What options exist for limiting current in this scenario?

I have been searching for an option and it looks like I need to reduce the field on the alternator but I don't understand enough to know if that will work on its own with a DC/DC charger.

At the moment the option seems to be to buy a DC/DC charger that does the number of amps I require (being 0.5C of the LiFePO4))


simulate this circuit – Schematic created using CircuitLab

EDIT: Added information per question/comment

The charger is a CTEK Smartpass120S coupled with a CTEK250SE. The 120S does up to 120Amps in a "passthrough mode". And the 250SE does 20Amps as a proper DC/DC charger and has selectable profiles.

When the battery can take a current of over ~25amps both the 120S and the 250SE are engaged and "doing their respective things". Once the 120S detects that its current is below ~6amps it disengages and then the 250SE continues and applies a proper charging profile.

https://www.ctek.com/au/all-products/leisure-sport/d250se https://www.ctek.com/uk/products/leisure-sport/smartpass-120s

NB: Interestingly there is no actual digital connection/signalling between the two units. They work in tandem or stand-alone and are designed to work as a pair for up to 140Amps

I don't currently have a LiFePO4 battery connected, I am still using AGM, the most current I have seen my AGM suck up is 75amps, and it's allowed as much as it wants. However, I am migrating to LiFePO4 Cells and I know that they will take all 140AH if they can. So looking for a way to limit that so I have my charging system sorted.

  • \$\begingroup\$ Also, title says 50 A, diagram says 100 A. Either way, these might not want to be the kind of problems you want to work on if you're not quite sure what you're doing. Even LiFe batteries can explode, and 50 A are a pretty solid current, even for an arc welder. Please take all precautions necessary to avoid fire, structural damage or blindness through arcing. \$\endgroup\$
    – mmmm
    Mar 16, 2021 at 3:28
  • \$\begingroup\$ 0.5C and 50A implies you have 100Ah battery pack right? 1mohm current sense @ 50mV amplified to a 200A switch shud do it \$\endgroup\$ Mar 16, 2021 at 3:34
  • \$\begingroup\$ Have edited the title - sorry for the confusion \$\endgroup\$ Mar 16, 2021 at 3:40
  • \$\begingroup\$ Can you post datasheet/documentation of the charger and LiPo battery? \$\endgroup\$
    – bobflux
    Mar 16, 2021 at 7:42
  • \$\begingroup\$ Have added to the question. Battery will be LFP, not LiPo. \$\endgroup\$ Mar 16, 2021 at 8:06

1 Answer 1


Your charger has built-in current limiting. Adjust that. Every other approach to limit current will confuse your charger.

  • \$\begingroup\$ This DC/DC charger doesn't. It has a few profiles to choose from, but limiting current is not an option. \$\endgroup\$ Mar 16, 2021 at 3:37
  • \$\begingroup\$ It does, otherwise it's not a charger. You'll need to figure out how to adjust it, or plainly will need a different charger - the option of putting a resistor in series with the battery is the only alternative I see, and that would be a very hot, very expensive, very much usefulness-of-having-batteries-limiting resistor. \$\endgroup\$
    – mmmm
    Mar 16, 2021 at 3:40
  • \$\begingroup\$ Interesting - so if I have understood you correctly, you're saying because it currently limits to 140amps there has to be a way to modify the circuitry so it limits to 100amps?? \$\endgroup\$ Mar 16, 2021 at 3:44
  • 1
    \$\begingroup\$ @RowanSmith that's more or less correct. In most cases for small current, feedback is provided by a burden resistor and changing the value of the burden resistor will change the output of the charger. For your charger the current might be high enough that they used something more advanced though. The challenge and risks of modification could be significant, but typically it is at least possible to get a device to run at lower than original ratings. Efficiency might suffer. \$\endgroup\$
    – K H
    Mar 16, 2021 at 3:57
  • 2
    \$\begingroup\$ Because of the extreme dangers posed by misusing Li-Ion batteries and the fact that you don't appear to be an engineer, you would have to take this project on with an abundance of caution, and by the time you solve the problem in a satisfactory way, if you value your time, it's easily possible you will have spent more time than the value of a properly chosen charger. If you solve it, but fail to do so in a satisfactory way, you could be looking at burning down everything near the battery and voiding your insurance in the process. Proceed with great caution. \$\endgroup\$
    – K H
    Mar 16, 2021 at 4:00

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