Choosing a Battery Chemistry for Your Project

If you are developing a product that needs a portable power supply, you might immediately jump for a popular lithium polymer battery or simple AA battery. However, is that really the optimal choice for your device? Let's take a look at the vast array of options out there, as well as other considerations when it comes to battery choices.

Critical Points to Consider

One often disregarded aspect of battery choice by engineers is how the product will be delivered to the end user. With shipping regulations for Lithium based batteries, it can be hard to get a product with lithium polymer batteries to your end user by mail. But shipping batteries is not just regulations, a lead acid battery for example is very heavy and could incur significant costs for delivery due to the weight. These disadvantages may outweigh any benefit you gain from either of those battery types.

Another consideration that is often overlooked is the environmental conditions present where the device is used. If the product is going to be used outdoors, or in an industrial environment, you might find it exposed to temperatures that are far lower, or higher, than some chemistries can handle. When you’re testing the product in your room temperature lab, all looks good, then the product ends up in Canada exposed to -40°C, or in Australia experiencing +45°C ambient and all of a sudden, the batteries don’t perform as expected. Batteries are based on chemical reactions, which slow down at -40°C, if the electrolyte doesn’t freeze completely and stop the battery from functioning. In the Australian example, a black box in the sun can easily reach over 70°C. Such high temperatures can cause some chemistries to fail in fiery ways.

If your product is going to be portable, the volume and weight of the battery may play a significant role in the choice. Hearing aids are a great example of this, as you wouldn’t expect to see a lead acid battery in a hearing aid, at least not in one meant for human use.

The voltage of the battery might also play a significant role in your choice. If you require multiple cells to achieve a voltage that is practical for your project, the pack might become too bulky or impractical. Closely related to voltage is of course current. Some batteries are capable of very high currents, and others may struggle to produce much at all. If you have high current demands for motors, bright LEDs, or processing power, you could rule out many chemistries immediately.

As an example of these considerations, I had an experimental autopilot crash a small unmanned crop photography aircraft some time ago. The crash was in the middle of a field which had not seen rain for 6 months, on a 42°C (107.6°F) windy day. The large 4 cell lithium ion polymer battery was crushed, and one cell started to fail. This cascaded through the other cells until the venting gasses caught fire and set fire to the surrounding grass. We were lucky we had a firetruck on hand for this eventuality, or it could have been a major disaster. From then on, we only flew LiFePO4 cells, as they do not exhibit this cascading failure mode, and are much more stable. I needed the energy density of a lithium secondary cell, but not the ability to cost millions in fire damage liabilities.

The fire may not look like much, but if this had happened at the far end of it’s flight area over by the trees in the distance this would have been a huge disaster by the time we were able to drive to it. I’ve crashed dozens of aircraft testing new hardware/firmware, this was the first time one ended up catching fire. It goes to show that despite a lot of testing, edge cases of battery behaviour can come out to bite you if the situation is just right.

Primary vs Secondary Cells

When looking for a battery, you need to consider whether a rechargeable battery is the right choice or not. Having charging circuitry in your device can be very convenient, but it can also come with significant regulatory requirements and safety approvals. Lithium based batteries are sensitive about how they are charged, and can cause a fiery disaster if not treated well. Other chemistries are happy with being over charged without transforming into a rocket engine.

If you have a device that will be in storage for a long time, and needs incredible reliability when it is used, then a rechargeable battery probably isn’t going to be the solution you are looking for. Examples you might have come across include Personal Locator Beacons and Automated External Defibrillators.

So what is a primary or a secondary cell? Simply, a primary cell is single use. The chemicals in the battery create a charge, but that reaction can’t be reversed by charging the cell. A secondary cell allows reuse by recharging.

Primary cells typically have relatively high energy densities and storage life compared to their secondary cell counterparts. Secondary cells can be more convenient, as they don’t need to be replaced after being drained, however, they can’t be stored in a charged state for a long period of time and may not have a comparable capacity for the same cell size to a primary cell.

Quick Comparison

Here’s a quick comparison of what I consider the important factors for each chemistry.

