How long can a 5Watt bulb work before draining a car battery

[Camelfilter]

Hi there,

Just wondering about how long a 5W bulb can run using an average car battery?

I have an idea for "homemade" work light / camp light. Modify a LED desk lamp with a cigarette lighter plug/cord.

Thanks!

[Towncivilian]

An LED desk lamp will undoubtedly want 120 volts as its input voltage, so your idea may not work. As for how long it'll last realistically, I have no clue.

[Camelfilter]

The lamp I was going to use is rated 12v. It has a wall plug ? Converter AC to DC. 120V AC to 12V DC. The lamp itself has a mini plug which plugs into the ? Converter. Would make an easy modify to a cigarette lighter plug.

[Towncivilian]

Oh, that should work, yes. An LED draws very little power, so I'd imagine a typical battery could power a single LED for days without a problem.

[Camelfilter]

...An LED draws very little power, so I'd imagine a typical battery could power a single LED for days without a problem.

I was thinking along the same lines, but just don't feel like looking up the physics which I've long since displaced.

-If I do this project I'll post up some pic's & info. If it works, I think it will make for some convenient portable external lighting (repairs, camping, tailgating etc etc).

[AZhitman]

Shouldn't faze the battery.

If you're *real* concerned, swap out to a deep-cycle (Optima) battery.

[ltsnotme]

Probably about 30-50 days.

[QX4ME]

5 Watt bulb on 12 V battery draw ~ 0.5 Amp. If a battery is rated 100Ah, then it can run 200 hours (just based on 100Ah definition). I am not sure how they claim 100Ah. Does it mean after 100Ah 12V starts to drop, or 12V goes down to half?

I have measured a couple of car standby currrent, all around 0.1A and all those car can sit for a month with no startup problem. With 0.5A consumption, I am not sure how long.

[BubbaFL]

Keep in mind, if you're using an inverter to run that desk lamp, the inverter by itself will probably draw five or ten watts (and could be more if it's a larger unit).

Best to get a light designed to run off of 12 volts. The second best option would be a USB-powered light, and a 12v-to-USB adapter.

Also, in my QX4 the key has to be in the accessory position for the lighter outlets to work, which means other accessories are energized and there is more load on the battery. If yours is the same, you'll have to directly wire your light to the battery to avoid this.

Easier to just buy a solar-powered rechargeable lantern...

[Chuck Tribolet]

5 Watt bulb on 12 V battery draw ~ 0.5 Amp. If a battery is rated 100Ah, then it can run 200 hours (just based on 100Ah definition). I am not sure how they claim 100Ah. Does it mean after 100Ah 12V starts to drop, or 12V goes down to half?

Not quite. Watts = Volts*Current, so Current = Watts/Volts = 5/12 = 0.416666... Amps.

And I think 100AH is on the high side.

[Camelfilter]

Keep in mind, if you're using an inverter to run that desk lamp, the inverter by itself will probably draw five or ten watts (and could be more if it's a larger unit).

Best to get a light designed to run off of 12 volts. The second best option would be a USB-powered light, and a 12v-to-USB adapter.

Also, in my QX4 the key has to be in the accessory position for the lighter outlets to work, which means other accessories are energized and there is more load on the battery. If yours is the same, you'll have to directly wire your light to the battery to avoid this.

Easier to just buy a solar-powered rechargeable lantern...

Good points. The desk lamp is 12V DC, there is an inverter it plugs into which plugs into household AC (at least the stickers and such attached to it say so).

- & great idea about direct wire to the battery!

Might give that a go (safely).

I have an automotive battery trickle charger which can clamp onto the battery. It also has a plug which you can bolt onto the battery terminals . The plug, if I recall, seemed pretty weather resistant and I think had a cover at the end of it.

[Camelfilter]

5 Watt bulb on 12 V battery draw ~ 0.5 Amp. If a battery is rated 100Ah, then it can run 200 hours (just based on 100Ah definition). I am not sure how they claim 100Ah. Does it mean after 100Ah 12V starts to drop, or 12V goes down to half?

Not quite. Watts = Volts*Current, so Current = Watts/Volts = 5/12 = 0.416666... Amps.

And I think 100AH is on the high side.

--Thanks for the info! Appreciate it!

[Chuck Tribolet]

Good points. The desk lamp is 12V DC, there is an inverter it plugs into which plugs into household AC (at least the stickers and such attached to it say so).

- & great idea about direct wire to the battery!

If it runs on 12V DC, that's a power supply (AC to DC) not an inverter (DC to AC).

I'd think about putting some big clips on the end of the wire and clipping it to the battery when required.

[BubbaFL]

K

- & great idea about direct wire to the battery!

Make sure you put a fuse inline, on the positive side, as close to the battery as possible.

[Camelfilter]

Make sure you put a fuse inline, on the positive side, as close to the battery as possible.

Great idea-thanks! I think I have a 12gauge inline fuse lying around somewhere.