Cheap Backup Option for running Modem during Powercut?

[ijsa]

you still will be doing DC --> AC --> DC . so no change but yes rather than say 500W regular UPS, you now have capacity of 60W so the losses will also reduce to that extent.

On the other do keep in mind that if you change battery the charger coming with this UPS circuit might not be powerful enough to charge your bigger battery, which will spoil your whole plan.

 

[varkey]

@ijsa How would it be doing DC --> AC --> DC? I'm confused, the only conversion is AC --> DC while the AC power supply is available.
The device I mentioned gives 12V DC output which you can use to directly power the modems and routers. It is basically a DC power supply with UPS function and battery charging circuitry.

This is not something taken out from a regular UPS, please refer to the data sheet and you will know. It can supply 1.5 A charging current which is good enough for a 12 AH battery from what I read.

Source

 

[ijsa]

Most of the modem/ routers I have seen, use adaptor with 5V DC output, so if your router/ modem requires 12V DC input then it is good.

And please check charging current again for 12AH 12V battery....... what I read 1.8A required for 7AH battery.

 

[SomeAB]

Your normal inverter would work. Just get a electrician to wire your socket (near modem) to inverter.

 

[varkey]

Most of the modem/ routers I have seen, use adaptor with 5V DC output, so if your router/ modem requires 12V DC input then it is good.

And please check charging current again for 12AH 12V battery....... what I read 1.8A required for 7AH battery.

Well, almost all my devices are rated at 12V DC, I think I have one which needs 9V but then it works with 12V as well. I haven't recently come across many devices which needs 5V, even if you need one, I think a simple step down converter would do and DC-DC step down is much more efficient from what I could see.

Regarding the charge current for a lead acid battery, when operating in a floating or standby mode, something around 10-30% of its capacity works. For cycling mode, it's higher.

So for 12 AH battery it can be 1.2 to 3.6 A or so. The only difference being it would take a bit longer to charge, plus for extending battery life the lesser the C rate the better.

I felt for a 12 AH battery, charging at around 1.5A seems like a good balance.

 

[varkey]

Your normal inverter would work. Just get a electrician to wire your socket (near modem) to inverter.

It certainly works, although I wouldn't say it's efficient. But then yeah if you already have a power backup system for your entire house, perhaps it makes sense.

But then if it's specifically for the routers and modems, I would say a DC UPS is way more efficient.

 

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[ijsa]

do keep posted about the project..

 

[Rehan Kumar]

@varkey correct me if I am wrong,
I don't think a DC-DC converter would have efficiency greater just based on the fact that it's only DC-DC converter because even a DC-DC converter would first convert the input signal to square/sin wave and then stepup or step down it and then back again to DC which is same as DC --> AC --> DC which would happen on normal UPS configuration
The only difference I think is that there will be 1 less transformer in the circuit and less amount of stepup/down is needed

 

[varkey]

@Rehan Kumar You'd need to step down the voltage only if you need 5V or so. Since most devices these days work with 12V or so you don't even need to do that. So it's just the output from the battery which goes to devices directly when AC input is absent. There is no voltage regulation happening AFAIK. And when running on AC input, it works as a regular power adaptor which outputs DC.

If you do need 5V, wouldn't you still be better off using a buck converter? Rather than make it 220 V AC, then attach the original power adaptor of the device which converts to 5V DC. Maybe I'm wrong here but a buck converter could reach pretty high efficiencies 80-90%?

 

[varkey]

So I went to purchase the battery today and had a good discussion with couple of battery shops and for a UPS kind of use case, all of them suggested to keep the charge current at around 0.1 CA (ie 10% of the battery capacity). It is OK if its charge current is a bit less, it would just take longer to charge, but keeping it above 0.1 CA would reduce the battery life.

Based on that I went ahead and purchased a 18 AH battery, Exide PowerSafe Plus. It cost me Rs 2250 incl GST (and was surprised to learn that GST for batteries is 28 % !! )

Even the warranty card which comes with the battery mentions that the max charge current should be only 0.2 CA ie 3.6 A for an 18 AH battery (would be 1.4 A for a 7 AH, 2.8 A for 12 AH, please note this is the max charge current). With the power supply I have giving 1.5 A, it would take a bit longer to charge probably around 15 hours in case the battery is fully drained.

The 18 AH battery uses a nut & bolt type of connector, so I got the necessary cable too from the same shop, costed me Rs 50.