What Is Battery Float Charge?

If you leave a backup battery connected for months, you’ll hear the term float charge. It sounds technical, but the idea is simple: once a battery is already full, a charger holds it at a lower, gentle voltage so it won’t self-discharge. Float is common in telecom rooms and UPS systems. It’s helpful for lead-acid batteries, but it’s usually not needed (and can even be harmful) for lithium unless you set it carefully.

Below is a clear, step-by-step guide you can follow—even if you’ve never set a charger before.

What does “float charge” mean?

• Definition: After the battery reaches full in the bulk and absorption stages, the charger drops to a lower, constant voltage called float.
• Purpose: Offset natural self-discharge so the battery stays ready without overworking it.
• Where it appears: UPS/telecom backup, emergency lighting, some solar/backup inverters.
• Not the same as “keep charging to 100% all day.” Float should be lower than the charging voltage used to fill the battery.

Charging stages (in order): Bulk → Absorption → Float (Equalize exists for some flooded lead-acid only.)

How is float different from trickle, bulk, absorption and boost?

Stage/Mode	What it does	Typical control	Use with lead-acid?	Use with lithium (LiFePO4)?	Main risk if misused
Bulk	Fast filling from low SOC	Constant current (rising voltage)	✅	✅	None if limits are correct
Absorption	Finish to 100%	Constant voltage (current tapers)	✅	✅ (short)	Over-time = heating/overcharge
Float	Maintain full, offset self-discharge	Lower constant voltage	✅ Standard for standby	⚠️ Often disable; or set low	Long high-SOC dwell → aging (esp. lithium)
Trickle	Very small current drip	Low constant current	⚠️ Easy to overdo	❌ Not recommended	Slow overcharge on full battery
Boost/Equalize	Higher-than-normal voltage to rebalance	Elevated voltage, limited time	⚠️ Only for some flooded	❌ Never	Gas, water loss, damage

Plain English: Float = low voltage hold. Trickle = small current drip. They are not the same.

What float voltage is good for lead-acid?

Float must be lower than the absorption voltage and adjusted for temperature. The numbers below are typical ranges at 25 °C; always check your battery manual.

Flooded / AGM / Gel quick chart (25 °C)

System	Flooded (V)	AGM (V)	Gel (V)
12 V	13.5 – 13.8	13.4 – 13.6	13.2 – 13.5
24 V	27.0 – 27.6	26.8 – 27.2	26.4 – 27.0
48 V	54.0 – 55.2	53.6 – 54.4	52.8 – 54.0

Rules of thumb (25 °C):
Flooded ≈ 2.25–2.30 V/cell, AGM ≈ 2.23–2.27 V/cell, Gel ≈ ~2.20–2.25 V/cell.

How does temperature change float voltage?

Lead-acid wants lower voltage when it’s hotter and higher when it’s colder. A common slope is –3 mV/°C per cell from 25 °C.

• Example, 12 V flooded (6 cells), set at 13.6 V at 25 °C:
  • At 35 °C (hot room): 13.6 V – (0.003 V × 10 °C × 6) ≈ 13.42 V
  • At 5 °C (cold room): 13.6 V + (0.003 V × 20 °C × 6) ≈ 13.96 V

Tip: Many chargers let you plug a temperature sensor onto the battery so the device adjusts automatically.

Should lithium (LiFePO4) be float charged?

Usually no. LiFePO4 doesn’t need long float because its self-discharge is very low, and staying at high state-of-charge (high voltage) for days can age the cells faster.

Better approach for LiFePO4

• Disable float if your charger/inverter allows.
• Use a “charge-to-full then stop” profile. Let the BMS terminate charging; only restart when the battery drops to a set SOC/voltage (e.g., restart at 40–60% SOC in off-grid solar).

If the menu requires a “Float” number for LFP

• Enter a low standby value clearly below your absorption voltage.
• Typical LFP examples (check your brand’s BMS guide):
  • 12 V LFP: Absorb 14.2–14.6 V; set “Float/Standby” around 13.4–13.6 V
  • 24 V LFP: Absorb 28.4–29.2 V; “Float/Standby” 26.8–27.2 V
  • 48 V LFP: Absorb 56.8–58.4 V; “Float/Standby” 53.6–54.4 V

The idea is not to hold lithium at 100% all day. Keep it ready, but avoid long high-voltage dwell.

When is float useful—and when can it hurt?

Useful

• UPS/telecom standby (mostly lead-acid): systems must be instantly ready; float keeps them topped without full-power charging.
• Emergency lighting and alarms with lead-acid.

