Battery physics 101

Voltage is the difference in electric potential between two points. If you would compare electricity to water flow, voltage would be the water pressure in a hose. Even small amounts of water have a big impact if the pressure is high enough, for example a pressure washer. The voltage that is considered safe for humans in most situations is 50V and lower. When working on a higher voltage level, safety measures need to be taken. Check the “safety” section. It is common for battery cells to have a voltage output range between 2.5 and 4.2V. For battery packs with higher voltages you need to chain batteries together in series: 10 batteries of 3.6V will provide 36V in series. One of the drawbacks of batteries is, that their voltage decreases when they hold less charge. A fully charged lithium-ion cell is 4.2V, while it could be 2.5V when almost empty. Therefore, It is commonly rated at 3.6V as an average voltage between full and empty. Note that the end of charge voltage is considerably higher when calculating system voltages and choosing components. A 360V nominal battery pack can reach 420V end of charge for instance.

Current is the rate of flow of electric charge past a point, also known as amperage. When you compare electricity to water flow, this would be the volume flow of water. Even at low speed, a huge amount of water can have a big impact, for instance waves at sea. While a high current at low voltage is not considered dangerous directly, the consequences can be huge. For example, current surges can cause extreme heat buildup which can lead to burns. Check the “safety” section. There is a wide variety of battery cell sizes available. Laptop 18650 cells can deliver a couple of amps, while large prismatic cells can deliver hundreds of amps. If you place batteries in parallel, you can increase the amount of current(A) in your pack.

A battery is an electrochemical cell with two external terminals which powers electric devices. The negative terminal is the source of electrons which will flow trough an electric device towards the positive terminal. While electrons are flowing to power the shown lamp, chemical processes are going on inside the battery. The ions are taken from the negative electrode (anode). After that the ions flow through the electrolyte to be added to the positive electrode (cathode) . The flow of electrons will come to a stop when all the material from the anode and cathode is converted: the battery is depleted. When charging, assuming the battery is equipped with chemicals that allow for charging, this process is reversed. Batteries were already used in the late 1800s for electrical vehicles. Thomas Edison had one, for instance. In the early 1900s 38%(!) of the cars in the US were electric! Edison: “Electricity is the thing. There are no whirring and grinding gears with their numerous levers to confuse, no dangerous and evil-smelling gasoline and no noise.”  

Capacity is the amount of current a battery can deliver for an amount of time, usually one hour. For larger batteries this is often stated in Ah (amperage hour), for smaller cells most of the time in mAh (milliamperage hour). For instance, a battery that is rated “2500mAh” can deliver 2.5A for one hour. This ratio can be shifted, it means it can also deliver 1.25A for 2 hours, or 5A for 30 minutes. There are batteries available with low and high capacities, ranging from 1500 mAh 18650 cells towards 300Ah (300.000 mAh) or even more. Most of the time this capacity will only be reached with a very low current, often only 20% of their rating. So, in the mentioned example the battery can deliver 0.5A for 5 hours. If you use a higher current, the heat generation in the battery will account for some losses and the rated capacity will not be reached. The smaller this deficit, the more suitable the battery is for high drain applications. Sometimes battery suppliers will provide you only with the capacity of the battery. If you know the voltage, you can figure out the amount of energy the battery has. If the voltage is unknown, for instance in battery pack assemblies, a major variable is unknown for you to judge the amount of energy the battery pack holds.

C rate is the rate at which a battery can be charged and/or discharged, and it is strongly related with the capacity of the battery. “C” however is not short for “capacity”! This “C” is useful to compare the current (dis)charge capability of various sized batteries. The capacity of a battery is commonly rated at 1C: a fully charged battery rated 2500mAh should provide 2.5A for 1 hour. You can use this C-rate to determine (dis)charge amperage versus time. For instance, a 2500mAh cell rated at a discharge rate of 3C, can be discharged at 7.5A. If the current is 3x higher, the duration is 3x shorter. So, theoretically the battery can be discharged with 7.5A for 20 minutes (drain losses and voltage drop will likely reduce this time by a couple of minutes).

