Why the battery choice decides the field life of the assembly

Solar garden lights are unusual rechargeable applications. They do not get a steady, well-regulated charge cycle; they get whatever the PV panel delivers, which is rarely a full charge in real outdoor service. They do not get a defined discharge cycle either; they run an LED at whatever brightness the battery can sustain through the night, then shut off when the cell voltage drops below the controller’s cutoff.

The two rechargeable chemistries that handle this kind of abusive duty cycle are Ni-CD and Ni-MH. Both deliver a nominal 1.2V per cell, both accept the same charger topology in principle, and both fit the AA form factor that almost every garden light battery compartment uses. They differ sharply in how they respond to the realities of solar garden light service: partial charge, deep discharge, cold weather, and long calendar life.

OEMs who treat the battery as a commodity — buying the cheapest AA cell that meets the 1.2V spec — usually end up replacing it within two years. OEMs who choose the chemistry against the actual service conditions get three to five years out of the same pack. The chemistry decision is more important than the cell brand or the cell capacity.

The two chemistries at a glance

Both Ni-CD and Ni-MH AA cells deliver a 1.2V nominal voltage and use a similar alkaline electrolyte with a nickel oxyhydroxide positive electrode. The negative electrode is what differs: cadmium in Ni-CD, a hydrogen-absorbing alloy in Ni-MH. That single difference drives most of the practical differences between the two.

The numbers above are nominal ranges across reputable cell suppliers; specific cells will fall inside or outside these ranges depending on the manufacturer and the production batch. Treat them as a comparison framework, not as a contract spec — the actual MTC from the cell supplier is what governs the build.

Why cold weather swings the decision to Ni-CD

Cold weather is the single largest reason Ni-CD remains the default chemistry for outdoor garden lighting in mixed and continental climates. The capacity numbers in the table above are room-temperature values; in real field service the temperature is rarely room temperature, and the difference between the two chemistries widens as the temperature drops.

Below about 0 °C, Ni-MH capacity drops because the metal-hydride negative electrode becomes kinetically slow at low temperature. The cell can still be charged, but the discharge capacity at -20 °C is often 40-55% of the room-temperature rating. That means a 2500 mAh Ni-MH cell rated for 14 hours of garden light runtime at 20 °C delivers only 6-7 hours at -20 °C — which is exactly the winter condition the solar garden light is supposed to handle.

Ni-CD capacity drops more slowly over the same temperature range. At -20 °C, an 800 mAh Ni-CD cell still delivers roughly 70-80% of its room-temperature rating, which translates to roughly 8-10 hours of garden light runtime on the same LED load. The Ni-CD cell has less nominal capacity, but in cold weather it has more usable capacity than the higher-rated Ni-MH cell.

For an OEM shipping to a single mixed-climate market, this is the deciding calculation. A garden light that works in summer and dies in winter is a warranty claim waiting to happen; the cell chemistry that survives the winter is the one that goes into the BOM.

Why partial-charge duty cycle also favors Ni-CD

Solar garden lights rarely reach a full charge in the field. Cloud cover, panel soiling, shorter winter days, and partial shading all conspire to leave the cell at 60-90% state of charge on a typical day, and well below that on consecutive overcast days. The two chemistries handle chronic undercharge very differently.

Ni-CD tolerates partial charge well. The cadmium electrode is more forgiving of incomplete charge cycles, and the memory effect that older Ni-CD cells suffered is largely under control in modern production cells. In a chronic-partial-charge duty cycle, Ni-CD loses capacity slowly and predictably.

Ni-MH is more sensitive to chronic partial charge. The metal-hydride electrode develops a higher internal resistance over time when repeatedly cycled at partial state of charge, which reduces both the capacity the cell will accept on charge and the capacity it will deliver on discharge. In garden light service, this shows up as a Ni-MH pack that performs well in the first year and visibly fades in the second and third year as the cells lose capacity.

The deep-discharge side of the same problem is also relevant. Solar garden light controllers typically cut off the LED when the cell voltage drops to about 1.0 V, which is near (and sometimes below) the safe discharge floor for Ni-MH. Ni-CD cells tolerate this deep discharge without permanent damage; Ni-MH cells driven below their voltage floor on a regular basis show accelerated capacity fade. Combine chronic partial charge with occasional deep discharge and Ni-MH loses roughly half its rated cycle life. Ni-CD in the same service loses a much smaller fraction of its cycle life.

The real cost comparison is cost-per-year, not cost-per-cell

The unit price difference between Ni-CD and Ni-MH AA cells is real, but it is not the right metric for an OEM deciding which chemistry to put into a 3-year or 5-year product. The right metric is total cost over the field life of the assembly.

A representative calculation for an OEM shipping 100,000 garden lights per year:

• Ni-CD AA cell: assumed low unit cost, 3-5 year field life, end-user-replaceable battery compartment.
• Ni-MH AA cell: medium-high unit cost, 2-3 year field life, sometimes end-user-replaceable but with higher failure rate.

