What internal resistance is
Every battery resists the flow of current inside it to some degree. That resistance comes from the plates, the electrolyte and the internal connections between them. As a lead-acid battery ages, changes such as grid corrosion, sulphation and loss of electrolyte tend to increase it.
That makes internal resistance a useful indicator of a battery’s internal condition, and it can be measured without taking the batteries out of service.
Instruments and monitoring systems measure it in different ways. Some apply an AC signal and report impedance. Some report conductance, which is the inverse. Others use a DC load method. The results are related but not interchangeable, so only compare readings taken by the same method.
Why one reading isn’t enough
Absolute values vary with battery design, capacity and manufacturer. They also shift with temperature and state of charge at the time of measurement. A value that’s normal for one battery can be a concern for another.
The meaning comes from comparison:
- Against the battery’s own baseline, ideally recorded when the batteries were new or when monitoring was first fitted
- Against other batteries of the same type and age in the same string
- Against the manufacturer’s reference values, where they’re published
What a rising trend can indicate
A slow rise across a whole string is expected as batteries age. The readings that deserve attention are a battery rising faster than the rest, or a sudden step change in one battery.
Either can point to a deteriorating battery. A loose or corroded connection also raises the measured value, so the next step is to investigate before replacing anything.
| Pattern | Possible causes | Typical next step |
|---|---|---|
| One battery rising faster than its neighbours | Battery deterioration, poor connection | Check connections, retest, consider a capacity test |
| Sudden step change in one battery | Connection fault, sensor issue, internal failure | Investigate promptly |
| Whole string rising gradually | Normal ageing | Track the trend and plan replacement |
| Readings moving with time of day | Temperature changes | Check room and cabinet temperature |
Alert thresholds are usually set as a percentage above baseline or above the string average. The right values depend on the battery and the manufacturer’s guidance. Your maintenance provider can help you set them.
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How monitoring changes the picture
Periodic testing gives you a reading at each maintenance visit, often months apart. Continuous battery monitoring records the trend for every battery and alerts you when a threshold is crossed, so you can act between visits.
It also removes a source of variation. Readings come from the same sensor, in the same position, under known conditions, which makes trends easier to trust.
Internal resistance won’t tell you everything. A discharge or capacity test is still the direct way to confirm what a string can deliver. Monitoring helps you decide when and where that testing is needed.
Key points
- Internal resistance indicates a battery’s internal condition and rises as lead-acid batteries age.
- Compare readings against a baseline and against other batteries. Single values mean little.
- Only compare readings taken by the same measurement method.
- A battery rising faster than its neighbours needs investigating, starting with its connections.
- Monitoring gives you the trend between visits. Capacity testing confirms performance.