Smart batteries help us get much more from portable power but what exactly are they and why have they evolved?

A smart battery includes one or more of the following:

• A Battery Management System (BMS) containing various levels of data about the battery
• A Protective Circuit Module (PCM) to avoid overheating from misuse or malfunction.
• A cell balancing system to keep all the cells within the battery at the same voltage which helps improve service life

In this article we’ll look at how and why they evolved and then dive into each one in more detail.

What do we know about a battery?

There are many aspects of batteries which are useful to know:

• How much charge is left? – or for how long will it continue to power various different devices
• What types of appliances can it power without being damaged or overheating?
• How old is it? – because all battery types degrade over time, even when not in use.
• Can it be recharged? And if so:
  • How long will it take to fully recharge?
  • Can it be ‘fast charged’?
  • How often has it been charged and discharged (because all batteries have a limited ‘cycle life’)
  • What is the battery’s expected cycle life (how many times can we charge and discharge it)

Surprisingly a simple (or ‘dumb’) battery can’t answer most of the above questions!

Without a label or technical specification sheet the only test we can do is to measure the voltage and this is actually how most basic battery testers work.

The basic problem of a basic battery

The voltage of all batteries reduces gradually as a battery discharges so, theoretically, if we know what the voltage is, we know how much charge is left in the battery.

To do this it’s important to understand that the voltage stated on the label of a battery is not the actual voltage. The real voltage is higher. A fully charged 12 Volt SLA battery will have an ‘actual voltage’ of between 12.5 and 13 volts (sometimes referred to as the ‘charge voltage‘).

We can see this in the discharge graph from a BatteryGuy SLA battery which shows the voltage of the battery over time when discharged at certain rates:

As you can see when fully charged this 12 volt battery is actually a 13 volt battery.

We’ll focus on the top line, when the battery is being discharged at a rate of 0.05C. The reduction in voltage over time is hard to see but with good testing equipment it can be read.

So theoretically if we found the voltage was 12.7 we might conclude that it has been discharging for 2 hours and has at least another 10 hours of power before the voltage will drop below 12 volts, a point at which it will probably no longer be able to power the appliance it is connected to.

But this reading can actually be very misleading.

Voltage is not an accurate state of charge indicator

This is because of how batteries change over time. Let’s take a typical 12 Volt Sealed Lead Acid battery as an example.

When it comes out of the factory fully charged its ‘actual voltage’ is around 13 volts.

But, every time the battery is discharged and recharged it never quite reaches the ‘factory fresh’ level again. In fact with every discharge and recharge the actual voltage degrades. After a number of discharge and recharge cycles it may only be capable of recharging to 12.5 volts.

As we mentioned earlier voltage also gradually declines as a battery discharges so if we measure the cross circuit voltage of a 12 volt battery and find it is 12.5 volts is it:

• A brand new battery that is slightly discharged?
• An older battery that is fully charged?

If it is a brand new battery we know that we can charge it further to give us longer power when we need it but if the battery is older attempting to charge it would be a pointless waste of time.

The only way to find out is to connect a charger to the battery and, after a period of time, see if the voltage increases.

Let’s say after a period of charging the voltage moves to 12.6 volts but no further. We can take this as a signal that the battery is now fully charged but, with very slight variants of actual voltage in factory production we still don’t know it this battery is new or not.

The final nail in the coffin of this method is that we need the temperature to be around 25C. If it is much colder, or hotter, than this we need a completely different graph in order to carry out the State of Charge test.

So a basic battery can only give us a very rough estimate on its state of charge.

Safety issues with basic batteries

Different types of batteries can discharge and recharge at different rates (known as the C-rate).

Sealed Lead Acid batteries can only discharge slowly while lithium batteries can provide far higher rates of power over short periods of time.

For example, a 40 amp hour lithium battery might be able to power a 40 amp device for one hour while a 40 amp hour lead acid battery can provide the same amount of power … but over a 20 hour period.