Over a 5-year warranty window, a Ni-CD-equipped garden light sees zero or one battery replacement; a Ni-MH-equipped garden light sees one or two. The aggregate warranty cost, including the cell cost and the service labor, comes out lower for Ni-CD on most climate profiles. The buyer is paying more for Ni-CD chemistry in absolute terms — the cells cost more per watt-hour — but they are paying less for total system ownership.

The exception is mild-climate installations where Ni-MH chemistry is operating in its preferred temperature range, the higher capacity per cell translates directly to longer runtime, and the partial-charge cycle is less aggressive because the PV panel harvests more energy per day. In those installations, Ni-MH can match Ni-CD on field life and win on runtime.

Low-self-discharge Ni-MH: logistics, not performance

Standard Ni-MH cells lose 20-30% of their charge in the first month after a full charge, which is a problem for OEM logistics. A garden light that ships from the factory in March with a fully charged battery may sit on a distributor shelf until June, by which point the battery has dropped to perhaps 40% of charge. The end user installs it on a cloudy week, the LED flickers for two nights, and the assembly is returned as defective.

Low-self-discharge (LSD) Ni-MH cells solve this problem by retaining 70-85% of charge after one year of storage at room temperature, using a more refined metal-hydride alloy and tighter cell construction. From a logistics standpoint, LSD Ni-MH lets an OEM ship pre-charged batteries without the pre-installation recharge step.

From a field performance standpoint, LSD Ni-MH is the same chemistry as standard Ni-MH. It has the same cold-weather limitations, the same partial-charge sensitivity, and the same cycle-life envelope. LSD Ni-MH is a warehouse and shipping improvement, not a service-life improvement, compared to Ni-CD. For OEMs that ship large volumes through long distribution chains, it solves a real problem. For OEMs whose garden lights go from factory to installation in a few weeks, it is unnecessary.

Standards that govern the choice

The standards that show up most often in solar garden light battery sourcing are:

• IEC 61951-1 — the international standard for portable Ni-CD cells. Defines test methods, marking requirements, and safety tests.
• IEC 61951-2 — the international standard for portable Ni-MH cells. Sister standard to 61951-1 with chemistry-specific differences.
• IEC 62133 — safety standard for portable rechargeable cells. Relevant for both chemistries; covers abuse, drop, vibration, and thermal stress.
• GB/T 22084 — the Chinese national standard equivalent to IEC 61951-1 for Ni-CD cells.
• GB/T 22083 — the Chinese national standard equivalent to IEC 61951-2 for Ni-MH cells.
• UN 38.3 — transport safety testing. Less directly relevant for Ni-CD and Ni-MH than for lithium chemistries, but cited in some import documentation.
• EU Battery Directive 2006/66/EC and RoHS 2011/65/EU — restrict cadmium content in portable batteries in the EU. Solar garden lights that ship into the EU consumer market must confirm RoHS compliance for the chosen chemistry.

OEMs shipping into the EU consumer market should specifically confirm with their cell supplier that the Ni-CD AA cells are manufactured under the solar-lighting exemption (or use Ni-MH if the destination application does not have an exemption pathway). RoHS exemptions for cadmium in Ni-CD batteries used in solar lighting applications exist but are periodically reviewed; confirming current exemption status before each production run is the safest approach.

Single cell vs. cell stack in 1.2V garden lights

Solar garden light assemblies are usually described as 1.2V, 2.4V, 3.6V, or 4.8V nominal, which corresponds to one, two, three, or four AA cells in series. The cell count changes the runtime and the cell-matching requirement but not the chemistry decision per cell.

• 1.2V single-cell garden lights are the simplest design. A single 800 mAh Ni-CD cell runs a white LED at low brightness for 8-12 hours. The cell selection is straightforward; the cell matching is not a concern.
• 2.4V / 3.6V / 4.8V stacks are more common in commercial garden lights with brighter LEDs or RGB color-changing LEDs. The cell matching requirement becomes important: cells in a series string should be from the same production batch and have similar capacity. Mixed batches cause the lower-capacity cell to reverse-charge during deep discharge and fail early.

For multi-cell stacks, the recommendation is to source cells as a matched set from the cell supplier — not to buy cells from inventory and assemble them into packs. The supplier can match cells by capacity, voltage, and internal resistance within tight tolerances, which keeps the stack healthy for the full service life.

Selection protocol for OEM sourcing

The decision sequence below is the one our application engineering team walks through with every garden light OEM that requests a battery recommendation. It is short because the decision points are predictable; the verification is what catches the exceptions.

1. Define the destination climate window. Lowest expected operating temperature for the deployment region. Below -10 °C, default to Ni-CD. Above 0 °C all year, Ni-MH is viable.
2. Calculate the nighttime load. LED wattage times hours of operation per night, with a margin for days where the battery did not fully charge. This sets the cell capacity requirement.
3. Confirm the charge window. PV panel wattage, latitude, average daily insolation at the deployment region, and the controller’s charge algorithm. Partial-charge operation favors Ni-CD.
4. Compare lifetime cost. Cell cost plus expected number of replacements plus service labor, across the warranty period.
5. Check regulatory compliance. RoHS and battery directive compliance for the destination market, particularly for Ni-CD in the EU consumer market.
6. Validate with a seasonal field test. At least one full season of outdoor testing at the destination climate before locking the BOM for mass production.

