteries.

There are 4 x 12v batteries connected in series and the charger for the SLA only has an output of 1.6A. The charger output voltage is 57.6v

Any advice on what charger to buy as I don't think the current charger output current will be enough.
 
Are they permanently connected in series? If so you'd be looking for a way to charge and balance 12s. Balancing is a must since the cells will drift apart and some may overcharge without a balance function. If you can disconnect them and charge 4 3s packs separately then a 4 port hobby charger might do the job.

What kind of batteries are you working with?
 
Thanks for your reply.

The 4 batteries are permanently connected in series. They are for my son's 48v scooter.

These are the batteries I'm installing

https://batteriesdirect.com.au/shop/product/36628/12lfp15.2.html
 
Had to dig for info on that battery.

https://www.catch.com.au/product/drypower-12lfp15-2-lithium-iron-phosphate-12-8v-15-2ah-rechargeable-battery-7280593/

"Built-in Circuit ProtectionIEC & UN38.3 Safety Certified at cell level and integratedBMS protection to ensure safety and prevent damage."

So you have 4 of those each with an individual BMS inside. I would try to contact the seller or MFR and find out the specs for that BMS. Also ask them what charger and charging method they recommend for 4 12v lifepo4 packs in series.

Specs say
Standard charge current: 3.04A
Max. cont. charge current: 15A

but then if you keep reading it says "Superior Charge & Discharge E iciencyFaster charge/discharge rates (C/2 LiFePO4 vs C/20 SLA)" which means 0.5C or 7.5 amps.
 
You're a legend. Thank you. So if I can find a charger with an output of 57.6v and 3 - 15 amps that should do the trick?

Sent from my SM-N976B using Tapatalk

 
I would strongly recommend contacting both the seller and the manufacturer for recommendations. Or at least getting some opinions here on the forum before purchasing a charger.

The issue I have with the BMS that are likely inside those batteries is this. Lifepo4 cells dont stay balanced when charging. So when one cell gets close to full the voltage ramps up in a steep curve. And the BMS will typically dissipate that extra charge thru some resistors in the form of heat. Balancing is only done once one cell gets close to 3.65v in this case.

I would much prefer a BMS with whats called active balancing. Taking current from the cell with the highest state of charge and using that to charge cells at a lower state of charge. Or balance charging with an Rc charger which isnt practical for those batteries as it would involve opening the cases and installing balance leads.

At the very least when you do get a proper charger, be sure to charge those batteries while you're awake. Or put them in a safe container so if worse comes to worse and they let the magic smoke out it wont cause too much damage.

Stay safe out there.

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Thanks. I'll give them a call.

About the charger, is any output voltage between 48v and 57.6v ok?

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4 cells x 4 batteries is 16 cells. So 3.65v x 16 = 58.4. But its not that simple. You'll need a battery charger that plays nice with the BMS inside each battery.

Remember the BMS is taking current from the cells with the highest SOC and your charger will have to have the right programming and charge curve so the BMS can keep up with cell balancing so none of the cells go too high.

In other words, I dont know for sure. I use Rc hobby chargers and/or active balancers for my packs. Not familiar with that particular product or batteries like it.
 
This is pretty straightforward:
Each of the 4 batteries are composed of 4 cells in series.
4 batteries in series = 16 cells in series. This pack is 51.2V nominal (also called 48V)
You don't need anything special for the BMS, just a regular 56.8VDC LiFePO4 charger:
3A charger
6A charger
15A charger
 
Coasta007 said:
the charger for the SLA only has an output of 1.6A. The charger output voltage is 57.6v

Any advice on what charger to buy as I don't think the current charger output current will be enough.

The charger you have should fill the batteries from empty in 10 hours or so. If that's not fast enough for you, then you can replace it with a more powerful one. But it will work.
 
Chalo said:
Coasta007 said:
the charger for the SLA only has an output of 1.6A. The charger output voltage is 57.6v

Any advice on what charger to buy as I don't think the current charger output current will be enough.

The charger you have should fill the batteries from empty in 10 hours or so. If that's not fast enough for you, then you can replace it with a more powerful one. But it will work.
Thank you. I'll try it out and see how it goes. If I get annoyed by the amount of time I'll upgrade to a higher output charger.

Sent from my SM-N976B using Tapatalk

 
A regular lead acid "dumb" battery charger also works for lithium iron phosphate.
 
Thanks guys, I wasnt sure. The last thing I want to do is give someone bad advice and it ends up going catastrophically wrong.
 
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