Battery Health is the single number most people open BatteryInfoView to find, and it is also the number most often misunderstood. It is not a score invented by the program, it is not a measure of how much charge you have right now, and NirSoft does not attach any official good, fair or poor bands to it. It is a simple, documented calculation based on two capacity values your battery reports — Full Charged Capacity and Designed Capacity. Once you understand those two inputs, the percentage explains itself, and so do most of the questions that follow from it, such as why a brand-new battery can read a little under 100% or why health and charge are never the same figure. If you have not tried the tool yet, get it from our download page — the main button there uses the official NirSoft file — and then work through the checks below. For the wider topic, see the Battery Health hub.
The documented calculation
BatteryInfoView calculates Battery Health according to the Designed Capacity and Full Charged Capacity values. In plain arithmetic: Battery Health % = Full Charged Capacity ÷ Designed Capacity × 100. Designed Capacity is the capacity the battery had when it was completely new, as designed. Full Charged Capacity is the capacity the battery can hold now when it is fully charged. Both values are usually shown in milliwatt-hours (mWh) in the main Battery Information view (F7), and the division cancels the unit out, leaving a percentage that compares present maximum capacity with original maximum capacity.
The official description puts the expectation plainly: the health of a new battery should be around 100%, and it gradually decreases over time. That wording matters. Around 100% is the starting point, not a permanent state, and a gradual fall is the normal behaviour described by the author — not, by itself, proof that a battery has failed. How far and how fast it falls depends on the battery and its history, which BatteryInfoView reports but does not judge.
A worked example, with example numbers only
The numbers in this example are illustrative examples chosen to make the arithmetic clear — they are not readings from any specific laptop. Suppose a battery reports a Designed Capacity of 50,000 mWh and a Full Charged Capacity of 42,500 mWh. The calculation is 42,500 ÷ 50,000 × 100, which equals 85%. That is the Battery Health figure BatteryInfoView would show for those two inputs.
- Designed Capacity (example): 50,000 mWh — what the battery could hold when new.
- Full Charged Capacity (example): 42,500 mWh — what a full charge holds now.
- Battery Health (example): 85% — the ratio of the two, expressed as a percentage.
- Capacity no longer available (example): 7,500 mWh — the simple difference between the two capacity values.
Try the same arithmetic with a newer battery: a Designed Capacity of 50,000 mWh and a Full Charged Capacity of 49,000 mWh gives 98%. With an older battery at 35,000 mWh against the same 50,000 mWh design figure, the result is 70%. Nothing else enters the formula — not your current charge, not how long the laptop has been running today, and not the program’s estimated time fields. If either input changes, the percentage changes; if both stay the same, it stays the same.
Health is not the same as current charge
The most common confusion is between Battery Health (field 10 in the official field list) and Current Capacity in percent (field 6). Current Capacity in percent is the same percentage value Windows itself displays: how full the battery is at this moment, relative to what a full charge currently holds. It rises while you charge and falls while you discharge, often many times a day. Battery Health compares that current full-charge ceiling against the original design ceiling, so it moves slowly, over months and years rather than minutes.
A concrete way to keep them apart: a battery at 85% health can still charge to 100% — but that 100% charge represents 85% of the energy the battery held when new. Conversely, a healthy new battery sitting at 40% charge is not at 40% health; it is simply partly discharged. If a reading surprises you, check which of the two percentage fields you are actually looking at before drawing any conclusion. Our tutorial How to Check Laptop Battery Health with BatteryInfoView walks through finding both fields step by step.
Wear is simply the opposite number
Before version 1.23, this same calculation was labelled Battery Wear Level. NirSoft renamed the caption to Battery Health in v1.23 because, as the version history notes, the wear level is the opposite value. The relationship is exact: wear = 100 − health. A battery at 85% health has 15% wear; a battery at 98% health has 2% wear. The feature itself was added much earlier, in v1.05, already calculated from Full Charged Capacity and Designed Capacity — only the label and the direction of the number changed.
This is why you should be careful when comparing BatteryInfoView with tools that headline a wear figure. The same battery can be described as 85 (health) in one program and 15 (wear) in another, with no disagreement at all between them. Our companion piece Battery Wear Level vs Battery Health goes deeper on that naming trap, and it is worth reading if you are cross-checking numbers between utilities.
How to read your own two inputs
Open BatteryInfoView and look at the Battery Information view. Note the Designed Capacity and Full Charged Capacity values first, and only then the health percentage. Reading the inputs protects you from two mistakes. First, if the health figure looks odd, you can do the division yourself and confirm the program is reporting the ratio correctly. Second, the absolute values tell you things the percentage alone cannot: two batteries can both read 85% health while holding very different amounts of energy, because their designed capacities were different to begin with.
Also give the readings a moment. The Battery Information view refreshes every 10 seconds by default, and the program’s calculated fields update only every 30 seconds or more, so a value captured the instant you unplug or plug in may not be settled. For a fuller picture over time, the Battery Log (F8) adds a line with battery status every 30 seconds by default — see the Battery Log guide — and the wider context for all of these readings is collected in the Battery Health hub.
What the percentage cannot tell you
Battery Health is a capacity ratio, and only a capacity ratio. It does not tell you how many charge cycles the battery has done (that is a separate field, shown only if the battery provides it), it does not report temperature or per-cell voltage (BatteryInfoView has no such fields), and it does not predict an exact remaining lifespan in months. NirSoft publishes no thresholds for it either, so any replace-it-now line you see elsewhere — including on this site — is general editorial guidance, not a BatteryInfoView feature or an official NirSoft rule. For that decision, read Designed Capacity vs Full Charged Capacity alongside this page, and then When Should You Replace a Laptop Battery?
Frequently asked questions
Can Battery Health be over 100%?
The calculation is a simple ratio of two reported values, and the official expectation is that a new battery should be around 100%. This fact base does not document a promised range, so treat any single reading as the ratio of the two values your battery is reporting at that moment, and sanity-check the two inputs themselves if a figure looks implausible.
Why did my health change between two readings?
Because one of its two inputs changed. Full Charged Capacity is a value reported by the battery and can settle after a full charge; if that input moves, the percentage moves with it, exactly as the formula says it should.
Is health the same as the percentage in Windows Settings?
No. The percentage Windows displays matches BatteryInfoView’s Current Capacity in percent field — how full the battery is now. Health is the separate comparison against Designed Capacity described above.