Understanding how battery internal resistance growth changes optimal charge algorithms over time

Solar-powered marine lanterns and remote navigation aids depend on batteries that must perform reliably through years of daily charge and discharge cycles, often in extreme temperatures and with no opportunity for regular servicing. Understanding how battery internal resistance changes over time is not a purely academic exercise for the engineers and operations managers responsible for these systems. As internal resistance grows, the charging behaviour that kept a battery healthy in its first year may actively accelerate its degradation in its third or fifth. This article builds a working understanding of battery internal resistance from first principles, then explains how that knowledge should inform charging strategy as batteries age.

What is battery internal resistance and why does it grow?

Battery internal resistance is the opposition to current flow that exists within the battery itself, distinct from the resistance of any external circuit. Every battery has it, and it is a direct measure of how efficiently the battery can accept or deliver electrical energy. A battery with low internal resistance can absorb a charge current or supply a load current with minimal energy lost as heat. A battery with high internal resistance wastes a greater proportion of every charge and discharge cycle as thermal energy rather than stored electrochemical potential.

The resistance arises from several physical sources: the ionic conductivity of the electrolyte, the contact resistance between electrode materials and current collectors, and the impedance of the solid-electrolyte interface (SEI) layer that forms on electrode surfaces during cycling. In a new battery, these contributions are small and relatively stable. Over time, each one increases.

The growth of internal resistance is driven by a combination of electrochemical and physical processes that are collectively described as battery degradation. In NiMH batteries, which are widely used in self-contained solar marine lanterns because of their wide operating temperature range, the electrolyte gradually degrades and the active electrode material loses surface area through crystallite growth. In sealed lead-acid (SLA) batteries, which provide higher capacity for larger lantern installations, sulfation of the negative plate and corrosion of the positive grid are the primary contributors. In all chemistries, elevated temperatures accelerate these processes, which is why a lantern deployed in a tropical port environment will typically exhibit faster internal resistance growth than an equivalent unit installed in a temperate region.

For example, a NiMH cell that begins service with an internal resistance of around 30 milliohms may reach 80 to 100 milliohms after several years of cycling in a demanding environment. That increase of roughly three times the original value has measurable consequences for every aspect of how the battery interacts with its charge source.

How rising internal resistance alters charge dynamics

The relationship between internal resistance and charging behaviour follows directly from Ohm’s law. When a charge current flows into a battery, the voltage measured at the terminals is not the true electrochemical potential of the cell alone. It is the sum of the cell’s open-circuit voltage and the voltage drop caused by current flowing through the internal resistance. As internal resistance grows, this voltage drop becomes larger for any given charge current.

This has two practical consequences that compound each other over time.

• Terminal voltage rises faster during charging. A charge controller monitoring terminal voltage will interpret the elevated reading as evidence that the battery is more fully charged than it actually is, causing it to taper or terminate the charge prematurely.
• More energy is dissipated as heat. The power lost to internal resistance equals the square of the current multiplied by the resistance. As resistance doubles, heat generation from a given charge current quadruples, accelerating the degradation mechanisms that are already driving resistance upward.

The result is a self-reinforcing cycle. Elevated internal resistance causes the charge algorithm to undercharge the battery, which leaves it in a chronically low state of charge. Operating consistently at a low state of charge accelerates the degradation of electrode materials, which further increases internal resistance. In a solar marine lantern operating through long winter nights or extended overcast periods, this cycle can shorten service life significantly compared to what the battery chemistry is inherently capable of delivering.

Why standard charge algorithms fail aging batteries

Most battery charge algorithms are designed and calibrated for batteries at or near their beginning-of-life condition. The voltage thresholds, current profiles, and termination criteria built into a standard constant-current, constant-voltage (CC-CV) algorithm reflect the electrochemical behaviour of a fresh battery with low internal resistance. As internal resistance grows, these calibrations become progressively less accurate.

The most common failure mode is premature charge termination. In a CC-CV algorithm, the controller switches from constant current to constant voltage when the terminal voltage reaches a defined threshold, then terminates the charge when the current falls below a defined minimum. With an aging battery, the inflated terminal voltage caused by higher internal resistance triggers the voltage threshold earlier in the charge cycle, before the battery has reached its target state of charge. The battery appears full to the algorithm but holds significantly less energy than its rated capacity.

A second failure mode is thermal runaway risk during bulk charging. Some algorithms apply a fixed high-rate bulk charge current regardless of battery condition. In a new battery, the cell can absorb this current efficiently. In an aged battery with elevated internal resistance, the same current generates substantially more heat. In chemistries such as NiMH, where the end-of-charge detection method relies on detecting a characteristic voltage drop or temperature rise, increased internal resistance can mask these signals, causing the algorithm to overcharge the cell.

It is a common misconception that a battery that accepts a charge and provides some useful output is performing adequately. In practice, a battery can appear to function while operating at a fraction of its design capacity, with an internal resistance profile that makes it increasingly vulnerable to both undercharge and thermal stress. Standard algorithms do not distinguish between this condition and healthy operation.