If you discharge a basic battery faster than its design parameters it is likely to overheat which in turn could cause a fire or an explosion. A basic battery will try to power whatever device it is connected to even if it leads to its own destruction.

The same issue is true for recharging. Try to recharge a battery faster than its capabilities and fire or an explosion are a real risk.

We’ll add on top of this the fact that all batteries are prone to manufacturing errors which could cause them to short out, once again leading to a fire or explosion.

Optimization Issues in Basic Batteries

A final blight on basic batteries is that they are not very good at providing an ‘optimal’ level of power.

Most batteries are made up of multiple cells. Traditionally battery chargers switch off when at least one cell reaches full charge. To continue charging would damage that cell. However it means all the other cells are undercharged.

• See also: How a Battery Works

Similarly a battery will stop being able to supply current as soon as the voltage on one cell drops too low, or it will drag energy away from other cells causing potential damage and shortening the battery life.

So in short a basic battery:

• gives us very little information about itself;
• can be dangerous enough to bring down a plane;
• is poor at delivering its full power potential.

The Evolution of the Smart Battery

Smart batteries evolved in three areas:

• advanced features which make them safer to use.
• an ability to provide more information about themselves
• cell balancing to optimize charging and discharging

Safety Concerns of Lithium Batteries

The introduction of lithium batteries, especially lithium-ion batteries, to commercial markets in the 1990s made mobile phones (brick like though they were) and mobile computing a possibility.

But lithium batteries packed far more power into much smaller spaces than previous battery designs. That meant that if they were to malfunction the results were far greater.

The heat, fire or explosion from one lithium cell could be enough to cause the cell next to it to fail, in turn catching fire and damaging any other cells around it. The process is known as ‘thermal runaway’.

The world’s media captured images and films of cars on fire and laptops bursting into flames while flight crash investigators concluded at least one plane accident that killed everyone on board had been caused by lithium batteries.

These events had been caused by small manufacturing errors or damaged batteries but there were plenty more due to misuse such as overcharging or rapid discharging.

For a while the whole technology was bought into question and even today the transportation of lithium batteries is heavily regulated.

• See also: Shipping Lithium Batteries

The need for more information from the battery

While safety concerns needed to be addressed lithium batteries seemed to good to just let go. They had a much longer life span than previous battery designs but they were expensive to produce.

This meant it was more important to answer those questions we saw back at the beginning of this article. How old is this battery? How often has it been used? What is it’s accurate state of charge?

So lithium manufacturers began looking at how to embed this in the battery itself.

Getting more from each charge

In a fiercely competitive market manufactures were also looking for ways to manage individual cells of a battery so that a fully charged battery meant every single cell was fully charged.

If they could deliver on this they could offer a greater weight to power ratio than their competitors.

So on three levels there was a need for a smarter battery:

• a battery that recorded things about itself from it’s date of manufacture to the number of times it had been discharged.
• A battery that could take action when it sensed misuse or malfunction.
• A battery that could manage cells at an individual level when it came to charging and discharging.

Smart Batteries – Level 1 (Safety) – Circuit Protection

In response to safety concerns, primarily about Lithium batteries, manufacturers developed the Protective Circuit Module (PCM).

PCMs are so small and compact that, even in the smallest of batteries, they can be hard to see or completely hidden under a wrapper or labeling.

The PCM is a small electrical circuit which usually monitors:

• The internal heat of the battery (to identify potential thermal runaway)
• How fast the battery is being discharged (to stop misuse through over discharge rates)
• How fast the battery is being recharged (also to stop misuse through over charging or over charging from faulty chargers)

If the PCM senses an issue it can:

• Isolate and cut off the internal components from the battery terminals
• Disconnects the battery from any external appliance or charger.