Electric power, like mechanical power, is the amount of work. It’s the multiplication of Voltage and Current. For instance, if your battery pack can deliver 500A at 400V, it can deliver 500A x 400V = 20,000W or 20kW. This is what you need to know to see if your battery pack can deliver the amount of power you require. Some battery suppliers only provide the absolute maximum their pack can deliver. Most of the time this is only usable for a couple of seconds, and sometimes they even give ratings that are beyond the design specification of the cells they use. So, check the fine print and ask questions: you always need to verify if the pack can provide the rated power for the amount of time you require.

There are various definitions of energy used in different fields. Here we will limit ourselves to the following: energy is the amount of power (W or kW) supplied for 1 hour. If it’s not given by a supplier, it can easily be calculated by multiplying the pack capacity with the voltage. For instance, a 500Ah pack at 400V nominal, is a 20kWh pack. Be sure to use the nominal voltage, not the maximum voltage. This is a very important piece of information as this determines the size of your battery pack, its price and what you can do with it. If the previously mentioned battery pack, that can deliver 20kW, only supplies this power for 5 minutes, it contains a lot less energy than a battery pack that can deliver 20kW for 5 hours. Often the amount of energy a battery pack can hold is referred to as “battery size” or “battery capacity”. Strictly speaking this is wrong as neither of those units are used for energy. Especially “battery capacity” is a tricky one as the capacity is indeed an important piece of information, but only in combination with the voltage its useful to determine the energy storage capability of a battery pack.

Especially in mobile applications it is often important that a battery pack is as light and small as possible and yet holds as much energy as possible. More energy means you can use a given power for a longer time, which in a vehicle means more range. When comparing various cells and batteries, you can calculate the amount of energy in relation to their weight (gravimetric) and size (volumetric). For instance, a Nissan Leaf 24kWh pack weighs 294 kg and has a volume of 494 liters. The gravimetric energy density is 24,000Wh / 294kg = 81 Wh/kg The volumetric energy density is 24,000Wh / 494 L = 48 Wh/L This is fairly low. Our 72Volt “range” pack for instance has the following specs: The gravimetric energy density is 190 Wh/kg The volumetric energy density is 316 Wh/L You can also calculate it the other way: if we would have a volume of 494 liter of our batteries, we would have 494L x 316Wh/l = 156,104 Wh or 156kWh of energy. That would be the same as 6 Nissan leaf battery packs. Please note that we are comparing battery packs, not bare cells! You would need to consider the mechanical casing and internal subsystems as well. Bare cells have better figures, but you can’t just toss them in the trunk, now can you? 

Power density is the amount of power you can get out of a given size or weight. Especially in high performance applications with limited available space, like motorcycles or go-karts, this is an important parameter. The same Nissan Leaf pack can deliver 110kW and weighs 294 kg and has a volume of 494 liters. The gravimetric power density is 110,000 / 294kg = 374 W/kg The volumetric power density is 110,000 / 494 L = 222 W/L This is also fairly low. Our 72V “race” pack for instance has the following specs: The gravimetric power density is = 1850 W/kg The gravimetric power density is = 2830 W/L Again, we can calculate the other way: if we would have a volume of 494 litre of our batteries, we would have 494L x 2830W/L = 1.398.020 W or 1398kW or 1.4 MW of power as opposed to the 110kW of the Nissan Leaf! That’s more than 12 times better. If you feel like building a megawatt sports car, talk to us! Also, it is good to keep in mind that we are comparing battery packs, not bare cells!

Just as in life, you can’t get it both ways. If you go for maximum power, you will lose some range, and when you maximise range you will have to make do with less power. If you want both, you’ll have to make a compromise. This is the single most important choice you need to make when it comes to battery selection. The reason for this is fairly simple: if you wish to drain a lot of power from a given cell, the metal poles in that cell will get hot. There is only so much current that can be handled with a given size. This can be solved by increasing the size of those poles. The extra space that these poles will need can’t be used for the actual battery chemicals, hence the loss of energy storage capacity.

The other way around is similar: if you don’t need much power you can optimise the battery for maximum energy, but you can’t drain it too heavily. If you attempt to do that anyway, you will stress the battery too much which will lead to a large voltage drop and a generation of a lot of heat. Best case your batteries will wear down quickly, worst case you are heading for a battery meltdown. Since heat development increases exponentially with amperage, this gets out of hand quickly.

