Batteries, UPS systems and inverters
Batteries, UPS systems and inverters
Batteries, UPS systems and inverters are different parts of the same energy chain. A cell or battery stores electrical energy chemically; a BMS monitors and protects suitable battery packs; a UPS is intended to keep a load powered when the mains supply fails; and an inverter can convert DC energy into usable AC power.
Technical framework: voltage → capacity → usable energy → current → runtime → charging → protection → DC/AC conversion → load. Looking at only one value can give the wrong result. Ah alone does not determine runtime, watts alone do not determine battery size, and two batteries marked with the same voltage are not automatically interchangeable.
The central question throughout this section is simple: how much energy is stored, at what voltage, how much power does the load require and for how long can the complete system supply it safely?
Why should mAh, Ah and Wh not be confused?
Ah and mAh describe charge capacity, while Wh includes voltage and therefore describes stored energy more directly.
A 12 V 10 Ah battery and a 24 V 10 Ah battery have the same Ah rating but do not contain the same nominal amount of energy. A simplified relationship is:
Wh ≈ volts × Ah
See what mAh, Ah and Wh mean for the complete explanation.
For the wider energy concept, continue with what kWh is and kW versus kWh.
Does a higher mAh rating always mean longer runtime?
At the same voltage, with similar chemistry and load conditions, greater capacity generally supports longer runtime. Comparing mAh values across different voltages, however, can be misleading.
Usable energy also depends on battery chemistry, discharge current, temperature, age, low-voltage cutoff and system efficiency.
Alkaline, rechargeable, lithium-ion and LiFePO4 are not the same
Primary alkaline and zinc-carbon cells have different chemistry and operating characteristics from rechargeable cells. See alkaline, zinc-carbon and rechargeable batteries compared.
The common question about 1.2 V rechargeable cells versus 1.5 V primary cells is also a chemistry issue. See why rechargeable cells are rated at 1.2 V and ordinary cells at 1.5 V.
For larger rechargeable systems, continue with lithium-ion versus LiFePO4.
What is an 18650 cell?
18650 describes a common cylindrical cell form factor rather than one single battery chemistry.
Capacity, discharge capability and protection can differ significantly between cells. See protected versus unprotected 18650 cells.
Why are batteries connected in series or parallel?
Series connection increases voltage. Two compatible 12 V batteries connected in series form a nominal 24 V arrangement.
Parallel connection keeps the voltage nominally the same while increasing total capacity and potential current capability.
Battery matching and protection are important. See what happens when batteries are connected in series or parallel.
Can battery state be determined from voltage alone?
Open-circuit voltage can provide useful information, but interpretation depends on chemistry and loading.
An aged battery may show a plausible voltage with no load and then collapse when current is drawn. See how a full or depleted battery can be identified.
What does a BMS do?
A battery management system can monitor cell voltage, current and, in many designs, temperature.
Depending on the design, it can provide protection against overcharge, over-discharge, overcurrent and other abnormal conditions and can help manage cell imbalance.
A BMS is not the same thing as a charger. The battery still requires charging appropriate to its chemistry. See battery chargers.
Why do batteries leak or swell?
Chemical ageing, unsuitable charging, excessive temperature, internal failure or mechanical damage can produce abnormal battery behaviour.
A swollen lithium battery is not normal and should not simply remain in service. See why batteries leak or swell.
AGM, gel or LiFePO4?
A “12 V battery” description is not enough for selection. AGM, gel and LiFePO4 batteries differ in charging requirements, cycling behaviour, weight and usable capacity.
See AGM, gel and LiFePO4 batteries compared.
For the wider product and charging structure, continue with batteries and battery chargers.
What does a UPS do?
An uninterruptible power supply combines energy storage and power electronics to keep a critical load operating when the mains supply is interrupted or unsuitable.
Its role is therefore broader than simply producing AC from a battery.
For the full product family, see uninterruptible power supplies and UPS types.
Online, line-interactive or offline UPS?
These architectures differ in how the load is supplied, how voltage is regulated and when the battery/inverter path becomes involved.
See online, line-interactive and offline UPS systems compared.
Why does a UPS keep beeping?
A UPS alarm does not automatically mean failure. Battery operation, low battery, overload and battery faults can use different alarm patterns.
For troubleshooting, continue with why a UPS keeps beeping or sounding an alarm.
Is a UPS the same as an inverter?
No. The core purpose of an inverter is DC-to-AC conversion. A UPS can combine batteries, charging, transfer or conversion architecture, control and alarms in one system.
See UPS versus inverter.
For the electrical foundation, see AC versus DC.
Pure sine wave or modified sine wave?
An inverter output marked “230 V” does not automatically have the same waveform as utility power.
Some motors, transformers and electronic power supplies can respond differently to waveform quality. See pure sine wave versus modified sine wave inverters.
How is UPS runtime calculated?
UPS runtime depends on battery voltage and Ah capacity, load watts, conversion efficiency, battery age and discharge behaviour.
For the UPS-specific calculation, see how UPS runtime is calculated.
For the battery-side calculation, see how battery runtime is calculated from Ah.
Does a 1000 VA UPS supply 1000 W?
Not necessarily. VA and watts are different. A UPS has an apparent-power limit and an active-power limit, and both must be respected.
See VA versus watts and kW versus kVA.
Does 100 Ah mean 100 hours of runtime?
No. Ah is an ampere-hour capacity rating, not a runtime label.
Voltage, load power, allowable depth of discharge and conversion efficiency must also be considered.
Adapter, charger, UPS or inverter?
An adapter or power supply converts electrical power into the voltage and current required by a device. A charger additionally manages charging according to battery requirements. A UPS provides continuity of power, while an inverter fundamentally converts DC into AC.
For the adjacent topic, see adapters, chargers and power supplies.
Where does solar power fit into this chain?
In an off-grid or backup system, solar panels generate energy, a charge controller manages the battery charging path, batteries store energy and an inverter converts DC for AC loads.
Continue with solar power and electric-vehicle charging.
What should be checked when selecting a system?
1. What is the load power? Determine the real watt requirement.
2. What is the system voltage? The current required for the same power changes significantly between 12 V, 24 V and 48 V systems.
3. How long must it operate? A few minutes of UPS backup and several hours of off-grid operation require very different energy storage.
4. What is the battery chemistry? AGM, gel, lithium-ion and LiFePO4 do not share identical charging and discharge limits.
5. Does waveform matter? Sensitive and motor loads may require consideration of pure versus modified sine wave.
6. Is transfer time important? Critical loads may require careful choice between UPS architectures.
The sentence to remember: Ah is not energy by itself, a UPS is more than a battery, an inverter is not automatically a UPS, and runtime requires volts, Wh, watts and efficiency to be considered together.