I’ve got a head scratcher that I hope some of you dopers who are smarter than I can help me figure out.
I’ve added a second 12v battery to my trailer, so that we can camp for longer periods of time without having to hook up to electrical power or a generator. The positive terminals of the batteries are hooked up to a switch similar to this one, so that I can switch between battery 1 or 2, or use them both in parallel.
The output of the switch goes to the 12v side of the trailer’s AC-DC converter/battery charger. The negative terminals of both batteries are connected to the trailer chassis.
What I would like to do is to install a pair of LEDs inside the trailer so that I can tell at a glance which battery (or both) the trailer is using. But I can’t for the life of me figure out where I can connect the leads of the LEDs to give me that information.
It seems like any of the terminals on the batteries or the switch will always have voltage present (unless one of the batteries is dead).
Can anyone figure out how I can wire this up to do what I want?
What you need is a two-pole switch; one pole to switch the batteries, the other to switch the LEDs.
ETA: But, that won’t work for both in parallel. I can’t think of a simple way to do this without logic. I’ll get back you at lunchtime, if no one else beats me to it.
The simplest way to do this would be to get a bunch of #4 or even #2 wire and put the switch itself inside the trailer. Depending on the layout of your trailer and the locations of the batteries and where you’d like the switch, this might need three feet of wire or fifty feet.
On my old trailer, the battery compartment was under a seat, so it would have been a very short detour to run them through a switch mounted through the base of that seat. My current trailer has the batteries out on the hitch, so a bunch more wire would be needed.
I do have the switch mounted inside the trailer, but it’s not readily accessible. There’s a small compartment underneath the step to the front bunk where the AC-DC converter is installed. I put the switch on the back wall of this compartment and ran wires to the battery on the tongue of the trailer. I can get to the switch by lifting up the compartment lid (aka step platform) but I’d like some kind of visual indication without having to go to that bother.
Yes, I suppose I could mount the switch somewhere more visible, but I’d prefer not to for aesthetic reasons.
Yeah, I can’t think of any way to do it without modifying the switch in some way. You really need two separate circuits; one to switch the batteries and one to set the LEDs. Perhaps someone makes a battery switch with accessory termianls for just this application?
One thing you could do is inect some high frequency into the plus terminals of each battery-side on the switch–use two different, nonharmonc frequencies–and then have a detector downstream detect which frequency(ies) is/are present and turn on the appropriate LED(s). Probably more complex than you want, but that’s all I got.
Try measuring the voltage between each battery’s negative terminal and the lug where (I assume) both batteries are grounded. If the ground wires have any resistance at all (and they probably do), there should be a voltage drop across each ground wire when (and only when) the trailer is drawing power from the battery it grounds. There might actually be enough voltage drop to drive an LED directly. If not, there’s probably enough to switch a transistor that drives the LED.
The voltage will vary depending on the current being drawn. Measure once in the trailer’s quiescent state (low power draw), and again with everything turned on. Pick an LED/resistor combination that can handle the highest voltage drop, or possibly tap the ground wire partway between the battery and ground lug. If the quiescent current is very low, you might find you have to turn something on for a minute when you want to check the LEDs.
If one ground wire is longer than the other, I’d tap the long wire at a spot that’s the same electrical distance between the two batteries’ negative lugs, and ground both LED circuits at that point. (I’m assuming both ground wires are the same gauge.)
I’m just a tinkerer, not an EE, so someone smarter may come along and shoot holes in this idea.
Thanks, all, for your input. Q.E.D., especially. Your replies got me to thinking that maybe what I need is two double pole switches, one for each battery, with one pole of each switch going to the load, and one pole going to the LED.
For now, though, I think I’ll bag the idea and get by with what I’ve got.
Sorry, mwbrooks, I didn’t see your post when I wrote my last response. In this case I don’t think that would work, because the negative terminal of battery 1 is connected directly to the chassis, and battery 2’s negative terminal is connected to battery 1.
ETA: Besides, I’m not enough of a tinkerer to start messing around with diodes, transistors, etc. If I can connect an LED to a wire, fine. Otherwise, I’m not messing with it.
There are remotely-controlled battery switches for marine applications, but like pretty much anything else meant to be used on a boat, they’re made of finely-machined compressed expensivum. (Not that RV gear is exactly cheap either!)
A quick Google gander (a googer, perhaps?) turns up such switches in the $200+ range and wiring diagrams that suggest professional installation is a good idea.
Something else you might want to wire in is a battery isolator - it’s a couple of hefty diodes that electrically isolate the batteries for charging purposes and prevent a dead battery from turning a fully-charged battery into a half-dead battery when the selector is turned to Parallel. There’s a similar device called a battery integrator that appears to be slightly more automatic in function, but they’re more complex to wire, more expensive and can’t be used with external chargers - they’re meant to exist within the closed environment of a boat’s batteries, starter, and alternator.
All told, you could easily spend $300-600 on this gear without installation.
I’m thinking of a wrapping of smaller gauge wire along a length of battery cable on each positive battery lead, and amplifying the derived current flow to activate the LED. But, like the other suggestions, it is probably more of a PITA than you want to become involved with.
If you did mess with it, you could still come up with something. With your setup, there should still be a slight voltage drop between the two batteries’ ground lugs when the second battery is in use. (The one most distant from the chassis ground.) If nothing else, you might hook up a cheap Harbor Freight VOM there and use it as an indicator.