The Dos and Don'ts of Charging Sealed Lead-Acid Batteries: A Manufacturer's Guide

The short answer: charge a sealed lead-acid (SLA) battery with an automatic charger that has a lead-acid or AGM setting, matches the battery's voltage, and delivers roughly 10% to 30% of the battery's amp-hour rating. Recharge soon after every use and leave the charger connected until it switches to float. Nearly every SLA battery that dies young does so for one of four reasons: the wrong charger, a charge that never finishes, a battery left sitting discharged, or two batteries in a series pair that have drifted out of balance.

AJC® has made replacement SLA batteries since 2006. This guide is written from the battery maker's side of the counter. It covers how to pick a charger that suits an AGM battery, what to do when a new battery arrives, how to charge 24V battery pairs, and how to diagnose a battery that won't take a charge.

The Short Version: Dos and Don'ts

How Do You Choose a Charger for a Sealed Lead-Acid Battery?

Choose a charger by checking six specifications against your battery. A charger that gets all six right will protect the battery for its whole service life; one that misses even one can shorten it by years.

1. Voltage. Match the charger to the battery or battery string: 6V, 12V or 24V. A 24V mobility scooter with two 12V batteries in series needs a 24V charger.
2. Charging profile. Look for a setting labeled lead-acid, SLA, AGM or VRLA. AJC SLA batteries are AGM, so an AGM setting is the best match. Gel batteries need their own lower-voltage gel setting.
3. Output current. Aim for about 10% to 30% of the battery's amp-hour (Ah) rating. CSB, for example, caps its 12V 7.2Ah GP1272 at 2.16 amps (0.3C), and Yuasa's storage recharge guidance uses 0.1C. For a 12V 7Ah battery, that means a charger of roughly 0.7 to 2.1 amps.
4. Automatic float. The charger should finish the charge and then hold the battery at a float voltage, typically 13.5 to 13.8V for a 12V battery, instead of switching off or continuing to push current.
5. Temperature compensation. A charger with a temperature sensor lowers its voltage when warm and raises it when cold. Battery University reports that temperature compensation can prolong battery life by up to 15%. It matters most for batteries in garages, sheds and outdoor enclosures.
6. Clean output and the right connector. Yuasa's NP series literature notes that "AC ripple reduces service life," so a well-filtered charger is worth paying for. Then check the connection: spade clips for F1 and F2 terminals, ring terminals for bolt posts, or the device's own charging port on mobility scooters and wheelchairs.

Can You Use a Car Battery Charger on a Sealed Lead-Acid Battery?

Sometimes, but check two things first. The charger needs an AGM setting, because older automotive chargers are built for flooded batteries and can run higher voltages than a sealed battery tolerates. It also needs a low-current mode: a 10-amp setting is far above the 0.3C limit on any SLA battery under about 33Ah, which covers most UPS, alarm, mobility and powersports batteries. Many modern automotive smart chargers have a 1 to 2-amp AGM mode that works well. Avoid any "desulfation," "recondition" or "equalize" mode on a sealed battery unless the battery maker approves it.

Can You Charge an SLA Battery With a Power Supply?

Only with an adjustable bench power supply that limits both voltage and current, and only if you know the right settings. Set the voltage to the battery's float voltage (13.5 to 13.8V for 12V) and the current limit to no more than 0.3C. A plain DC adapter without current limiting can overload the battery. For most people, a purpose-built automatic charger is safer and costs less than a good bench supply.

Should You Charge a New AJC Battery Before Installing It?

Usually not. AJC batteries are delivered charged, sealed and ready to install. But every lead-acid battery loses some charge while it waits on a shelf or in transit, so a two-minute voltage check is worth doing, especially for backup equipment that has to work the first time.

1. Inspect the case. Look for cracks, swelling or damaged terminals. Contact us before installing a battery that arrives damaged.
2. Measure the open-circuit voltage. With nothing connected, read the voltage across the terminals with a multimeter. Yuasa's NP series guidance says open-circuit voltage should be above 2.1 volts per cell, which is about 12.6V for a 12V battery.
3. Top it off if it's low. If the reading is below about 12.6V, give the battery a full charge on an AGM or SLA setting before installing it. Yuasa recommends a supplementary charge for batteries stored six months or more from their production date.
4. Install it, then let the device finish charging. UPS units, alarm panels and emergency lights will bring a nearly full battery the rest of the way on their own float charge.
5. Write the install date on the battery. A marker date on the case makes replacement planning easy, especially when you manage more than a few devices.

How Do You Charge Batteries Wired in Series?

Many 24V devices, including most mobility scooters and power wheelchairs, run on two 12V batteries wired in series. AJC's mobility scooter listings note how many batteries a full replacement needs for exactly this reason. Charging a series pair works well as long as the two batteries stay matched.

