How to Choose a Lithium Battery for a Solar Inverter

Choosing the right lithium battery for a solar inverter is not simply a matter of selecting the highest Ah capacity. The battery must match the inverter’s DC voltage, power requirement, charging capability, BMS limits, chemistry and available installation space.

For most solar energy-storage applications, LiFePO₄ (LFP) is a commonly considered lithium chemistry because of its cycle-life and operating characteristics, but the correct choice depends on the complete system design.

1. Check the Inverter Battery Voltage

The first step is to identify the inverter’s required DC battery voltage.

Common systems include:

The battery’s voltage range must be compatible with the inverter’s specified DC input and charging voltage.

For larger systems, higher battery voltage reduces DC current for the same power, which can reduce cable size and losses.

Example

A typical 51.2V 100Ah LiFePO₄ battery has:

51.2V × 100Ah = 5,120Wh

or approximately:

5.12 kWh nominal energy

2. Check the Inverter Power

Next, check the inverter’s continuous and peak power.

For example:

Inverter = 5,000W

At approximately 51.2V, the theoretical DC current at 100% efficiency would be:

5,000 ÷ 51.2 ≈ 97.7A

Actual battery current will be higher because an inverter has losses.

Therefore, a 5kW inverter requires a battery and BMS capable of supplying the required continuous current and any applicable surge current.

Battery and inverter manufacturers specify these limits, and they need to be matched rather than relying only on the battery’s Ah rating.

3. Calculate Required Battery Capacity

Determine how long you want the battery to operate your loads.

The basic calculation is:

Battery Energy (Wh) = Voltage × Ah

For example:

51.2V × 100Ah = 5,120Wh

So a 51.2V 100Ah battery has approximately 5.12kWh nominal energy.

Actual usable energy will depend on the battery’s usable state-of-charge range, BMS settings, inverter efficiency and operating conditions.

Example

Suppose your backup load is:

1,000W

A 5.12kWh battery does not necessarily provide 5.12 hours of AC backup because inverter losses and the usable battery-energy limit must be considered.

4. Consider Your Daily Energy Requirement

For solar systems, calculate how much energy you normally consume.

For example:

A battery should then be sized according to the required backup/autonomy and the amount of solar energy available for recharge.

5. Choose the Battery Chemistry

LiFePO₄ / LFP

LiFePO₄ is widely used for stationary energy-storage applications.

Advantages can include:

• Long cycle life
• Good thermal characteristics
• High usable energy
• Suitable for repeated charge/discharge applications
• Available in large prismatic-cell configurations

NMC Lithium-ion

NMC can provide high energy density and may be useful where weight and physical size are important.

For a stationary solar inverter, however, the decision should be based on the complete application rather than chemistry alone.

6. Check the BMS Current Rating

The BMS is one of the most important parts of a lithium battery.

Check:

• Maximum continuous discharge current
• Maximum peak current
• Maximum charging current
• Overcurrent protection
• Overcharge protection
• Over-discharge protection
• Temperature protection
• Cell balancing

For example, if the inverter requires around 100A DC at a particular operating point, a battery with a BMS that cannot continuously support that current is not an appropriate match.

The battery’s maximum charge current also needs to accommodate the combined charging sources, including the inverter/charger and solar charge controller where applicable.

7. Check Solar Charging Current

Do not look only at the inverter’s output power.

You should also check:

Maximum solar charging current + AC charging current

against the battery’s allowable charging current.

For example:

• Solar charger: 40A
• Inverter charger: 30A
• Possible combined charging current: 70A

The battery/BMS must be designed to accept the required charging current, or the charging system must be configured to stay within the battery’s limits.

8. Check Inverter–Battery Communication

Some modern lithium solar systems use communication between the battery BMS and inverter.

Possible interfaces include:

• CAN
• RS485
• Proprietary communication protocols

Communication can allow the BMS to provide information such as:

• State of charge
• Charge-current limit
• Discharge-current limit
• Charge-voltage limit
• Cell voltage
• Temperature

For example, some Victron-compatible battery systems use BMS communication to transmit charge/discharge limits and battery information to the inverter system.

