DIY Battery Pack

Most deployable oceanographic instruments use alkaline battery packs for long term power. These battery packs are usually just a bunch of off-the-shelf 1.5 volt D cell batteries soldered together in series/ parallel to output the desired voltage. It seems a little anachronistic that these kind of high tech instruments are using such a basic battery technology, but I see 3 issues that keep this technology in use:

1- Deploying and recovering instruments takes time, and as researchers we want to maximize the amount of time instruments can stay out in the field, and minimize the number of retrievals. Rechargeable battery packs still don't match up to alkaline cells for battery life, so alkaline packs are the choice for lengthy deployments.

2- Lithium battery packs are great for electronics, but don't hold up as well when driving a motor. So if an instrument uses a sampling pump to move water through the housing (and many oceanographic instruments do), Lithium batteries don't hold up as well during a deployment. But if your instrument has a passive sensor and is just recording data, then Li battery packs may be the way to go for long deployments.

3- Solar/ Wind methods for recharging are difficult to implement on the ocean surface. And if you are using a bottom mounted mooring, it may be logistically impossible to get power to your instrument.

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If you find yourself using Alkaline battery packs, purchasing them from commercial vendors can be cost prohibitive. Considering these battery packs are nothing more than D-cells soldered together, the mark-up on these battery packs is many fold over what you might pay for the batteries alone.

I've had luck making my own battery packs, so I've put together a short tutorial on how to assemble your own alkaline battery pack below. And while I'm using my battery packs for oceanographic instrumentation, battery packs like these can be used for any application that requires long term battery life.

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HOW TO MAKE YOUR OWN BATTERY PACK:

Step 1- Rough up the ends of your batteries

I'm using standard Duracell D-cell batteries for my pack. In order to solder them together, I'll need to rough up the ends of each battery so the solder will adhere properly to the battery.  I used a dremel tool with a grinding tip to rough up my batteries, but sand paper or a file could work as well.

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Step 2- Shrink wrap batteries

I want my battery pack to output 6 volts, so I will need to wire up 4 of my D cell batteries in series in order to get 6 volts. The easiest way to do this is to use shrink wrap hold the batteries in place, in essence creating a new "mini-pack" 6 volt battery cell out of four 1.5 volt cells.

To do this, I line up the batteries in the configuration I will want them in to create a 6 volt cell. Then I cut the shrink wrap to a length just larger than the size of the batteries. The idea is to have a little overlap on the top and bottom size of the mini-pack in order to pull the batteries together, but not so much as to cover the ends of the batteries. 

I've found this shrink wrap works well for two D-cells placed side to side.

Use a heat gun to shrink the wrap, causing the battery terminals  to maintain good contact.

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Step 3- Solder battery tabs to bottom end of battery pack

The configuration I have chosen for each mini-pack is two batteries pointing down, and two batteries pointing up. In order to wire up all 4 batteries in serial, I will need to connect one of the ends with a soldering tab.

The red you see on the ends of the batteries is flux. I put a dab of flux on each end of the battery before I soldered the tab on. The flux will improve your solder contacts, and quickens the time it takes to melt the solder. This is important as we want to minimize the amount of time the solder iron is in contact with the battery, as it is possible to damage the battery by overheating it with the iron.

After you have put together all of the mini-packs you will need for your main battery pack, check each tab to make sure you have a good solid solder connection.

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Step 4- Compress/ stabilize mini-packs together

I have chosen to use hose clamps for this step, but most commercial companies will use a combination of glue internally with an outer layer of shrink wrap to keep their mini-packs together. The important thing is to make sure all of the mini-packs are compressed and will stay together as one unit. It's imperative that the mini-packs not move in relation to each other as they will be soldered together in the next step, and if there is even a small amount of movement between the mini-packs the solder connections can be compromised

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I decided to use hose clamps as opposed to shrink-wrap and glue to compress my mini-packs as it will make it easier to break down my main pack for recycling. For my particular pack I am including  a spacer (grey inserts in the middle) and I want to be able to re-use this spacer. Using hose clamps allow me to easily disassemble my used battery pack to recover the spacer.

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Step 5- Wire up the mini-packs in parallel.

Once the mini-packs are stabilized, they can be wired up in parallel. Looking at the below image, you can see I used a combination of more battery tabs (for short lengths) , and insulated wires (for longer stretches between mini-packs) to connect the mini-packs.

The end result is a battery pack that still outputs 6 volts, but with a much larger amp hour capacity.

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Step 6- Always Use Protection

The last step is to include some sort of circuit protection. I've chosen to add a diode, as well as a fuse to protect my electronics down stream in case of an issue with the battery pack. The diode will protect against any instance of the battery pack being hooked up with reverse polarity (though any diode will also drop the voltage by a bit). And the fuse will protect against any sudden increase in current in case of a short in the pack.