If you’ve ever sourced shrink pack batteries for industrial, medical, or off-grid applications, you’ve likely heard the term “internal resistance” thrown around. But as someone who’s spent over a decade working directly with shrink pack batteries—first as a tech in a lab testing prototype packs, now as a supplier helping clients cut through the confusion of battery specs—I can tell you most people outside of electrical engineering don’t actually grasp what this number means, why it matters for shrink packs specifically, and how to use it to pick the right battery for their needs. Today, I’m breaking this down straight, no jargon, no fancy marketing fluff. Shrink Pack Batteries
First, let’s get one thing clear: shrink pack batteries aren’t the same as the AA or AAA you grab from the grocery store. They’re custom-assembled, usually with individual cylindrical or prismatic cells wrapped in a tight plastic shrink film, often wired in series or parallel to hit a specific voltage, capacity, and form factor for things like portable medical pumps, emergency lighting rigs, or commercial power tools. And their internal resistance (we call it IR for short) is one of the most underrated specs there is—right up there with capacity and cycle life.
So, what exactly is internal resistance, anyway? At its core, it’s the total opposition a battery puts up to the flow of electrical current, measured in milliohms (mΩ) for small batteries like the ones we supply. Think of it like the narrow spot in a garden hose: the wider the spot (lower resistance), the more water (or current) can flow through, with less energy lost as heat. In a battery, that opposition comes from four main parts: the resistance of the individual cells’ electrodes, the electrolyte (the liquid or gel that carries charge between the electrodes), the connections between cells, and the shrink film wrapping—wait, yes, the shrink film can actually impact IR if it’s too tight or conductive, but that’s a story for later.
Now, here’s where it gets specific to shrink pack batteries, not off-the-shelf cells. Most shrink pack suppliers take off-the-shelf cylindrical cells (like 18650s) and assemble them into a pack, but we do this our way: we spot-weld every cell connection instead of using cheap tabs, and we use a thin, non-conductive polyolefin shrink film that doesn’t add extra resistance. That means our shrink packs have consistent, well-controlled IR, not the random spikes you might get from a supplier that cuts corners on assembly. For example, a typical 12V shrink pack made with high-quality 18650 cells should have an IR of around 150 to 250 mΩ. If you’re getting a pack with 400 mΩ or higher, that’s a red flag—either the cells are old and degraded, the connections are bad, or the shrink film is wrong.
Why does this number matter so much? Let’s use a real example from a client I worked with last year: a small medical device maker that was sourcing shrink packs for portable insulin pumps. Their old supplier’s packs had an IR of 380 mΩ, and the pumps were overheating mid-use, draining batteries 20% faster than expected, and failing their FDA thermal testing. When we swapped them for our packs with IR in the 210 mΩ range, the overheating stopped, battery life met their specs, and they passed their audit on the first try. That’s not an isolated case.
Internal resistance affects three big areas of shrink pack performance: current output, heat generation, and efficiency. First, current output: a battery can’t deliver more current than its IR allows. If your application needs a burst of power—say, a power drill that needs to spin up fast, or a defibrillator that needs to deliver a shock—high IR will limit that current, making the tool underperform. Second, heat: every time current flows through a battery, energy is lost as heat from IR. The formula is simple: Power Lost = I²R, where I is current and R is IR. So if you have a 5A current draw, a 100 mΩ IR pack loses 2.5 watts of heat, while a 400 mΩ pack loses 10 watts—enough to overheat small medical devices, melt shrink film, or even shorten cycle life drastically. Third, efficiency: a pack with low IR will deliver almost all its stored power to your device, while a high IR pack wastes a chunk of that power as heat, meaning you need a higher capacity pack to get the same runtime. For industrial clients running 24/7 power systems, that adds up to thousands of dollars in extra costs over the life of the pack.
Now, let’s talk about what affects the IR of a shrink pack, because that’s how you know you’re getting a quality product. First, cell quality: the individual cells that make up the shrink pack are the biggest factor. New, high-grade lithium-ion cells (which is what 90% of the shrink packs we supply are, for their high energy density) have much lower IR than used, low-grade cells. A brand-new Samsung 18650 has an IR of ~100 mΩ, while a used cell that’s been charged/discharged thousands of times can jump to 300+ mΩ. That’s why we test every cell before assembly, and only use cells with less than 110 mΩ IR for our shrink packs—we won’t risk bad cells ruining the pack’s performance.