Always stay within the recommended specified amperages and beware for battery suppliers which claim they can do both maximum power and maximum energy, this is physically impossible. Think of it like letting a weightlifter run a marathon and let a runner do weightlifting. They will both be bad at each other’s specialism, and the runner will get hurt trying to lift too much, like a range pack will get damaged when trying to extract too much power. Looking for somebody who could do both is possible, like a decathlon athlete. However, he will never run a marathon as fast as the runner, nor will he be able to do the weightlifting to the extent of the specialist. Our “performance” pack is comparable to the decathlon athlete. In graph you can see how our solutions are related to power and energy.

This is the indicator of how “full” the battery is, 100% would be full where 0% is empty. There are various ways of measuring this, the two most common are the voltage method and the current integration method.

Voltage method: since an empty battery has a lower voltage than a full battery, it seems logical to determine the SOC based on the voltage. However, the voltage doesn’t go down in a linear fashion. When at 100% the voltage drops quickly, then remains fairly constant and drops quickly again when getting near 0%. This means that between 80% to 20% SOC it is difficult to determine, especially since the voltage varies under load. In cheaper battery management systems (BMS), you can see that the SOC behaves unpredictable because of this effect.

Current integration method: The current (amperage) can be measured in most systems. Since the capacity of the battery is known, and the current variation over time is known, you can “count down” to zero quite accurate. However, when load varies in a system, the capacity of the battery also varies a bit. So, when a battery is discharged at a faster rate than what the system is designed for, the battery can be empty before the SOC indicates such, resulting in unexpected early cutout from the battery management system. Another disadvantage is that this system will drift over time, so you need to reset this frequently. Most commonly the SOC will reset to 100% when the charger is finished charging.

Kalman filtering: to overcome the inaccuracy of both stated systems, an algorithm can be used to combine the data and make a more accurate approximation of the SOC. These algorithms are widely used in signal processing systems. Our sophisticated battery management system uses this.

This is basically the inverse of the SOC, 100% is empty and 0% is full. This is not a logical way to show the state of charge, but its mainly used in relation to cycle life. The deeper a battery is discharged, the shorter the cycle life of a battery will be. The traditional lead acid battery in a petrol car, should not be discharged below 50% or it will dramatically shorten its lifetime. Most modern lithium-based cells are rated for 100% DOD without affecting the rated lifetime. However, avoiding the last 20% of your batteries charge will considerably increase the lifetime beyond the rated amount.

A charger is a device that forces an electrical current into the battery so its state of charge will increase. This may sound simpler than it is, because lithium batteries are actually quite sensitive to temperature, voltage and current. Assuming temperature and voltage are within the normal operating window, the charging principle contains two stages:

• Stage 1: Constant current: the charger will supply a predetermined current to the batteries. The amount of current is depending on the application and what the batteries can take. A safe number for normal charging would be 0.5C. This charging will continue while the voltage of the battery pack slowly increases. At some point the maximum voltage of the pack is reached, which for lithium-ion cells is normally 4.2V per cell. When we would continue to charge like this, the voltage would keep going up and the battery will get damaged. This 4.2V is usually the point the batteries are at 80% of their SOC. Now the second stage of charging comes into play.
• Stage 2: Constant voltage: When the maximum voltage per cell is reached, the charging current will be lowered to keep the batteries at this maximum voltage. The current will keep dropping until almost zero up to the point that the batteries are fully charged. Because of the dropping current, it takes almost 50% of the time to charge the last 20% of the battery. This is also the reason a lot of EV manufacturers specify their charging time until 80%. When fast charging on the road, it doesn’t make much sense to charge that last 20% as well, in most cases it is sufficient to charge up to 80% and continue your travel.

Both these stages are referred to as “CCCV” charging which is the only way to properly charge a lithium battery. Any other “wizardry” is unnecessary or even hazardous: the memory effect like in NiMH batteries does not exist for lithium-ion, and trickle charge is a great way of shortening your battery’s cycle life. Conditioning a battery is unnecessary: lithium batteries are at their peak capacity when delivered. If the charger is not specifically made for lithium batteries and doesn’t clearly work on the CCCV principle don’t even consider it.

