Practical Battery Knowledge

Practical Battery Knowledge: Charging, Discharging & Real-World Realities

When it comes to batteries, there's a big difference between lab results and what really happens out in the field. At Element 82, we see firsthand how batteries perform under load, in the weather, and through the seasons. This article expands on our earlier insights and digs deeper into charging conditions, generator use, DC charging, and the limitations imposed by both physics and modern Battery Management Systems (BMS). Whether you're off-grid, on the water, or just want a system that works, here’s what you need to know.

Usable Capacity and Depth of Discharge (DoD)

Batteries are rated for a theoretical 100% depth of discharge, but that’s rarely how they’re used—or should be.

  • Lithium Batteries: Technically capable of 100% DoD, but in practice, most BMS systems restrict use of the top ~10% and bottom ~10% of capacity to preserve cell health. What you actually get is around 60–80% usable capacity.

  • AGM Batteries: While they can be discharged fully, this drastically shortens lifespan. Most users will get better results cycling between 50–80% DoD.

Charging Conditions Matter

Charging is not a one-size-fits-all situation, and how (and when) you charge your battery makes all the difference.

  • Lithium: Quick to recharge, but temperature-sensitive. Most lithium chemistries can’t accept a charge below 0°C. If charging continues below freezing, lithium plating can occur, permanently reducing capacity. A quality BMS will cut charging to prevent this, but it also means you might be without power until the battery warms up.

  • AGM: More tolerant of cold, and although charging takes longer, it doesn’t require BMS protections. AGM batteries will accept a charge at colder temperatures and are more forgiving to mismatched chargers, though temperature compensation is still important.

Generator & DC Charging Considerations

Generators paired with lithium batteries can be tricky. Lithium wants high current until it’s full, then no current. But many generators don’t respond well to this sudden drop in demand and may surge or stall. You’ll need a smart charger or DC-DC converter that buffers this.

For vehicles, DC charging (from alternators) is common. However:

  • Smart Alternators: These often drop voltage to save fuel, meaning lithium batteries won’t charge unless you install a DC-DC charger.

  • AGM Batteries: More adaptable to alternator voltage swings, and many older vehicles can charge AGMs directly.

Charger Sizing and Timed Stages

Battery chargers often use timed stages. If your charger is too small (like 5A for a 150Ah battery), it may never reach the thresholds required for switching stages within the time limit, triggering an error or incomplete charge. Ideally:

  • Use a charger rated to at least 5–10% of your battery’s Ah rating (e.g., 15A for a 150Ah battery).

  • For maintenance charging, 1/20th of capacity can suffice—but only if the battery is already full.

Charging slowly also has the benefit of reducing heat buildup, which can extend battery life for both AGM and lithium chemistries.

Standards and Specifications: What Really Matters

Too much focus is placed on headline specs without considering the test conditions or limitations:

  • CCA, MCA, HCA Ratings: These cranking numbers are important but are all based on different temperatures. Make sure you're comparing like for like.

  • Reserve Capacity (RC): Often overlooked, RC is crucial for applications like RVs, boats or off-grid homes. It represents the time a battery can deliver a constant load before voltage drops below a usable level.

  • BMS Limitations: Many warranties only allow one full cycle per day—something internet forums rarely mention. Some BMS units also throttle performance in high heat or freezing temps to protect the cells.

Myth Busting: Online Hype vs. Reality

  • 100% DoD daily? Sure, in theory. But your warranty likely disagrees.

  • Fast charge any time? Only if heat and BMS cutouts aren’t a concern.

  • Plug-and-play lithium swap? Only if your entire system was designed for it.

  • Temperature compensation is standard? Not always. Many chargers don’t truly adjust charge voltage based on ambient temperature unless paired with an external sensor.

Real-world performance depends on wiring, charger specs, climate, and user habits—none of which get factored into those glossy marketing brochures.

Final Thoughts: There’s No Perfect Battery

Whether it’s AGM, lithium, gel, or something else—every battery chemistry has trade-offs. The right choice comes down to:

  • Your application

  • Your environment

  • Your budget

  • Your ability to manage the system

AGM offers simplicity and robustness. Lithium offers performance and energy density—but only when supported by the right infrastructure.

At Element 82, we look at the whole system—not just the battery—because no component works in isolation. Understanding the limitations and capabilities of each technology is key to designing a reliable and safe power system.

Need the Right Advice? Talk to us at Element82.co.nz. We’re here to match real-world problems with real-world solutions, not just hype.