EV Fundamentals: Battery Dynamics
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Battery Dynamics
06.36
| 00:00 | Over the last few modules we've touched on how batteries react when they're charged and discharged to their limits, but now let's talk about how they react in real time to large dynamic changes in current. |
| 00:09 | The first concept we need to get a good understanding of is voltage sag. |
| 00:12 | When a battery is charged or discharged its voltage changes because it effectively has a resistance to accepting or releasing electrical current. |
| 00:20 | When charging a battery for example, the voltage of the battery will go up proportionally to the current. |
| 00:25 | The more current you try and force into the battery the higher the voltage will become as the battery resists accepting this current. |
| 00:32 | Similarly when discharging a battery quickly the voltage will drop because the cell's pent-up internal pressure, its voltage, is being allowed to bleed out. |
| 00:39 | When that ability to flow stops the pressure builds back up again. |
| 00:43 | Let's take a look at some data to get a better idea. |
| 00:45 | Here you can see that as the current increases the battery voltage decreases, but this is just a temporary phenomenon and the battery voltage will return to close to its original level once the discharge is over. |
| 00:57 | Other than being proportional to current the amount of voltage sag is also significantly affected by battery temperature as well as obviously the size of the battery. |
| 01:05 | What that means is that the total amount of power a battery can output is relatively related to the voltage sag. |
| 01:12 | At some point the voltage sags below the safe lower limit that the cell is designed to handle and in addition the voltage is dropping so quickly that the increase in current is resulting in smaller and smaller increases in power. |
| 01:24 | For example if a 400 volt battery drops to 320 volts at a thousand amps, which is 320 kilowatts of power but then drops again to 290 volts at 1100 amps that's 319 kilowatts now. |
| 01:36 | We've effectively pushed the battery that much harder adding 100 amps more discharge but the battery power has decreased because the drop in voltage outpaced the increase in current. |
| 01:47 | This is a little bit of an extreme example but it's not too dissimilar to raising the boost in a turbocharged engine. |
| 01:53 | At some point you're adding a lot more stress for a very little amount of additional power. |
| 01:58 | The same thing happens during charging and it's important to understand that this is the main limiting factor in terms of fast charging and why batteries will always charge at a slower rate when they're at a high state of charge. |
| 02:08 | So, how do we know how much voltage sag or increase for that matter we should expect from our system? This all comes down to understanding the internal resistance of the battery and making a few simple calculations from there. |
| 02:19 | Firstly, by observing the voltage before and during a discharge or charge pulse the internal resistance can be calculated. |
| 02:24 | It's a very simple calculation that uses ohm's law. |
| 02:27 | Whatever the voltage changes by divided by the current applied gives you the resistance. |
| 02:31 | It's really it's that simple. |
| 02:34 | So, for example if the voltage sags 30 volts under 300 amps of current the battery resistance at that moment is 30 divided by 300 or 0.1 ohms. |
| 02:42 | Once an estimation of the battery's internal resistance has been made we can then use this to estimate what the voltage would be at a particular current and you'll quickly realize that at a high state of charge when the voltage of the battery is already close to its maximum there isn't much current that can be shoved in before the battery will reach that maximum allowed voltage. |
| 03:02 | For example with an internal resistance of 0.1 ohms if a battery is resting at 380 volts it can only accept 200 amps of current before it will reach 400 volts. |
| 03:11 | So, that's a 20 volt difference between 400 and 380. |
| 03:14 | Following that if the battery is at 390 volts it can only accept 100 amps before it will reach 400 volts. |
| 03:20 | So, with all that being said the internal resistance of the battery will change based on temperature as well and this is why modern EVs will preheat their batteries prior to fast charging. |
| 03:29 | If the internal resistance drops by 10 percent from preheating this means that now the battery will be able to accept 10 percent more current and thus charge power. |
| 03:38 | This is also why for drag racing and short races a hot battery is generally desirable over a cold one. |
| 03:43 | Less voltage sag means the inverter gets higher input voltage and the motors are able to produce more power. |
| 03:49 | So, let's now move on to the dynamics of constant current constant voltage charging, which is how most lithium batteries are charged. |
| 03:55 | The concept is actually really simple the charger will just charge at its fixed current, which is usually the limit of what it can output until the battery reaches its maximum allowable voltage usually right around 4.2 volts per cell. |
| 04:07 | Once the battery reaches that voltage the current has to be reduced otherwise the cell voltage would exceed that allowed maximum. |
| 04:14 | So, the charger monitors the voltage and continues to reduce its current in order to maintain that voltage. |
| 04:19 | Once the current drops below a cutoff threshold the charger stops the rest voltage of the battery will now closely match that cutoff voltage at , which point the battery will be considered fully charged. |
| 04:29 | Of course, it's also possible to terminate charging earlier if a full charge isn't necessary. |
| 04:33 | The same concept also applies when discharging. |
| 04:36 | The battery can discharge at very high current levels until the voltage drops to a critically low point at which point the current must be reduced to prevent the voltage from dropping further. |
| 04:45 | Before we finish up with battery dynamics there's one more important aspect to cover voltage recovery. |
| 04:51 | After a pulse or charge event ends the battery will return very close to its rest or open circuit voltage. |
| 04:57 | However, it then takes some time for the voltage to fully come up to that true rest voltage and even longer where the battery is cold. |
| 05:04 | There's also some hysteresis to this. |
| 05:07 | The voltage will be slightly above the true open circuit voltage after a charge event and slightly below after a discharge event. |
| 05:14 | So, in terms of determining the state of charge of the battery from voltage this can really only be done if the battery has been resting for some time so that this rest voltage is an accurate representation of the true rest voltage of the battery. |
| 05:27 | Typically a BMS will record the voltage when it wakes up and then use current counting also called coulomb counting to calculate the state of charge while the battery is working dynamically, because an accurate rest voltage measurement is difficult to do. |
| 05:40 | Once the battery goes back to sleep the BMS will update its state of charge once it wakes back up to correct any inaccuracies that may have built up during a previous drive cycle. |
| 05:48 | Summarizing this module it's important to get a decent understanding of the dynamics involved when a battery is cycled aggressively and one of the key behaviors is voltage sag, which occurs when a battery is charged or discharged and as voltage, which can be also thought of as pressure changes. |
| 06:03 | Working out how much sag we should get from our system comes down to understanding the internal resistance of the battery and making some very simple calculations from there. |
| 06:12 | Though it is worth noting that temperature has a big effect on how much sag we'll see so that internal resistance is not a fixed number. |
| 06:18 | This is why OEMs tend to preheat their battery packs to drop internal resistance before fast charging. |
| 06:23 | Charging and discharging also need to be tightly controlled by the BMS in order to ensure that the battery doesn't go outside its maximum and minimum allowable voltages, as this can damage the battery cells. |
