Cc/cv converter as current limiter

Thread Starter

bigjoncoop

Joined Feb 1, 2019
185
I see many people online talking about using a cc/cv buck or boost converter in order to limit the current when charging 12v Lifepo4 Batteries using a car alternator...

What I don't understand is normally when using such a device, the voltage input must be slightly higher or lower then the target output voltage.

But a 12v lifep04 battery is fully charged at 13.8v the same as a car lead acid battery. The car alternator is also only outputing 13.8v also... So how would you use a buck or boost converter to fully charge the battery?

Now granted it's better off to normally not completely fully charge lifep04 batteries since it will greatly increase their lifespan. But as far as I remember a buck or boost converter needs a voltage higher or lower usually by a couple of volts correct? If so that's way to much of a difference...

Now I do know that obviously the best solution for this is to just use a dedicated DC to DC charger that is designed for this application. But they are normally fairly expensive.

But I see tons of people online talking about doing it this way. And they're normally just using those large red 1500 boost converters. (photo below)

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MisterBill2

Joined Jan 23, 2018
18,176
In an automotive alternator charging system the voltage is regulated to control the charging current. It does not seem reasonable to use a similar system for any type of Lithium technology battery systems.
 

Ya’akov

Joined Jan 27, 2019
9,071
A "12V" LiFePO4 battery is actually a 12.8V battery (four 3.2V cells in series). This means the optimal charge voltage is a little higher than the 13.8V the car's system is designed to provide the PbSO4 battery that is normally installed.

While charging with a lead acid charging is usually OK, if it is a high quality charger and properly maintains voltage and current, the best charger for a LiFePO4 battery is going to be just a bit hotter than the car's system.

The biggest danger is a bad alternator with poor voltage regulation. The battery's BMS could respond to a momentary over voltage by disconnecting the battery, possibly not in an easily resettable way. Using a buck converter is one way to deal with this, but if you are going to put a DC to DC converter in place, why not use a buck-boost so you can usually get the more ideal 14-14.2V charge?

This is an alternative to a better 13.8V voltage regulator which will also work, and might be cheaper, but will slightly undercharge the battery.
 

Danko

Joined Nov 22, 2017
1,829
But I see tons of people online talking about doing it this way. And they're normally just using those large red 1500 boost converters. (photo below)
Exactly what you need, especially for lithium battery:
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1685454256787.png
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