Laptop failing to charge from car cig lighter

[Gareth Magennis]

Actually, I'll take that back, it most probably isn't wire, but may itself need a minimum voltage to operate at its full output or spit its dummy out.

Gareth.

[John Weston]

A 12v car battery is seldom 12V, unless it only has some 30% charge left in it. Fully charged, it will be around 12.8V and up to around 14V whilst being charged. It drops to 11.6V when fully discharged assuming deep discharge wet cells. Typically, this is too great a range for a laptop normally designed to run on its own batteries having a closer specification. Unless they are designed for car battery use, you will need some form of regulated power supply.

My laptops have had DC to DC switching-mode converters for external power, usually to boost the voltage up to something like 18V to charge their batteries. They would run with an external regulated 12V but wouldn't charge the internal batteries. These converters will function for input voltages down to some 7V, so will work fine over the discharge range of typical car batteries. The awkward designs were those expecting

12V input. For these the converter had to be of buck/boost design.

[<me9]

It is exactly what it is on my Nissan Primera. Minimally specced cable going the long way round (not mentioning the route of the return).

1971 to be precise.

If I ever get round to doing anything about it I would use at least 2.5mm^2 to reduce the voltage drop. Fuses also drop enough on a 12 volt system to be significant.

I'm happy with an average of 5A, but the ability to supply nearer 12volts at occasionally higher loads would be desirable.

I've also found the headlights on this car rather dim and yellow, also due to voltage drop in the wiring.

[Ian]

The Eee needs 9.5V, and car have horrible nasty 12-ish to 14-ish volts with added spikey goodness. It'll be a DC-DC convertor, or at least some sort of regulated supply.

has some pictures of the bits that let the magic smoke out when you supply an Eee with

12V ...

Ian

[Lobster]

Thanks for all the responses to the above query. As anticipated, there was no 'in-the-field' solution, so my kids gave me another taster of the hell that me and my siblings put my parents through on long drives a generation ago...

So, back home now, and some responses to the queries raised:

1. The cycling occurred with the engine running; same problem with it switched off.
2. AFAICS the 12V adapter the PC OK when the PC is powered down and the engine's running (ie the charging LED stays lit), but haven't put this to the test yet (it wouldn't be a solution since the PC needs to be on for use as DVD player).
3. The adapter I bought was just a cheapo ebay item from China, though FWIW(!) was sold as bespoke for the ASUS EEE PC900, and has a label with the correct 12VDC/3.0A rating, and definitely has the correct plug.
4. Have just tested out my 240v inverter with the PC; that does seem to charge the PC OK while the PC's running, so that would seem to be a solution for next time.

So, I presume that the problem's just a crappy adapter, would that be right?

Is there any reason *not* to just use my inverter for this task in future, in terms of personal safety, damage/wear and tear on the car/PC/battery or whatever? It would be used exclusively while on the move (which makes me slightly uneasy having 240V mains around - is that just irrational?)

Thanks David

[Duncan Wood]

Connect inverter chassis to car chassi, plug rcd in inverter output.

[Lobster]

Oh yeah, forgot to mention: when using the 240V->12V PSU with the inverter, the PSU buzzes moderately loudly whereas it's silent when used in the house. Is that an issue?

David

[John Henderson]

I shoudn't think so. But a cheap inverter will output "modified square wave" (with lots of harmonics) rather than sine wave, and this often causes an audible buzz in electromagnetic components downstream. The PSU may run a little warmer, but shouldn't let the harmonics through.

John