Creating a Portable 6V 1A Battery Power Supply for a Modem: Where to Start?

[jtu]

I'm trying create a battery power supply to create a portable modem. The modem runs off an AC wall adapter, and on it, its written to have 6V 1A output. My power supply needs to have the same output characteristics, but be battery powered. Can someone please point me in the right direction? Thanks

 

[fedaykin]

Perhaps you could use 4 D batteries in series and then parallel them quite a bit?
I wish for someone to verify this though, and having a massive battery pack somewhat defeats the purpose of portable modem.

Wall-warts, (which I assume are what's powering your modem) are usually over-engineered, but you'll still want to parallel whichever battery type you use to account for ESR.

 

[Redbelly98]

One option is to get a 6V sealed lead-acid (SLA) battery and a recharger. Google the following to find numerous suppliers:
6V SLA

They do make 6V alkaline batteries, commonly called "lantern batteries". But if the current draw is really 1A, you'd only get 1 to a few hours of useful life. See the figure labeled "constant current discharge" here:
http://rocky.digikey.com/WebLib/Energizer/Web%20Data/EN529.pdf

If you could measure the actual current draw of the device, it might be useful to know. Your supply can deliver anything up to 1A.

 

[NoTime]

Two things.
1) For the simple unregulated wall-wart that 6v is a RMS voltage.
The peak voltage would be about 8.5v.
Depending on the modems internal regulator headroom requirements the battery pack voltage may need to be higher than 6v.
6v worth of batteries could work, but you may need 7.5 volts.
Note: Some wall-warts are regulated and you need to check this. If it is regulated then you may need to add a regulator to your battery pack.

2) The 1 Amp is a max allowable current draw.
The modem is unlikely to use more than 1/2 of the max draw.
If you measure the actual current the modem uses then you can choose the battery size based on how often you want to change them vs how much weight you want to carry around.

Warning: When selecting batteries the actual voltage can vary quite a bit from the nominal stated voltage. Depends on the chemistry and stated application. For example a 7.5v rechargeable RC battery pack can put out 12v with no load while a 6v rechargeable pack might produce only about 5v.

 

[Redbelly98]

Two things.
1) For the simple unregulated wall-wart that 6v is a RMS voltage.
The peak voltage would be about 8.5v.

Depends if the output is AC or DC, which jtu did not specify. I just assumed it's 6 Vdc, since most of them are, but you make a valid point and it would be good if jtu can confirm to us if the output is AC or DC.

When he wrote "AC wall adapter", I was thinking the AC refers to the input.

Edit added: if the modem requires AC power, then a battery or any other DC supply would not work.

 

[deakie]

So he would need a circuit that chops the dc supply from the battery to a cycle consistent with his country's supply frequency...50 or 60 hz...a chopper circuit should do it.
For simplicity, could use a power transistor fed by a wein bridge oscillator...for a more sine output...

 

[klystronman]

try the local electrical store and see what gel cel batteries are available. some emergency back up lights use this size of battery. you could use a 6v and then take it home and charge it up. the most important is you pick up one of adequate current rating to deliver your current after a length of time you will use the modem. otherwise it will be useless to you.
good luck

 

[NoTime]

Depends if the output is AC or DC, which jtu did not specify. I just assumed it's 6 Vdc, since most of them are, but you make a valid point and it would be good if jtu can confirm to us if the output is AC or DC.

When he wrote "AC wall adapter", I was thinking the AC refers to the input.

Edit added: if the modem requires AC power, then a battery or any other DC supply would not work.

This applies to DC wall-warts.
Most only have a CT transformer and two rectifiers in them and spec the resultant RMS voltage rather than peak.
Put a cap on the terminals and measure the DC voltage.

Edit: DC can work just fine. You don't have to worry about polarity.

 

[klystronman]

oops... I assumed a dc supply was required. Please disreguard my note above for the dc battery. sorry