Keep cordless tools running by charging each lithium-ion pack with its own charger from an inverter fed by a solar panel or a running vehicle, rotating several packs so one is always charged, and never letting a pack discharge fully or sit in heat. Do not wire a lithium pack directly to a 12 V source, because it needs its charger’s control circuit, and do not charge a pack that is swollen, hot or frozen.
Part of Tool Identification & Repair
A cordless drill, saw or grinder is only as good as its newest charge, and the three common battery chemistries age, self-discharge and fail in different ways.
Chemistry Decides How You Treat The Pack
The parent article describes three kinds:
- Lithium-ion (Li-ion). Modern tools. Holds charge for months; 300-500 charge cycles. A full discharge kills cells permanently.
- Nickel-cadmium (NiCd). Older tools. Self-discharges in weeks, is virtually indestructible and can be revived from a full discharge.
- Nickel-metal hydride (NiMH). Moderate performance. Self-discharges in days to weeks.
Check the label on the pack for its chemistry, voltage and amp-hour rating. A pack with no readable label should not be charged until you know what it is.
What You Need
- The tool’s own charger. Charger and pack are designed together; a mismatched charger can overcharge.
- An inverter, pure sine wave preferred, though a modified sine wave works for most chargers (parent article).
- A power source: a vehicle with a running engine, or a solar panel with a charge controller and a battery bank. See Inverter Basics and Charge Controller.
- Several packs, to rotate.
- A cool, dry place to store them.
Method: Charge, Rotate, Store
Step 1: Identify and inspect each pack. Discard any that are swollen, cracked, leaking, hot or smell sweet or acrid.
Step 2: Charge from a vehicle. Run the engine, connect the inverter to the vehicle battery, plug in the charger. The parent article gives 15-30 minutes of engine running to charge one battery. Run the vehicle outdoors.
Step 3: Charge from solar. A 100 W panel with a charge controller can charge tool batteries through an inverter; it takes longer but costs no fuel (parent article).
Step 4: Rotate. Keep one pack in the tool, one charging and one spare. Stop using a pack when the tool slows, not when it stops, to avoid deep discharge.
Step 5: Store properly. Store packs partly charged, cool and dry. Do not leave them in a hot vehicle, and let a frozen pack warm to room temperature before charging.
Worked Example
A pack’s energy is voltage times amp-hours. For an example pack labelled 18 V and 2 Ah: 18 × 2 = 36 Wh. Suppose a 100 W panel delivers its rated power for 4 hours of strong sun, an assumed figure: 100 × 4 = 400 Wh before losses in the controller, inverter and charger. 400 ÷ 36 is about 11 packs of this size, an upper limit, since losses and cloud reduce it. Read your own pack’s label and your own conditions before planning.
On Charging Straight From A 12 V Source
The parent article notes that some cordless batteries can be charged directly from 12 V with the right adapter, and that 18 V and 20 V packs need a boost converter, and warns of fire risk. This page recommends against it for lithium-ion packs: they rely on a charger’s voltage and current control and, in some packs, on built-in protection circuits. Use the original charger on an inverter. Older NiCd packs are more tolerant but still need current limiting.
Common Mistakes
- Running the pack flat. Lithium cells damaged by full discharge cannot be revived.
- Charging in the sun or in a closed vehicle. Heat damages packs.
- Using one charger for any pack. Voltage and chemistry vary.
- Leaving a charger plugged into the inverter all night. It drains the main battery.
Safety
A damaged lithium pack can catch fire
A pack that is swollen, cracked, hot, smelling of solvent or deeply discharged can overheat and burn fiercely, and the fire is hard to put out. Charge on a non-flammable surface, outdoors or in an open space, away from fuel and bedding; never charge a damaged pack; and never connect packs to a power source with improvised wiring. Keep metal objects from bridging battery terminals. Sparks near a lead-acid battery used as the source can ignite hydrogen.
What’s Next
- Learn next: Generator & Solar Options — when running a generator makes sense
- Combine with: Manual Alternatives to Power Tools — saving charge for the jobs that need it