Posted in

What is the state – of – health assessment for stacked lithium batteries?

by tyrone•

Hey there, let’s cut to the chase: if you’re working with stacked lithium batteries—whether you’re an OEM, a fleet manager, a solar install tech, or just someone who’s had to troubleshoot a wonky battery pack for your RV—you’ve probably heard someone mutter “SOH” at some point. SOH, or State of Health, is that vague but critical metric that tells you if your battery’s still performing like it should, or if it’s on its way out. But when we’re talking about stacked lithium batteries specifically (the ones we manufacture and supply, for the record), SOH isn’t just a generic buzzword—it’s the backbone of how we make sure our products deliver for you, cycle after cycle. Let’s break this down like we’re chatting over a coffee, no stuffy jargon, I promise. Stacked Lithium Batteries

First, let’s get one thing straight: stacked lithium batteries aren’t your standard pouch or cylindrical cells rolled together into a pack. These are cells that are literally stacked on top of each other, usually with custom busbars, heat sinks, and insulation that fits into a slim, space-efficient enclosure. They’re super popular for things like electric delivery vans, stationary solar storage, heavy equipment, and even medical devices because their design cuts down on wasted space and boosts energy density compared to traditional packs. But that stacked layout also changes how we measure SOH—we can’t just use the same one-size-fits-all method we’d use for a bunch of 18650 cells taped together. That’s a key point most people miss, so let’s anchor the rest of this to that unique stacked design, not generic lithium-ion theory.

So what even is SOH, exactly, in plain terms? If SOC (State of Charge) is how full your battery is right now, like a gas gauge, SOH is how healthy it is overall. Think of it like a phone battery that used to last 12 hours but now dies after 5—its SOH dropped. For batteries, SOH is usually expressed as a percentage where 100% is when it’s new off the line, and anything below 80% is generally considered end-of-life for most applications (that’s the threshold the big automakers and energy storage folks use, fyi). But for stacked lithium batteries, we measure SOH across a bunch of unique variables that come from their stacked build, not just the total capacity.

Let’s start with the big one: capacity fade. That’s the most basic SOH metric, right? It’s just how much charge the battery can hold compared to when it was new. For our stacked cells, we test this by doing a full charge-discharge cycle at a consistent, controlled temperature (usually 25°C, that’s room temp, easy to work with in a lab). We charge it up to our set voltage limit, discharge it at a standard rate (like 1C, meaning it would fully drain in 1 hour), and measure how much energy comes out. If that number is 85% of the original capacity, SOH is ~85%—simple enough. But wait, because these are stacked, we can’t just test the whole pack in a vacuum. Each individual cell in the stack has to check out too. If one cell’s capacity is off, it drags the whole stack down, which is a problem no one wants. That’s why when we do SOH testing on our products, we do cell-level and pack-level checks, not just a pack-wide average. For example, if we have 10 stacked cells, and 9 are at 90% capacity, but 1 is at 70%, the pack SOH isn’t 80%—it’s lower, because that weak cell will cause imbalances during charging and discharging that kill performance faster. That’s a quirk of stacked design that standard SOH tools don’t always account for, which is why we’ve fine-tuned our testing process specifically for our products.

Next up: internal resistance. This is another huge SOH metric, and it’s extra important for stacked lithium batteries because of how the cells are connected. Internal resistance is basically how much the battery fights against the flow of current—high resistance means the battery wastes energy as heat, drains faster, and can’t deliver power when you need it (like when an electric delivery van is pulling away from a stop sign). For new stacked cells, internal resistance is really low, usually a few milliohms per cell. As the battery ages, that resistance creeps up—from things like electrode degradation, lithium plating, or the small gaps that can form between stacked cells over thousands of cycles. When resistance goes up by 20% from new, that’s a red flag for SOH, even if capacity is still holding steady. But here’s where stacked design makes this tricky: resistance can vary between cells in the stack because of how we align the busbars (the metal strips connecting cells) or how uniform the pressure is between stacked cells. If one cell is pressed a little tighter than the others, its internal resistance will be lower, and that mismatch causes current to flow unevenly, which speeds up fade. So when we test SOH, we don’t just measure total pack resistance—we do a resistance check on every single cell in the stack, which is a step that a lot of smaller battery suppliers skip because it takes a little more time. But for our customers, that means they get a more accurate SOH reading, not a misleading average.

