Hey fellaz,

I've got a very large 4s 12v LifePo4 battery that I need to charge using my car's alternator while driving.

Obviously I can't just connect it directly to the cars 12v System since this battery will pull hundreds of amps from the alternator if the battery is fairly depleted.

So normally the obvious solution would be to use a CC/CV BOOST or Buck Converter, but the issue I think I'm going to run into going that route is normally there needs to be at least a couple of volts difference between the input and output in order for the boost or buck converter to work properly.

The voltage of my alternator is somewhat low and usually outputs around 13.5v. The LifePo4 battery voltage is between 10v completely discharged(not very often) and 14.4 fully charged.

The other option would be to use a MPPT or Pwm solar charge controller but I'm not sure either of those are able to charge a battery with the input voltage being so close the batteries voltage...

The only other thing I can think of is to use a boost converter to boost the cars voltage and then use a cc/cv buck converter to charge the battery.... I know I'm going to lose a ton of the efficiency that way but that's not really an issue in this situation

I'm curious what your guys thoughts are about this. I know they make a car DC DC battery Chargers but there fairly expensive...

Thanks

 

I think the MPPT controller is a nice idea but way more than you need for a battery charger. I think a current limited Boost would work well.

 

To avoid an explosion and fire, a Lithium battery is charged by a charger that is designed for that battery.
A Lithium battery charger circuit does many things to prevent an explosion and fire.

 

To avoid an explosion and fire, a Lithium battery is charged by a charger that is designed for that battery.
A Lithium battery charger circuit does many things to prevent an explosion and fire.

Hey Audio Guru,

You're absolutely correct and I appreciate you mentioning it. The battery has a a BMS installed in order to the balance the cells and is able to cut off the charge and discharge of the battery if individual cell voltages or the entirety of the pack voltage goes to lower too high.

So all the safety concerns for taking care of.

As long as the battery has a BMS you do not need to use a dedicated lithium battery charger. You can use a buck or boost converter as long as its constant current / constant voltage and obviously those two settings are set correctly.

 

You likely could use a buck-boost converter which will operate with the input voltage either above or below the output voltage..
What's the maximum charging current you want?

 

Hey fellaz,

I've got a very large 4s 12v LifePo4 battery that I need to charge using my car's alternator while driving.

The voltage of my alternator is somewhat low and usually outputs around 13.5v. The LifePo4 battery voltage is between 10v completely discharged(not very often) and 14.4 fully charged.


Thanks

Can you double check what you stated above?
A 4S Li-Po battery charging voltage cuts off at 16.8V. The terminal voltage of the battery should never be allowed to go below 12V.
A 3s Li-Po battery charging voltage cuts off at 12.6V. The terminal voltage of the battery should never be allowed to go below 9V.
If you figures are correct, you have a damaged battery, whether it is 3s or 4S.
If it is 3S, you are charging it to too high a voltage (14,4V) which will severely limit its life.
If it is 4S, you are discharging it to too low a voltage (10V) where it may not recover.

 

LiFePO4 are different than lithium-ion. 3.6V/cell is full, so 14.4 is full for a 4S, basically the same as with a 12V lead-acid.

Renogy makes various DC-DC chargers, some include solar charge controllers.
https://www.renogy.com/battery-chargers/dc-dc-battery-chargers/

Alternatively, a low-ohm power resistor between the alternator/car battery and the LiFePO4 would limit charging current. You could have a NO relay to isolate the LiFePO4 when the engine isn't running.