Turning one power supply into two isolated power supplies

Thread Starter

wachanna

Joined May 26, 2021
12
Hello manipulators. Is there a way to use one 12 volt battery and make two seperate/isolated 12 volt power sources from it with some sort of circuit? Or is it best/cheapest to just use two batteries? It just seems more practical to use one battery and some sort of circuit to turn one power supply into two, to use as if it were two batteries. Any ideas or suggestions leading to a practical, cheap, easy solution is welcomed. Thanks
 

Veracohr

Joined Jan 3, 2011
761
It probably depends on the power level. There are isolated power supply modules you can buy. If you’re looking for high power, it may be easier to just use two batteries unless you’re going for small footprint.
 

AnalogKid

Joined Aug 1, 2013
9,498
The power levels / amperages are important to answer your question. Without that information, all is guesswork.

There are many fully isolated DC/DC converter modules.

By "isolated". do you mean fully isolated / galvanically isolated / no common ground?

ak
 

MisterBill2

Joined Jan 23, 2018
9,877
A.K. does ask the important questions. For cars and trucks there ate "isolator" devices that allow one alternator to charge two batteries at once. Those devices sort of work, but only one battery gets fully charged. Also, the battery negative is not at all isolated.
So how much isolation is an important consideration, and how much power controls how complex and expensive the isolation will be.
 

shortbus

Joined Sep 30, 2009
8,971
Those devices sort of work, but only one battery gets fully charged.
This needs explaining. How does one of the batteries not get charged,fully, as long as the alternator is run for the time it needs to charge both of them? Again, things must be very different on the planet Michigan.
 

MisterBill2

Joined Jan 23, 2018
9,877
This needs explaining. How does one of the batteries not get charged,fully, as long as the alternator is run for the time it needs to charge both of them? Again, things must be very different on the planet Michigan.
The reason is that the alternator only senses the voltage on one battery circuit, So the second battery will not be charged quite as much, no matter how long the alternator runs. Close, but not as much.
 
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shortbus

Joined Sep 30, 2009
8,971
The reason is that the alternator only senses the voltage on one battery circuit, So the second battery will not be charged quite a much, no matter how long the alternator runs. Close, but not as much.
Maybe on your homemade one but not if you get a real one made to do just that. Today they don't use diodes like on planet Michigan.
 

DickCappels

Joined Aug 21, 2008
7,989
Perhaps a flying battery is the solution.

1624896248131.png
The concept realized using an LMC555.

And if you need more current:
1624896400856.png
A CMOS based converter. The left-most NOR Schmitt trigger is an oscillator.
Adapted for use with a wall wart. Note: The inductors and regulators may be omitted if all you need is a switching type power supply splitter.
 

Tonyr1084

Joined Sep 24, 2015
6,462
I had dual batteries in a car back in the 90's. One battery was isolated by a starter solenoid. It was jumped with a 60 amp diode. It would charge whenever the engine was running, but due to the forward voltage of the diode the second battery would not see the same voltage the first battery did. When I needed extra power I could push a momentary button to activate the starter solenoid to connect it to the main battery. If ever the main battery was down on power the second battery could kick in the needed wattage (volts and amps). So I can understand how @MisterBill2 claims the second battery won't be fully charged. Full enough for sure, but not fully charged.

OK, to answer this thread, I'm thinking something like PWM. Only, with an additional flip-flop circuit to turn one 12V circuit on and charge a cap, then turn it off and turn the second 12V circuit on and charge a cap. However, unless the switching device or circuit is able to fully disconnect the first before connecting the second - and vice versa, something's going to get very hot. Possibly leak smoke. But I'm only guessing at this. For years I've wondered if there was a way to charge two 12V batteries in series using just one 12 volt charging source. There probably is - but it's beyond me. Something has to turn off before the other turns on.
 

N5KS

Joined Apr 17, 2016
6
Not sure what your definition of isolated is, but let's go simple. Use 2 SPST switches, both switches have a pole connected to the positive terminal and the other pole of each switch are ready to feed 2 circuits that have a common negative terminal.
 

MisterBill2

Joined Jan 23, 2018
9,877
Aside from some nasty comments , there has been no discussion of HOW ISOLATED the supplies are needed to be. And the assumption that they are automotive batteries is just a guess. The TS did not say.
 

PaulNewf

Joined Mar 24, 2020
12
Q's:
- Current/Wattage of each isolated output?
- Input/Output exactly 12V or what operating range?
-- Check specs carefully, your input has to handle the full range of input voltage,
-- for a car battery that could be 10V(drained) to 14V(Charging).
- Do you need both isolated? (or just battery isolated from the other)
-- Definition of isolated? Just a switch so one load doesn't drain other and to reduce noise, or fully electronically isolated without a common ground reference?
- Board mount or standalone?
- Is the output 12V going to power regulators and such - if yes then just get an isolated DCDC that outputs the final voltage (Lower V = lower cost)
- Few (recommend DCDC module) or large quantity (build a DCDC circuit from components to save cost).
- Ensure you include "over voltage protection" and "reverse polarity protection" if appropriate to your application.
- Also what level of isolation (Minimal, 100V, 1000V, 5000V, more?)

Here are some "off the shelf" solutions (Cost varies significantly by Output Wattage):
a) Isolated 12VDC 100mA = https://www.mouser.ca/ProductDetail/Murata-Power-Solutions/MEJ2S1212SC?qs=pitIP5Ybo/8NQvyGI%2BcuLg==
b) Isolated 12VDC 30A = Samlex https://www.samlex.com/product/idc-360a-12/

Electrosonic has good prices on the bigger ones, Mouser on the smaller.
https://www.mouser.ca/c/power/dc-dc-converters/isolated-dc-dc-converters/?sort=pricing

Paul
 
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