Primary Cells

For discharge current, C is capacity. So a 0.1C discharge of a 2500mAh battery would be 250mAh.

Secondary Cells

Nickel Cadmium is banned for new applications within Europe.

Alkaline

Approximate 80% of batteries manufactured are alkaline cells, so they are likely to be the battery chemistry you have had the most exposure to. They are primary cells, meaning they are non-rechargeable. You will find them in many common forms, such as letter sizes (AAA, AA, C, D), button cells, or packs of cells (9v battery). These common forms are not exclusive to alkaline batteries, but are the most standardized form you will find an alkaline battery in.

The nominal voltage of an alkaline battery is 1.5v. However a new battery will vary from 1.5v to 1.65v depending on it’s quality. A fully discharged cell will have a resting voltage of around 0.8v to 1.0v.

This voltage range is fairly convenient for most electronics, as three cells with a very low dropout regulator can run a 3.3v device. At the end of the cell’s capacity, the voltage will drop lower, but most ICs will handle the lower voltage gracefully. However, due to the fact that the same cell sizes are used with secondary cells that have a nominal voltage of 1.2v, a four cell pack is generally the minimum you would want to use to power a 3.3v device.

The discharge current from an alkaline cell is relatively low, and the usable capacity is directly related to the current draw. With a 25mA draw from a AA sized cell, you can expect around 2700mAh. However, at a 500mA load, you will see just around half that capacity usable.

Alkaline batteries do not have any restrictions for transport by air, and are available at virtually every grocery store, convenience store, and hardware store in the world, which makes replacing exhausted batteries very simple. The cost of name brand alkalines can be quite high, but low cost brands and store brands can be exceptionally cheap with very little lost capacity, and may even hold a greater capacity than the name brand option.

One of the key downsides to alkaline batteries is that they are prone to leak. Some name brand cells are advertised as 100% leak proof with a guarantee, and depending on your application, may be worth paying the brand name premium for. Leaks are caused as the battery discharges and generates hydrogen gas. This gas can cause the isolation between the case and cap to fail, or other safety devices such as vents to open. Once the seal has failed, acid will leak out as a crystalline growth that will corrode most metals it comes into contact with.

Alkaline batteries are readily recycled, with many grocery and office supply stores around the world (especially in Europe) offering recycling bins for them.

Compare Alkaline batteries on Octopart.

Lithium (Primary Cells)

There are two major chemistries of consumer lithium primary cells, lithium manganese dioxide (Li-MnO2) and lithium iron disulfide (Li-FeS2). Lithium manganese dioxide cells have a 3-3.3v nominal voltage and are typically found in a button cell packaging. Lithium iron disulfide is most commonly found in high discharge/capacity alkaline replacement cells in AAA/AA sized batteries.

If you’re working in aerospace or military applications, lithium carbon monofluoride is a viable option for low self discharge at elevated temperatures, and is qualified for applications in space. The high energy density makes it ideal for such applications, however it’s cost is prohibitive for consumer products.

Lithium batteries of all chemistries are restricted for air transport. Some airlines, couriers, and postal services will no longer carry them at all, or may restrict transport to batteries installed in consumer equipment. Unfortunately, this is not from an abundance of over-caution; several freight aircraft have been lost due to both lithium primary and secondary cell fires when cells have been poorly packaged or been faulty. There have been multiple fire incidents involving lithium batteries, which you can find if you search through NTSB and CTSB records.

Lithium Manganese Dioxide

This chemistry of lithium cell is the most common on the market. You’ll likely find them running watches or real time clock batteries due to their low self discharge and high energy density. At elevated temperatures, the self discharge rate rises rapidly, making it most suitable to room temperature applications.

The nominal voltage of the cell is 3.0v, but a new cell will offer open circuit voltage of around 3.3v. Once fully discharged, the cell will have an open circuit voltage of about 2.0v. In a 2500mAh cell, discharge rates between 5mA and 100mA have a negligible effect on usable capacity. However, under a 200mA load, the cell will only have 1700mAh of usable charge, and under a 300mA load, this drops to around 1300mAh. Lithium manganese dioxide cells handle short pulses of high current well, but not continuous load. Their usable current also varies significantly with temperature. For example, at 60°C, a 40mA load on the previously mentioned 2500mAh cell will allow the full 2500mAh to be consumed, but by 0°C, this figure drops to about 2200mAh. It further rapidly drops between -10°C and -20°C, from 1800mAh to just over 1000mAh.