We can provide you with chargers that are a great match with our batteries and battery management system: fully programmable to suite every possible situation. Remarks: Keeping a battery at its peak voltage shortens the lifetime. It’s not advised to trickle charge a lithium battery and keep it at its voltage top. More information about this can be found at the section “cycle life” of the battery. Charging as fast as possible during the first CV stage doesn’t really shorten the time until the battery is fully charged. It only decreases charging time until 70%, and the time to charge from 70% to full will increase. So fast charging is only important when you wish to continue your trip quickly and accept a shorter range. In most cases this is the practical way to go. If you plan your trip along chargers its best to charge when the vehicle is fairly empty: those charges are the quickest and you are on your way again soon.

For racing you want the batteries at maximum charge to use their maximum energy capacity. Even with fast charging you would still need 1.5 hour or more if you want to get the last few percent in: it would be a waste to drag around the weight and not use all the energy it could hold. The time these batteries are at their maximum and minimum voltage is low anyway, providing they are fully charged at the last moment and immediately charged to storage capacity after the race.

Prolonged high temperatures are bad for batteries. They can occur during fast charging and therefore measures are required to cool batteries during fast charging. Our batteries are fluid cooled and can easily transfer the heat out of the pack. So, they can safely be charged with the maximum current the cells are specified. If a battery pack is stressed to its limit and on the verge of overheating, its best to postpone the charging until its cooled down. Our BMS can manage that process to ensure maximum lifetime of your battery pack. Lithium batteries can handle cold temperatures rather well. That means, as long as they are not used. Charging below zero degrees Celsius is not permitted. That includes regenerative braking as that is basically charging the batteries with your drive train! The BMS must be set to prohibit low temperature charging. Our BMS has this feature, and its also capable of preheating our fluid cooled battery pack before charging as this cooling system can also be used as a battery pack heating system for the winter. Often the BMS controls the charger through the CAN bus connection: it will consider battery status, SOC and temperature and will determine the proper setting, which in turn the charger will carry out. Our charger and BMS are an ideal combination and will come custom preset to function optimal in your application.

Charging should start at low current and should take a couple of minutes to reach the full current level. This reduces stress on the battery prolonging its life. Lithium batteries don’t suffer from the memory effect like the older NiMH batteries do. You can charge them at any SOC, and they don’t need to be fully charged. You can charge them until full or only 10 minutes just to reach your destination, no problem. Manufacturers of some chargers claim they can recover batteries which are deemed unable to charge by the BMS. This is impossible: lithium batteries can’t be regenerated. The capacity loss is irreversible. What they mean is that they can charge batteries which are depleted below their normal voltage. They give the battery a boost, so the batteries are brought back above their normal operating voltage, at which point the safety systems of the BMS give the “all clear” to commence normal charging. However, batteries that were below their minimum voltage for a prolonged time are best case damaged, and worst case a fire hazard.

Be sure to charge your battery regularly, even when not in use, and never let it dwell below its minimum voltage. This can happen over time since lithium batteries lose their charge slowly even when not used at all. When batteries are not in use for a prolonged time, try to store them slightly below room temperature and around 50% SOC. Never store them fully charged or fully depleted for prolonged time. When stringing cells to form a high voltage battery pack, it is mandatory that each and every cell in this series connection remains within the proper voltage range. So, each one of those cells must have a BMS chip to monitor it. If this wouldn’t be monitored, and one cell would be at a higher voltage, that cell will raise above a safe voltage during charging while the total of the cells would still seem ok. The same is true for a lower voltage cell which will dip below its operating voltage. If cells aren’t used within their operating range, they will get damaged and will lose capacity rapidly. You have to think of batteries in series like links of a chain: the strength of the chain is defined by the weakest link. The same is true for battery cells: the capacity of the weakest cell defines the capacity of the whole pack. This is the reason why we monitor every single cell, and the BMS will tell the charger to stop charging if even one cell is above its allowed voltage and it will warn or even stop discharging if even one cell is below its allowed voltage.

So, what to do if cells get unbalanced? The BMS chip is able to dissipate power from the cell with a higher SOC, thus balancing it with the others. Our BMS is automatically correcting during the charging process without the need for you to do anything at all. When a battery pack is properly built from quality cells with all the same capacity, this is hardly an issue. It will take up very little time to balance, and your pack’s capacity remains ok. However, when a battery pack is abused or gets old, the imbalance gets larger. This will take up more time to get balanced, and the pack as a whole loses capacity.

This is the accepted way of properly charging a lithium battery as explained under “battery charging”.