DC to DC charging circuit

[David Lowy]

Anyone want to help me design and build a DC to DC circuit to keep a 12V battery charged from a much larger 12V battery with a variable voltage?

Details:

I have a homebuilt RV6A airplane with a 12V Odyssey PC680 battery that runs down when not used for a while.

The aircraft is stored in a hangar with no AC power. The available power source is a large 12V Marine battery which is kept charged by solar panels. The marine battery's voltage drops when used by other things faster than sun can charge it, and, presumably the circuit goes higher than 12V when the battery is full and the charger is peaking.... I'll have to test it, but let's say for now that the input voltage for our circuit will range from 8V to 17V DC, but it is ok if it stops working at some floor level.

The goal, of course, is to always have the airplane's battery as full as possible, so I can start its IO-360 engine whenever I feel like going flying.

I plan to rig a cable (perhaps on a bungee cord) that can be pulled down from the ceiling and connected to the aircraft via its installed Anderson SB50 connector, which is connected directly to the battery via less than a foot of 6AWG wire. The total required cable distance from battery to plane is about 35' long but the new circuit can be within a few feet of either end if doing so will reduce the cable cost or increase the efficiency.

I considered getting a DC to AC inverter and connecting a conventional AC to DC charger, but that somehow seemed wrong and inefficient.

Thoughts?

Dave
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[Aaron]

I was doing something this once, keeping a 12 volt battery topped off from another DC source.  I also have to be as efficient as posable.  After a lot of research I used there:

http://www.vicorpower.com/cms/home/products/brick/mini-maxi-micro-converters

Vicor units pretty much rock when it comes to a wide range of input voltage and efficiency.  I recommend trimming the DC to DC at the bottom of the battery float charge rating.  

Aaron Thul
http://www.chasingnuts.com

[Chris Cudahy]

A couple thoughts - I don't know the specific batteries, but I'm assuming the smaller one is a lead acid that is < 100Ah. If the self discharge is around 1%/day you really only need 50ma or so to keep the battery topped up. Round that up to 200ma to give you some extra to cover actual usage. 6 gauge wire is massive overkill for that, 22 gauge is more like it.
One source of small, cheap current sources like that are boost converters used for LED lighting. http://www.ledsupply.com/boostpuck.php is one example - perhaps with a pwm dimmer on it.
I'd double check, but I believe you can leave a charger running with such a small fraction of the total battery current connected indefinitely.

Cheers,
Chris

[Loial Otter]

This might seem a little simplistic (especially coming from me) but what about just putting a schottky diode from the other charge circuit. When the other battery gets near full charge the charger would top up the aircraft battery as well. It would require the solar charger to fully charge the other battery every now and then but it'll keep the airplane battery at least reasonably charged at all times.

[David Lowy]

I installed 6 gauge wire for those times when the battery is completely dead and I have to jump start the plane. I want to use this existing connection for the trickle charger, so I don't add anything else to the plane.

[David Lowy]

Thanks Loial.  I thought of that, and I may try it out, but  I figure if I am going to go to the trouble of installing something, it might as well work even on cloudy days and days when the old battery is being used by other things and  never gets fully topped up.

Dave

[Loial Otter]

A step-up switchmode power supply would probably work for what you want. One interesting thing about them is when the input voltage goes above the output voltage they conduct. This is normally considered a failure mode as the current is not limited (there's an inductor and diode in it's path only). It'd be a good idea to disconnect the circuit before attempting to start the engine or at very least put a resettable fuse (PTC) in it.

To keep it regulating the output when the input can be lower or higher than the output, you don't have as many options of power supplies. SEPIC, Inverting, flyback if you use a transformer and i think CUK power supplies should work. Inverting would be the easiest with flyback being the second easiest. Since the airplane should be fully isolated, you can use an inverting regulator. Though since you're not drawing all that much power, even just doing a step-up to something like 15V and a linear to drop it back down (as inefficient as that is) wouldn't be all that terrible.

Can hack something together if we are in the space at the same time. Can show how to wind inductors too, hehehehe, and characterize them.

[Chris Cudahy]

If you do go the diode route be sure to run the math on it. If the smaller battery is somehow in a discharged state the only thing really limiting the current is the internal resistance of the much larger marine battery. This could easily be 100's of amps. Verify that the smaller battery can take this amount of current, not to mention the diode! (forgetting to plug in could cause this case the next time you do plug in.)  C --- On Mon, 11/19/12, David Lowy <Da...@Lowy.com> wrote: