The most common reason a solar light stops working is a dead battery, and the most common reason people throw away a working solar light is that they do not know the battery is replaceable. I have salvaged hundreds of fixtures that were headed for the trash, simply by swapping the battery. The LED still works. The solar panel still works. The sensor still works. Only the battery, a $2 component, has failed.
This guide covers everything you need to know about replacing solar light batteries. I will cover the battery types, how to identify what you have, how to open the compartments (including the sealed ones), where to buy replacements, and how to get the most life out of the new batteries.
Why Solar Light Batteries Fail
Solar light batteries fail for predictable reasons, and understanding the failure mode helps you choose the right replacement.
Charge Cycle Degradation
Every battery has a finite number of charge-discharge cycles. A solar light battery cycles once per day, charging in the day and discharging at night. Over a year, that is 365 cycles. Most NiMH batteries are rated for 500 to 1000 cycles, which means 1.5 to 3 years of life. Most lithium-ion batteries are rated for 500 to 2000 cycles, depending on quality.
The battery does not die suddenly. It loses capacity gradually, holding a little less each day, until the runtime drops below what is useful. A light that ran for 8 hours when new might run for 3 hours after two years, and for 30 minutes after three years.
Heat Damage
Heat accelerates chemical degradation. A battery in a solar light in a hot climate, where the fixture interior reaches 120 degrees or more, degrades faster than the same battery in a cool climate. In desert and Southern states, expect battery life at the short end of the range. In Northern states, expect the long end.
Over-Discharge Damage
If a solar light is left in a shed or garage for months without sun, the battery self-discharges to zero and stays there. This deep discharge damages the battery chemistry, and even when recharged, the capacity is reduced. A battery that has been stored dead may not recover at all.
Cold Damage
Cold temperatures reduce battery capacity temporarily, but they do not usually cause permanent damage. A battery that performs poorly in January may perform fine in May. However, freezing a discharged battery can cause internal shorts, so storing lights for winter with dead batteries is a recipe for failure.
Identifying Your Battery Type
Before you can replace a battery, you need to know what type you have. There are four common types in solar lights.
NiCd (Nickel-Cadmium)
The oldest type still in use. NiCd batteries are cheap, tolerant of overcharge, and work in cold weather. They have a memory effect, meaning they lose capacity if not fully discharged regularly, and they contain toxic cadmium.
NiCd batteries are being phased out but are still found in cheaper solar lights. They are typically AA size, 1.2V, and labeled “Ni-Cd” on the wrapper.
You can replace a NiCd battery with a NiMH battery of the same size, and you should. NiMH is better in every way.
NiMH (Nickel-Metal Hydride)
The standard replacement for NiCd. NiMH has higher capacity, no memory effect, and no toxic metals. They are slightly less tolerant of overcharge and high heat, but in solar light applications, they perform well.
NiMH batteries in solar lights are typically AA size, 1.2V, with capacities from 600mAh to 2000mAh. Higher capacity means longer runtime, but only if the solar panel can charge it fully. A high-capacity battery in a light with a small panel will never reach full charge and will perform no better than a low-capacity battery.
Look for “Ni-MH” on the wrapper. The capacity, in mAh, is also printed there.
Lithium-Ion (Li-ion)
Found in higher-end solar lights, especially spotlights and security lights with high LED output. Li-ion batteries have high energy density, meaning they pack more charge into a smaller space. They are 3.7V nominal, which is three times the voltage of a NiMH cell, so they are not interchangeable with NiMH.
Li-ion batteries require a protection circuit to prevent overcharge and over-discharge. Solar lights designed for Li-ion have this circuit built in. Do not put a Li-ion battery in a light designed for NiMH, and do not put a NiMH battery in a light designed for Li-ion. The voltages are wrong and the light will not work, or worse.
Li-ion batteries in solar lights are often in a specific form factor, such as an 18650 cell or a custom pack. Check the label for “Li-ion” and the voltage, 3.7V.
LiFePO4 (Lithium Iron Phosphate)
A newer lithium chemistry that is safer and longer-lasting than standard Li-ion. LiFePO4 has a lower voltage, 3.2V nominal, and a much longer cycle life, 2000+ cycles. It is found in premium solar lights.
LiFePO4 is not interchangeable with standard Li-ion, because the voltage is different. Use only the chemistry the light was designed for.
How to Identify the Battery Without Opening the Light
Sometimes you can identify the battery type from the outside, before you open the compartment.
Check the manual or the packaging, if you still have it. The battery type is usually specified.
Look at the light specifications on the manufacturer website. Search the model number.
Check the battery compartment cover. Many lights print the battery type and size on or near the cover.
Measure the voltage of the solar panel output with a multimeter in full sun. A panel that outputs 2V is designed for a 1.2V NiMH battery. A panel that outputs 5 to 6V is designed for a 3.7V Li-ion battery.
If none of these work, you have to open the compartment and look.
