I am 99.9% sure that the center pin is positive and the outer part is negative. At least that's what a test light has always shown me in the last ~20 years in every vehicle that I have worked on.

 

Positive in center and negative/ground on the outside is typical. Maybe not in England.

Think light bulbs. Do you have a multimeter? They come in handy, use one frequently for testing all sizes and types of batteries (1.5V, 6V, 9V, 12V, whatever).

Test light? finger in socket?

 

finger in socket?

That's the BEST method! Doesn't work in vehicles, though.

 

Unless its wired improperly, it's as Carbuff and Jeepman said: Negative ring, positive center pole. This is the same across every vehicle ever made.

 

I think the OP put their finger in the socket and now is RIP. Sorry for giving that advice. 😔

 

Or just get a battery bank? I can't imagine lugging a lead acid up a mountain, or if you're near the van the battery should be capable of many phone charge cycles:

----Math---

Figure a car battery is probably 40-60 Amp/hours
~50Ah × 12v = 600watt/hours

Cellphone battery is roughly 3000-4000 mAh, or 3-4Ah, at 3.7 volts.
3.5Ah × 3.7v = ~13W/h

Most common smartphone charger is 2.4amp at 5 - 5.2volts.
2.4A × 5.1v = 12.25watts




600W/h ÷ 13W/h = 46
Your car battery holds roughly 46 times the energy of a phone battery

600W/h ÷ 12.25W = 49 hours
Your battery could run a phone charger(charging a fully dead phone) for ~50 hours.

 

Figure a car battery is probably 40-60 Amp/hours

"Deep cycling" a car battery will cause it to degrade quickly and fail prematurely...

"Group 34 batteries are medium-large, but still rather powerful batteries, featuring 20h capacity in the 50-75 Ah range"
Automotive wet Pb battery cycle is designed at ~20% (beyond that the plates start irreversible degradation), so regularly pulling over ~15Ah from your car battery will lead to capacity loss and a need to replace the battery.

 

WOW! A firefighter that's good at math too.

I like those calculations.

 

Don't most modern chargers auto-switch polarity anyway? I know AC chargers do this (especially in US).

I agree with Edy that a purpose-built power bank is a much better alternative for a "portable" device. If you had a static assembly being used for solar or something, then maybe PbAcid is a cost effective option. But for just a cellphone, get a large battery bank, and if it's in the budget, get one with an AC output, if needed. Lots of those available in the $125US/£100 range.

Otherwise, steer clear of lead-acid unless you're prepared to really build a stable/safe enclosure. Li-Ion is here now (see "Jackery"), but LiPo is the way of the future...and better for temperature swings found outdoors in camping environments.

 

I don't know of any 12v accessory that switches polarity aside from the devices to check if they are operating. USB C chargers will switch on the output side only.

 

Probably not for DC, you're right.

 

Deep_Truck,
Thank you for the reply, and thank you for taking the time.
I do apologise for my late response, I am off wifi in the widerness in vancouver island
(parked outside a McDonalds now)
Thank you - all the best

 

The great Canadian "Golden Arches" wilderness

 

Yep, Cottage Country McDonalds. Sour dough bread and rabbits. Lumberjack burgers, axe included.