I have a DC-DC converter which converts 9-36V to 13.8V with a 20A rating. I would like to use this converter to charge a 100aH LiFePo4 (with BMS, see: https://ca.renogy.com/renogy-smart-lithium-iron-phosphate-battery-12-volt-100ah/) from my car's 12V SLA battery.

From what I understand, the 13.8V output of the DC-DC converter works for charging my LiFePo4, but I don't believe that the converter has any current limiting built in. I assume that this means that when I connect the batteries using the converter, the LiFePo4 will immediately try to draw as much current as possible.

The converter has over-current protection, which I assume means that it will shut off at >20A.

What is the simplest way to add current limiting to my setup? Or... can I use the DC-DC converter as is?

  • 1
    \$\begingroup\$ If it is a power supply, and not a lithium charger, do not connect it to lithium batteries. Ever. \$\endgroup\$
    – Justme
    Commented Dec 3, 2020 at 21:32
  • 2
    \$\begingroup\$ The only safe way of adding current limiting to your set-up is putting a proper LiFePo4 charger between the DC/DC converter and the battery, which, incidentally, is also the only safe way of stopping the charging when it should be stopped. \$\endgroup\$
    – ocrdu
    Commented Dec 3, 2020 at 22:04
  • \$\begingroup\$ @ocrdu My LiFePo4 battery has a built in BMS, I have added a link to the battery I am using. \$\endgroup\$
    – BBales
    Commented Dec 4, 2020 at 0:14
  • 1
    \$\begingroup\$ A BMS is not a charger; the company that sells that battery also sells the appropriate charger, it is mentioned in their blurb. \$\endgroup\$
    – ocrdu
    Commented Dec 4, 2020 at 12:00

1 Answer 1


A simple supply is not a good choice for charging liFePO4 batteries for a number of reasons.

Initially when charging LFP you supply it with current and the battery will decide what the voltage is. Practically for you this means that you need a supply that can provide a constant current to the battery. You say that you assume your supply will shut off at 20A. Assuming anything around these batteries can be a very expensive mistake and this does not give you a constant current source.

When the battery reaches the bulk voltage limit at 13.8V you need to let the battery absorb at that voltage until the current drops to C/20 (that would be 5A for a 100AH battery), and then STOP CHARGING. You can ruin your batteries by allowing them to float with a 13.8 charge voltage. Your BMS will not protect you against that.

I know links are frowned upon but there is a lot of good information on Nordkyn design's website.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.