Then there’s cycle life, which is tied directly to SOH but is a bit more of a long-term measure. A cycle is one full charge and discharge, right? For stacked lithium batteries, we expect them to last 2,000 to 6,000 full cycles before SOH drops below 80%, depending on the chemistry we use (we offer LFP and NMC, fyi). But SOH isn’t just counting how many cycles it’s done—it’s tracking how much capacity fades each cycle. For example, a battery that fades 0.05% per cycle will hit 80% SOH at 4,000 cycles, while one that fades 0.1% per cycle will hit that threshold at 2,000. Our SOH assessment includes calculating that fade rate, which helps our customers plan for replacement or refurbishment long before the battery dies, instead of getting surprised mid-job or mid-project. That’s a big deal for our delivery van customers, for example—they can schedule battery swaps during scheduled maintenance, not as an emergency.

Wait, I should also mention the role of temperature here, because it’s a huge factor that ties into SOH, especially for stacked packs. The stacked design is supposed to help with heat dissipation, but if a pack is running too hot or too cold for long periods, that messes up SOH fast. We test SOH at different temperatures because a battery that shows 90% SOH at room temp might be at 85% at -10°C, which is important for customers in cold climates or places with really hot summers. Our SOH assessment includes temperature correction factors, so our customers get a real-world reading, not just a lab number. We even build thermal management systems into our stacked packs, which is part of our SOH testing too—we monitor how the heat spreads across the stack, because hot spots can cause cells to degrade unevenly, which we already talked about being a problem.

Now, let’s get into how we do SOH assessment for our stacked lithium batteries, because that’s what matters most if you’re working with our products or looking to source stacked batteries for your business. We don’t just plug a battery into a tester and get a number (though we do that too). Our process is a mix of automated testing and hands-on quality checks, tailored specifically for our stacked design. First, every batch of cells we use gets pre-tested for capacity and internal resistance before we stack them, so we’re only putting cells that are within 1% of each other in a single pack. That cuts down on the cell imbalance that ruins SOH later. Then, once the stack is built and the pack is assembled, we run three main tests:

  1. Full Cycle Capacity Test: We do 3 full charge-discharge cycles at 25°C, measure the total energy delivered, and compare that to the original rated capacity. If it’s below 90% out of the box, we scrap the pack—no cutting corners here. For customers who already have our packs in use, we offer a portable SOH tester that does the same quick capacity check without having to take the pack out of service. It’s small enough to fit in a toolbox, which is great for our field service clients.

  2. Impedance Spectroscopy Test: This is the fancy one that measures internal resistance across different frequencies, which lets us spot early signs of degradation that capacity and resistance tests miss. Like, if a cell is starting to develop lithium plating, it might not show up in a basic resistance test until it’s bad, but impedance spectroscopy catches it early. We use this for our quality control, and we also provide customers with a simplified report that explains the SOH percentage and any potential issues, so they don’t need a PhD in battery science to understand it.

  3. Thermal Uniformity Check: We use thermal cameras while we run the charge-discharge cycle to make sure all cells in the stack are running at the same temperature. If there’s a hot spot of more than 2°C, that’s a red flag—we adjust the heat sink design or the stack alignment to fix it before the pack ships. That’s a big part of why our stacked packs have such consistent SOH over their lifespan, compared to competitors that skip this step.

Now, let’s talk about why SOH matters for you, the customer, not just us as suppliers. Let’s say you’re a fleet manager with 50 electric delivery vans all using our stacked lithium batteries. If you can track SOH for each pack, you can spot which vans are starting to fade before they lose range, so you don’t have to pull the whole fleet out of service all at once. You can also plan for battery replacement budgets more accurately, instead of being hit with a big unexpected cost when a pack dies prematurely. For solar storage customers, SOH tells you when it’s time to add a new battery or upgrade your system, so you don’t end up with a system that can’t power your home or business during an outage.

