Introduction
A project specification says:
2V 1000Ah OPzV, C10
The customer then asks:
“How many hours can this battery bank run my system?”
It is tempting to answer:
1000Ah ÷ load current = backup hours.
Unfortunately, real battery sizing is not that simple.
A C10 rating describes battery performance under a particular laboratory discharge condition. Actual project autonomy depends on several additional factors:
- Discharge rate
- End-of-discharge voltage
- Battery temperature
- Battery age
- DC system losses
- Inverter efficiency
- Cable voltage drop
- Continuous and intermittent loads
- Required end-of-life capacity
Two projects using exactly the same 2V 1000Ah OPzV cells can therefore achieve very different backup times.
For professional B2B battery selection, the correct question is not:
“How many Ah do I need?”
It is:
“What battery can support this specific load profile for the required time to the permitted final voltage under the expected site conditions?”
What Does C10 Mean?
C10 means the battery capacity is measured over approximately a 10-hour discharge period under specified conditions.
For a simplified example:
1000Ah C10
corresponds approximately to:
100A × 10 hours
under the manufacturer’s stated:
- Reference temperature
- Final voltage
- Test procedure
But the “1000Ah” figure should never be separated from those conditions.
A current industrial OPzV product range, for example, specifies its European capacity basis using:
C10 + defined end voltage + defined reference temperature.
This is the correct way to read a capacity rating.
C10 Does Not Mean the Battery Always Provides 1000Ah
Consider one battery: 2V 1000Ah C10
Now imagine three applications.
Application A
Load current: 50A
Expected duration: approximately 20 hours
Application B
Load current: 100A
Expected duration: approximately 10 hours
Application C
Load current: 300A
Expected duration: approximately 3 hours
The available Ah is not necessarily identical in all three cases.
Lead-acid batteries generally deliver lower usable Ah capacity at higher discharge rates.
This rate effect is one reason the manufacturer publishes separate discharge tables.
The Discharge Table Is More Important Than the Large “1000Ah” Label
For engineering projects, the battery data sheet may list current available for:
- 1 hour
- 2 hours
- 3 hours
- 5 hours
- 8 hours
- 10 hours
- 20 hours
at different final voltages.
This table should be the main sizing tool.
If the customer requires:
5-hour backup
use the manufacturer’s 5-hour discharge data.
Do not simply divide the C10 Ah figure by five.
End-of-Discharge Voltage Changes the Available Capacity
A battery can supply more energy if it is allowed to discharge to a lower final voltage.
For example, a manufacturer may publish different performance values to:
- 1.85V/cell
- 1.80V/cell
- 1.75V/cell
- 1.70V/cell
depending on discharge duration and application.
Therefore: 1000Ah to 1.80V/cell is not directly equivalent to: 1000Ah to 1.70V/cell.
A buyer comparing different quotations should make sure suppliers are using the same end voltage.
The DC Equipment Determines How Low You Can Go
You cannot select final voltage only from the battery perspective.
The critical load also has a minimum operating voltage.
For example, a 110V DC system may contain:
- Protection relays
- Breaker trip coils
- SCADA
- Communication equipment
- Control electronics
The battery may technically be capable of discharging further, but the equipment may stop operating first.
Therefore:
Battery end voltage must remain compatible with minimum system voltage.
Cell Count Also Matters
Suppose a system contains: 55 × 2V cells
If the selected final voltage is: Ve volts per cell
then: Total final battery voltage = 55 × Ve
The project engineer should then subtract:
- Cable voltage drop
- Connection losses
to confirm the voltage still available at the actual DC load.
Temperature Can Significantly Change Real Backup Time
OPzV and OPzS capacity is normally specified at a reference temperature.
At lower temperatures:
- Electrochemical reaction slows
- Available capacity decreases
- Voltage under load may fall sooner
A battery that provides adequate runtime at 20°C may not deliver the same autonomy at 0°C.
Current OPzV manufacturer guidance provides different capacity correction factors for different temperature ranges and discharge durations.
This is particularly important for:
- Outdoor substations
- Mountain telecom sites
- Northern Europe
- Central Asia
- Cold warehouses
High Temperature Creates a Different Problem
Higher temperature may initially increase available discharge capacity slightly.
However, it also accelerates lead-acid battery aging.
Therefore, a hot-climate design should not simply reduce battery size because initial capacity appears favorable.
The project must consider: short-term capacity + long-term service life.
New-Battery Sizing Should Include Aging
A battery bank should not be designed to meet the required autonomy only on Day 1.
Suppose a customer requires: 8 hours backup
and the new battery provides exactly: 8.0 hours.
There is no allowance for future aging.
As capacity declines, the project requirement will quickly be violated.
Professional sizing therefore includes an aging or end-of-life factor.
The exact design factor depends on:
- Project specification
- Applicable standard
- Required replacement threshold
Design Margin Is Different from Aging Margin
These are often confused.
Aging Margin
Allows for battery deterioration during service life.
Design Margin
Allows for uncertainties such as:
- Future load additions
- Calculation error
- Cable losses
- Operating variations
Do not simply add random percentages.
Use the project’s engineering requirements.
Inverter Efficiency Matters in AC Backup Systems
Suppose the battery bank powers an AC load through an inverter.
The simple calculation:
Battery Energy = Voltage × Ah
gives nominal DC energy.
But the AC load receives less because the inverter has conversion losses.
A preliminary calculation can use:
Required DC Energy = AC Load Energy ÷ Inverter Efficiency
Example:
AC load: 10kW
Backup: 5 hours
AC energy: 50kWh
If inverter efficiency under the relevant load is 92%:
Required battery-side energy before other corrections ≈ 54.3kWh
This still does not include:
- Battery discharge-rate effects
- Temperature
- End voltage
- Aging
Inverter Efficiency Is Not Always Constant
An inverter may have different efficiency at:
- 20% load
- 50% load
- 100% load
For accurate project design, use the inverter manufacturer’s efficiency data at the expected operating point.