Second, assembly: how you connect the cells matters more than most people think. If a supplier just tapes cells together and cranks on the shrink film, the connections between cells will be loose, adding unnecessary resistance. We use automated spot welding to connect every cell in the pack, ensuring tight, consistent connections that don’t add extra IR. We also test each pack’s IR twice: once before shrink wrapping, once after, to make sure the shrink film didn’t pull on connections and alter resistance. Third, shrink film and pack design: as I mentioned earlier, the film we use is non-conductive and thin, so it doesn’t add resistance. Some cheaper suppliers use thick, conductive shrink film to cut costs, which can add 50+ mΩ to a pack’s total IR—hard to detect unless you test it, but a huge performance killer.
A common question I get from clients is: “Is lower IR always better?” The short answer is yes, but only up to a point. There’s no such thing as IR that’s too low, but a pack with unusually low IR (like less than 50 mΩ for a 12V pack) is probably cutting corners somewhere—maybe using unregulated cells or dangerous materials. The sweet spot for most shrink pack applications is 100 to 300 mΩ for 12V packs, 200 to 400 mΩ for 24V packs, depending on your current draw. If you’re running a low-current application, like a portable sensor, even 350 mΩ is fine, but if you need high current (like power tools), you’ll want to stay under 250 mΩ.
Another thing to watch out for: how IR is tested. A lot of cheap suppliers will test IR at 0.1A, which is a very low current, and that number is useless for real-world applications. We test our packs at the actual current draw the client will use, whether that’s 1A for a medical pump or 10A for a power tool, because IR changes at different current levels. A pack might have a great low-current IR, but a terrible high-current IR—something that would cause big problems in the field.
I’ve been in this long enough to see suppliers cut every corner on IR to quote a lower price, and it never ends well. Last year, a construction contractor came to us after buying shrink packs for their job-site lights from a competitor. The packs were $20 cheaper per unit, but they failed in half the time, and the lights would dim mid-shift because the high IR couldn’t deliver enough current. When we tested those competitor packs, their IR was 420 mΩ, compared to our 200 mΩ, and their cells were over two years old, already degraded. The contractor ended up paying 20% more per pack for ours, but they only had to buy half as many, and the lights lasted twice as long. That’s the value of knowing what IR to look for.
If you’re in the market for shrink pack batteries, here’s my quick checklist to make sure you’re getting a product with solid, controlled internal resistance:
- Ask for the IR of the full pack, tested at your expected current draw, not just the individual cells’ IR.
- Avoid any supplier that won’t share their IR test results—legitimate suppliers test this every time.
- Match your application to the IR range: low current = up to 350 mΩ, high current = under 250 mΩ for 12V packs.
- Make sure the assembly is done with spot-welded connections, not just tape or cheap tabs.
At the end of the day, internal resistance isn’t a complicated spec that only engineers need to worry about. It’s the difference between a shrink pack that works when you need it, lasts as long as it should, and doesn’t waste your money, and one that causes headaches, delays, and extra costs. If you’re not sure what IR your application needs, or you want to test how our shrink packs perform compared to what you’re using now, we’re here to walk you through every step. We offer free sample packs for testing, so you can see the IR and performance for yourself before placing a full order.
Don’t waste money on underperforming shrink packs that cut corners on internal resistance. Reach out to our team to discuss your power needs, get a custom quote, and answer any questions you have about IR, pack design, or performance. We’re here to help you find the right shrink pack battery for your application.
Alkaline Battery References
- Linden, D., & Reddy, T. B. (2002). Handbook of Batteries (3rd ed.). McGraw-Hill.
- Power Sources Manufacturers Association. (2019). Shrink Pack Lithium-Ion Battery Assembly Best Practices.
- Wang, Y., et al. (2021). “Effect of Internal Resistance on Li-Ion Battery Thermal Performance in Portable Applications.” Journal of Power Sources, vol. 498, p. 229987.
Shenzhen Pkcell Battery Co., Ltd.
Shenzhen Pkcell Battery Co., Ltd. is one of the most professional shrink pack batteries manufacturers and suppliers in China, also supports customized service and OEM&ODM service. Please feel free to wholesale bulk CE approved shrink pack batteries made in China here from our factory. Welcome to contact us for quotation.
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E-mail: sales@pkcell.net
WebSite: https://www.pkcell.net/
