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I have looked up my requirements in all possible sources (Google, StackExchange, DiY boards, etc.) but I have not been able to find a clear answer to my problem.

What I have is a device that needs 12V 2Amp DC Current and is currently powered by a AC SMPS which provides that. What i need to do is convert it to a battery powered device as where I am going is off grid but I have easy access to both Sealed Lead Acid Batteries and Li-On batteries that provide DC current well over 12V (13V-14V). I have been able to run the device by replacing the battery of an old UPS and running it off that but the size of the Unit becomes too large especially with the transformer (which is not such a big issue) and I need the device to be as effecient as possible.

What I have found out is I need a DC-DC buck-boost SMPS (nobody has written this directly, but it is what I have gathered so far) to regulate, be effecient and to be able to handle the variation in voltages etc without harming the device (Please correct me if I am wrong here.)

So basically the question(s) is:

  1. Is it possible ? (my current knowledge says, it should be)
  2. How would I go about it?
  3. What kind of a circuit (with components) am I looking at to do the job?

PS: I humbly request all of you knowlwgable guys to be as forgiving as possible, as this is only my 2nd project involving power related stuff and 1st if u consider battries.

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3 Answers 3

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Assuming you will still have automobiles in your off grid location; a very simple and cheap alternative is to buy one or two extra auto batteries. The car already has the voltage regulated charging system in place. And at a 2 amp draw, a typical car battery will run your device for quite awhile.

Nevertheless, aside of offering an alternative, back to the question itself:

1 Yes it is possible. Even with building your own battery fed regulator, you have some options. You could make a battery pack that places several batteries in series til you have 3-5 volts above your rgulated output and use a cheap and simple linear regulator. Or you could build a switch mode power supply as you mention.

2 & 3 Pick a switch mode regulator chip and read its data sheet and app note. Build the app note as a first step to gain experience building a power supply. Here is a youtube video that will walk you through using the MC34063. The 34063 in boost mode will be easily able to handle your requirements and it has been used for some time in USB phone chargers and hubs, so there are a lot of schematics on the web easily found. https://www.youtube.com/watch?v=qGp82xhybs4

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... be effecient ...

Not at such a low drop. Switching and linear will be neck-and-neck for efficiency here, and linear will be a lot simpler to use for the price. Just make sure it is a LDO regulator, since you only have about a volt to squeeze by.

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  • \$\begingroup\$ But will the (pardon my noobness) LDO regulator be able to manage once the battery starts to droop to say 10V? And all this while supplying the device 2 Amps (at max power draw)? EDIT: Also I need to increase the hours (Days/months - Parallel Batteries) I can squeeze out of the battery. \$\endgroup\$
    – JackMack
    Commented Nov 1, 2014 at 3:30
  • \$\begingroup\$ A 12V SLA is considered empty at 11.8V and a 4S Li-ion battery is empty at about 12V, so you may be running a little lean at the end. And a TO-3 package can handle 2A easily enough with a smallish heat sink. \$\endgroup\$ Commented Nov 1, 2014 at 3:37
  • \$\begingroup\$ @mkeith Li-ion battery pack will need to be contructed. I will be trying this setup out on a SLA 1st. If and when all of it is working in order I will replace the SLA with a Li-ion Battery pack. Do you recommend this approach? \$\endgroup\$
    – JackMack
    Commented Nov 1, 2014 at 15:01
  • \$\begingroup\$ Efficiency of the linear regulator will drop to 71% at full charge. But there's no need to charge fully if you want to extend battery lifetime. Using 4S LiFePO4 is another option, but has slightly lower energy density. \$\endgroup\$ Commented Nov 1, 2014 at 15:16
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According to your requirement, simple solution -

1) If input is 12VAC, You can convert it to DC by simple bridge and filter. Then follow second or third stage.

2) If input is 12VDC, You can use DC-DC switching regulator with 2A or more current ratting(according to your requirement). Use operating freq. as high as possible to reduce space.

3) If you want to design battery charging/ discharging(we say Battery management) circuit then Simply you can use MCP73833/4 Stand-Alone Linear Li-Ion / Li-Polymer Charge Management Controller(see datasheet http://www.microchip.com/TechDoc.aspx?type=datasheet&product=mcp73833). It will work very easily.Keep in mind Vbat track should be enough broad with PTH.

Be free to more help.

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