The PCM can be manufactured to only reconnect in certain circumstances such as:

• after a certain number of seconds have passed
• only once the battery has been disconnected from external appliances or chargers.
• only when the internal temperature of the battery has returned to a certain level

Where overheating is detected the PCM might operate vents (although these can also be mechanical) which will allow hot gases to escape rather than build up in the battery. It will also ensure that the battery cannot be connected to an appliance or charger.

PCMs were a forced measure. In order for the advantages of Lithium batteries – their lighter weight and small size – to be realized in modern appliances the safety issues had to be addressed.

Despite this boot leg lithium batteries do still exist on the market today and if you hear stories of replacement cell phone batteries which have ‘melted down’ the appliance they were supposed to power you are probably looking at a lithium battery without a PCM or with a cheap and unreliable PCM.

Smart Batteries – Level 2 – Battery Management

Battery Management Systems (BMS) usually have a PCM but may also include any of the following:

• Data on the voltage and capacity of the battery (in case the label is damaged)
• Who manufactured the battery
• The date the battery was manufactured
• How many times it has been charged and discharged
• To what level it has been charged and discharged each time
• How fast the battery has been, and is currently being, discharged at

A BMS can then aggregate all this information together and provide an accurate estimate of

• how much charge the battery has (State of Charge)
• how long it is expected to last at the current rate of discharge
• how long it would (or will) take to recharge
• if the battery can currently accept a ‘fast charge’
• if any of the internal parts of the battery are showing signs of defects.
• how much longer (in days, months or years) the battery is expected to last before needing to be replaced.

Communicating with a BMS

All this information is only useful if it can be accessed.

A modern smartphone will have an inbuilt application which can read the BMS of its own battery as shown in the screenshots below.

For other batteries a Bluetooth connection is usually included in the BMS so the information can be shown on a computer or smartphone via an application provided by the manufacturer.

Battery Management Systems can be found in many BatteryGuy Lithium Iron Phosphate (LiFeP04) batteries and allow for a smarter use of batteries as well as protection against misuse which in turn leads to a longer service life.

Fast Charging and the BMS

One of the key frustrations all battery users have faced is the time it takes for a battery to recharge.

In reality all discharged batteries can recharge faster to start with but their ability to accept charge slows down the closer they get to fully charged.

If we apply a fast charge to a battery which is only slightly discharged this will cause the battery to overheat causing damage to the cells and potentially risking fire or explosion.

But if we apply a slow charge to a battery that is nearly empty we’re wasting time because the battery is able to accept a faster charge.

A Battery Management System can also manage this aspect. It will allow a higher current when it senses the battery can accept it but reduce the current as the battery nears full charge.

In the image below we can see the power level of a charger to a smartphone when the battery is 40% full and the power level when the battery is 90% full.

This is achieved through a mix of data (knowing the basic data and history of the battery) and monitoring the temperature to avoid overheating.

Cell balancing chips

As we have seen basic batteries never get used to their optimal potential because chargers switch off as soon as one cell is fully charged. This avoids overcharging the cell which would damage it.

But it means that all the other cells are never fully charged and when the battery is put into use again they drag down the fully charged cell, potentially damaging it to some degree.

This damage is minor but it can mean the difference between a battery lasting, for example, 3 years or 5 years.

A cell balancing chip and circuitry provides the capability to manage each cell individually during a recharge. As each cell reaches it’s full voltage it is disconnected while the other cells continue to recharge.

This ensures that all cells achieve the same voltage, reducing internal damage on each charge and discharge cycle and so giving the battery a longer service life.

A cell balancing chip can be a standalone feature of a battery or an integrated part of a Battery Management System.

Smart Battery Summary

As we have seen the term ‘Smart Battery’ can mean many different things but it is a battery with one or more of the following:

• a cell balancing chip
• a Protective Circuit Module (PCM)
• a Battery Management System (BSM)

Remember that there are various levels of complexity both in Protective Circuit Modules and Battery Management Systems from very basic features to highly advanced aspects.

Not all ‘smart’ batteries have the same smart features and not all PCMs and BMSs are the same.