Important: Do not assume that every CAN or RS485 battery is automatically compatible with every inverter. Check the specific battery/inverter compatibility documentation.

9. Check the Battery Charging Voltage

The inverter’s lithium charging profile must match the battery.

For example, a typical:

16S LiFePO₄ battery = 51.2V nominal

has a full-charge voltage around:

58.4V

But the exact charging voltage, absorption/float settings and BMS limits should be taken from the battery manufacturer’s specifications.

Different lithium batteries can have different recommended voltage settings, so the inverter should be configured according to the specific battery manufacturer’s instructions.

10. Consider Battery Size and Installation Space

Before ordering a battery, measure:

• Length
• Width
• Height
• Available floor/wall space
• Cable routing
• Terminal location
• Mounting arrangement

For example:

Available space: 600 × 450 × 200mm

The battery enclosure should fit within that space while leaving appropriate clearance for installation and ventilation according to the manufacturer’s requirements.

11. Decide How Much Backup You Need

Ask yourself:

How many hours of backup do I need?

For example:

2–3 hours

A smaller battery may be sufficient if the backup load is modest.

5–8 hours

A larger battery bank may be required.

Overnight backup

Calculate the total energy consumed during the night rather than choosing the battery based only on inverter VA rating.

Longer off-grid autonomy

You may need additional battery capacity and sufficient solar generation to recharge it.

12. Consider the Inverter’s Surge Load

Some appliances require high starting current.

Examples:

• Refrigerator
• Water pump
• Air conditioner
• Compressor
• Motor
• Power tools

The battery and BMS must be capable of supplying the required DC current when the inverter handles these loads.

The inverter’s surge capability and the battery’s peak-current capability therefore need to be considered together.

13. Example: Choosing a Battery for a 5kW Solar Inverter

Suppose you have:

Inverter: 5kW
Battery system: 48V class
Battery chemistry: LiFePO₄
Required battery: 51.2V 100Ah

Energy:

51.2 × 100 = 5,120Wh

So the battery has approximately:

5.12kWh nominal capacity

At a 5kW load, the theoretical DC current is approximately:

5,000 ÷ 51.2 = 97.7A

Allowing for inverter losses, the actual battery current can be higher.

Therefore, you should verify that the battery cells, BMS, cables, connectors and protection devices can support the required current.

The inverter manufacturer’s recommended battery capacity and current limits should also be checked. Some manufacturers publish specific minimum battery sizes for particular inverter models.

Lithium Battery Selection Checklist

Before ordering, check these specifications:

Common Mistakes to Avoid

❌ Choosing battery only by Ah

A 100Ah battery is not automatically suitable for every inverter.

❌ Ignoring BMS current

A battery may have sufficient kWh but still be unable to supply the inverter’s required current.

❌ Ignoring charging current

A large solar array can potentially exceed the battery’s permitted charging current if the system isn’t configured correctly.

❌ Assuming all 48V lithium batteries are identical

Different batteries can have different voltage limits, BMS settings and communication requirements.

❌ Ignoring inverter compatibility

For smart lithium systems, verify the exact battery/inverter communication and compatibility requirements.

Custom Solar Lithium Batteries from Lion Battery

Lion Battery manufactures custom LiFePO₄ and lithium-ion battery packs for solar, UPS, inverter and ESS applications.

Custom battery specifications can include:

• 12.8V lithium batteries
• 25.6V LiFePO₄ batteries
• 48V/51.2V LiFePO₄ batteries
• Custom Ah capacity
• Custom BMS
• Smart BMS
• CAN / RS485 communication
• Wall-mounted ESS batteries
• Custom battery enclosures
• OEM battery packs

For a custom solar battery, provide your inverter model, inverter power, battery voltage, required backup time, solar-panel capacity and available battery space.

Phone / WhatsApp: +91-9724991737
Email: info@lionbattery.in
Location: Umargam, Valsad, Gujarat