Opening the Battery Compartment
Most solar lights have a battery compartment that is accessible, but the access method varies. Here are the common configurations.
Screw-Down Cover
The best design. A cover on the bottom or back of the fixture is held by one or two small screws. Remove the screws, lift the cover, and the battery is inside.
Use the right screwdriver. These screws are usually Phillips #0 or #1. Using the wrong size strips the head, and then you have a problem. If the screw is tight, press down firmly while turning to avoid cam-out.
Keep the screws safe. They are tiny and they disappear into grass and carpet. Put them in a small dish or magnet.
Twist-Lock Cover
Common on path lights. The cover twists a quarter turn and pops off. No tools needed.
The twist lock can stick, especially after a year of weather. Grip the cover firmly and twist. If it will not turn, tap the edge gently with a screwdriver handle to break the seal.
Slide-Off Cover
The cover slides in a track, like a drawer. Push down on a tab and slide the cover off.
These covers jam if dirt gets in the track. Rinse the area with water and let it dry before sliding.
Sealed Compartments
Some solar lights have no accessible battery compartment. The battery is sealed inside the fixture, intended to be non-replaceable. This is common in cheap string lights and in fixtures potted for waterproofing.
Opening a sealed fixture voids any warranty, but if the light is dead anyway, you have nothing to lose. Here is how I do it.
For a fixture with a seam, such as two plastic halves glued together, run a thin blade, a utility knife or a spudger, along the seam to break the adhesive. Work slowly around the entire seam. Once the adhesive is cut, the halves pry apart.
For a fixture with no seam, potted in resin, the battery is not accessible. The fixture is disposable. Do not attempt to cut into potted electronics, because you will damage the circuitry and possibly cut yourself.
Once you have opened a sealed fixture, you can identify the battery, replace it, and re-seal the fixture with silicone adhesive. The re-seal will not be as waterproof as the original, so use the fixture in a sheltered location or accept a shorter life.
Removing and Replacing the Battery
Removing the Old Battery
The battery is typically held in a clip or a sleeve. Note the orientation before removing, because you will need to install the new battery the same way. The positive terminal, the nub end, usually faces up or toward a marked “+” on the compartment.
If the battery is corroded, with white or green crust on the terminals, clean the terminals before removing the battery. Use a cotton swab dipped in white vinegar to dissolve the corrosion, then dry with a clean swab. Corrosion on the battery terminal means the battery leaked, and the new battery will fail quickly if the corrosion is not cleaned.
Remove the old battery by lifting it out of the clip. Do not pry with a metal tool, which can short the terminals. Use a plastic spudger or your fingernail.
Installing the New Battery
Insert the new battery in the same orientation as the old one. The positive terminal goes to the positive contact, marked “+”. Reversing the battery will not damage the light in most cases, because the circuit has reverse polarity protection, but it will not work.
Seat the battery fully in the clip. It should click or snap into place. A loose battery will vibrate and lose contact, causing flickering.
Close the compartment. Replace the cover and screws. If the cover has a gasket, make sure the gasket is seated and apply a thin film of silicone grease to keep it soft.
Testing the New Battery
Before you put the light back in the yard, test it. Place the light in direct sun for a full day to charge the new battery. New batteries ship with a partial charge but not a full charge.
After the day in the sun, bring the light into a dark room or cover the panel. The light should turn on within 30 seconds. If it does, the replacement was successful. If it does not, check the battery orientation, check the contacts for corrosion, and try another battery to rule out a defective cell.
Sourcing Replacement Batteries
Hardware and Electronics Stores
Big-box hardware stores and electronics stores carry AA and AAA NiMH batteries in the battery aisle. These are consumer-grade batteries and they work, but they may not be optimized for solar light use. Solar light batteries are designed for the daily charge-discharge cycle and for high-temperature tolerance.
Look for batteries labeled “solar” or “high temperature.” These have formulations that withstand the heat inside a solar fixture better than standard consumer batteries.
Online Retailers
Online, you can find batteries specifically marketed for solar lights. These come in multi-packs and are priced lower per battery than retail. The quality varies, so read reviews and buy from a seller with a return policy.
I buy in bulk, 20 or 50 at a time, because I maintain many lights and the per-battery cost drops significantly. A battery that costs $3 at retail costs $1 in bulk.
Specialty Battery Suppliers
For Li-ion and LiFePO4 replacements, specialty battery suppliers have the best selection and the best quality. These suppliers sell to makers and hobbyists, and they carry the specific form factors used in solar lights, such as 18650 cells with and without protection circuits.
When buying Li-ion, buy from a reputable supplier. There are counterfeit cells on the market that have far less capacity than claimed and that can be unsafe. A name-brand cell from a specialty supplier is worth the premium.
Salvaging From Other Devices
You can salvage NiMH batteries from other dead devices, such as old cordless phones or power tool battery packs. These are often the same AA or sub-C cells used in solar lights. Test the salvaged battery with a multimeter to confirm it holds a charge, and check for physical damage or leakage before using it.