• Use a charger rated for the full string. Two 12V batteries in series need a 24V charger with a lead-acid or AGM profile. Mobility scooters and wheelchairs usually come with one that plugs into the chair's charging port.
• Replace both batteries at the same time. A new battery paired with a worn one ends up overcharged while the weaker battery stays undercharged. The pair wears out at the speed of the weaker battery.
• Match the batteries exactly. Use the same capacity, brand and age. Don't pair a 35Ah battery with a 33Ah one.
• Check each battery separately once in a while. After a full charge and a few hours' rest, both batteries should read within about 0.1 to 0.2V of each other. A bigger gap means the pair is drifting out of balance.
• Rebalance with a 12V charger if needed. Charging each battery individually on a 12V AGM charger can bring a drifting pair back into line. If one battery keeps falling behind, replace both.

How Do You Know When an SLA Battery Is Fully Charged?

An automatic charger tells you directly: its indicator changes from charging to float, maintain or full. The more reliable test is the battery itself. Disconnect the charger, let the battery rest for several hours (ideally a day), then measure the open-circuit voltage. A fully charged 12V AGM battery typically rests at about 12.8V or higher.

Don't measure right after charging. The surface charge makes the battery read high for several hours, so a battery that reads 13.2V on the bench can settle back to 12.4V overnight, which means it was never fully charged. Also remember that the indicator light only means the charger has finished its program. If the charge was cut short by a timer or a power interruption, the battery may only be partly charged.

Why Won't My Sealed Lead-Acid Battery Charge?

Most charging problems come down to the battery, the charger or the connection. Use this table to narrow it down.

Can You Leave a Sealed Lead-Acid Battery on the Charger?

Yes, if the charger has an automatic float stage. Float charging holds the battery at a gentle maintenance voltage that offsets self-discharge, which is exactly how UPS units and alarm panels keep their batteries ready for years. No, if it's a basic trickle charger that never reduces its output. A constant trickle slowly overcharges a sealed battery, dries out its electrolyte and corrodes its plates. If you aren't sure which kind you have, check the manual for the words "float," "maintain" or "automatic."

How Often Should You Charge an SLA Battery You Aren't Using?

Recharge an idle SLA battery about every six months at room temperature. Battery University recommends a topping charge every six months for lead-acid batteries in storage and notes that AGM batteries can stretch that to six to twelve months. Heat shortens the interval: CSB says its batteries can be stored for more than six months at 25°C (77°F), but that "for higher temperatures the time interval will be shorter." Keeping a stored battery on an automatic maintainer removes the guesswork. For more on storage temperature, orientation and manufacturing dates, see how to get the maximum shelf life from an SLA battery.

Charging Dos and Don'ts for Businesses and Fleets

Facilities, healthcare providers, security installers and equipment rental fleets often manage dozens or hundreds of SLA batteries at once. A few habits keep a large battery pool reliable:

• Do date every battery on receipt and at installation, and log the dates with the equipment record.
• Do rotate spare stock first in, first out so no spare sits on the shelf long enough to need a recovery charge.
• Do check spares' voltage on a schedule and recharge any that fall below about 12.6V.
• Do standardize chargers by battery size and profile, so a technician can't pair a 10-amp charger with a 5Ah battery.
• Don't top up one battery in a series pair and return it to service next to an older one; replace pairs together.
• Don't stack charging batteries in a closed cabinet or crate. Leave space for heat and gas to dissipate.

AJC's wholesale battery program supports volume orders for fleets and facilities.

Frequently Asked Questions

What charger do I need for a 12V sealed lead-acid battery?

A 12V automatic charger with a lead-acid, SLA or AGM setting, an automatic float stage, and an output of about 10% to 30% of the battery's amp-hour rating. Temperature compensation is a useful extra for batteries kept in unheated spaces.

How many amps should I use to charge a 12V 7Ah SLA battery?

About 0.7 to 2.1 amps, which is 10% to 30% of 7Ah. A 1 to 2-amp smart charger is a good fit. Higher currents heat the battery and shorten its life.

Do AJC batteries come charged?

Yes. AJC batteries are delivered charged, sealed and ready to install. Check the voltage before installing a battery for critical backup equipment, and top it off if it reads below about 12.6V.

Can you charge two 12V batteries in series with a 24V charger?

Yes, as long as the two batteries are the same capacity, brand and age, and the charger has a lead-acid or AGM profile. Replace both batteries together so they stay balanced.

Why does my charger say full but the battery dies quickly?

The battery has most likely lost capacity from age, deep discharges or sulfation. The charger fills the little capacity that's left, so it reports full quickly. Check the resting voltage or have the battery load tested, and replace it if it can't hold a load.

Can you leave a sealed lead-acid battery on the charger all the time?

Yes, on an automatic charger or maintainer that switches to float. No, on a basic trickle charger that never reduces its output, because that slowly overcharges the battery.

Is it safe to charge a sealed lead-acid battery indoors?

Yes, in a normally ventilated room away from sparks and flame. Sealed batteries release very little gas under normal charging, but never charge one inside a sealed container.

Shop AJC Replacement Batteries

AJC® batteries are AGM, UL Recognized Components, delivered charged and backed by a 12-month warranty with 30-day returns. Find an exact match in AJC sealed lead-acid batteries, or shop by equipment: UPS, mobility scooter, wheelchair and powersports batteries. For the basics of charger types and charging stages, see our guide to charging SLA batteries.