Old/new compressor

Just out of interest, what connections are there on this split-charge unit?

You could use diodes, you'd only lose about 0.6v over them, but the big disadvantage is the fact they are always in circuit, so your second battery is always pulling from your primary - not just when the alternator is running. And big ones aren't too cheap either...

Personally, I'm not keen on running a relay off the battery light circuit (like it doesn't show!) A little one would need less current to work, but it's still an extra load.

I'd still run a relay as a 3055 is only rated to 15A and if it blows would be harder to replace (just getting at it and soldering rather than a few blade connectors) - If you were to say use the radio in the 'van, along with lights and maybe a heater control for most of the night, you could easily be looking at a 25amp or more charge demand for a short time. I'd say a relay would be more forgiving of a short term overload.

Basically you've got big power in cable (positive) small switching wire (to ignition lamp) 2 outlets (one does battery charge, the other does the fridge. My fridge is 100w, but others are more powerful). Take into account the fridge runs flat out all the time it's connected (no thermostat on the 12v side). Then you've got a combined earth.

I was thinking about using a transistor to power the relay(s), as even if I got a big enough transistor or FET it would need one hell of a heat sink, & like you say would be more difficult to access. But using a transistor to power the relay, along with the correct resistor, could make less of an impact on the field circuit & possible stop that relay flutter you mentioned above (probably the reason mine has gone so quickly).

My caravan also has a unit called a "UniCharger", this takes 12v from the car battery, boosts it to around 13.5v (battery charging voltage) & trickle charges my caravan battery from the car over night. It's limited to around 4amps, so it won't run the car battery down. These units were very expensive when new (this one was fitted by the previous owner around 1999-2000 at a cost of over £95 just for the unit!) mine seems to have stopped working, but I haven't had a chance to test it yet. But when working this helps keep my battery topped up, but I suppose should be disregarded for the purposes of this conversation.

So what do you reckon? Use a transistor to switch the relays, or try not to use the battery light at all, maybe go for voltage sensing circuit or something?

Also, I don't wanna hijack your thread too much.

Don't worry about the hijack, it's going to be a while before I get time to actually 'do' anything and it's all related anyway.

I did realise I forgot something though - a split-charge relay works exactly opposite to what I want to do - it energises the relay when the lamp goes out... I suppose I could use the 'other' terminal on a changeover unit, but I still don't really like the idea... Even though people have been using them for decades!

If my idea works and you want to steal it, you'd have to use a changeover relay to do a split-charge. (I say my idea, it's probably been done loads in the past...)

If it's just to switch 'normal' relays (like the type you get for aftermarket spotlight kits) then you could get away with a smaller transistor to do the job - or much less of a heatsink on a bigger one.

The voltage sense systems obviously work, you could make one but it's a lot of design if you are starting from scratch and the components (if you have to buy them) probably wouldn't be much less than a retail unit.

Ok missed the bit about your pressure switch, easy enough to fix.

RLY 1 to ground, correct it will be at 0v potential in a non running state but once the alternator gets excited it will come up to 12v canceling the bulb throwing the relay to open cutting out the starter. This relay is also in conjunction with the starters own soleniod switching the 12v feed to the actuator (ACT in diagram).

Guys don't worry to much about wandering off subject most of the threads on here do but they get back to the main topic in the end so all is well. If it's a problem I can delete or move somewhere else if you want me to.

Stu.

stationary stu wrote:Guys don't worry to much about wandering off subject most of the threads on here do but they get back to the main topic in the end so all is well. If it's a problem I can delete or move somewhere else if you want me to.

Stu.

I think it if gets too long and meandering then I'll probably do a new 'compressor build-up' thread. A lot of the information so far is interchangeable anyway as I'm using a mish-mash of small engine and car parts - plus nothing has been completely decided yet.

Right, this thing has to get done soon. I've been fiddling a bit but it's annoying trying to work with it while everything is dirty.

I've been trying to clean up some parts and it's bugging me I can't get them clean. I need my blaster.

I'm setting the cabinet up over the next few days but I'm going to have to use my tiny compressor. Then I can get this big one cleaned up and sorted, figure out the electrics and have a decent air source at last.