This is not ideal, because salvaged batteries have unknown history, but it is free and it works in a pinch.
Choosing the Right Capacity
Batteries of the same physical size come in different capacities. A AA NiMH battery ranges from 600mAh to 2500mAh. Which do you choose?
Matching the Original
The safest choice is to match the capacity of the original battery. If the light came with a 1000mAh battery, replace it with a 1000mAh battery. The solar panel was sized to charge that capacity in a typical day, and a larger battery may not fully charge.
Upsizing for Longer Runtime
You can install a higher-capacity battery for longer runtime. A 2000mAh battery in a light that came with 1000mAh will run roughly twice as long, if the panel can charge it fully.
The risk is that the panel cannot charge the larger battery in a single day, especially in winter or partial shade. The battery then never reaches full charge, and the runtime is no better than with the original battery. The light also draws a fixed current, so a larger battery does not make the light brighter, only longer-running.
Downsizing for Reliability
In a light with a small panel, a lower-capacity battery may actually perform better, because the panel can fully charge it each day. A 600mAh battery that reaches full charge will outperform a 2000mAh battery that only reaches 40 percent.
If your light struggles to charge, try a smaller battery. The runtime will be shorter, but the light will be more reliable, turning on every night rather than intermittently.
Maximizing Battery Life
Initial Charge
When you install a new battery, give it a full initial charge before the first use. Put the light in direct sun for 2 to 3 days, with the light switched off if it has a switch. This conditions the battery and ensures it starts at full capacity.
Annual Replacement
Even if the battery still works, replace it annually in hot climates and every two years in cool climates. A battery at 70 percent capacity is not long for this world, and proactive replacement prevents the gradual dimming that most people accept as normal.
Seasonal Storage
If you store lights for winter, remove the batteries and store them separately in a cool, dry place. A battery left in a stored light will self-discharge to zero and may be damaged by the time you retrieve it in spring. Store batteries at about 40 percent charge, not fully charged and not empty.
Cleaning Contacts
Corrosion on battery contacts increases resistance, which reduces charging and discharging efficiency. Clean the contacts annually with a cotton swab and white vinegar, then dry and apply a thin film of silicone grease to prevent future corrosion.
Avoiding Overcharge in Hot Weather
In very hot climates, the battery can be overcharged by a panel that is too large for the battery capacity. Overcharge generates heat and degrades the battery. If you notice the battery compartment is hot to the touch in the afternoon, the light is overcharging. There is no easy fix for this, short of shading the panel partially in the afternoon, but knowing the cause helps you understand why batteries fail faster in hot climates.
Troubleshooting After Replacement
If the light still does not work after a battery replacement, the problem is not the battery. Here are the next steps.
Check the Panel
A dirty or damaged panel will not charge the new battery. Clean the panel with a damp cloth and inspect for cracks. A cracked panel may still produce some voltage but not enough to charge.
Check the Sensor
The light sensor, usually a small photoresistor on the fixture, tells the light to turn on at dark. If the sensor is dirty or damaged, the light may not turn on even with a charged battery. Clean the sensor with a cotton swab.
Check the LED
LEDs rarely fail, but they can. If the LED is dark even with a known-good battery, the LED may be dead. Replacing an LED requires soldering and is rarely worth the effort for a cheap fixture. Recycle the fixture and buy a new one.
Check the Circuitry
The charge controller and the driver circuit can fail, usually due to water intrusion. If the compartment has moisture, the circuit may be corroded. Inspect the board for green or black corrosion. If the corrosion is minor, clean with vinegar and dry. If it is extensive, the fixture is done.
A Note on Disposal
Do not throw batteries in the trash. All battery types used in solar lights contain materials that should be recycled.
NiCd batteries contain cadmium, a heavy metal. They must be recycled, not landfilled. Most hardware stores have a battery recycling bin.
NiMH batteries are less toxic but should still be recycled. The same bins accept them.
Li-ion batteries must be recycled. They can catch fire if damaged or punctured, and they should never go in the regular trash. Tape the terminals before recycling to prevent shorting.
Check your local recycling options. Many communities have household hazardous waste drop-off days, and many battery retailers accept old batteries for recycling.
Final Thoughts
Replacing a solar light battery is the single most cost-effective maintenance you can do. A $2 battery brings a $30 light back to life. A bulk pack of 20 batteries, $20, can revive an entire yard of dimming fixtures.
The process is not complicated. Identify the battery type, open the compartment, swap the cell, and test. The main barriers are knowing what battery to buy and knowing how to open the compartment, both of which this guide covers.
Keep a stock of spare batteries on hand. Check your lights in spring, after the winter dormancy, and replace any battery that does not hold a charge. Do this every year and your solar lights will outlast their expected lifespan by years, running reliably night after night on nothing but sunlight.
