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Battery capacity is quoted two ways. Charge capacity (Ah or mAh) counts how much current the battery can deliver over time. Energy capacity (Wh or kWh) is what actually does work, and depends on voltage: Wh = Ah × V. Comparing batteries only by Ah is misleading unless they share the same voltage — a 100 Ah 12 V battery (1200 Wh) stores the same energy as a 25 Ah 48 V battery.
| Quantity | Formula |
|---|---|
| Energy | Wh = Ah × V |
| Charge | Ah = Wh / V |
| Milliamp-hours | mAh = Ah × 1000 |
| Runtime | t = (Wh × DoD × η) / Load |
Usable energy is less than the nameplate figure: you rarely discharge 100% (a depth of discharge of 80–90% is typical for lithium, 50% for lead-acid), and an inverter or converter loses a few percent. The runtime tab applies both factors.
Cells combine in two ways, and they do different things:
Either way the energy is identical — 29.6 Wh — which is the clearest demonstration of why watt-hours is the honest way to compare packs. A 4S1P and a 1S4P arrangement of the same cells store exactly the same amount of energy and simply deliver it at different voltages.
Three separate things reduce the figure on the label to the figure you can use:
Cells in parallel share current according to their internal resistance, so a lower-resistance cell takes more than its share and ages faster — which lowers its resistance relative to the others and concentrates still more current on it. Mixing cells of different age, capacity or manufacturer in parallel is therefore a genuine reliability problem rather than merely inefficient. Series strings have the complementary issue: the weakest cell limits the whole string, which is why balancing circuits exist.
Multiply amp-hours by the battery voltage: Wh = Ah × V. A 100 Ah battery at 12 V stores 1200 Wh (1.2 kWh).
Divide watt-hours by the voltage: Ah = Wh / V. A 5000 Wh pack at 48 V has about 104 Ah of charge capacity.
First convert mAh to Ah by dividing by 1000, then multiply by voltage: Wh = (mAh/1000) × V. A 3000 mAh cell at 3.7 V is 11.1 Wh.
Because energy depends on voltage. A 100 Ah 12 V battery and a 100 Ah 24 V battery hold very different energy (1.2 kWh vs 2.4 kWh). Compare in Wh or kWh for a fair comparison.
DoD is the percentage of the battery you actually use before recharging. Lithium batteries tolerate 80–90% DoD, while lead-acid is usually limited to about 50% to preserve cycle life.
Runtime = (Wh × DoD × efficiency) / load power. For a 1200 Wh battery, 80% DoD, 90% inverter efficiency and a 300 W load, runtime is about 2.9 hours.
No. The mAh is measured at the cell voltage (about 3.7 V). After boosting to 5 V USB and conversion losses, the usable output is roughly 60–70% of the nominal Wh.
Use the nominal voltage: about 3.6–3.7 V for a lithium cell, 3.2 V for LiFePO4, 12/24/48 V for packs. Using the fully-charged voltage overstates the energy slightly.
Cold reduces the deliverable capacity (a battery may give only 70–80% of its rating near freezing), while high temperatures raise capacity slightly but accelerate ageing. Ratings are usually given at 25 °C.
The C-rate sets how fast you charge or discharge. Discharging faster than the rated rate can reduce the effective Ah delivered (Peukert effect), especially in lead-acid batteries.
Divide the pack voltage by the cell voltage. A 48 V lithium pack uses about 13 series cells (13×3.6 V≈48 V); paralleling cells increases the Ah.
Nominal is the nameplate rating at full discharge. Usable is what you can safely draw — the nominal energy times the depth of discharge — and is the figure to use for real runtime and range.
Series adds voltage and leaves ampere-hour capacity unchanged. Four 3.7 V 2,000 mAh cells in series give 14.8 V at 2,000 mAh, which is the same total energy as the parallel arrangement.
Parallel adds capacity at unchanged voltage. The same four cells in parallel give 3.7 V at 8,000 mAh — 29.6 Wh either way.
Battery management systems reserve capacity at both ends of the charge window to protect life, usable capacity falls at higher discharge rates and in cold conditions, and every pack degrades with age. Usable energy is commonly 80–90% of nominal.
It is strongly inadvisable. Parallel cells share current according to internal resistance, so a stronger cell takes more than its share, ages faster, and progressively concentrates even more current on itself.
Because the weakest cell limits the whole string: it reaches the cut-off voltage first on discharge and the full-charge voltage first on charge. Balancing circuits equalise the cells so the pack uses the capacity it actually has.
By watt-hours, unless both packs are at the same voltage. Ampere-hours measure charge only, so the comparison is meaningless across different voltages.
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