Steps 1-3 set the technical answer; steps 4-5 set the commercial and regulatory answer; step 6 confirms both. Skipping step 6 is how OEMs end up with a garden light that works in the lab and fails in the field after the second winter.

Common failure modes and what they mean

When a solar garden light comes back as a warranty claim, the failure mode usually tells you which battery problem it was. The three most common modes are worth recognizing.

• LED flickers after a few hours of nighttime operation, recovers on a sunny day, fails again the next night. This is partial-charge capacity fade. The cell is reaching the cutoff voltage early because its effective capacity has dropped below the load demand. Ni-MH shows this faster than Ni-CD; the fix is to replace with a higher-capacity cell or to switch to Ni-CD chemistry.
• LED never lights even after a full day of charging. This is cell reversal or internal short. One cell in a multi-cell stack has reversed or shorted, dragging the rest of the stack down. The fix is to replace the entire cell stack with matched cells, not just the failed cell.
• LED lights for full first season, then runtime drops sharply in the second season. This is chronic capacity fade from partial-charge cycling. Ni-MH is more prone to this than Ni-CD. The fix is to confirm the cell supplier’s cycle-life test data under partial-charge conditions, not just under full DOD test conditions.

All three modes point back to cell selection at the OEM design stage. Once the failure mode is in the field, the only fix is replacement, which is why the decision in this article matters more than the cell brand or the cell price.

A short summary for the OEM buyer

If you are specifying a battery for a solar garden light assembly, the chemistry question is not “Ni-CD vs Ni-MH” in the abstract. It is “Ni-CD vs Ni-MH at this destination climate, with this PV charge profile, for this warranty period.” In most mixed and cold-climate applications, Ni-CD remains the safer choice because it tolerates the partial-charge cycle and the cold weather better. In mild-climate applications where the capacity-per-cell advantage translates directly to longer runtime, Ni-MH is the better pick. Ni-CD is the default; Ni-MH is the upgrade when the operating envelope justifies the higher unit cost.

For OEMs that need either chemistry in production volumes, our Ni-CD AA rechargeable battery and Ni-MH AA rechargeable battery ranges cover the standard solar garden light capacity bands (Ni-CD 600-1000 mAh, Ni-MH 1300-2500 mAh, with LSD variants on request). For sizing, application review, or matched-cell-pack requests, the application engineering team is reachable through our contact page; a destination climate, LED load, and PV panel spec is enough to start a sized recommendation.

Frequently asked questions

Is Ni-CD or Ni-MH better for a solar garden light?

For mixed and cold climates, Ni-CD is the better default because it survives sub-zero temperatures and partial-charge cycling with less capacity fade. For mild climates where the capacity-per-cell advantage translates to longer runtime, Ni-MH is the better pick. Most OEM solar garden light assemblies shipping to mixed markets default to Ni-CD.

What capacity Ni-CD AA cell should I use for a 1.2V solar garden light?

For a single-cell 1.2V garden light with a 0.5 W to 1 W LED and 8-12 hours of nighttime operation, 600-800 mAh Ni-CD AA is the standard. For 2-cell and 3-cell stacks (2.4 V or 3.6 V), the same cell capacity is used per cell and the runtime scales with the cell count.

How does cold weather affect Ni-CD vs Ni-MH in solar garden lights?

Below about 0 °C, Ni-MH capacity drops sharply — usable capacity often halves at -20 °C compared to room temperature. Ni-CD capacity drops more slowly, retaining roughly 70-80% of rated capacity at -20 °C. For solar garden lights installed in continental or northern climates where winter temperatures regularly fall below freezing, this is the single largest reason Ni-CD remains the default chemistry.

Why does Ni-CD last longer than Ni-MH in solar garden light duty?

Solar garden lights run on partial-charge cycles that are not friendly to Ni-MH chemistry, with occasional deep discharge below the safe Ni-MH floor. Ni-CD tolerates both modes with much less capacity fade. In real garden light service, Ni-CD typically lasts 3-5 years and Ni-MH typically lasts 2-3 years before noticeable runtime loss.

Are Ni-MH LSD cells a viable alternative for solar garden lights?

LSD Ni-MH retains 70-85% of charge after one year of storage, which solves OEM logistics for pre-charged shipments but does not solve the cold-weather or partial-charge limitations. It is a logistics win, not a field-performance win, against Ni-CD.

Can I mix Ni-CD and Ni-MH cells in the same battery pack?

No. Mixing the two chemistries in a series string causes reverse-charging in the lower-capacity cell and permanent damage. The replacement protocol is to replace all cells in a pack with cells of the same chemistry, capacity, brand, and ideally production batch.

What standards govern Ni-CD and Ni-MH AA cells for outdoor solar use?

IEC 61951-1 for Ni-CD, IEC 61951-2 for Ni-MH, IEC 62133 for portable battery safety, GB/T 22084 for Chinese Ni-CD standards, GB/T 22083 for Chinese Ni-MH standards. EU consumer products must also confirm RoHS compliance for the chosen chemistry, including any current exemptions for solar lighting applications.