Can hurt

• High temperature + high float voltage: speeds up water loss and dry-out (flooded/AGM/gel).
• Lithium kept at high SOC: accelerates side reactions and capacity fade.
• Small chargers mis-set as trickle: a tiny current for months can still overcharge.

Best practices

• Use the right float range for your chemistry.
• Enable temperature compensation and mount the probe on a battery case, away from heat sources.
• Provide ventilation for lead-acid rooms.
• For lithium off-grid, prefer float disabled with smart restart thresholds.

How do you set float in solar controllers and UPS menus?

1. Find the menu: In MPPT/hybrid inverters look for Bulk/Absorb/Float/Equalize. In UPS, look under Battery settings → Float/Standby.
2. Enter values:
   • Lead-acid → use the table above; add temperature compensation.
   • LiFePO4 → disable float or set low standby; reduce absorb time.
3. Set re-bulk / re-charge: Don’t restart charging for tiny drops. For lithium, many users choose a lower restart SOC (e.g., 50%).
4. Save and test: Watch the actual terminal voltage with a meter for 10–15 minutes to confirm.

Five mistakes to avoid

1. Keeping lithium at a lead-acid float value all day.
2. No temperature probe on lead-acid—hot rooms need lower float.
3. Absorption time too long → battery stays hot and gassy.
4. Long trickle on a battery that is already full.
5. Measuring at the charger only—cable drop can hide an over-float at the battery.

Does float charging damage a battery if it stays connected?

• Lead-acid: Safe when voltage and temperature compensation are correct. Check electrolyte (flooded), and keep the room cool.
• Lithium (LFP): Long high-SOC can shorten life. Use disable/low-standby strategies instead.

Warning signs

• Battery case feels warm at rest
• Noticeable smell (lead-acid gassing)
• Capacity seems to drop month by month

How do you check float voltage safely?

1. Tools: Digital multimeter (DMM), insulated probes, eye protection.
2. Where to measure: Measure directly at the battery terminals. Don’t rely on the charger display alone.
3. When to measure: After the system has been in Float for 5–10 minutes with normal loads.
4. What to record: Voltage, battery temperature, room temperature.
5. Adjust: If numbers don’t match your target (table above), tweak the menu and retest.

What real-world examples make this clear?

• Hot UPS room (32 °C) with 2.30 V/cell: Water loss and early failure. After enabling –3 mV/°C/cell compensation, float drops ~0.18 V on a 12 V block and lifespan improves.
• Off-grid solar with LFP: Float disabled. Absorb ends at 14.4 V, then the inverter powers loads; re-charge only when SOC drops to 50%. Result: fewer high-SOC hours, longer life.
• RV flooded battery on maintainer: Float 13.6 V at 25 °C; once a month check electrolyte and, if the maker recommends, run a brief equalize to desulfate (flooded only).

Float Charge Faq

What is a safe float voltage for a 12 V AGM?
Around 13.4–13.6 V at 25 °C, with temperature compensation.

Is float charging the same as trickle?
No. Float controls voltage at a low level; trickle feeds a small current. Float is safer for standby.

Can float revive a sulfated lead-acid?
No. Sulfation needs proper equalize (flooded only) or a dedicated reconditioning method—follow the battery maker’s instructions.

Does float charging damage lithium batteries?
Long high-SOC can speed aging. Prefer disable float or set a low standby value and use restart thresholds.

What is the 80/20 rule—does it apply here?
That rule is a charging habit (charge at 20%, stop at 80%). It’s not a float setting. Float is for standby after full.

How do I set float for 24 V and 48 V lead-acid?
Double or quadruple the 12 V values respectively (see the table).

Where should I place the temperature probe?
On a battery case in the middle of the bank, away from hot air or heatsinks.

How often should I equalize?
Only for flooded batteries and only as the manufacturer recommends. AGM/Gel/Lithium: do not equalize.

Why does my charger show float but the battery reads higher?
Cable drop or wrong sensing point. Measure at the terminals and enable remote sense if available.

Can I leave a maintainer on my car all winter?
Yes for lead-acid if the maintainer is smart and follows proper float ranges. For lithium car retrofits: follow the pack maker’s guidance.

Download: float quick chart and a simple calculator

Want an easy worksheet? Ask us for a PDF quick chart (lead-acid float vs temperature) and a one-cell mV/°C calculator you can keep on your phone.

Need help setting a charger?
We configure batteries and inverters every day. If you’re moving to LiFePO4 home storage, we’ll share a float-free profile that protects cycle life while keeping your lights on.