Battery Selection for Smart Locks: Alkaline vs. NiMH Rechargeable Batteries

To understand the long-term impact of a smart lock and IoT devices it’s essential to consider the cost, energy efficiency, and environmental impact. This analysis evaluates the suitability of alkaline and NiMH rechargeable AA batteries in a smart lock, focusing on:

1. Initial Cost Comparison

2. 5-Year Cost Analysis

3. Waste Generation and Environmental Impact

4. Impact of Temperature Fluctuations (-30°C to 40°C)

5. Conclusion and Recommendations

Assumptions:

• A typical igloo bluetooth smart lock uses 4 x AA batteries per year in average conditions.

• A typical competitor WiFi based smart lock uses 4 x AA batteries every 3 months.

• Average cost for a 4-pack of alkaline AA batteries is $4 and $15 for a rechargeable NiMH kit (4 x AA batteries + charger).

1. Initial Cost Comparison

Alkaline Batteries: $4 for a 4-pack, replaced once a year in normal conditions.

NiMH Batteries: $15 for a rechargeable kit (4 x AA batteries + charger), with a lifespan of around 500 charge cycles, far exceeding 5 years if recharged annually.

NiMH batteries are more expensive upfront but provide long-term value through reusability.

2. Cost Analysis Over a 5-Year Span

The following table illustrates the year-by-year costs of alkaline and NiMH batteries over a 5-year period assuming a bluetooth smart lock with 1 year lifespan, showing cumulative costs and the point where NiMH batteries become more cost-effective.

Break-Even Point: NiMH rechargeable batteries break even with alkaline costs by Year 4 and yield a total savings of $5 by Year 5.

While this may not seem much on the individual scale, when you multiply this over larger deployments, the savings add up. For example, managing 10 properties with NiMH batteries would yield a total savings of $50 over the same span of time.

3. Environmental Impact and Waste Generation Over 5 Years

Quantifying waste generation reveals the environmental impact of each battery type. This includes battery and packaging waste as well as any hazardous materials.

Waste Calculation Breakdown

Alkaline Batteries:

• Battery Weight: Each AA alkaline battery weighs ~23 grams.

• Battery Use Over 5 Years: Under normal conditions, this results in 4 batteries per year x 5 years = 20 batteries.

• Total Alkaline Battery Waste: 20 batteries x 23 grams = 460 grams

• Packaging Waste: Each 4-pack typically has 5 grams of plastic/cardboard packaging. Over 5 years: 5 packs x 5 grams = 25 grams

Total Alkaline Waste (batteries + packaging): 485 grams (0.485 kg)

NiMH Rechargeable Batteries:

• Battery Weight: Each NiMH AA battery weighs ~27 grams.

• Battery Use Over 5 Years: Since each NiMH battery can handle around 500 recharges, no replacements are needed in 5 years. Total: 4 batteries.

• Total NiMH Battery Waste: 4 batteries x 27 grams = 108 grams

• Charger Waste: One NiMH charger (~200 grams) is assumed to last 5+ years, contributing to disposal weight if replaced.

• Packaging: Minimal single-pack packaging for the NiMH kit (5 grams).

Total NiMH Waste (batteries + charger + packaging): 313 grams (0.313 kg).

Summary of Waste Comparison

With the disposal of single use batteries, one saves approximately 77% in terms of battery mass, 80% in terms of packaging and 35% overall (when including the charger) When managing properties at scale these numbers add up to massive savings showing how rechargeable power sources reduce the impact on our environment.

4. Impact of Temperature Fluctuations (-30°C to 40°C)

Temperature variations in North America, -30°C in winter to 40°C in summer, significantly affect battery performance.

Alkaline Batteries:

• Cold Temperatures (-30°C): Alkaline batteries lose up to 50% of their capacity in subzero temperatures (below -20°C). This requires more frequent replacements in cold months, potentially doubling usage to 2 sets per year (8 batteries).

• Hot Temperatures (40°C): Elevated temperatures accelerate self-discharge, though to a lesser degree than cold. This could slightly shorten battery life, contributing to the increased replacement frequency.

Rationale for Twice-Yearly Replacement: Under extreme cold, batteries deplete faster in smart locks due to increased internal resistance and reduced efficiency. Field tests and battery manufacturer data suggest that alkalines may need replacement twice a year in very cold climates. For this analysis, we assume 2 replacements per year under these conditions.

NiMH Rechargeable Batteries:

• Cold Temperatures: NiMH batteries also suffer in cold but retain more capacity than alkalines at low temperatures. A recharge may be required more often in winter, but replacements are not needed.

• Hot Temperatures: High temperatures increase self-discharge, but since NiMH batteries are rechargeable, this impact is mitigated by recharging rather than replacement.

Revised 5-Year Cost Comparison (With Temperature Effects)

Break-Even Point with Temperature Impacts: NiMH batteries become cost-effective by Year 2, with cumulative savings of $25 by Year 5. As before, scaling this across 10 properties would yield approx ~$250 in savings.

Revised Waste Calculation with Temperature Effects

When including the effects of extreme temperatures, NiMH batteries reduce overall waste by ~68% over alkaline batteries, provide much more predictable discharge curves, and avoid frequent replacements. It is important to note that while no solution is perfect, by weighing the pros and cons of each, scaling deployments become a lot more cost effective.

5. Conclusion and Recommendations

Cost Savings: Using NiMH rechargeable batteries results in cost savings by Year 2 when accounting for extreme temperatures. By Year 5, total savings amount to $25 compared to alkaline batteries.

Environmental Impact: Over 5 years, NiMH batteries reduce waste by 35% under normal conditions and by 68% in colder climates where alkaline replacements are doubled. This reduction also decreases exposure to hazardous chemicals, as fewer batteries are discarded and NiMH batteries contain recyclable metals (nickel and cobalt) compared to potassium hydroxide found in alkalines.

Temperature Resilience: NiMH batteries are better suited to temperature extremes, maintaining more stable performance and avoiding the frequent replacements necessitated by alkalines, especially in subzero temperatures.

For smart locks, NiMH rechargeable batteries offer substantial long-term cost savings, environmental benefits, and superior performance in fluctuating temperatures. For climates with seasonal extremes, NiMH batteries are the optimal choice, reducing both financial and environmental costs over the life of the smart lock.

Summary Table of Findings

This analysis underscores that while NiMH rechargeable batteries require a higher initial investment, they deliver significant cost and environmental advantages in both standard and extreme temperatures over a 5-year period. For eco-conscious consumers and those in areas with temperature fluctuations, NiMH batteries are a better choice for smart lock power.

Another factor to consider is how these numbers spin out of control when using locks with embedded WiFi. WiFi is extremely power hungry and directly contributes to battery consumption. Many WiFi lock models have reported battery lifespans of only 3 months, when factoring in extreme temperatures this could result in 16-20 Alkaline batteries in a year! This amount of battery waste is completely unsustainable in terms of cost and very damaging to the environment. Why do people still buy WiFi embedded locks? Primarily because they want the ability to generate temporary pincodes for visitors, guests and non regular access. At igloo we are able to provide the same functionality without the need for WiFi via our algoPIN technology. Not only does this technology save time & money, but it also massively reduces energy requirements of the smart lock and hence, the environmental impact as well. 

We recommend that you select the right power source for your application as every deployment is different. If you would like to learn more about our locks and technology, view our products here.