Measuring and tracking internal resistance over time

Tracking internal resistance over the service life of a battery provides the most reliable early warning of degradation and the most actionable basis for adjusting charge strategy. The measurement itself requires only that a known current be applied and the resulting voltage response be observed, but the method and timing of measurement matter considerably.

Direct measurement methods

The simplest approach is DC internal resistance measurement: the battery is subjected to a defined discharge pulse, and the instantaneous voltage drop is divided by the current to yield resistance in ohms. This is accessible with basic equipment and provides a useful trend metric over time, though it captures only part of the total impedance picture.

Electrochemical impedance spectroscopy (EIS) applies a small alternating current signal across a range of frequencies and analyses the complex impedance response. EIS separates the contributions of electrolyte resistance, charge-transfer resistance, and diffusion-related impedance, giving a far more detailed picture of where degradation is occurring. This technique is more commonly used in laboratory and research settings, but is increasingly available in advanced battery management systems deployed in field applications.

Practical tracking in field applications

For solar marine lanterns and remote navigation aids where direct measurement access is limited, internal resistance can be inferred from operational data. A battery management system that logs terminal voltage, charge current, and temperature continuously can calculate internal resistance from the voltage response to known current steps. Sealite’s solar marine lanterns with satellite connectivity and the Star2M platform, for example, provide remote access to operational data that can support this kind of trend analysis across an entire network of installed assets.

The key is establishing a baseline measurement at commissioning and recording periodic measurements under consistent conditions, particularly at a consistent state of charge and temperature. Temperature has a strong effect on measured resistance, and comparisons made at different temperatures will produce misleading trends. A doubling of internal resistance from the commissioning baseline is a widely used threshold for considering a battery to be at the end of its useful service life, though the appropriate threshold varies by chemistry and application.

Adapting charge algorithms as internal resistance increases

Adapting the charge algorithm to compensate for internal resistance growth requires addressing the two core problems identified earlier: premature termination caused by inflated terminal voltage, and excessive heat generation from high-rate charging. Both can be managed through targeted adjustments to charge voltage thresholds, current rates, and termination criteria.

Adjusting voltage compensation

The most direct correction for inflated terminal voltage is internal resistance compensation, sometimes called IR compensation or voltage compensation. The charge controller calculates the voltage drop across the internal resistance in real time by multiplying the measured current by the known or estimated resistance, then subtracts this value from the measured terminal voltage to obtain a corrected estimate of the true cell voltage. Charge thresholds and termination decisions are then based on the corrected value rather than the raw terminal voltage.

This approach requires either a measured internal resistance value or a dynamic estimate updated from operational data. A fixed compensation factor applied at installation will become progressively less accurate as resistance continues to grow, so systems that can update the compensation parameter over time provide the most reliable long-term performance.

Reducing charge current as resistance increases

Reducing the bulk charge current rate as internal resistance increases directly addresses the thermal problem. Because heat generation scales with the square of current, a reduction in charge rate from C/5 to C/10, for example, reduces heat generation by a factor of four for any given resistance value. The trade-off is a longer charge time to reach the same state of charge, which must be weighed against the available solar energy window and the energy demands of the application.

For solar-powered systems, the available charge current is already limited by solar panel output, and the charge window is constrained by daylight hours. This means that reducing charge rate may not always be a practical option without also increasing solar panel capacity. The design relationship between panel capacity, battery capacity, and expected daily energy demand must be re-evaluated as the battery ages, rather than treated as a fixed parameter set at commissioning.

Extending absorption and equalisation phases

Extending the duration of the absorption phase, during which the battery is held at a constant voltage while charge current tapers naturally, allows more energy to be delivered to an aged battery without increasing the peak current or voltage. This compensates in part for the premature current tapering that occurs when resistance is high, by giving the battery more time to absorb charge at the reduced rate that the elevated resistance naturally produces.

Periodic equalisation charging, in which a controlled overcharge is applied to reverse sulfation in lead-acid batteries or to balance cell voltages in multi-cell NiMH packs, becomes more important as batteries age. The frequency and duration of equalisation cycles should increase as internal resistance trends upward, not remain fixed at the schedule established for a new battery. An adaptive battery management system that triggers equalisation based on measured resistance or capacity fade, rather than a fixed calendar interval, will consistently outperform a static schedule in extending usable battery life.

For port and harbour operators and coast guard organisations managing large networks of solar-powered aids to navigation (AtoN), the cumulative effect of these adaptations is significant. A battery that would otherwise reach the end of its useful service life in four years under a fixed charge algorithm may deliver six or more years of reliable operation under an adaptive strategy that tracks resistance growth and adjusts accordingly. Across a network of hundreds of installed lanterns and buoys, that difference represents a substantial reduction in maintenance cost and replacement frequency, and a measurable improvement in the long-term reliability of the navigation marking infrastructure that vessel traffic depends on.

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