Using lifepo4 charger for li-ion battery

[csheep]

Hey,

I am currently using a 43.8V 2A lifepo4 charger for my 36V lifepo4 battery (12S of 26650 A123-like cells). I am building a new battery for my latest project. It is going to be a 10S4P pack made of SAMSUNG INR 186650-25R cells protected by a 30A BMS from elifebike (http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2011-4Q-MW0Q.1NEXH). Apparently, these cells can absolutely not be charged over 4.2V.

My question is: can I use my 43.8V lifepo4 charger to charge my new 36V/10Ah pack by counting on the fact that the BMS will protect the cells from being charged over 4.2V or do I absolutely have to buy a new 42V li-ion charger?

Thanks a lot!

Csheep

 

[cycborg]

The BMS will protect your battery, but you might be a little short of a full charge. The BMS will cut off charging while the charger is still in constant current mode, not allowing it to transition to constant voltage mode to complete the charge. The charge you'll lose this way is tiny, probably less than 5%.

You should be able to adjust the output of the charger to the right voltage, though. Open the case and look for a potentiometer next to the output, and tweak it while monitoring the open-circuit output voltage. If you don't have the capability to do this, don't worry about it.

 

[csheep]

Thanks a lot for the reply. I would like to keep using the charger for the lifepo4 battery pack as well so I will go with the first option: losing 5% of charge on the li-ion pack!

 

[dnmun]

open your lifepo4 charger and post up pictures from inside so we can show you where to adjust the output voltage. if you have a 12S lifepo4 pack then you should be charging to 12x3.60V or 43.2V instead of 43.8V.

if you charge your 10S lipo pack to 43.2V then it will be overcharged. you cannot chose to just undercharge the lifepo4 to 42V because the BMS will not balance the pack at less than the 43.2V required.

but you can add a resistor to the feedback circuit that controls the output voltage so that you can have two separate output voltages by selecting to include the resistor in a parallel circuit to the feedback resistor in the output of the lifepo4 charger.

then you would put the resistor that you added on a switch so that it can be deselected to charge your lifepo4 pack to 43.2V and then when it is switched into the circuit in parallel it will have the required 42.0V on the output.

we should be able to help you depending on your skills. lemme know if you wanna try it.

 

[dogman dan]

Second charger the simplest thing you could do. EM3ev sells a charger you can switch to two different voltages, if you want that in your next one.

 

[csheep]

Thanks for the replies. Dnmun, I will come back in a few days with the pictures, I'd be happy to give it a try... if only for the fun of it!

 

[dnmun]

won't be fun. it will require thinking and learning so it is not for the normal stay at home buy a new one style.

the charger that paul sells does not balance the pack if it is the one i have worked on in the past. that has a latching output that turns the charger off when it reaches final voltage.

we can do similar but with a balancing current. paul's charger can be hacked to repair the latching output too.

 

[dogman dan]

Go for it if you are up to it!