cheap(er) 2nd battery setup

[photo_van]

I've been thinking about a second battery in my camper van for sometime, but didn't want to fork out the money for a sealed lead acid battery. It would just be needed for some lighting and to run a 700 watt inverter now and then to charge a computer or phone that we failed to charge while driving.

I carry around one of those jumpstart/compressor Powerstation units so I thought; "Why not use it as the battery?"

I dove into my memory banks from high school electronics class (40 yrs ago) and came up with the following schematic to create an isolator circuit and AC circuit:

I would need a switch, two normally open relays and one normally closed relay. I had recently scavenged an under the hood fuse box from an '03 Astro that had just that (and more!) The box has three lugs (two visible at top), that were easily isolated to provide: main DC in, Aux DC in and DC out to inverter. Other fused circuits: lights and two DC lighter type ports.

This is the top of the box after reconstruction; the grey relays are closed, the black ones open. It should look pretty familiar to anyone thats looked under their hood.

I was also in the process of pulling out the rear heat and building a small cabinet behind the drivers seat. The in dash rear heat switch and harness plug very conveniently worked for switching from main to aux power. Since the switch is only powered when the ignition is in the "run" position, the isolater automatically switches to aux battery when the engine is off.

This is the inside bottom;

Since most every circuit in the van runs through here, there are a lot of wires.
It had me scratchen for sure, but after a while I could trace the circuits I needed. The DC output lug is now visible in the lower right

[MountainManJoe]

Interesting. I'm not sure what's going on in the center of the diagram, but are the relays rated for the 60A that your inverter can draw?

Why is there AC going to the powerstation? Is that to charge it? Does it not charge from DC?

Also, can't you get adapters for charging the phone and computer through a cig lighter socket?

I like the McGyver-ness of this, and I think I'm going to "borrow" the idea of using a salvaged fusebox.

[photo_van]

Interesting. I'm not sure what's going on in the center of the diagram, but are the relays rated for the 60A that your inverter can draw?

Why is there AC going to the powerstation? Is that to charge it? Does it not charge from DC?

Also, can't you get adapters for charging the phone and computer through a cig lighter socket?

I like the McGyver-ness of this, and I think I'm going to "borrow" the idea of using a salvaged fusebox.

The 700 watt inverter is an overkill for my needs, but the price was right at $39. If I do need full power for some reason I can easily jump straight from the battery to the inverter. From the best of my research the relays seem to be rated at 25 amps and I have them fuse protected.

Apparently sealed lead acid batteries can be easily ruined by over charging. My unit recommends avoiding DC charging to prevent over charging. Although they do say in a pinch you can, but not to exceed 2 hrs. I'm guessing there is a regulator on the AC charge system and 14+ volts DC from the alternator is too high. The volt meter on the unit shows max charge at 13 volts.

Yes you can buy DC chargers for your electronics, they cost money and every phone and computer we have (and three different camera bodies) require a different one!

[photo_van]

Sorry I dont have pics of everything, I was doing this entire project two days before we were leaving on a two week road trip.

This is inside the top cover showing circuits and components.

Another bit of connivence is the fact that the power circuit to all the dome lights is in the piller right behind the drivers seat. When I pulled all the plastic paneling for the cabinet the harness was exposed right where I needed it. Just grab one of the four orange wires and hope for the best. Again, due to the isolator circuits, the lights are powered by the main battery while driving and automatically switch to the aux battery while parked.

Here's the setup installed:

still need to tidy up the wires.

Here's the cabinet with the PowerStation. Inverter is mounted vertically behind the seat.

[MountainManJoe]

I guess if the unit says not to charge it that way, then don't. But the starter battery is sealed lead-acid and it seems to do OK on the 14+ V from the alternator. Once full, a battery naturally stops drawing current. At rest, it will also drop to around 13V, but to charge it, it needs at least 13.9V. Maybe auto batteries are just designed to handle high current better.

How does the automatic switching work?

[photo_van]

I really don't know anything about any of this, I just winged it; wung it?

Aren't car batteries vented not sealed? Perhaps I have my battery terminology mixed up, but it seems that I've read (other than in the instruction manual) that this type of battery is extremely sensitive to over charging.

I did run the large wire to the inverter that came with it so that I could jump and bypass the isolater should I need to for some reason. Also you said something about 60 amps? Isn't 700w @ 120v = 6amps? Again, I don't really know so insight is great. No where on the inverter did it say how much it draws

The relays also didn't have an amp rating on them, but I kinda found some reference somewhere that suggested 25amps, but that's not solid data. I haven't had a melt down yet (well actually I did when I thought I needed a diode in the circuit, it melted.....but that's another story)

The auto switching works only if the selector switch (rear heat) is in the ON position. When it is ON and the ignition is ON, relay #1 closes the main battery circuit and relay #3 opens the Aux battery circuit. When the ignition switch is off the relays are at rest with the continuity in the aux bat circuit. Both circuits feed the DC distribution, so whatever draw that's there is automatically switched with the circuits. As well, when the system is at "rest", relay #3 is open so the aux battery doesn't try to "charge" itself thru the inverter; a loosing proposition.