DC Loads Should Be Calculated Differently
Substations and telecom systems often use direct DC loads.
Examples:
- 48V telecom equipment
- 110V control systems
- 220V DC switchgear
In these cases, inverter efficiency may not apply.
However, still consider:
- Cable loss
- DC-DC converters
- Auxiliary devices
Continuous Load and Momentary Load Must Be Separated
A substation load profile may include:
Continuous Load
15A for the full backup period.
Momentary Loads
- Breaker trip: 80A for several seconds
- Closing operation: 100A briefly
The momentary load may contribute little total Ah but can cause significant instantaneous voltage drop.
The battery must satisfy both:
energy capacity and power/current capability.
Example: Preliminary 110V OPzV Sizing
Assume:
- Continuous load: 20A
- Required autonomy: 8 hours
Simple Ah requirement:
20A × 8h = 160Ah
But selecting a 160Ah battery immediately would be incorrect.
The engineer must next consider:
- Applicable 8-hour discharge data
- Final voltage
- Temperature
- Aging requirement
- Future load
- Peak current
The selected battery might therefore need a nominal capacity considerably above the simple 160Ah calculation.
Example: Solar OPzS Application
Suppose a remote site consumes: 40kWh per night
Battery nominal voltage: 48V
A simple calculation gives: 40,000Wh ÷ 48V ≈ 833Ah
But this does not yet account for:
- Planned DOD
- Inverter loss
- Discharge rate
- Temperature
- Battery aging
- Cloudy-day autonomy
If only 50% of nominal capacity is intended for routine use, the required battery bank already increases substantially.
This demonstrates why simple Wh ÷ V calculations should be treated only as a starting point.
C10 vs. C20 in Supplier Quotations
Supplier A: 2V 1000Ah C10
Supplier B: 2V 1000Ah C20
These should not be compared as identical batteries.
For lead-acid chemistry, the slower C20 discharge typically produces a higher Ah result than the faster C10 test on the same basic battery.
Ask every supplier to quote capacity under the same:
- Duration
- End voltage
- Temperature
UPS Projects Should Consider Constant-Power Tables
UPS loads may behave more like constant-power loads.
As battery voltage decreases, the inverter can draw more current to maintain the same AC output power.
Therefore, constant-power battery data expressed as:
Watts per cell
can be more useful than nominal Ah for certain UPS applications.
Calculate the Whole String, Not One Cell in Isolation
For a series string:
- Voltages add
- Ah remains the same
Example:
55 × 2V 1000Ah: 110V nominal, 1000Ah
not: 110V, 55,000Ah.
This is an important mistake to avoid when customers calculate stored energy.
Parallel Strings Increase Ah
Two identical 110V 1000Ah strings in parallel: 110V 2000Ah
Three: 110V 3000Ah
However, current sharing and charger capacity must be considered before adding multiple parallel strings.
Battery Autonomy Should Be Verified After Installation
A calculation provides the design.
Commissioning verifies the real installation.
After installation, check:
- Actual continuous current
- Peak current
- Battery temperature
- Cable voltage drop
- Charger settings
If the project is critical, perform the agreed acceptance or capacity test.
Common Calculation Errors
Using C10 Ah as Universal Capacity
Actual capacity varies with discharge rate.
Ignoring Final Voltage
Supplier data may use different test endpoints.
Ignoring Temperature
Cold sites can lose substantial usable capacity.
Assuming 100% Inverter Efficiency
Real conversion losses reduce runtime.
Using 100% Nominal Capacity
May create excessive DOD and poor life.
Forgetting Aging Margin
The battery may meet requirements only when new.
Adding Series Ah
Series connection increases voltage, not Ah.
Ignoring Peak Loads
A battery can have enough energy but still suffer excessive voltage sag.
Information to Send a Battery Supplier
For accurate OPzV or OPzS sizing, provide:
- Nominal DC system voltage
- Continuous load
- Peak load
- Required backup time
- Minimum permitted DC voltage
- Ambient/battery temperature
- AC or DC load
- Inverter efficiency if applicable
- Required standard
- Aging/design margin
For solar projects also provide:
- Daily energy consumption
- Daily DOD target
- Required autonomy days
- Charging source
Frequently Asked Questions
Does 1000Ah C10 mean the battery can supply 100A for exactly 10 hours in every system?
No. The rating applies under the manufacturer’s specified test conditions.
Is C20 better than C10?
Neither is inherently better. They describe different discharge rates.
Why is actual runtime shorter than Ah ÷ current?
Possible reasons include higher discharge rate, inverter losses, temperature, cutoff voltage and battery aging.
Can I use nominal kWh to predict runtime?
It is useful for preliminary calculations but insufficient for final lead-acid sizing.
Should a new battery bank be sized exactly to the required runtime?
Normally the project should consider aging and appropriate design margin.
Which data are most useful for final sizing?
The manufacturer’s discharge table under the actual required duration, temperature and final voltage.
Conclusion
The large Ah number on an OPzV or OPzS label is only the beginning of a proper battery calculation.
Real project autonomy depends on:
discharge rate + final voltage + temperature + aging + system losses + actual load profile.
For EPC contractors and distributors, the safest approach is:
Start with the load requirement and work backward through the manufacturer’s discharge table.
Do not start with:
“We want 1000Ah because the old project used 1000Ah.”
A correctly selected 2V battery bank should meet the customer’s required backup time both when new and later in its planned service life.
