How supply a DC 8V 3Amp device from battery

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Joined May 23, 2021
I have a POS device that input voltage is DC 8V 3A that uses an adaptor device to works but I want use a battery for it (because our main power outage every day in this f*** country). How can I use it ?because the input current is 3 amp and I know a normal 9Volte batteries is not supply 3 amp. Also I prefer to use a rechargeable battery.



Joined Dec 29, 2008
Purchase 6 silicon rectifier diodes, rated at 10 amps or something like that. Buy a 12 volt motorcycle battery, or possibly a small 12 volt car battery. ... A 3 amp load would cause excessive current drain on a smaller battery. Connect the 6 rectifier diodes head to tail, in series fashion, negative (circle band) end to positive end (unmarked usually). Then connect the battery positive terminal to the positive end of the diode chain. ... This arrangement should provide approximately 8 volts, and allow the load device to consume 3 amps for a reasonable time before the battery requires recharging.. It may be a good idea to place a small knife switch in line with the diode chain for convenience.
... The diodes connected in series will decrease the 12 volts from the positive battery terminal by about 4 volts, resulting in 8 volts going to the load device.
... The only thing to be careful about is that the plug going into the desired load will have a positive metal contact and a negative metal contact. The positive plug contact goes to the terminal end (it will be marked negative with a ring) of the diode chain, and the negative plug contact goes to the 12 volt battery negative terminal.
... If this arrangement is not perfectly clear, then ask someone for assistance, in order that you do not damage the device in question.
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Joined Nov 6, 2012
Keep in mind that your POS-Device doesn't draw 3-Amps continuously,
if it did, it would get too hot to hold in your hand.
It probably only draws ~3-Amps while the Thermal-Printer is working,
and the rest of the time it probably draws less than 0.5-Amps.
So, if You can do business without printing a receipt,
You could probably use a much smaller set of Batteries.

You could just run your Power-Brick off of a standard Computer-UPS-Power-Supply,
it would probably run your device continuously for several hours.

This is the smallest unit that the APC Company makes,
(more money = more run-time),
and it should run your POS Unit for at least ~2-hours, probably more like ~4 hours ..................


Joined Nov 6, 2012
For Charging the a 12-Volt "Gel-Cell" Battery,
you just need a cheap Automotive "Trickle-Charger",
also known as a "Battery-Maintainer".
Which is exactly what is already inside of a Computer-UPS-Power-Supply.


Joined Mar 30, 2015
How can I use it ?because the input current is 3 amp and I know a normal 9Volte batteries is not supply 3 amp. Also I prefer to use a rechargeable battery
Before you can pursue a battery powered solution, you need to determine what voltage your device will operate from.

What is the output voltage from the existing power source? From that, you need to determine what voltage range the device will actually tolerate. Once you have that information, you need to determine whether the drain is a constant 3A or if that's some conservative figure.

Once you have that information, you can start looking for appropriate batteries.

POS has multiple meanings. Point Of Sale wasn't what came to mind first... If the device wants to send information to another device that doesn't have power, will the device buffer the transaction and send it/them when the receiving device comes back online?


Joined May 30, 2018
LowQCab is headed in the correct direction. Size and buy a UPS that can support both the card reader and the computer / displays / modem / router supporting it (APC Smart-UPS and Back-UPS series come to mind). Make sure the output is pure sine wave not just stepped approximation. Both of these series have models that can do that. Next time the power takes a dump, You can finish whatever sales are in process before having to close up shop until power returns. In my home, I have multiple UPS units that keep the Cable and the Internet as well as the computers that use them alive through most storms around here. The local cable and telephone systems seem to be immune to storms. I think it would take a nuke to stop them. LOL