Running laptop and router/modem from 12v battery?

elia

I'd like to be able to run the following during power outages (which tend to occur during storms in our rural area during winter, but which last winter were usually corrected within 4-8 hours):

A laptop (Kill-a-watt readings of up to .65 amps and usually around 75 watts but occasionally and briefly to 90 watts)
A 12V wireless router/modem (readings .05 amps, 4 watts).

If I can get away with running them off just a battery without an inverter, I'd like that. I've read a couple of older posts from people saying they do it, but setup details weren't given.
Is this feasible?, and if so:

How many minimum ah do I need to look for in a battery or batteries, realizing that more ah (I believe) mean longer run time?
How would it be set up?


I'm thinking:

Attach (ring terminals, or clamps?) a DC outlet to the battery terminals, wiring in a low amperage fuse on (the positive wire?).
Get this DC to DC adapter for the laptop: http://www.amazon.com/X15l-1224els-X15z-5834els-X15l-1024els-X15l-1786slv-X15z-10417els/dp/B007PKCMRC/ref=wl_it_dp_o_pC_nS_nC?ie=UTF8&colid=10CTMOZSJ0FNF&coliid=ID5TBHX1XG9BA and plug it into the DC outlet. Is a cutoff switch needed behind the outlet?
Could something like this http://www.amazon.com/Cigarette-Splitter-SPLITTER-UNIVERSAL-suitable/dp/B001IUVJHO/ref=sr_1_42?s=electronics&ie=UTF8&qid=1377298715&sr=1-42&keywords=double+dc+outlet be used to plug into the DC outlet so both laptop and router could be accommodated, or is there a better way to do that?

Or would the router need a separate battery? It's normally powered with a circular plug-in jack at the back, its cord having a DC to AC adapter at the plug, which is plugged into an AC outlet. How would it be attached to a DC outlet?

AN AGM battery would be my first choice if I can afford the size necessary, since ideally I'd like this set up inside the house. But also as light a battery as possible, so it could be taken along on a graveyard shift and charged during the night at work (where there is always power). Am I dreaming?

Any constructive suggestions appreciated.

Cariboocoot

Re: Running laptop and router/modem from 12v battery?

Whether or not you can run "direct DC" is a matter of the equipment you want to run. Some things may operate with a wide enough Voltage range to take the 14+ found when charging, some can be accommodated with simple or more complex Voltage regulation, and some laptops actually have 12 VDC adapters for car use. First step is to have a look at the equipment and see what its power requirements are, followed by measuring the actual Voltage output of the power supplies. Then you have to do a lot of figuring out.

But yes the direct method will use less power for not having to feed an inverter and then reduce the 120 VAC back down to "12 VDC".

When you've got power consumption numbers you can multiply by hours used and get an idea of how many Amp hours capacity you need - remembering you do not want to discharge below 50% so any Amp hours number has to be X2 at least.

Always use secure mechanical connections to the battery, not clips. You'd be amazed how poor of a connection they really are.

vtmaps

Re: Running laptop and router/modem from 12v battery?

elia wrote: »

A laptop (Kill-a-watt readings of up to .65 amps and usually around 75 watts but occasionally and briefly to 90 watts)
A 12V wireless router/modem (readings .05 amps, 4 watts).

You've done the first step: know your loads. It looks like you should plan on drawing 90 watts on a continuous basis. That works out to about 7.5 amps at 12 volts. If you run your system for a hour, it will draw 7.5 ampHours from the 12 volt battery.

The size of the battery depends on how long you want to be able to draw that 90 watts (7.5 amps). You shouldn't draw your battery below 50% state of charge (SOC). Therefore you need 15 ampHours of battery for every hour that you draw down the battery.

Doesn't your laptop have its own internal battery that can run it through a few hours of power outage? Perhaps a spare laptop battery would be a good idea... certainly easier to carry to work for charging. Also, 90 watts seems a bit high for a laptop... I think newer laptops are more efficient.

--vtMaps

PNjunction

Re: Running laptop and router/modem from 12v battery?

Elia - you are not dreaming and it can be done. I'm doing that very thing right now, although I'm powering a small desktop computer, monitor, dsl modem attached to a dual-purpose agm near the operating position, but using a Samlex PST-155 12A pure-sine-wave inverter for convenience. (I prefer to feed my computer gear psw rather than cheaper msw inverters but that's covered elsewhere..)

By all means strive for a dc-dc connection to get greater efficiency, but it's not the end of the world if you must use an inverter, especially if you are not going solar and have the time to charge. More on that later...

Vtmaps brings up a good point - that 90w sounds high, but part of that might be due to trying to recharge the laptop battery. If your sole interest is just getting through a few hours, then perhaps you can rely on the laptop battery for part of the time, and your new setup for the rest without actually charging the laptop battery by pulling it so you are not actually recharging it from the new agm battery system.

OR, do you want to operate as a stand-alone with no backup laptop battery (other than maybe in the drawer)? If so, pull the battery and then measure your wattage with the Kill-A-Watt if you don't mind operating this way. Of course try using the laptop's own power-management features to a point that you are comfortable with and take measurements again. Perhaps make sure the hard drive is spinning when taking the measurement to give you some headroom.

What is critical is knowing how many hours you realistically want from the system. The hours, along with your wattage measurement, determine the capacity of the battery needed, BUT will also determine how heavy your agm battery will be generally, and could be a make/break decision for you. Realistically, unless your hours and power are very short, we're talking in the range of 30 to 65 pounds. Handles help but only to a point.

If you ever intend to go solar, at your particular location an agm makes a good choice due to limited solar-insolation and an agm's ability to charge faster than a flooded. However, to take advantage of that, you'll want to supply the maximum current your agm can handle. In other words, don't fool around with low power panels in Oregon! Just something to keep in mind on the back burner without going too deeply into solar. Yet - I've got a feeling you'll go there with us eventually.