When designing in an ultracapacitor it is common to use more than one in a system. In order to do this you will need to assembly them together. This article provides some best practices when connecting together a pack or module of ultracapacitors. For information on balancing the packs visit this article.
FastCAPs capacitors come in a variety of sizes and can be built into a variety of modules to form whatever housing is needed. These methods have been used by FastCAP to withstand 20Grms (and higher) and temperature variations of -40C up to 165C.
Some typical layouts are below. These are just examples, as the number of cells, parallel or series arrangement is determined based on the application.
To connect one cell to another FastCAP typically uses Nickel 201 Annealed tabbing material. Here are some best practices for the tabbing
- Parallel resistance weld to the Pin/Bottom of the ultracapacitors. They are both Stainless Steel and have favorable resistance welding properties to Nickel.
- Use at least a 0.005” thick piece of Nickel tab, using a thicker piece will improve resistance and produce less heat, but may be difficult to bend or weld.
- Only bend the Nickel tabs once when assembling. Too many bends can weaken the metal and it may break.
- Insulate the Nickel tabs in order to protect from shorting against the can, housing, or any other metal. FastCAP preferably will use a PTFE heat shrink around the tabs, or in more narrow spaces Kapton tape.
- Kapton tape is useful to use as an insulation for everywhere as it is a good insulator, works well at extreme temperatures and is also very thin and cheap.
- The tabs can be SHARP, especially when bent, so be mindful of this if the tab is against a piece of insulation, rubbing on a material, against potting etc.
1.2 Individual Capacitors
Depending on the layout of the module each arrangement may require different techniques. The most important is isolating each capacitor from one another, the tabs, the housing and any other sources
- It is important to isolate every capacitor because the pin is the positive terminal and the entire can, bottom, and top cover are the negative terminal.
- Insulate every cell with heat shrink, tubing, or similar. FastCAP prefers to use 0.020” or 0.040” wall thickness PTFE heat shrink, but has also used polyimide tubing, FEP heat shrink and a last resort is Kapton Tape.
- A good redundancy to isolate the interconnects is to use a thin Kapton or PTFE washer that goes around the pin and further isolates the tab material coming off the pin from the top cover.
1.3 Module Assembly
When building the module the potting and amount of potting will depend on the form factor you are fitting and design constraints you need. Here are some common best practices for module design.
- Typically FastCAP uses a silicon elastomer, such as Sylgard 184, for potting.
- The best case is for potting to have no air bubbles or voids. Voids create cavities that have a higher rate of failure during shock and vibration. Cover the entirety of the can, and interconnects with potting.
- Most importantly there are to be no voids around the tabs and connection points. During vibration if a tab is not fully potted it will vibrate in air causing fatigue and most likely break.
- Potting compounds will expand at high and low temperature. It is important to factor this in when designing the module. FastCAP has often made certain expansion slots (such as removed part of a housing) or enough space at the end in order to allow for expansion.