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?
- Find the menu: In MPPT/hybrid inverters look for Bulk/Absorb/Float/Equalize. In UPS, look under Battery settings → Float/Standby.
- Enter values:
- Lead-acid → use the table above; add temperature compensation.
- LiFePO4 → disable float or set low standby; reduce absorb time.
- Set re-bulk / re-charge: Don’t restart charging for tiny drops. For lithium, many users choose a lower restart SOC (e.g., 50%).
- Save and test: Watch the actual terminal voltage with a meter for 10–15 minutes to confirm.
Five mistakes to avoid
- Keeping lithium at a lead-acid float value all day.
- No temperature probe on lead-acid—hot rooms need lower float.
- Absorption time too long → battery stays hot and gassy.
- Long trickle on a battery that is already full.
- 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?
- Tools: Digital multimeter (DMM), insulated probes, eye protection.
- Where to measure: Measure directly at the battery terminals. Don’t rely on the charger display alone.
- When to measure: After the system has been in Float for 5–10 minutes with normal loads.
- What to record: Voltage, battery temperature, room temperature.
- 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.
