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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?

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    \$\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
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    \$\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
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    \$\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

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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.

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