Portable DC power for a Nikon D40
I shoot long exposures with my Nikon D40 for astronomy. That means I have to leave the shutter open for long periods of time for most of the night. One battery will not last the whole night. You can get spare batteries but the Nikon batteries are expensive and the generic batteries don’t last as long and eventually fail.
My solution at the time was to buy an AC power supply for the D40 (EH-5A and EP-5) but that setup still required me to use an AC inverter to change 12 volts to 110 volts. Using an inverter on 12 volts wastes a lot of energy from my precious deep cycle marine battery. I need to conserve this battery to run my laptop and the rest of my remote astrophotography equipment. Who wants to spend over $100.00 on an AC power adapter and plug?
I needed a DC power source for my camera that could run off of a deep cycle marine battery (12 volt battery) without having to use an inverter. My total setup minus a battery cost about $20.00. It’s lighter, smaller, and way more efficient. This should work for a Nikon D40, D40X, D60, D5000, and D3000. Check your battery and adapter voltage specs for other camera applications. The process will be similar, but the parts will vary. As a word of caution, even though the cameras listed above should work, still test the voltages and look up the specs. And make sure you test everything several times before you use it.
Here’s the part list:
• DC12V to 9V 3A Power Adapter (buck) -ebay $6.00 free shipping
• Nikon EP-5 Replacement Power Supply connector for D40 -Amazon $13.00 shipped
• 12 Volt DC cigarette light plug –used free
• Wire -used free
Heat shrink tube
Digital multi meter
Step 1: Testing
First I tested the Nikon battery to see what voltage it put out. The battery says it’s 7.4 volts. It tested at 7.61 volts from the positive terminal to the negative terminal. I also checked the voltage between positive terminal and the “S” terminal. Here it was 7.54 volts. On the AC adapter for this camera at the battery adapter, the voltage was 9.38 volts between positive and negative, and 9.29 volts between positive and “S.” That showed me that the camera accepts between 7.4-9.29 volts. As far as how much current is needed I guessed around 3 amps max for this camera. This is based on some tests I’ve seen posted online. The actual draw is probably half that, but to be safe I used a 3 amp power source.
Step 2: Wire the Input on the DC Adapter
On your 12 volt DC adapter you will need to run the negative of the 12 volt wire labeled input to the negative wire on your cigarette lighter plug. You will also need to run the positive from the adapter to the positive wire on the plug. Solder and heat shrink all connections.
Step 3: Wire the Output of the DC Adapter
You first need to cut the plug from the battery adapter. You will notice that there are only two wires here: a positive (red) and a negative (black). Now you can solder the negative output of the DC adapter to the negative (black) wire on the battery adapter. After that solder the positive output from the DC adapter to the positive (red) wire on the battery adapter.
Make sure you slide on your heat shrink tube before you make the connections!
Step 4: Test Output
After all connections are made and protected , you should test the output voltages and polarity. First connect the 12 volt cigarette light plug into your power source and then test at the battery adapter.
I had positive 9.11 volts across the negative and positive terminals, and positive 9.02 volts across the positive and "S" terminal. Make sure the polarity is correct. Test this against your regular working battery to verify the results are similar.
Step 5: Insert Adapter!
After you have tested the output of the battery adapter and are 100% sure of the polarity, it's now time to use it. Note that this generic battery adapter plug shows up as a fully charged battery. When the Nikon AC adapter with the battery adapter are used you see an AC plug icon displayed where the battery icon is. My guess is that this has something to do with the chip in the generic battery adapter as it's not a genuine Nikon product.
I tested this by holding down the shutter and and letting it take a bunch of shots. I also used the autofocus and took more photos. Everything worked fine on my tests and I'll be using for astronomy from now on. No more AC to DC inverters!!!