Battery Self-Discharge Calculator

Understand how much charge a stored battery may lose

Batteries do not stay at full charge just because they are sitting on a shelf. Even when nothing is connected, internal chemical reactions slowly drain stored energy. That quiet loss is called self-discharge. It matters when you store backup batteries, seasonal equipment packs, electric vehicle accessory batteries, lab instruments, emergency lighting packs, or any other battery that may sit unused for weeks or months before you need it. This calculator estimates how much capacity remains after storage so you can decide whether a battery will still be ready for service, whether it needs a maintenance charge, and whether your storage conditions are reasonable.

The most important idea behind the calculator is that time and temperature work together. A battery stored for a long time loses more charge than one stored briefly, and a battery stored hot generally loses charge faster than one stored at room temperature. That means the same battery can behave very differently in a cool basement, a climate-controlled warehouse, or a hot garage. A six-month storage plan that looks safe at 25 °C can become much less forgiving if the real temperature is 35 °C or higher.

This page keeps the math simple enough to use quickly while still capturing the main trend you care about. You enter four values: the battery's starting capacity in amp-hours, its monthly self-discharge rate at 25 °C, the actual storage temperature, and the number of months in storage. The calculator then adjusts the monthly loss rate for temperature and compounds that loss over the storage period. The result tells you both the remaining capacity in amp-hours and the percentage of the original capacity still retained.

What each input means in plain language

Initial Capacity (Ah) is the amount of charge the battery starts with, expressed in amp-hours. If a battery is rated at 50 Ah and is truly full when storage begins, that is the number you would enter. If you already know the battery entered storage only partially charged, you should use the actual starting amount instead of the nameplate value. The output can only be as realistic as the starting point.

Self-Discharge Rate (% per month at 25 °C) is the baseline monthly loss, usually from a battery data sheet, maintenance guide, or field experience. Different chemistries behave differently. Some batteries lose only a small amount per month at room temperature, while others lose charge faster. The label in the form matters: the rate should be monthly and specifically referenced to 25 °C. If your source is weekly, daily, or annual, convert it before entering it.

Storage Temperature (°C) is the average temperature during storage, not just the temperature on the day you check the battery. For short storage windows, a simple average is often enough. For long storage windows with major swings, you may want to test a few scenarios such as a cool case, a normal case, and a hot case. That gives you a realistic range instead of a single optimistic number.

Storage Time (months) is how long the battery sits before use or inspection. Because self-discharge compounds over time, small monthly losses add up. A rate that looks harmless for one month can become significant across six or twelve months. If you maintain batteries on a schedule, entering the interval between maintenance charges is often the most useful way to use this tool.

How the calculator models self-discharge

The specific battery model used here has two parts. First, it adjusts the monthly self-discharge rate for temperature. Second, it applies that adjusted rate repeatedly over the number of storage months. The formula follows a common engineering rule of thumb: each 10 °C increase above 25 °C roughly doubles the self-discharge rate, while cooler storage reduces it.

Here, r₂₅ is the self-discharge rate at 25 °C, T is storage temperature, C₀ is the initial capacity, and m is the number of months. The result C_m is the estimated capacity after storage. This is a practical estimation model, not a chemistry-specific laboratory simulation, so it is most useful for planning, maintenance intervals, and quick comparisons.

The page also preserves the general mathematical view of calculators shown below. Those formulas are more abstract, but they express the same idea: a result is a function of several inputs, and each input contributes differently depending on the model.

In other words, the battery estimate is not magic. It is a repeatable process. When you change the temperature or the storage time, the result should move in a direction that makes physical sense. That is exactly why this calculator is helpful: it lets you compare scenarios consistently instead of relying on vague intuition.

Worked example with realistic battery numbers

Suppose a battery starts at 50 Ah, its self-discharge rate is 3% per month at 25 °C, the storage temperature is 35 °C, and the storage time is 6 months. Because 35 °C is 10 °C above the 25 °C baseline, the rule of thumb doubles the monthly loss. That turns the 3% monthly rate into an adjusted rate of roughly 6% per month.

