can you use a DC motor controller to charge car battery from a higher voltage battery
 

well, can you use a DC motor controller to charge car battery from a bigger higher voltage battery?

say you wanted to maintain 14.5v and didn't want the cars voltage to drop with your batteries depth of charge like with the single deep cycle battery. or you had several higher voltage batteries available and didn't want to cut them up. instead you could use the 24v or 48v and then use a DC motor controller to PWM it down to 12v.

would the 48v square wave mess with the cars electronics? or would the 12v car battery adsorb them if the controller was set low enough.

What you're looking for is a DC to DC converter. Possibly a solar charge controller might work (and are fairly cheap), although I'm not sure they can handle much amperage.

For just charging up the 12V system, I'd go with a DC/DC. It's simple and cheap.

I'm fairly sure this was discussed on the EVDL in the past, and I believe it's possible. Try searching the EVDL archives, and if you can't find it, DM me and I'll give you the email address for the person I believe had the definitive answer on the EVDL discussion. In that case, they were talking about using a higher voltage pack of flooded golf cart batteries to charge up their lower voltage "power pack" of Optimas. That way, you could get the range you wanted by using cheap golf cart batteries, and the power output you wanted with the higher amperage Optimas.

you'll probably loose about 7 to 10% of the energy but a DC to DC converter is probably the best thing to use. Look in to a buck converter.

I actually have that idea to offload the alternator. A buck converter is surprisingly cheap and easy to build. At those voltages, over 90% efficiency is not unreasonable and 95% or more is possible.
BTW, universal input power supplies that operate on 100-240v AC will almost always work without modification on 150-370v DC. Some Prius owners used that approach to operate various equipment on-the-go.

I have thought about building a circuit to provide just enough boost to allow a 12v deep cycle battery (11.5-12.8v) to charge a regular car battery (13.8-14.4v), but such small boosts are tricky to design. I'll send a message to an engineering friend of mine, however. (She knows a lot about switching power supplies.)

95% or more efficiency is a pipe dream at high current levels and fluctuating loads. In the 11 switchers I have designed the most I got out of them has been 92%. I also had low loss capacitors and low voltage drop diodes and low RDS MOSFETs to work with.

Most of the losses are on the low voltage side. Using Litz wire, oversizing the windings, and using synchronous rectification all help efficiency. Low switching frequencies also reduce switching losses but increase size of parts.

Getting high efficiency at low voltages is a big challenge. Nonetheless, 95% efficiency is certainly possible at low voltages. The PTV08T250W is 95% efficient when outputting 3.3v at 50A from a 12v supply. Getting that kind of efficiency when outputting 14.4v from a 20v+ source should be a piece of cake by comparison.

Okay a team of engineers came up with a module using a 3 phase buck converter can hit 95% at 3.3V when used with huge caps that have an ESR of 0.005 Ohms. Good luck finding caps rated at 16V with an ESR that low.

Coming up with a suitable and efficient inductor for this application is going to take more than just throwing thick wire at it or using Litz wire. With larger windings you have magnetic leakage between the coils. Also the outer windings have a larger distance to travel potentially increasing resistive losses. Litz wire is only good for high frequencies. An inductors job is to resist change in the flow of current. In a proper buck converter the change in current in the inductor is minuscule compared to the current of the load. The inductor's current is essentially DC and Litz wire won't help. You would have to go with wire made of silver.

Dropping the frequency of switching speed also increases the output ripple voltage. An increase in ripple voltage increases the loss in the capacitors caused by the capacitors' ESR.

In short, designing an efficient switch DC to DC converter takes careful matching of the efficiency curves of all the components and operating parameters. Don't expect to just slap one together. My hat is off to the TI engineers that hit 95% efficiency as it is a true work of engineering.

if i wanted to use a switched mod psu as a DC converter (48v to 12) how could i mod it so that it accepted 48v and didn't refuse to switch on because it was below 110v?

No practical way. Best bet is to just build a buck converter just for that purpose.