charging two 12v lifepo4 in series

[kcampbell]

Hello all!

I recently killed one of my 8 cell 24v lifepo4 batteries i made 4 years ago. I am replacing it with pre made two 12v 100ah batteries, Same brand, etc. I currently have a victron 24v 8a charger.

My question is can i charge these batteries in series with this charger or is it a bad idea?

 

[rmaddy]

Once the batteries are properly balanced and put in series you can charge them with a 24V charger. But ideally you will need a 12V charger to initially balance the two 12V batteries before putting them in series. Failing to properly balance them before putting them in series will lead to issues.

Using a 12V charger, fully charge each battery separately to 100% SOC. Then connect the two batteries in parallel (not series) and fully charge them together, with the 12V charger, to get them to the exact same SOC. When in parallel be sure to connect the 12V charger correctly. Put the positive lead on one battery and the negative lead on the other battery.

Once this is done you can connect the two batteries in series into your system. Now any 24V charging source (SCC, charger, etc.) can be used to keep the batteries charged.

 

[kcampbell]

Thank you @rmaddy. this is exactly what i was looking for. I did a search but couldn't find this exact answer. I do have a 12v charger for my other 12v lifepo4 so thats perfect. Appreciate the help!

 

[Jamr]

I have a similar question.
Looking for some help here from the very smart people in this forum.
I recently was givin some electric Razor scooters.
The stock batteries were dead so I bought some LIFePo batteries to replace the stock batteries. They were both 12vdc and both have battery built in BMS.

The question I have is charging.
I want to use the stock charging port on the bottom of the scooter to charge the batteries once they are in the scooter.
The new LiFePo batteries are 12 volt and they are in series to the speed controller. Unfortunately the charging cables once inside the scooter also go into the speed controller which makes me think there is some circuitry in the controller for the charging process. I am thinking of cutting the wires there and just running them to the batteries directly and using my RC charger to charge this thing.

My first question is. Since both batteries have a BMS, do I need to charge each of them separately before installing them and connecting then to the controller?
Where do I attach the leads from the charger to the batteries?
How do I determine how big the charging wires should be?
My second question is since I will be using my RC charger to charge these, do I charge them at 14.4 as recommended on each of the batteries? or since they are in series, I charge them at 28.8?

Please ask if yo need any clarity.
Thanks in advance.

 

[Supervstech]

I can help with the wiring, but you should know LFP drop in batteries are a terrible choice for this...

Those batteries rarely can output over 10A... the scooter will not be able to push itself out of a wet paper bag...

 

[Jamr]

I can help with the wiring, but you should know LFP drop in batteries are a terrible choice for this...

Those batteries rarely can output over 10A... the scooter will not be able to push itself out of a wet paper bag...

Thank you for the reply.

Well you were right. The scooter had very little push.
So what spec am I missing or not getting?
The original batteries state;
Initial Current; Less than 2A
The LiFe's put out a discharge current of 5A and a surge of 7.5A. That should be enough???
My LiPo RC batteries have a C rating which is a basic discharge rating and not on either of these batteries. Is that what I am missing here?

 

[MisterSandals]

My LiPo RC batteries have a C rating which is a basic discharge rating and not on either of these batteries. Is that what I am missing here?

Those LiFePO4 batteries are 5Ah and say they have a max discharge of 5A. This is 1C.
If they could discharge 10A, that would be 2C.

 

[Supervstech]

Thank you for the reply.

Well you were right. The scooter had very little push.
So what spec am I missing or not getting?
The original batteries state;
Initial Current; Less than 2A
The LiFe's put out a discharge current of 5A and a surge of 7.5A. That should be enough???
My LiPo RC batteries have a C rating which is a basic discharge rating and not on either of these batteries. Is that what I am missing here?

You either need to build a battery with 25C cells… or switch back to SLA…
The motor in that scooter likely pulls 30A or more during acceleration.
Plenty of RC crowd batteries have the output, but the batteries are explosive.
The huge cylindrical cells have the C output, but, they are bulky.

 

[Jamr]

Thanks for the help. I decided to get some SLA's like the originals.