Chargers and Power Supplies
Choosing and using chargers for different battery types.
Can I charge an AGM battery with a regular charger?
Short answer: Use a charger with an AGM setting or one the maker approves for AGM, because AGM batteries need controlled voltage and can be damaged by chargers that overcharge.
Absorbed glass mat batteries are sealed and sensitive to overcharging. Older chargers without voltage regulation may push voltage too high for too long, drying the battery out and shortening its life. Modern smart chargers include AGM modes that follow the right charging profile.
Check the battery label and the charger manual. Set the correct battery type, connect per the instructions, and let the charger complete its stages. Many vehicles with AGM batteries also use battery registration, so if you replace the battery, register it with a scan tool so the charging system adjusts.
Lithium starter batteries, found on some vehicles and powersports equipment, need their own chargers or modes. Never charge a frozen, swollen, cracked or leaking battery. Charge in a ventilated area away from sparks and flames, and wear eye protection.
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What is the difference between a battery charger and a programming power supply?
Short answer: A charger varies voltage and current to recharge a battery through stages, while a programming power supply holds a steady voltage at high current to support the vehicle during long electronic procedures.
Battery chargers are designed to restore a battery's charge efficiently. Smart chargers move through bulk, absorption and float stages, changing voltage as they go. That is great for charging, but the changing voltage and limited current can cause problems during module programming.
A programming power supply holds a steady, selectable voltage and can deliver enough current to run the vehicle's electronics during long procedures with ignition on, fans running and modules awake. Vehicle makers often specify supply voltage and current for programming.
Some units do both jobs with selectable modes. If you do programming, diagnostics with the key on for long periods, or showroom demonstrations, a true power supply mode is worth having. Connect it to the battery or designated jump points, keep cables secure and away from moving parts, and avoid sparks near the battery.
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Why won't my smart charger charge a completely dead battery?
Short answer: Many smart chargers need to detect a minimum voltage before they start, as a safety feature; a deeply discharged battery may read below that, and it may also be damaged beyond recovery.
Microprocessor chargers check the battery voltage before charging so they do not push current into a short, a reversed connection, or nothing at all. A battery drained far enough can read below the charger's detection threshold, so the charger shows an error or does nothing.
Some chargers have a manual or force mode for this situation, and some shop chargers have a recovery mode. Use them only as the charger maker describes and stay with the battery while it charges. Check for heat, swelling, leaking, or a sulfur smell; stop at once if you find any.
A battery left fully discharged for a long time often will not recover fully even if it accepts charge. Charge it completely, then test it with a battery tester. Charge in a ventilated area away from sparks and flames, connect clamps before switching the charger on, and wear safety glasses.
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Should I charge a battery in the vehicle or take it out?
Short answer: Most modern batteries can be charged in the vehicle with a smart charger, following the vehicle maker's instructions for the charging points; remove it when the maker requires it or the battery is damaged.
On many vehicles, removing the battery means resetting radios, windows and learned values, and some batteries sit under seats or in the trunk with remote charging posts. Charging in place with a modern smart charger is usually fine if you follow the maker's guidance, connecting to the designated posts or the negative ground point the manual lists.
Take the battery out if it is cracked, swollen, leaking, frozen, or if the maker requires removal for a certain battery type. Never charge a frozen battery.
Use a charger setting that matches the battery type, such as AGM, EFB or flooded, and avoid high boost rates in the vehicle, since voltage spikes can stress electronics. Charge in a ventilated area, keep sparks and flames away, connect the charger before plugging it in, and wear safety glasses.
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What is a battery maintainer and when do I need one?
Short answer: A maintainer holds a battery at full charge with a low, controlled current during storage or long periods of sitting, which prevents the self-discharge and sulfation that shorten battery life.
Every battery slowly discharges, and modern vehicles draw small amounts of current even when parked. A vehicle that sits for weeks, such as a stored classic, a seasonal vehicle or a car waiting on parts in the shop, can end up with a flat battery that never fully recovers.
A maintainer, sometimes called a float charger, monitors voltage and supplies just enough current to keep the battery full without overcharging. Many smart chargers include a maintenance mode, so one tool can do both jobs.
It is not a substitute for a charger on a dead battery or for a power supply during programming. Choose one with settings for the battery type. Mount it where it cannot fall, keep cords away from moving parts and pinch points, and use it in a ventilated area away from fuel and sparks.
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General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.