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    Step down a LiPo battery voltage

    Hello,

    What would be the most light-weight and minimalist way to step down a 6 cell LiPo battery (25.2V fully charged) to the area of 24V-24.5V.

    That is, to step it down 0.7V-1.2V. The battery is connected to a device that consumes 0.5A.

    Thanks.

    •   Alt7th December 2016, 21:41

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    Re: Step down a LiPo battery voltage

    Hi,

    Maybe placing a MOSFET in series, looking for one that will drop about that amount at that load due to RDSon value, or something with a (medium power type?) BJT; or two or so rectifier or Schottky diodes?


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    Re: Step down a LiPo battery voltage

    Quote Originally Posted by d123 View Post
    Hi,

    Maybe placing a MOSFET in series, looking for one that will drop about that amount at that load due to RDSon value, or something with a (medium power type?) BJT; or two or so rectifier or Schottky diodes?
    i have here 1N4007 and if i'm reading correctly the datasheet, they can handle a device of ~20V 0.5A.

    The problem is that the battery wire is so thick, while the diode wire is so thin. It makes me think if i am missing some kind of essential data here..i guess it's a kind of a stupid question, but the battery was pricey and it took it 3 weeks to arrive, so i'd better ask even stupid questions..



    •   Alt7th December 2016, 22:41

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    Re: Step down a LiPo battery voltage

    6 LiPo cells in series will drop to about 3V each when discharging (18V) when they should be disconnected from the load. If you reduce the voltage 1V then the total voltage will drop to 17V.
    The wires on a Lipo battery are thick because a LiPo battery can produce many amps of current. Thin wires on a diode melt with a high current.
    A 1N4007 can block up to 1000V, pass 2A continuously but getting very hot dropping 1.5V max or pass 30A for a short duration. The typical voltage drop at 0.5A is 0.87V.



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    Re: Step down a LiPo battery voltage

    Hi,

    Stupid questions show wise procedure, eh!

    I got that too, about 0.9V at 0.5A, the 4007 should be fine, it's about the right voltage drop, unless you know you will have large surge currents. An alternative is something like the 1N5408, that has much thicker leads, but it only drops about 0.7V at 0.5A...

    You could combine a rectifier and a Schottky in series to get approx 1 - 1.2V, not sure if there are special reasons not to do that.

    Just in case: if the circuit operating temperature is known, and not going to get extremely hot, fine, as diodes have poor tempco performance compared to transistors.

    - - - Updated - - -

    ...I can't help but think, don't know what a waste of invested time searching for a suitable part it could turn out to be, but what about going from "poor man's" to more sophisticated solution and splashing out on a suitable LDO, as usually drop out is influenced by a) load and b) Vin - Vout, and in your case b) is only about 1V..., and if a suitable LDO showed up, then the thick battery wire to thin diode lead problem can be eliminated by wide PCB track(s) ((and possibly also adding a suitable screw or spring clamp connector block)).



    •   Alt8th December 2016, 11:31

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    Re: Step down a LiPo battery voltage

    If your application can get along with a DC-DC Buck (switching) regulator, I think such a module would be the simplest (and cheapest) solution:

    Output voltage is adjustable. See here its features:

    Perhaps with output voltage display

    Or may be such ones.

    Just read Audioguru's post: the LiPo accu can get down to 18V at end of discharge. In this case you'd need a Boost Buck DC-DC Step Up Down Converter, which can convert a changing input voltage above or below the desired output voltage to this adjustable fixed output voltage.
    Last edited by erikl; 8th December 2016 at 23:15. Reason: Amendment



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    Re: Step down a LiPo battery voltage

    Quote Originally Posted by roineust View Post
    What would be the most light-weight and minimalist way to step down a 6 cell LiPo battery (25.2V fully charged) to the area of 24V-24.5V...
    Even if the battery shows 25.2V fully charged, it will not stay at that voltage for long.

    Within a rather short time, the voltage will drop to 3.6 to 3.7V per cell or about 22V for the assembly. If you need a supply between 24-24.5V with 0.5A current capacity, you need to consider a regulator.



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