DC to DC 12v chargers

[Kayos]

for use with a constant current draw like a inverter you”re going to want a higher amp output than the one i linked...

The idea is it can run off the 110ah as needed but if I need constant power I will switch over to the main battery and start the engine.
Ive had it running my tracksaw all day without the engine running so that will be rare, as long as the battery is kept topped up

 

[qwakers]

then the one i have will do the job nicely.

 

[Barking Mat]

@sako243 post #14, thank you, very well explained.

What's PWM please?

 

[Seadog]

Pulse width modulation

 

[Barking Mat]

Ahhh? That clears that up, I'll Google that later. Well Bing is now my preferred search engine.

 

[sako243]

All it means is that you're switching something on and off for a varying amount of time.

From the electronics side of things devices like MOSFETs are much better at switch on and off rather than partially switching on. They heat up very rapidly if they're partially on so it's much more efficient to "hard" switch on and off.

A switch mode power supply (SMPS) will do this. Lets imagine you have an AC 240V input, you can rectify it with some diodes and smooth it with some caps to get ~110V DC output. If you wanted 50V then you can turn on and off a switch (MOSFET) at ~50% duty cycle and if you then smooth the output with a capacitor you get 50V output. If you wanted 25V then you simply chop it at about 25% duty cycle.

The faster you switch the smaller the smoothing capacitors need to be but the more losses you get due to switching. Nothing switches instantaneously so you get some losses whilst the MOSFET is transitioning from on to off. NOTE ideally you are still switching the FET on and off as fast as you can but you are switching more often so you get more losses.

Bigger caps are more expensive but also take a longer time to charge up / discharge which is why you get some power supplies still powering equipment for a period after the mains has been switched off. So it's all a balancing act.

There is a flip side - the battery is basically a massive capacitor so you don't actually need any smoothing capacitors from an alternator. Also mechanical devices such as motors and alternators are slow so you don't actually need to smooth the PWM input to them because they simply don't notice it and only see the average.

Very similar effect to how videos work - if you change the picture >~24fps then the cones in human eyes don't notice the individual pictures because they're too slow to react. Slow it down to 10 fps and you notice the jerkiness.

If you slow the switching frequency right down on a motor you'll begin to hear it stuttering / juddering as it begins to be able to respond quickly enough.

 

[sako243]

Incidentally efficiency is also the name of the game why manufacturers started putting electric motors to drive the power steering hydraulic pumps. Yes it's less efficient but overall on the average journey it can be made more efficient.

E.g. at higher speeds you don't need that much assistance from the power steering so rather than waste energy pumping fluid around a circuit for no benefit (in a classic power steering setup) you can simply turn off the electric motor and therefore have no load on the engine. Unless you have a clutch on the hydraulic pump there's no way to be as efficient and cutting a clutch in and out on the steering is less desirable.

When you disconnect load from the alternator all you really have is bearing losses from turning it around so you can minimise those quite easily. You don't actually need a clutch or anything like that - if you don't supply power to the field windings then it simply doesn't add any additional load other than the rotating mass of the windings.