[MountainManJoe]

Thanks. That's pretty clever.

Aren't car batteries vented not sealed?

I'm not 100% certain myself. But if it were vented, then you would lose water over time. Older batteries needed to be topped up, but modern ones are supposed to be maintenance free.

Isn't 700w @ 120v = 6amps?

Correct, a 700W appliance will draw 6A at 120V. But that's your high side. On the low side, your inverter will be drawing 700W @ 12V = approx 58A. Actually, since the inverter isn't 100% efficient, it will be more.

In Europe, mains voltage is 220V, so appliances need only half the current ours do. As a result, residential wiring can be thinner and cheaper and there is less loss.

I'm not surprised your diode melted. The big ones in the rectifier in your alternator pass up to ~30A, and they have huge heat sinks on them.

[photo_van]

Correct, a 700W appliance will draw 6A at 120V. But that's your high side. On the low side, your inverter will be drawing 700W @ 12V = approx 58A. Actually, since the inverter isn't 100% efficient, it will be more.:

That's good to know. I was wondering why the input cables to the inverter were so big! At 25amps for the relays I guess I should be good then to ~300W or 2 amps @120: enough to run one laptop which is our biggest draw.

Also note that the manual switch (rear heat) isn't really needed. it was simply a very convenient way to get ignition switched power to the relays

[photo_van]

Aren't car batteries vented not sealed?

[quote-"timeless"] I'm not 100% certain myself. But if it were vented, then you would lose water over time. Older batteries needed to be topped up, but modern ones are supposed to be maintenance free.[/quote]

I'm pretty sure "maintenance free" is a misnomer. Modern batteries still have vent caps, they just aren't as obvious as the old style. They should still be check and "maintained" if you want to get 10-15 or more, years out of them (which I get out of mine). The only true maintenance free lead acid batteries I believe are gell cells or some other type that suspends the acid in cloth or some such thing between the plates. These are much more expensive then the standard lead acid battery.

Again, I really have no idea what I'm talking about, but I think the above is somewhat accurate.

[MountainManJoe]

Cool. Let us know how it performs. Some cheaply made chargers and devices might have a problem with the "dirty" sine wave inverters produce. (the main reason I prefer DC chargers). Just watch out for excessive heat or interference (might show up as flicker on the laptop).

By the way, what is the capacity of the powerstation?

[MountainManJoe]

Modern batteries still have vent caps...

Yes, I believe you're right; They are valve regulated. Still, the caps are seldom removable which makes maintenance impossible (aside from equalization). I guess they're designed to minimize gassing, and they probably contain enough water to last their lifetime (or until a few days after the warranty expires).

So back to your powerstation. I'm going to guess that the charging restriction is a safety consideration. Exposure to alternator voltage could generate a bit of hydrogen off-gassing. Under the hood, this is does not normally pose a problem, but this thing is intended for indoor use.

[photo_van]

PowerStation is 400a (1000a peak). My brother had one of these apart after the battery fried. I think he said it was 12 amp/hr. Unless one is running kitchen appliances, this seems like it might work out as a reasonably priced alternative and a compresser ta boot!

I re=read the instructions: "only use DC charging when 120v ac is not available. DC charging may over-charge the battery causing product failure or reduced capacity. Limit DC charging to two hours"

Don't know what to say re. vent caps; I have four vehicles and the batteries all have removable caps in strips of three. Takes a screw driver to easily pry them out...

[MountainManJoe]

OK so the PowerStation probably dries out if charged too long. 12Ah should run your computer for an hour or two.

You must be lucky. My starter battery is AC Delco and it's all sealed up. So was the OEM one in my mother's VW. My deep cycle auxiliary battery has removable caps, thankfully. I had a little spill

[mdmead]

You must be lucky. My starter battery is AC Delco and it's all sealed up. So was the OEM one in my mother's VW. My deep cycle auxiliary battery has removable caps, thankfully. I had a little spill

I'd bet the caps are there... hidden under a decal. This seems to be the norm on all the maintenance free batteries I've seen. (You can usually cut the decal along the edge of the caps if you are so inclined.)

[MountainManJoe]

I'll take a closer look next time. But I'm positive about the VW one because I looked under the decal.