# I need help on backup power system

Discussion in 'General Electronics Discussion' started by level3, Mar 28, 2013.

1. ### level3

4
0
Mar 28, 2013
Hi,

I have an internet modem which requires 12V DC power input to operate. In my area everyday there's 2/3 hours blackout/load shading occurs. Is it possible to use 2x 6V battery as backup connected as series and use that for 2/3 hours?

Please guide us as I have no idea on electronics and I wanna build my own DIY system. I need experts help. Thanks,

Here is a link of my modem's power adapter:

2. ### CocaCola

3,635
5
Apr 7, 2012
If your 6V batteries are rated upwards of 8Ah at a 2A drain you should be OK... Not all batteries are created the same, so you will need to find batteries equal to or that exceed what you will need...

3. ### john monks

693
2
Mar 9, 2012
Sure if both batteries are identical.
Do you intend to charge them when the power is on?
What batteries do you have in mind?
Are they 6 amp hours or better?

4. ### level3

4
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Mar 28, 2013
Cocacola and John, thanks for quick reply. I was wondering if each battery with 6V 4.5Ah can serve my purpose? Yes, I would like to charge 'em when power is on. The circuit should work as instant backup power supply. And overcharge secure. I also heard something about deep cycle battery. which one is good?

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
4.5Ah is probably insufficient. Then again, the device probably doesn't draw the full 2A all the time.

The second issue is that even on a float charge, you probably want more than 12V to charge these batteries -- they will only be partially charged at that voltage (i.e. 6V each).

The third problem is that when the power comes back on, the plugpack will need to provide power to the device *and* charge the batteries. It is quite possible that the total load will exceed 2A, and that's not going to be good for the batteries.

You might be better off with a battery and charger suited to it, and use a relay (powered by the 12V plugpack) to switch your router between the plugpack and the battery

The problem is what happens while the relay switches over -- will the device drop out? This is something to test.

6. ### level3

4
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Mar 28, 2013
Ok, to minimize the complexity, I think I just need a backup power source that can feed my wireless router (which consumes 12V, 2.0A) and should last at least for 3 hours.

Which means, I need a single 12V battery -or- 2x 6V batteries with 8.0A capacity. - Correct me if I am wrong!

I don't mind manually switching the my routers power source in the event of blackout.
Only one problem left,

1. how to charge those batteries when main power is on with AC input?
2. I came to know there are lots of variations of battery - which one should I pick to serve my purpose best?
3. What circuit do I need to make a charger?
4. I need to make it 'over charge secure'

Thanks again.

Last edited: Mar 29, 2013
7. ### CocaCola

3,635
5
Apr 7, 2012
It's a guess, I posted 8Ah capacity because it gives you a significant margin of error, as said 6Ah technically should be enough and possibly even a little less...

But, the batteries you choose have to be rated at your current drain, a 2.0A drain is significant and some small 6v batteries rated for lets say 6Ah @ 100ma drain are not going to cut it since you would be drawing 20 times that current and thus reducing run time... Look at the datasheets or the manufactures specs of the batteries you choose and make sure they can sustain a 2A drain for the duration you require...

As for the charger, if they are lead acid batteries it's pretty simple, lots of low cost wall wart chargers you can purchase that will work fine as a slow trickle charger... Other battery chemistries will require special charging circuits...

8. ### level3

4
0
Mar 28, 2013
So, which one would be best for my application? Lead acid or deep cycle battery?

9. ### CocaCola

3,635
5
Apr 7, 2012
Deep cycles batteries are almost always lead acid...

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
A deep cycle lead acid would be best, but a higher capacity "normal" lead acid battery would also be fine as you would not be cycling it so deeply.

Have you measured the *actual* current your device requires?