Now apply the storage formula. The estimated remaining capacity becomes 50 × (1 − 0.06)⁶, which is about 34.5 Ah. That means the battery retains roughly 69.0% of its original capacity over that storage period. If the same battery were stored at 25 °C instead, the estimate would be 50 × (1 − 0.03)⁶ ≈ 41.6 Ah, or about 83.3% retained. The temperature difference alone changes the planning outcome by more than 7 Ah.

This comparison is the real value of the calculator. The exact final number is useful, but the difference between scenarios is often even more important. You can quickly see whether cooling the storage room, shortening the storage interval, or choosing a battery with a lower self-discharge rate would materially improve readiness.

Quick comparison table

The table below uses one common example: a 50 Ah battery stored for 6 months with a base self-discharge rate of 3% per month at 25 °C. Only the storage temperature changes.

Even if your own battery has different numbers, the pattern stays the same: warmer storage usually reduces the remaining charge much faster than many people expect.

How to use the result wisely

When the calculator returns a number, read it as an estimate of stored readiness. If the result says 18 Ah remains from an original 20 Ah pack, the battery is likely still near full usefulness. If it says only 8 Ah remains, that does not necessarily mean the battery is ruined, but it does mean your planned standby time, startup reserve, or available run time may be much shorter than expected. The result is especially helpful when compared against a minimum capacity requirement for your application.

It is also worth separating capacity retained from battery health. A battery may still be healthy but simply not fully charged after storage. Conversely, a degraded battery may have started below its nameplate capacity even before storage began. This calculator focuses on charge remaining due to self-discharge. It does not diagnose aging, sulfation, cell imbalance, internal resistance growth, or damage from deep discharge, overcharge, or freezing.

If you are using the tool for maintenance planning, try three runs instead of one. Start with a normal temperature, then test a cool scenario and a warm scenario. Next, adjust the storage time. That small exercise quickly reveals whether your maintenance interval is robust or fragile. If a slight temperature increase pushes the retained capacity below your required threshold, the storage plan may be too optimistic.

Assumptions and limits to keep in mind

This calculator uses a simple temperature-adjusted compounding model. That makes it fast and transparent, but like any simplified model it has boundaries. Real self-discharge depends on chemistry, age, state of charge, manufacturing differences, and storage history. Some chemistries do not follow the temperature rule of thumb perfectly across the full temperature range. For that reason, the best source for the baseline self-discharge rate is always the battery manufacturer or measured field data.

Another limitation is that the formula assumes the adjusted monthly loss stays physically meaningful. If you enter an unusually large self-discharge rate together with extreme heat, the model may imply losses so high that the estimate is no longer realistic for practical storage decisions. In everyday use, that is usually a sign to revisit the inputs, check the data-sheet units, or split the analysis into more realistic storage windows.

Finally, this calculator does not replace storage guidance from the manufacturer. Some batteries should be stored partially charged, some should be periodically topped up, and some are more sensitive than others to high heat or long inactivity. Use the result as a planning estimate, then combine it with the storage instructions for the exact battery chemistry and product family you are working with.

Practical storage tips that pair well with the calculator

If the output shows that a battery loses more charge than you expected, the easiest first question is whether the storage area is too warm. Reducing temperature often has a bigger effect than people realize. The second question is whether the maintenance interval is too long. Shorter intervals can keep standby batteries within a comfortable readiness band even when perfect temperature control is not possible.

It also helps to record the assumptions you used: starting capacity, monthly rate, temperature, and months in storage. That way you can compare actual inspection results against the estimate later. Over time, those comparisons let you tune the input rate to better match your own fleet, environment, and storage habits. A calculator becomes much more powerful once it is paired with even a little real-world feedback.

Use the form below to estimate the remaining capacity for your own scenario. Then, if you want a quick hands-on way to feel how temperature control affects storage loss, try the optional mini-game further down the page.