Battery charger for my 18650 12 volt input

I have a nice new service truck that I am setting up and would like to permanently mount a charger for my 18650's that runs off the vehicle 12 volts. I require two extra batteries on any given day. i would like to get away from the 12 volt accessory plug if possible. Ideally I would use the service truck charger exclusively for my work flashlight batteries in turn maximizing each cycle. In addition, should I be concerned that the ambient temperatures in a service truck in the Chicago area can be extreme both hot and cold.

For a simple solution, why not get a couple of 18650 chargers that are small and can be powered from 5V USB (such as the XTAR MC1), attach them to a multi-out 12V->5V USB adaptor and use that? Such an adaptor can be opened up and modified to run directly from a 12V feed. XTAR also make a model that can charge 2x 18650 from USB in a fairly slim profile. They also make several that operate directly from unfiltered car 12V supply (NOT 24V! many trucks are 24V!).

Yes you should be concerned about temperature. The inside of a vehicle in a hot day can reach very high temperatures, and you should not really be charging unattended either. Most of the commercially available 18650 compatible chargers do NOT feature temperature cutoff for undertemperature (freezing weather) or overtemperature (hot sunny day). These are both dangerous conditions for charging lithium ion.

Please don't charge unattended.

I am less concerned about price than I am safety, what I currently do is change batteries after my day and I am SOL if I run dead during the day, it would be nice to have a set of charged batteries in my truck just for a heavy day and not have to keep track of my spares because they would always be with me. I can burn through 3 headlight batteries in a single day when conditions are right. My eyes aren't what the used to be and I often need my headlight on super High. I also have the option of plugging into my inverter with a 110 charger, but with a modified syn inverter I question how safe of an option that would be. I am hoping for a good 12 volt option. As far as the USB charger option, I do not have any experience charging 18650 with a USB charger, I would think that it would take a long time to fully charge defeating my purpose. Is there no safe way to charge high quality protected 18650 batteries unattended?

Slow and fast are relativt things. A Nitecore F1 or XTAR MC1 would be compact, and charge relatively fast. They would Also give you choice in charging from 12V USB charger (Anker has good ones), or even power bank.

Which batteries are you using though? Protected cells with over temperature protection could be an idea.

And what's okay charge time? Need it done in less than an hour? Or would four be okay?

What I'd probably do, is get tree Solarforce V2 battery cases, and fill them with cells. Two in the car, the one you're using, and the next you'll be using, and the last case for batteries being charged at home. That'd give the safety that you always have what you need, even if you forget to charge or to bring a fresh case for the cabin, you'd still have another pack, still enough cells and still extras for backup.

The I'd add a Nitecore F1 or F2, just to have the ability to car-charge in an emergency. They're surprisingly fast for their size and price.

A larger charger might not help you much, since both cold and hot temperatures mean you should charge more slowly, if at all. There are no chargers currently on the market where I'd feel comfortable with unattended charging. The SkyRc MC3000 comes closest, in large part because of decent temperature monitoring, but it's hugely overkill, overpriced (for this job), and oversized.

For 12V itself, most of the chargers on the market support charging from either USB or 12V, so either would work off of a 12V supply.