Transformer problems, Output 15.8volt not 12 Volt

Hi, I have just wired 2 x 12 volt outlets to allow us to charge our phones overnight. I wired them into the 12 volt light circuit and foolishly did not check the voltage just assumed it would be 12 volt.
When I put an 12 volt USB adaptor it fried immediately. So in a genuine male approach I then checked the voltage and it was 15.8 volt with nothing else on and 14.3 volt with the internal lights on.
The 12 volt lights seem to work OK, I have not tried any other appliance to see if they work.
Can anyone comment about the sensitivity of a USB charger adaptor to the extra few volts. I am goint to try a 12v-24v USB adaptor to see if this copes.
Also the sockets are wired centre positive, outer housing negative, I believe this is correct polarity

Have you thought about a solar portable phone charger like one of these on ebay?

You can have it charging the battery bank in your van, and later on pop your phone onto it, and charge it from the inbuilt battery overnight.

don't know what circuit you've plugged all this stuff into but I stick to the cigarette lighter outputs that are regulated by the battery,
couple of options available. the high tech option is a 4 socket board that you attach somewhere under your dash that usually has a voltmeter, cigarette lighter output, a double high or one high and one low usb outlet and a switch. the switch you can wire up any which way you want. I usually set them up as voltmeter always on. This way, when you plug your dash mount solar panel in you know what your battery voltage is just by looking through the window. Most importantly, allows battery to charge with ignition off
The usb, cigarette lighter plug I route through the switch. ie,. accessories work when the switch is on, regardless of whether accessories is on or off.
Another option is to purchase a multiboard outlet. They all have a voltmeter built into the plug these days so it's like a free voltmeter for the cars' electrics.
option 2; Using a USB plug in that goes into the cigarette lighter is a safe bet. Been using them for years without a problem.
Plugging stuff into those inverters is a worry if that's what you're doing. Their 240v ac output is not sinusoidal but a 'flip flop' or square wave and that can play havoc with any load that likes a sinusoidal wave form, like synchronous motors. eg, fridges, exhaust fans. it will just burn them out and they won't even turn.