Batteries of incorrect voltage

Discussion in 'The Projects Forum' started by summersab, Nov 13, 2012.

  1. summersab

    Thread Starter Senior Member

    Apr 8, 2010
    Sorry for the short and nondescript title - there's apparently a bug that limits some people from posting with long titles.

    Here's my problem. I have a few devices that use some pretty unique batteries, but they're expensive and hard to come by. There are other batteries that will fit and work, but they're more than too powerful for the device. Obviously, the batteries are connected in series to the device itself. I know the relationship between the batteries mathematically. I attached a schematic of what I'm trying to do. For simplicity, I just used and LED as the "device" that I'm trying to power and used some simpler voltages than the exact ones I'm using. The top shows the device being powered by either two or three (or more) batteries. The bottom is what I would like to accomplish.

    So, is there an IC or a regulator that I could put in series to this that would produce the correct output regardless of how many batteries I have in series? That way, I could have a standard circuit that I could use between all devices. Is that possible?

    Thanks a million!
  2. SgtWookie


    Jul 17, 2007
    You may be confusing things by showing an LED as a load, because an LED requires current regulation, not voltage regulation; that is unless your actual load requires current regulation instead of voltage regulation.

    You didn't specify the load current range; this is important because otherwise recommendations won't match your requirement. Give your actual current requirement range.

    You'll also need to specify the input voltage range. It's quite possible to design/build a buck-boost type of switching power supply, but if you need a really wide range, the cost of it will increase.
  3. MrChips


    Oct 2, 2009
    Devices (or loads) either demand constant voltage or constant current, never both constant at the same time.

    You determine what type of load it is and then specify the voltage or current.

    Yes, you can have a regulator that produces a constant voltage or constant current.
    Take your pick.

    Edit: Wookie beats me to it again. He can type faster than I can.
  4. summersab

    Thread Starter Senior Member

    Apr 8, 2010
    Well, I guess that would be part of the problem and why I didn't quite know where to start. The current range is whatever the batteries can supply, really - I'd like to be able to use this on various devices if possible, and all of them are rated for different loads. As far as the voltage range . . . say, 5 of these batteries tops?

    What I was thinking of doing was using a LM3914 battery monitor to detect how many batteries I have present. Based on this, the output would be directed to a variable regulator (perhaps in combination of another step-down circuit). This would a) detect how many batteries there are, and b) restrict the voltage to the correct limit.

    However, I figured there would be a better way to do it.
  5. Audioguru


    Dec 20, 2007
    Some little NiCad cells can supply many AMPs (for a few minutes).
    The tiny LiPo battery in my RC airplane can supply many AMPs (for a couple of minutes).
    The boy scouts use an unknown battery to supply a few AMPs (for one second) to ignite their rocket motors.
    A car battery can supply hundreds of AMPs (for a few seconds) to start a cold engine.

    Since you refuse to give any details then we don't know what you are talking about.
  6. Doktor Jones

    Active Member

    Oct 5, 2011
    This is only true if your device is trying to draw as much or more than the batteries can supply. I can run a cheapo 2.2V 5mA red LED with a resistor off a 12 volt car battery that's capable of up to 800A peak current output. This won't fry the LED, and certainly is not indicative of my circuit requiring 800A.

    The reason people are asking about your current requirements is because power regulation components have max specifications, and if your load exceeds those specs, you'll end up letting the smoke out. If you just have a little LED flashlight that's got a 100mA load, a cheapo LM317 circuit would be overkill; on the other hand if you've got something with motors or heating elements or other heavy loads, that LM317 circuit will probably go out with a bang.

    We know the voltages you're looking for (at least some), so that's half the battle... but the war's not over, and we need more intel before we can advance :p
  7. summersab

    Thread Starter Senior Member

    Apr 8, 2010
    I understand. Well, I'm actually looking at Li-ion cells that just so happen to fit some of the oddball batteries that companies were obsessed with making 10-15 years ago.
  8. Dollarday

    Active Member

    Jan 25, 2012
    Depending on the current consumption of your LED's you can use an LED driver such as this: (Wide input voltage, adjustable current from 0mA to 100mA)

    Otherwise, if you do not want to use an LED driver, you can use a linear voltage regulator to fix the voltage, and a current limiting resistor after that to make sure the LED's do not exceed a certain amount of current:

    For example, if the battery voltage of your previous system was 5V, you can use an LM317 linear voltage regulator and any number (or different chemisry battery) that provides a voltage of between 8V and 25V. (See circuit diagram)

    You can adjust the LM317-ADJ to any output voltage between 1.2V and 37V with a maximum input voltage of 40V. Your input voltage must be 3V higher than the set output voltage of your regulator. (PS: If your LED's draw more than 100mA current you will need a heatsink)