Most common coin cell sizes are readily available around the world at convenience and grocery stores. Unfortunately, these can also be right next to alkaline replacements in the same packaging, which are cheaper. An end user may unknowingly use an alkaline cell as a replacement which may cause your device to not function to specifications if you require high discharge pulses or the much greater capacity of the lithium cell.

Compare Lithium Manganese Dioxide batteries on Octopart.

Lithium Iron Disulfide

If you want a battery for very low temperature, not much can compete with a lithium iron disulfide battery. These are commonly found as Energizer Lithium/Lithium Advanced AAA or AA sized cells. The price per watt hour is very high for these cells, relative to an alkaline battery. However if the application requires a long service life, or it will be difficult to swap discharged batteries, they might be the only viable option. I have used these in an application which required performance at -50°C, and they were the only battery that worked.

These cells have a nominal voltage of 1.5v with a fully charged open circuit voltage of around 1.7v. When fully discharged, the cell will drop to around 0.8v open circuit. In addition to the low temperature capabilities of these batteries, they handle relatively high continuous discharge rates exceptionally well. An Energizer Ultimate Lithium cell in an AA form factor has almost double the capacity of an alkaline cell, and most notably, this capacity barely drops when under a 1amp load. Under a 1amp load, the cell will retain almost all of it’s 3500mAh capacity, where an alkaline battery would have under 1/3rd of it’s rated low-current capacity usable.

You will find these cells in most larger stores around the world. Smaller convenience stores will not always have them due to their relatively high cost.

Compare Lithium Iron Disulfide batteries on Octopart.

Zinc Oxide

Sometimes called a zinc-air battery, you will find these batteries in a limited range of sizes. Primarily, these batteries are used in hearing aids and have tremendous capacity, but once activated they have a very short life. Zinc-air batteries have a sticker over one side of the cell to prevent air from entering. Oxygen in air forms the cathode, so once the sticker is removed the battery can function. The anode of the battery is saturated with an electrolyte that will attract atmospheric moisture and drop in effectiveness, as well as react with carbon dioxide that reduces it’s conductivity. These properties give the cells a service life of about 7 to 12 days once exposed, regardless of usage. If you have an application that allows frequent battery changes with a very small form factor, this battery might be for you.

Zinc oxide batteries have a nominal voltage of 1.4v, and will have an open circuit voltage of around 1.05v when fully discharged. While the chemistry has the highest energy density on the market, the discharge rate is quite limited. Energizer consider a 24mA pulse every 2 hours, with an 8mA continuous drain on the cell, to be high drain, and with a 5mA draw to be standard on a 600mAh cell. The discharge capacity is also highly dependent on temperature, with the chemical reaction not being practical below around -10⁰C.

You can buy hearing aid batteries at most stores that sell any sort of battery, and at pharmacies around the world. The ready availability of these batteries could make them very attractive despite the short service life.

Silver Oxide

You will only find silver oxide batteries in a coin cell form and they are relatively expensive. Alkaline batteries of the same dimensions and voltage are readily available, however have significantly lower capacity. If you need a compact, low current solution that provides years of service life and high capacity, you might consider a silver oxide battery.

The nominal voltage is slightly higher than an alkaline cell’s at 1.55v, and the cell can be discharged to 1.2v. Capacity drops off in a linear fashion from room temperature down to -20⁰C, where the cell has around 50% of its room temperature usable capacity. A silver oxide battery has extremely low discharge performance, with most datasheets providing discharge curves for just 0.2mA, with no demonstration of pulsed load capability.

Silver oxide batteries are harder to find than the same size alkaline batteries. When looking in local stores in the United Kingdom, I was only able to find alkaline and lithium button cell batteries. They are readily available online, but are probably not something you’ll be able to pick up a replacement cell for while doing your grocery shopping.

Compare Silver Oxide batteries on Octopart.

