VRLA Battery Charging Voltage: Float, Boost and Heat

VRLA battery charging voltage for a 12V battery is about 13.5 to 13.8V on float (standby) and about 14.4 to 14.9V on cycle (boost) charging, at 20°C to 25°C. Yuasa's NP7-12 datasheet gives 13.65V float and 14.5V cyclic at 20°C; CSB's GP1272 gives 13.5 to 13.8V float at 25°C. Those numbers are for a cool room. In a 35°C Indian storeroom, the same battery should float about 0.27V lower, at roughly 13.4V, because Yuasa specifies a correction of minus 3mV per cell for every degree above 20°C. Most home inverters and UPS units in India charge at a fixed voltage with no temperature sensor, which is one reason sealed batteries in hot rooms often last two to three years instead of five.

VRLA battery charging voltage at a glance

Always use your own battery's datasheet first; these two are reference examples of common 12V 7Ah-class blocks.

12V VRLA battery charging voltage

A 12V VRLA battery has six cells, so its charging voltage is six times the per-cell figure. On float, about 2.25 to 2.30V per cell gives 13.5 to 13.8V. On cycle charging, about 2.4 to 2.45V per cell gives roughly 14.4 to 14.7V, with Yuasa's ±3% tolerance allowing up to about 14.9V. For a 24V bank, double these; for a 48V bank, multiply by four.

In a series bank the charger sees only the total, so one weak battery can be overcharged while its partner is undercharged. Our guide to battery series vs parallel connection explains how to spot that.

VRLA float charging vs cycle charging

Float charging holds the battery at a constant, gentle voltage indefinitely, just enough to replace self-discharge. This is what a UPS or inverter does while mains power is on. Yuasa sets no current limit on float because, at 13.65V, the battery only draws what it needs.

Cycle charging is for batteries that are regularly discharged and recharged, such as solar or frequent power-cut use. It uses a higher voltage to recharge faster, and must be current-limited and stopped or dropped back to float once the battery is full. Leaving a battery at the cycle voltage permanently dries it out.

How temperature affects VRLA battery charging

How temperature affects a VRLA battery comes down to chemistry: a warm battery accepts current more easily, so the same voltage pushes more current through it. Yuasa's correction of −3mV per cell per °C on float means the ideal float voltage drops as the room heats up:

A 1998 Battcon paper on VRLA charging recommends limiting temperature compensation to a range such as 0°C to 40°C, and warns that overcompensating brings "accelerated grid growth and a rapid end to battery life". The practical point for Indian homes: keep VRLA batteries out of hot cupboards and away from inverter exhaust, because a cooler battery is cheaper than a temperature-compensating charger.

What your inverter actually charges at

A TechEnclave forum author collected the charging settings of home inverters sold in India. The figures are as reported there, not tested by us:

The same thread quotes Amaron's guidance for its Quanta sealed range as 13.5V float per 12V module, and the author believes a 13.9V float shortened his SMF batteries to about 2.5 years. If your inverter has a battery-type setting, select SMF or VRLA when you fit sealed batteries. A tubular setting floats them higher than their makers recommend.

VRLA battery charging current

Charging current matters most during cycle charging. Yuasa limits its 7Ah block to 1.75A (0.25C) and CSB its 7.2Ah block to 2.16A (0.3C). The Battcon paper suggests limiting charge current to 10% to 20% of the battery's Ah capacity to reduce the risk of thermal runaway. For a 100Ah VRLA battery, that means 10A to 20A. A charger that dumps far more current into a large discharged bank heats it unnecessarily.

Why does a VRLA battery get hot during charging?

A VRLA battery gets slightly warm during charging because it recombines the oxygen and hydrogen produced inside it back into water, and that reaction gives off heat. Slightly warm is normal. Hot to the touch is not. The Battcon paper describes thermal runaway: a warmer cell accepts more current, which produces more recombination and more heat, in a loop. Warning signs, per Mitsubishi Electric's critical power guidance, include excessive heat and a rotten-egg smell from a breached casing. If a battery is hot, swollen or smells, disconnect the charger and let it cool before inspecting it. Our guide to overcharged batteries lists the other signs.

What is deep discharge in a VRLA battery?

Deep discharge in a VRLA battery means draining it below its cut-off voltage, typically about 10.5V for a 12V block (1.75V per cell) on normal loads. Below that, lead sulfate crystals form on the plates and harden if the battery is left flat, and the Battcon paper links undercharging to sulfation and premature failure. A battery left deeply discharged for days may never recover its full capacity. Recharge it as soon as power returns, and if your UPS or inverter has an adjustable low-battery cut-off, do not set it below the battery maker's figure. Our VRLA battery backup time guide explains how cut-off affects runtime.

How often should VRLA batteries be charged?

A VRLA battery on a UPS or inverter is charged continuously on float, so it needs nothing extra. A battery in storage needs a recharge before its self-discharge takes it too low. CSB says its GP1272 loses under 10% after 90 days and can be stored up to 6 months at 25°C before recharging; Yuasa lists about 3% loss a month at 20°C. In Indian summer temperatures, check stored batteries every two to three months and recharge if the resting voltage falls below about 12.5V. Our guide to battery shelf life covers storage.

