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I have four 12V batteries (all with the same watt-hour rating), and I would like to achieve dual voltages by interconnecting them, while achieving the following:

  1. 48V at the full amp-hour rating of the combined batteries.
  2. 12V at the full amp-hour rating of the combined batteries.

I am building a robot for my thesis and have the following constraints:

  1. Space limitations - I cannot put as many batteries as I want to achieve the high amp-hour ratings with multiple voltage levels.
  2. I have a 12V inverter (12Vdc to 120Vac) that is feeding 120Vac to my stepper drives - this means that I need lots of amp-hours as I can't run out of power after a short time. Essentially I need 12V with the full amp-hour rating of at least 2 batteries (connected at a single location - a true parallel).
  3. The 12V to 120Vac conversion is wasteful, and in general steppers are very wasteful as well, so I bought servos which are powered with 48Vdc - this means that I need 48Vdc at the full amp-hours of my battery bank.

Solutions that do not fly:

  1. Connecting the 4 batteries in series and grabbing a single 12 volt tap form one of the batteries to feed the inverter. This means that the 12V will have watt-hour ratings of only one of the batteries which will not be enough.
  2. The inverter takes only 12V. I cannot change the range of the voltage input to the inverter.

I am fresh out of ideas. I am hoping someone can think of a novel idea with connecting high watt diodes.

If I may provide any additional information please let me know.

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Simple mathematics tells you that with 4 off 12v batteries, you cannot have 48v by connecting all 4 in series and more than 1 battery in parallel for 12v duty.

If you already have a use for 48v DC for the servo motors, and you still need your 120v AC, then I suggest you buy a 48v to 120v inverter. This is likely to be slightly more efficient than a 12v to 120v inverter.

A better solution would be to ditch the requirement for 120v, and build/buy something to run your steppers from 48v (using switching for constant current, not linear constant current, for better power consumption).

If you have some hi watt diodes, you could try selling them, and putting the money so raised in the kitty for the 48v inverter!

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  • \$\begingroup\$ Thanks for your answer Niel. Unfortunately it's exactly what i was afraid of - more money to spend... Thanks for your help! \$\endgroup\$ – Mike Apr 29 '16 at 17:39

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