I'm designing a small battery powered system, actually I'm battery powering a module which already has all the switching regulators it needs. This provides me with a wide input range of 5-10V. I was thinking of connecting the batteries directly to the module instead of using a regulator because I don't want to pay an efficiency penalty, especially in low power mode. I was worried about inrush current though when the batteries are first turned on. I know the internal resistance of the batteries should limit it some but should I add some kind of current limiting or soft-start circuit to protect the batteries from damage or overheating?

That'd be easy to do with a regulator, not sure what I'd use without one. Maybe a high side switch with some time based control. Should I even be concerned about this? Target batteries are LiOn that won't be recharged in the device itself.

  • \$\begingroup\$ The Vref in the switching regulator can be slowed down to soft start the current too,,depending circuit details with pulse to pulse current limiting. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 17 '16 at 16:10
  • \$\begingroup\$ If your skills are new then a short circuit battery fuse to prevent explosion is wise, but otherwise, how much do you trust the module fault modes, usually they fuse open on failure. but we can only guess how it is used \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 17 '16 at 17:03

You haven't said anything about the current draw of your system, how much inrush you expect, nor the type of battery, so the question can't be answered directly. However, it is unlikely that inrush current from occasionally turning something on is going to matter to the battery or the power supply input circuitry. If you are just powering a few 10s of mA of ordinary electronics, then you are overthinking this.

  • \$\begingroup\$ I think we need more xtal balls. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 17 '16 at 17:00
  • \$\begingroup\$ Sorry yeah I think I am overthinking it. It's a CPU module that pulls about 10 Watts fully active. I haven't measured inrush or anything like that I'm just in the conceptual phase. Thank you. \$\endgroup\$ – confused Oct 17 '16 at 17:25

I do not think that there will be any problem when connection your batteries. Even an inrush current of 20 or 30 A does not hurt given the time required to charge your input circuit.

Overheating of the battery will therefore also not take place because therefore the energy loss in the battery is far to low.


Just beware that high Capacitance or low ESR loads may cause extra inrush currents. Be careful how it is used with nonlinear loads. Add a curent shunt R to monitor current if in doubt on ground side with 50mV=5A or 10 mOhm shunt and monitor I with a DMM.( or higher R to suit desired range)

If you buy something with No Specs and No idea how it works, How can we tell you how it will perform?

e.g. Fusing the input may be too slow if your output has a very low ESR load.

For example, this "looks" like a good variable bench supply using Series LiPo cells that are pre-balanced, but we can only guess.

THe good news is it's incredibly cheap. $6 so you can reverse engineer it, measure hot spots, and look up datasheet for obvious tips on current limits.

  • e.g. (For step-up To prevent damage to the switch, its current must be externally limited to 6.0A.)

enter image description here

  • Input voltage :3-34V
  • Output voltage: continuously adjustable (4-35V)
  • Output Current: 2.5A (MAX)
    • ( IMHO... not at all voltages)
  • Module size: 49 (mm) x26 (mm)
  • Input mode: IN + input positive level, IN-input negative

I would try to design a precision external current limiter using a 50mV shunt drop R and Small RdsON MOSFET switch with comparator.

  • or do not try to run non-linear loads such as high current 2A motors or 3000 Farad supercaps or heavy pulsed loads without active current limit protection. Usually the part will get hot and give some warning of this before damage unless very high surge into a low ESR load..

I think it is better to use in step-down mode with Vbat greater than output. You can evaluate the IC performance doing load drop regulation and source voltage regulation tests and efficiency vs Vin-out drop.

Hint buy 1 spare.


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