Float Battery Charging Tips: How to Make Your Lithium Batteries Last Longer

Float charging keeps batteries ready in backup, solar, telecom, RV, and marine systems.

Lithium batteries need different handling. Some, like LiFePO4, may allow a low float voltage, while others should stop charging when full. Always follow the battery’s chemistry and manufacturer guidelines.

Key takeaways

• A float battery is not a special type. It is a battery kept at a stable voltage after charging.
• Many lithium batteries do not need float charging. Use it only if the manufacturer allows it.
• Some 12.8V LiFePO4 batteries use about 13.4V–13.6V as float voltage. This does not fit all 12V lithium batteries.
• Most 3.7V lithium-ion and LiPo cells use CC/CV charging with charge cut-off, not continuous float.
• Do not use lead-acid modes like equalization or desulfation on lithium batteries.
• The charger sets the charging profile. The BMS protects the battery but should not fix wrong charger settings.

Part 1. What is a float battery?

A float battery stays connected to a charger after it is full. The charger keeps a steady voltage so the battery stays ready.

The terms floating battery and float battery are common in UPS, telecom, and backup systems. In these setups, the battery often works in parallel with the charger and load.

A float battery is not a new chemistry. It can be lead-acid, LiFePO4, or others. Each type needs its own charging method.

What is float charging?

Float charging is a maintenance stage. After full charge, the charger lowers the voltage.

The charger then supplies only the needed current. If there is no load, current is very low. If devices run, the charger may power them.

The float voltage is lower than the main charge voltage. It is not the same as the maximum voltage.

What is a float charger?

A float charger can reduce voltage after charging. It keeps the battery in standby mode.

• Battery type selection
• Adjustable voltage
• Auto charge cut-off
• Float or standby mode
• Low-temperature protection
• Voltage and current monitoring
• BMS communication

When people ask about a floating charger, they usually mean a charger that can stay connected without overcharging.

Part 2. Do lithium batteries need float charging?

Most lithium batteries do not need float charging. This is different from lead-acid batteries.

Most lithium cells use a CC/CV charging method for lithium batteries. First, the charger gives constant current. Then it holds a fixed voltage while current drops. Charging stops when current is low enough.

Keeping a battery at max voltage is not proper float charging. Lithium batteries need charge cut-off. This guide from Analog Devices explains lithium charge termination.

Some LiFePO4 batteries in solar or backup systems may allow a low float voltage. Always follow the battery datasheet.

Part 3. How does float charging work?

Float charging usually has these steps:

1. Bulk: Current flows, voltage rises.
2. Constant voltage: Voltage stays fixed, current drops.
3. Charge cut-off: Charging stops or reduces.
4. Float stage: Voltage lowers for standby.

Current is not fixed in float mode. It changes with load.

This is useful for systems that must stay ready:

• Backup power
• Telecom systems
• UPS
• Solar storage
• Security systems
• RV and marine systems

Part 4. What is the right float voltage for a lithium battery?

There is no single float voltage for all lithium batteries.

It depends on:

• Chemistry
• Cell count
• Nominal voltage
• Max charge voltage
• BMS design
• Balancing method
• System needs
• Manufacturer data

Float voltage for a 12V LiFePO4 battery

Some 12.8V LiFePO4 batteries use about 13.4V–13.6V. Around 13.5V is common.

For example, Victron uses 13.5V in some systems. But this is not universal. Always check specs. See Victron lithium charger settings guide.

These are examples only. Always follow the battery spec.

What is the float voltage for a 3.7V lithium battery?

A 3.7V lithium cell usually charges to about 4.2V. Then charging stops.

Do not keep it at 4.2V for long. This can speed up aging.

Most devices stop charging and restart later. This is not true float charging.

A “12V lithium battery” can mean different things

The term “12V lithium battery” can be confusing.

A 12.8V LiFePO4 uses 4 cells. An 11.1V lithium-ion pack uses 3 cells. They need different voltages.

Always check chemistry and cell count first.

Part 5. Why a lower float voltage helps

High voltage for long time can age lithium batteries faster.

High temperature makes this worse. NREL research on lithium battery aging shows this clearly.

Designers must balance:

• Readiness: More stored energy
• Life: Less stress

Use the lowest approved float voltage that still meets system needs.

Part 6. Float charging vs. trickle charging

These two are different.

Some chargers use “trickle” for pre-charge. This is short and safe. It is not long-term charging.

Part 7. Where is float charging used?

For solar systems, LiFePO4 batteries for solar storage are popular due to long life and safety.

Part 8. How to set up a float charger

Start with the battery datasheet.

What you need

• Datasheet
• Battery type
• Charger
• BMS
• Multimeter
• Temperature sensor

Part 9. Common float charging mistakes

1. Using one voltage for all

13.5V is not universal.

2. Using lead-acid charger

It may damage lithium batteries.

3. Using max voltage as float

This is incorrect.

4. Relying only on BMS

Battery Management System basics show it is for protection, not control.

5. Wrong temperature settings

Do not use lead-acid compensation.

6. Charging in cold

This can damage cells.

7. Keeping full charge too long

This reduces life.

8. Ignoring balancing

See lithium cell balancing explained.

Part 10. Can you float charge forever?

Only if the system is designed for it.

Check:

• Manufacturer approval
• Correct charger
• Right voltage
• BMS active
• Temperature control

For storage, do not use float. Follow storage guidelines.

Part 11. How temperature affects float charging

Low temperature

Charging in cold can damage cells.

High temperature

Heat speeds up aging.

Use correct limits

Always follow datasheet.

Part 12. Float charging best practices

• Use correct charger
• Follow datasheet
• Avoid high voltage too long
• Control temperature
• Use BMS
• Check voltage at terminals

Read more in battery life improvement tips.

Part 13. Float charger selection checklist

Part 14. Choosing the right battery for float charging

Consider more than voltage:

• Runtime
• Temperature
• SOC
• Life target
• Charger match
• BMS

For custom needs, see custom lithium battery design services.

Part 15. FAQs about float battery charging