Lead Acid

Lead acid batteries are very cheap per watt-hour, but are quite bulky and very heavy. You will commonly find them in automobiles and alarm systems. If you need to run a system in a fixed remote location potentially with solar charging during the day, a lead acid battery might be just what you’re looking for. The batteries are not particularly fussy about charging and are quite safe, and a 100amp-hour leisure battery will run most systems for an extended period very reliably at minimal cost.

A lead acid battery has a nominal voltage of 2.1v per cell, but are rarely offered in a single cell. Typically, they are available in 3, 6, or 12 cell configurations, with alarm batteries being 3 or 6 cell, automotive and leisure batteries having 6 cells, and truck batteries having 12. Discharge rates for short periods of time are quite impressive; a typical light truck battery will discharge over 7C when cold.

Disposal of the battery can be difficult due to the lead and sulfuric acid construction, and can be highly hazardous if damaged. Typically, when buying a new battery, you will be able to trade in the old one for recycling.

Compare Lead Acid batteries on Octopart.

Nickel Cadmium

Nickel cadmium batteries are an older technology that has been almost completely replaced with NiMH (discussed next), and the chemistry is banned for all new applications in Europe. The cells are very inexpensive and can handle very high discharge rates, which is attractive, however, the environmental hazard negates the minor cost advantages of this chemistry.

Due to the restrictions of use in Europe, this chemistry can be considered obsolete and not for use in any new design.

Nickel Metal Hydride

Unlike nickel cadmium, nickel metal hydride cells are available everywhere in the world in great abundance. If you’re working on a consumer device, NiMH batteries are a very strong contender for a secondary cell. They don’t have the same energy density as a lithium based secondary cells, but also don’t have restrictions on transport, won’t catch fire if you don’t charge them right, and are extremely cheap. Nickel metal hydride cells are not suitable for high discharge applications, and have high self discharge characteristics. There are newer chemistries with low self discharge, however, the energy density is lower still in these cells.

The nominal voltage of a NiMH battery is lower than a same-sized alkaline battery, at 1.2v rather than the 1.5v the alkaline has. This can cause problems in a circuit designed for the higher voltage of the alkaline. When fully discharged, the cell will have an open circuit voltage of around 0.9v. Whilst the chemistry isn’t suitable for continuous high discharge current, it is still able to handle a 2C discharge.

Self discharge can be a major problem with NiMH cells. Newer chemistries advertised as low self discharge (LSD) can lose as little as 1% capacity per month, which is similar to a primary cell. This comes with a penalty of around 8-10% less capacity in the cell. On the other hand, non low self discharge chemistries can lose 20% of their charge on the first day after charge, and up to 4% per day thereafter. For applications that have little current draw, the loss of capacity from a low self discharge cell can be more than made up for in extended service life.

NiMH cells are widely available, however it is well worth checking their packaging for the capacity. In the larger cell sizes, such as C and D, the big name brands have been known to mount a smaller cell in a plastic case which gives the cell a small fraction of the expected capacity, at a higher price than a less well known brand. This means you can easily find AA, C, and D cells all with the same capacity and similar weight from a brand such as Energizer.

Compare Nickel Metal Hydride (NiMH) batteries on Octopart.

Nickel Zinc

If nickel metal hydride’s lower-than-alkaline voltage makes them impractical for your application, nickel zinc might be what you’re looking for, owing to its higher voltage. The invention of nickel zinc dates back to 1901, but it’s only recently that commercial options have become viable after resolving the very limited battery life. Now, NiZn cells can reach similar number of cycles as NiMH ones. Unfortunately, the cells do have quite substantial self discharge, which is reported to increase considerably after around 30 cycles.

The NiZn chemistry gives you a 1.65v nominal, which however can be as high as 1.85v after charging. A design expecting a NiMH or Alkaline cell might find the voltage beyond the rating of some components depending on how many cells are in series. Fully discharged, the cell will be left at 1.1 or 1.2v. The cells typically have manufacturer datasheet graphs with 3C or greater discharge showing only a negligible drop in discharge capacity, making them very attractive for high current devices, or devices with high current pulses.

