Leisure batteries and split charge relays

[Trig]

I have a van and would like to run some lights in the back of it, charge electrics, maybe even run a small fridge.

I was looking into the leisure batteries and wiring it up so that it gets charged as the vehicles running, but then a thought occurred.

I may be missing something here, but rather than all the split charge stuff, why not just get a battery pack that you can charge off the 12v or usb socket? Plug it in when driving, unplug it when not.

Why bother with all the split charge relay and wiring etc? Do you get more of a charge than the 12v or usb could provide?

 

[Broch]

In short, Yes. If you're fitting a full automotive or leisure battery you will want to charge it as fast as possible when you get running again. If you use a split charge system with a crowbar relay you will be charging at full alternator capacity; orders of magnitude more than you will get down a 12v fused socket. Even a solid state switch can give you tens of amps of charge (the one I had was over 100 Amps; I now use a relay unit)

Always fit an automatic switching system no matter which method you go for; forget to disconnect just once and you could be stranded. Believe me (I speak from experience ) when I say that the solenoid 'clunk' from a discharged automotive battery is more than disconcerting if you're trekking in the middle of nowhere. On some systems you have the ability to force the two batteries together via the crowbar relay for a couple of minutes; that can be used if for any reason your automotive battery is low (on a freezing morning for example).

Be very careful what fridge you get; this is an area where you get what you pay for. A good quality trekking 12v fridge consumes very little current compare to a cheap Halfords 'cooler' (other suppliers available )

 

[srod]

Yes, but a split charge relay saves you having to remember to plug in your charging system when the engine is running, and unplug it again when the engine is stopped, thus reducing the risk of ending up with either battery flat.

Also, wired in correctly, a split charge relay to a leisure battery will allow you the maximum charging performance your alternator can deliver, which can be 10's (or 100's) of amps, which means your battery will get charged quicker.

Your USB port will only put out an 1 or 2 amps at best, your 12V socket (cigarette lighter) a lot more but limited to the size of fuse or how heavy duty the wiring to is is, either option will be messy.

EDIT: post was in reply to Trig, ... and yes agree with Broch!

 

[Trig]

Alright, thanks.

Not got a clue about all the amps,whats and volts etc so figured it couldnt be as simple.

Its something ive been thinking about for a while, but the fridge is the only thing that i would really need it for.
Without it the lights could be battery powered, and i have a small powerpack to give my electrics a charge or two anyway, so i need to give it bit more thought and see whether its worth it pretty much just for a fridge. Not often we get this weather, usually the van is the fridge

 

[Bishop]

Broch pretty much nailed it there. For charging efficiency (and safety) nothing is going to beat using a relay to connect the spare battery straight to the alternator. Yes it does mean messing about with a chunky relay and wrestling heavy cable like old 32amp cooker flex. However there's way less risk of accidental snagging and wires or plug-in regulators overheating.

A fridge though is going to be the true pain. Avoid the ones that use a Peltier cooler, these will suck the life out of the battery fairly quickly. If you can stretch for it seek out the 12v/gas powered units. Though the specs will say 12v DC automotive 12 volts is typically anywhere between 13-18 volts and these fridges simply do not care. There's no sensitive electronics or pump motor inside just a heating element.

 

[Broch]

Not got a clue about all the amps,whats and volts etc so figured it couldnt be as simple.

Don't be afraid to ask; this was my 'metier' for over 30 years (well, applied to military vehicle anyway).

 

[gonzo_the_great]

I have a split charge relay on my second battery on my LR. It's wired to the main battery, rather than to the alternator, using fat (16mm^2 ?) cable. I do have a 100amp fuse inline of the relay, just in case.
The physics of the batteries tend to limit the initial charging current. So I wouldn't worry too much about that. And high initial chatge current only lasts for a short time, before the volteges come up.
The relay works by sensing the voltage on the main battery. Once that is up to charging voltage (about 13.5V if I recall correctly) it switches in the secondary battery.

I also have a battery isolator switch, one of the types with a red flag key, connected across the split charge relay (and fuse). That allows me to optionally tie the batteries together. And given the thick cable I used, I can quickly jump start the car from the second battery, if required. (And it has saved me a good few times!)