When More Capacity Is Not the Better Battery

Consider a manufacturer updating the battery in a compact industrial pump. The motor and controller are staying. The battery compartment cannot grow. The new pack needs to support repeated periods of operation before returning to the charger.

Two sodium-ion cells have reached the shortlist. One is a higher-capacity layered-oxide cell. The other uses NFPP, an iron-based phosphate–pyrophosphate cathode, and has a lower capacity rating but a higher permitted discharge current.

On the initial comparison, the higher-capacity cell looks attractive. More amp-hours in the same space could mean longer operation or fewer cells.

The lower-capacity option appears harder to justify. Why accept less capacity in a product whose customers already care about runtime?

Then the engineering team looks at a line in the requirement: 4C continuous discharge.

Before either cell is selected, that requirement needs translating back into what the pump actually draws.

The same C-rate can mean a different test

Take two illustrative cells rated at 1.5 Ah and 1.0 Ah. At 4C, the larger-capacity cell is supplying 6 A. The smaller-capacity cell is supplying 4 A.

Both could pass their respective 4C tests, but they have not been asked to supply the same current.

Now suppose the proposed pack arrangement requires each cell to provide 6 A during a particular part of the pump’s operating cycle. That remains 4C for the 1.5 Ah cell. For the 1.0 Ah cell, it becomes 6C.

The pump’s requirement has not changed. Only the way it is expressed has changed.

This is why the cell-selection brief should retain the original load in amperes or watts, together with its duration. C-rate remains useful for describing how hard a particular cell is working, but it should not replace the equipment requirement.

The smaller cell may be entirely comfortable at that operating point. It may also need more parallel cells to bring the current per cell within its approved limits. The answer comes from the specific product data, not from giving both candidates the same C-rate label.

A candidate that cannot support the required current within its approved limits needs a different pack arrangement or should leave the shortlist. Driving it harder simply to make the comparison look equal is not a solution.

Starting the pump is only the first part of the job

The pump must first start, then continue running under load. A battery that supports the starting event has demonstrated something useful, but not everything the application needs.

A peak-current rating qualified for a few seconds does not establish the current that can be sustained throughout a longer run. Published sodium-ion specifications distinguish these conditions and show that available pulse capability also changes with temperature and state of charge.

During the run, the important measurement is the voltage the cell maintains while delivering the required current. A cell can contain remaining charge yet reach the equipment’s minimum operating voltage under load. When the load is removed, some voltage recovery may occur, but that does not mean the pump could have continued operating at the required output. Electrical characterisation therefore needs to consider loaded behaviour, temperature and state of charge together, rather than capacity in isolation.

For the manufacturer, the useful result is how much work the pump completes before a permitted voltage or temperature limit is reached.

That is also why the comparison needs delivered energy, not just amp-hours. Cells with different operating voltages can deliver different amounts of energy for the same amount of charge.

The second run may be more revealing

Imagine that the pump completes its first run, pauses briefly and starts again.

The second run begins with less charge available and, potentially, a warmer battery. Whether the pause is long enough for meaningful cooling depends on the pack construction and its surroundings.

A cell tested on its own does not necessarily experience the same cooling conditions after it is placed between other cells inside a compact enclosure. Experimental pack-level thermal studies have measured differences between cell positions, with inner cells experiencing different temperatures from those nearer external surfaces.

For this pump, that changes the test programme. A single discharge from a fully charged, room-temperature cell is not enough to represent repeated work.

The comparison should follow the intended sequence of operation and rest. It should also include the conditions in which the first job begins: a cold workshop, a warm enclosure, or a battery that has not been fully recharged.

The practical question is whether the proposed configuration can complete the required sequence without relying on an unplanned cooling break, an early recharge or reduced equipment output.

Where NFPP becomes relevant

The lower-capacity NFPP candidate deserves consideration when its power capability changes what the complete battery can do.

There is published evidence supporting that possibility. In one 2026 evaluation, a specific 5.5 Ah NFPP cell completed a 30C discharge at approximately 22°C. The study did not establish long-term cycle life, and its result should not be transferred to other NFPP products. It does, however, illustrate why a cell with a modest energy rating can still be relevant to a power-intensive application.

The chemistry label alone does not settle the choice. Within a cell, electrode loading and thickness influence the balance between stored energy and rate capability. A higher-energy design and a higher-power design can therefore differ for reasons beyond their cathode material.

For the pump manufacturer, NFPP becomes commercially interesting if the actual candidate maintains the required output with a useful operating margin, fits the enclosure and provides sufficient runtime.

If the higher-capacity layered-oxide candidate already satisfies those conditions, its additional capacity remains a genuine advantage. There is no benefit in buying power capability that the product does not need.

A better cell does not always produce a better pack

Suppose the higher-capacity candidate cannot support the intended load with the initial number of parallel cells. Adding parallel branches would reduce the current assigned to each cell, assuming appropriate current sharing.

That might solve the electrical problem. It would also change the number of cells, interconnections and the space required inside the battery.

The lower-capacity power cell may meet the current requirement with fewer parallel branches. But if it then needs additional cells to achieve the required runtime, its apparent advantage could disappear.

This is where the purchasing comparison becomes more useful.

The relevant proposals are no longer “one cell with more capacity” and “one cell with a higher discharge rating”. They are two buildable battery configurations, each designed for the controller’s voltage range, the same operating duty and the same installation constraints.

One may be limited by energy: it can comfortably supply the load, but cannot run for long enough.

The other may be limited by power: it has sufficient nominal energy, but cannot deliver the required output throughout the job within its operating limits.

Those two problems call for different changes. A higher capacity rating does not automatically solve the second, just as a higher peak-current rating does not automatically solve the first.

What the next sample test should decide

For this illustrative project, the next test should help the manufacturer choose between those configurations.

The work would begin with the pump’s measured electrical demand, including starting, sustained operation and the pauses between runs. Each candidate would then be assessed at the load allocated to it by its proposed pack arrangement, within the manufacturer’s approved limits.

The comparison should record delivered energy, loaded voltage and temperature through the operating sequence. It should explain why a run ended, rather than reporting only a final capacity number.

A cell that completes the duty with little margin would require further investigation before the design is committed. Representative module testing would then establish whether the proposed arrangement behaves as expected inside its enclosure. Initial screening would not replace the later safety, durability and product qualification work.

The manufacturer may ultimately select the higher-capacity cell. It may select the power-oriented NFPP option. It may find that neither offers enough improvement within the existing housing.

The value of the assessment is that the decision can be explained in terms of the product customers will use: how long it works, whether it maintains output, and what battery configuration is required to deliver that performance.

Discuss your cell-selection project with IONERAS. Bring the equipment’s operating profile, installation limits and candidate specifications so the comparison can begin with the job the battery needs to do.