The self discharge of these cells is the biggest detractor for them in my opinion. It’s quite substantial at over 10% per month! If your application requires batteries to last for months, this could rule out a NiZn cell. If you need high amperage or a higher voltage than NiMH, and can charge the batteries more frequently, it may not be an issue.

Currently, NiZn cells are most readily available in AAA and AA forms, and I have only found them online. Electronics and photography stores in the United Kingdom did not stock them where I visited.

Lithium (Secondary Cells)

Just like their primary cell counterparts, Lithium secondary cells are heavily restricted for travel due to their propensity to turn into rocket engines of fiery doom. You’ve likely heard stories from the media of phones, laptops, or tablets turning into fireballs! Well, that’s due to the lithium battery. Air freight in many countries is a no-go, and even ground freight can be restricted. This can make it very difficult to sell a product with an integrated rechargeable lithium battery. I have a lot of experience with lithium secondary cells, and feel a lot of the danger of fire is exaggerated, but I have had fires, and it is certainly something to keep in mind.

Lithium cells have very attractive energy density and tremendous discharge rates in some chemistries. Yet, this volatility means they are very sensitive to being over discharged, overcharged, overheated, and having too high current draw. If you are using a lithium secondary cell, you should be sure your charging and battery protection circuitry are suitable. It is very common to find thermal sensors attached to lithium cells in designs to allow the device to shut down if the battery is getting too hot from discharging or charging.

There are a lot of chemistries available for lithium batteries, and you may not actually know what you are buying. The most common one you will see is lithium cobalt oxide (LiCoO2), which is typically labeled ‘ICR’. Gaining popularity is lithium manganese oxide (LiMn2O4) which is typically labeled ‘IMR’. Manganese is significantly cheaper than cobalt, and results in a higher cell voltage (3.9v nominal vs 3.7v nominal). However, manganese cells have a lower energy density. High discharge cells may be lithium nickel manganese cobalt oxide (LiNixMnyCo1-x-yO2), which are labelled as ‘INR’. INR cells also have very good energy density, and are what you might find in an electric vehicle. These are all lithium ion technologies, which are also available in a lithium ion polymer construction. Lithium ferrous phosphate (LiFePO4) is discussed separately.

Lithium Ion vs Lithium Ion Polymer

The major difference between the two is construction method. Lithium polymer cells use a thin microporous polymer membrane with a gel electrolyte, which results in it’s higher energy density and higher discharge rate potential. This thin polymer membrane is also what makes polymer cells more volatile, as it is easier for a cell to short circuit or for overheating to cause problems. This, when combined with the higher energy density, allows for a more energetic failure.

You will find both constructions available in cylindrical cells, as well as prismatic (pouch) cells. Costs are typically lower for the lithium ion, as the construction is less complicated.

The 3.7v nominal chemistries all have peak charge voltages of 4.2v, and should never be discharged to 3.0v. A battery discharged to below 2.8v per cell will sustain damage and it’s life span will be reduced, with a greater risk of becoming unstable during charging or heavy discharge.

Lithium Ferrous Phosphate

LiFePO4 is the calmer, slightly lower density, lower voltage cousin to the other lithium ion cells.

Lithium ferrous phosphate gives you a nominal voltage of 3.2v, and should be discharged to no lower than 2.2v. Discharging till 2.0v is risking damage to the cell. Compared to lithium ion, and especially lithium ion polymer, they have about 20% lower peak discharge amperage and capacity for the same weight/volume. If your application requires high discharge, but also improved safety over other lithium options, this cell could be for you.

As a note on my story about the fire, I was crashing a plane or two a week at that point working out bugs. Generally, the cells could end up looking like a banana and be fine, but it only takes one event where the cell internally shorts and causes a fire to cause a lot of damage. Smaller capacity lithium ion polymer cells are rather hard to set fire to. I’ve tried very hard to physically damage 100-200mAh rated packs to the point of a fire without any success. Overcharging even a small battery however is quite likely to end poorly.

Compare Lithium Ion batteries on Octopart.

We hope you found this article useful! If you'd like to have content like this delivered to your inbox, sign up for our monthly newsletter!