Charging L5 in a car/camper

guru · February 15, 2026, 8:43am

Hello,

My son and I we own an old firefighter Mercedes car and we are building a camper vehicle in its cabine. The voltage on board is 12V and we will have USB A and C sockets at the walls.

Is it save to charge the L5 directly with 12V?

Thanks

irvinewade · February 15, 2026, 10:11am

I would say “no”. Not safe and might not work even if it doesn’t destroy the phone.

In my car I use a cigarette-lighter-slot adapter that gives a (hopefully) compliant USB-C PD power source and that works fine with the Librem 5. The difference is though that this is an off-the-shelf car. I am not building it myself. That said, I have no idea what voltage the cigarette-lighter-slot provides.

dos · February 15, 2026, 10:21am

Technically, the Librem 5 is able to handle up to 14V on VBUS.

In practice, it’s a very bad idea to expose 12V on a USB-C pin (and even worse on USB-A) without performing PD negotiation, and the L5’s PD controller config is set to never negotiate more than 5V because of the risk to the 5V-rated CC lines (which are next to VBUS in USB-C) in case of a short. Older batches were very susceptible to damage from that; on Evergreen the risk is small, but AFAIK still non-zero (say thanks to TI).

Photon · February 16, 2026, 6:04pm

So far as I know, all “new” (1970+?) vehicles use 12v, negative ground. My cousin had a 60’s Austin that was positive ground (positive “earth” UK).

My old military trailer was wired for 6v, so that old fire wagon could be anything. Check the battery!

irvinewade · February 16, 2026, 9:46pm

Yes, I think you are right. So if the OP wants to wire in a car charger like mine (which offers a USB-C PD port for the Librem 5 (and for more recent iPhones) and a USB-A port (for older iPhones), maybe it would work. So the car charger handles the conversion from 12V to whatever is the negotiated phone charging voltage. However that might not apply in the EU and it might not apply to the vintage of car that the OP is working with.

guru · February 17, 2026, 6:17am

Just for the background two photos below. The one shows the car the other the inner part in construction.

Technically the car has two batteries: one lead liquid battery for starting the engine (100 Ah) and another one for the “home” (200 Ah LiPo4Fe). This one is connected to the starter battery over a relais and gets charged only when the engine is producing electricity. Later it will be charged by solar panels. Both are 12V.

The idea was/is to mount USB (A+C) sockets to the wooden walls for charging cellphones, laptops etc. I see now that they should have something in addition behind the sockets to produce at least the correct voltage or even protocol. I will look for it.

irvinewade · February 17, 2026, 6:51am

Does the siren still work? (although I would be fairly confident that it is not legal to use it)

guru · February 17, 2026, 7:09am

The siren must be electrically unmounted. Apart of this it’s still working. The blue lights on top must be covered. They’re still working below the cover and you can see it if you take a closer look

guru · February 17, 2026, 7:10am

@irvinewade , can you please specify what this is or point me to a source. Thanks

biketool · February 17, 2026, 9:01am

It is important to remember that the starter is directly wired into the electrical system with a solenoid especially on older vehicles. When that solenoid breaks the starter connection to the battery(you turn back the key to stop the cranking of the starter motor) the high amp connection the voltage spikes into the whole system with only the battery acting like a very slow capacitor to sponge up that spike. Automotive power needs at a minimum a voltage regulator though if you use a good automotive 12v to USB-C circuit it should contain the required protection.

guru · February 17, 2026, 9:38am

The USB sockets will only be connected to the serarate 200Ah battery and this is connected through a relay with the starter battery which connects both plus poles only when the alternator gives for some seconds enoungh voltage to the starter battery. Even this will be cut again once the solar panel are installed and feeding this second battery.

guru · March 9, 2026, 6:59am

Meanwhile we’re a little further with the USB sockets. It turned out, that the sockets itself has electronic inside and adjust the 12V DC to USB. Its specification says:

material        CNC-milled aluminium
voltage         input DC12V-24V
dual connector  USB Typ-A + USB Typ-C PD + QC3.0
output          5V/4,8A, 9V/2,7A, 12V/2A```

With a L5 connected, the following happened in that order:

L5 connected with USB-C cable, does not start charging, also not after turning the USB-connectors on both ends
Using an USB-C to USB-A adapter (as shown in the photo), charging starts fine.
Removing the adapter, charging starts now also but makes at start a scratching noise for 1-2 secs and then the seconds of the clock in the bar or lock screen give with any new second a tick like an old wall clock does.

What does this mean? I will for safety only charge using this USB-C to -A adapter.

irvinewade · March 9, 2026, 7:10am

Doesn’t sound great, mate.

I would be looking at the relevant /sys information to see what is actually being negotiated in each scenario. (For example, yes, OK with USB-A but what current? May be slow as - but at least safe.)

There may be merit in using a USB meter to get some information that is independent of the Librem 5. (Obviously you want a USB meter that supports both USB-C PD and USB-A whatever.)