I also want to clear up a common misconception: a lower SOH doesn’t always mean the battery is useless. For example, if a battery has 75% SOH, that still might be enough for stationary storage that only needs to power a fridge and a few lights, or a van that only drives short routes. Our team works with customers all the time to adjust their usage and charging habits to extend SOH, instead of just telling them to replace the battery. Like, we’ve had customers switch to slower charging rates, which can boost SOH by 10-15% over the pack’s lifespan. That’s the kind of support you don’t get from big generic battery suppliers, and it’s part of why we focus so much on SOH education as much as SOH testing.

Wait, should I mention how SOH for stacked batteries is different from other battery types? Let’s make that clear. Cylindrical cells, like the 18650s in Tesla packs, are wired in parallel and series, but they’re not stacked face-to-face like our products. Their SOH is mostly based on cell matching, but because they’re spaced out, heat is more evenly distributed, so cell imbalance is less of a big deal. Pouch cells are sometimes stacked, but they’re thinner and use simpler busbars, so their internal resistance and heat uniformity are easier to control. Our stacked design is optimized for energy density and space, but that comes with tighter cell spacing, so SOH testing has to account for that tight layout to avoid degradation. That’s the differentiator here—we didn’t just take a standard SOH formula and slap it on our stacks; we built our SOH assessment around the unique design of our products.

Now, let’s talk about the future of SOH for stacked lithium batteries. We’re working on integrating SOH tracking right into the battery management system (BMS) that comes with every one of our packs. That means our customers can check SOH in real time via an app or a dashboard, instead of having to run a separate test. The BMS will also alert you if SOH is dropping faster than expected, so you can troubleshoot before it becomes a problem. That’s something we’ve been refining over the last year, and we’re launching it for all our new packs starting next quarter. It’s all about making SOH accessible, not just a technical metric that only engineers understand.

Before I wrap this up, let’s make sure we cover the most common question we get about SOH: how do I know when my stacked lithium battery needs to be replaced? As I mentioned earlier, 80% SOH is the general threshold, but it depends on your use case. If you’re using your battery for heavy, high-drain applications like construction equipment, you’ll want to replace it when SOH hits 85% because you need that extra power. For light-duty applications like golf carts or small solar setups, you can go down to 75% SOH before replacing. The good news is that our SOH testing is accurate enough to give you that specific recommendation for your application, not a one-size-fits-all rule.

At the end of the day, SOH for stacked lithium batteries isn’t just a number—it’s a way to make sure your investment lasts, your equipment runs reliably, and you don’t get stuck with a lemon. We built our entire business around making high-quality stacked lithium batteries, and SOH assessment is the core of that. We don’t cut corners on testing, we don’t average out bad cells, and we don’t treat all batteries the same.

If you’re looking for stacked lithium batteries for your next project, whether that’s a fleet, a solar system, or heavy equipment, we’d love to chat. We can walk you through our SOH testing process, show you how it translates to real-world performance, and help you pick the right setup for your needs. No pressure, no sales pitch that’s too hard to follow—just real talk about batteries that actually work.

Lifepo4 Battery References:

  1. Liu, K., Li, Y., & Zhang, C. (2021). A review of lithium-ion battery state of health estimation methods. Journal of Power Sources, 485, 229367.
  2. Wang, H., et al. (2020). State of health assessment for stacked lithium-ion battery packs considering cell imbalance and thermal effects. Applied Energy, 275, 115382.
  3. US Department of Energy. (2022). Lithium-Ion Battery Test Manual for Electric Drive Vehicles. DOE/GO-102022-5897.
  4. Chen, Z., et al. (2019). Impedance spectroscopy for lithium-ion battery state of health monitoring: A review. Electrochimica Acta, 316, 121-136.

Dongguan Ritano New Energy Co., Ltd.
With abundant experience, we are one of the most reliable stacked lithium batteries manufacturers and suppliers in China. We warmly welcome you to wholesale customized stacked lithium batteries made in China here from our factory. If you have any enquiry about quotation, please feel free to email us.
Address: Building 27, Jewelry Park, Huanchang North Road, Changping Town, Dongguan City, Guangdong Province
E-mail: Umi.sales@ritano.cn
WebSite: https://www.ritanopower.com/