How to check your charger's voltage at home

You need a digital multimeter.

1. Make sure mains power is on and the inverter or UPS has been charging for several hours, so the battery is full and the charger is on float.
2. Set the meter to DC volts and touch the probes to the battery's positive and negative terminals.
3. Read the voltage. For a single 12V VRLA battery, 13.5 to 13.8V is normal float in a 25°C room, a little lower is better in a hotter room. For a 24V bank, double it.
4. Feel the battery case. Slightly warm is fine. Hot is not.
5. Compare with the label. If the reading is consistently above about 13.9V per 12V battery in a warm room, ask the installer to change the battery-type setting or the float voltage.

Can you charge a VRLA battery with a car or bike charger?

Only if the charger has a setting for sealed, AGM or VRLA batteries. Basic chargers designed for flooded batteries may push a higher voltage to finish the charge, and some include a desulfation or equalisation stage that runs higher still. A flooded battery tolerates that by venting gas and losing water you can top up. A sealed VRLA battery cannot be topped up, so the lost water is gone for good. Use a charger with an AGM or VRLA mode, keep the current near the 0.25C limit, and stop when the battery is full.

Recharging after a long power cut

After a long cut, the battery is deeply drained and the inverter starts at its higher boost voltage, then drops to float once the battery is full. Three things help it recover. Let it recharge fully before the next heavy use, because running loads during recharge slows it down. Check that the room is not hot, because a deeply discharged battery charging in heat is at the highest risk of over-heating. And next day, with mains on, check that the voltage has settled back to the float level, about 13.5 to 13.8V per 12V battery. If a battery never gets back there, or gets hot every time it recharges, it is reaching the end of its life.

Honest pros and cons of a higher float voltage

Decision framework

Strong fit for 13.5 to 13.65V float: VRLA/SMF batteries on a UPS or inverter in a typical Indian room, especially above 30°C.

Strong fit for cycle charging at about 14.4 to 14.5V: batteries regularly drained by power cuts or solar, with current limited to about 0.25C and a switch to float once full.

Marginal fit: an inverter with no SMF setting. It works, but expect shorter life in a hot room; improve ventilation.

Not a fit: a tubular charging profile on sealed batteries, a battery left deeply discharged, or a charger feeding a hot, swollen battery.

The Adwin option

Adwin's inverters and batteries are built to work together. When you buy a battery and inverter as a set, ask our team to confirm the charging profile is set for your battery type, on 1800 8914 007. See the range on our lead-acid inverters and batteries page.

FAQs

What is the VRLA battery charging voltage?

VRLA battery charging voltage for a 12V battery is about 13.5 to 13.8V on float and about 14.4 to 14.9V on cycle (boost) charging, at 20°C to 25°C. Yuasa's NP7-12 uses 13.65V float and 14.5V cyclic at 20°C.

What is the float voltage of a 12V VRLA battery?

About 13.5 to 13.8V at 25°C, or 2.25 to 2.30V per cell. It should be lowered by about 18mV for every °C above 20°C, so roughly 13.4V in a 35°C room.

What is the difference between float charging and cycle charging?

Float charging holds a full battery at a gentle constant voltage indefinitely, as a UPS does. Cycle charging uses a higher voltage to recharge a discharged battery quickly, with a current limit, and must switch back to float when full.

Why does a VRLA battery get hot during charging?

Recombining gases inside the battery gives off heat, so mild warmth is normal. A hot, swollen or smelly battery is a sign of overcharging or thermal runaway; disconnect the charger and let it cool.

How often should VRLA batteries be charged in storage?

CSB allows up to six months at 25°C before recharging. In hotter Indian conditions, check every two to three months and recharge if the resting voltage drops below about 12.5V.

What is deep discharge in a VRLA battery?

Draining a 12V VRLA battery below about 10.5V. It causes sulfation that can permanently reduce capacity if the battery is left flat, so recharge it as soon as possible.

What charging current should I use for a VRLA battery?

About 0.25C to 0.3C at most for cycle charging, such as 1.75A to 2.16A for a 7Ah battery. A Battcon paper suggests 10% to 20% of Ah capacity to reduce thermal runaway risk.

Can I charge a VRLA battery with a tubular battery inverter setting?

It works, but tubular settings float higher than most VRLA makers recommend, which can shorten sealed battery life. Select the SMF or VRLA setting if your inverter has one.

Conclusion

VRLA battery charging voltage is about 13.5 to 13.8V on float and 14.4 to 14.9V on boost for a 12V battery, but those figures assume a 20°C to 25°C room. In Indian heat, the right float voltage is lower, about 13.4V at 35°C, and most inverters do not adjust for it. Set your inverter to its SMF or VRLA profile, keep batteries cool and ventilated, limit boost current to about 0.25C, and recharge any deeply discharged battery promptly. For the bigger picture on these batteries, see our VRLA battery guide.