The Ah of a battery may depend quite a lot on the current drawn from it; I'd look for a datasheet, it probably has an Ah/current graph.
You need a pretty high-power switcher to feed that current at 19.5VDC, in electronics-land that's a TON of power. Definitely something with an external power FET and a big-ass inductor. Niftier switchers these days might get the physical size of the inductor down a bit, but it has to be rated at the max current you'll be drawing. Do-able, but enough work that unless you can get a travel charger to solve that part it might not be worth the headache - It Depends (TM). One-shot, or do you need 20 of them? --DTVZ On Mon, Aug 18, 2008 at 6:24 PM, Ben Scott <[EMAIL PROTECTED]> wrote: > Bringing this thread over here from gnhlug-org... > > Someone brought up the idea of powering a laptop from DC sources, > rather than the traditional AC power bricks. This is strictly about > external power, not the laptop's built-in batteries. > > Obviously, many laptops come with "travel adapters", designed to > plug into 12 VDC automobile "cigarette lighter" power sockets. If you > have one of those, and it works, use it. But things are not always so > easy. > > Case-in-point was a Dell Vostro 1700. It has a 17" display and > sucks power like a black hole sucks... everything. Dell doesn't offer > a travel adapter for it. According to the sticker on the bottom, it's > rated for 19.5 VDC at 4.62 A. Doing the math, that's 90 watts. > Dell-branded travel adapters seem to top out at around 65 watts for > some reason. 90 watts at 12 VDC is only 7.5 A, which doesn't seem > that big to me. I believe my car cig lighter socket is rated at 12 A. > > I know laptops can vary their power draw quite a bit. In > particular, running the laptop while also charging the internal > battery is a major pig. So that 90 watts may be overly high. > > I know a lot of travel adapters are misleading in their specs. A > travel adapter rated at 90 watts may only be able to deliver that when > running on AC. On DC, they may provide considerably less. This was > not mentioned in the published specifications. Again, I don't know > the reason for the limit, but I know it's caused me grief. When I > attempted to use such adapters to power a Gateway laptop rated at 80 > watts, the laptop would continuously switch between line-power and > internal-battery, at about 0.5 Hz. Apparently the adapter was going > in and out of over-current shut-off. Happened with two different > adapters, one of which I remember was Kensington brand. > > Inverters are an option, of course, but the DC->AC->DC conversion > tends to waste a lot of energy. Not a big concern if you're running a > car engine anyway, but if you're running solely off the battery, it > matters. Given that the discussion arose from a situation where a car > wouldn't be convenient (but a car battery in a box would be), I'm > curious about the battery-only scenario. > > Looking quickly, I can't seem to find amp-hour ratings from car > battery makers/sellers. Google finds various third-party claims, but > they're all over the map (25 to 100 Ah on the first page alone). > Assuming 7.5 A, and again doing the math, that is anywhere from 3 to > 13 hours of operation at full power. > > I know if one was planning on doing this on a regular basis, a deep > cycle battery would be highly recommended. For a one-shot, though, > pulling the battery from a car might be feasible (?). > > Anyone have additional information they'd like to share? Inquiring > minds... probably don't care. But I'd like to know. :-) > > -- Ben > _______________________________________________ > gnhlug-discuss mailing list > gnhlug-discuss@mail.gnhlug.org > http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/ >
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