EV Fundamentals: On Board Charger
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On Board Charger
04.50
| 00:00 | If you've spent any time around EVs, you've no doubt seen the wall-mounted chargers that have a cable that connects to the vehicle. |
| 00:05 | What you may not know is that these aren't actually chargers at all. |
| 00:08 | They're just simple electronics with a couple of relays that connect the building's electrical power to the car's onboard charger. |
| 00:15 | We'll get into how all that works in the next section of the course, but for now, know that these wall chargers are nothing more than expensive relays, and the actual charger that converts the building's AC power to DC is actually inside the vehicle. |
| 00:27 | This onboard charging device can be found on nearly all EVs, the exception being race cars, which, for weight reasons, often keep their charging equipment separate from the vehicle. |
| 00:36 | Onboard chargers range in size. |
| 00:37 | Entry-level plug-in hybrids might have as little as a 1.2 kilowatt charger, whereas some cars designed for three-phase grids, like the Renault Zoe, at one point came with an optional 43 kilowatt onboard charger. |
| 00:48 | That's a ton of power and is almost DC fast charger level. |
| 00:51 | The industry has now settled on around 10 to 12 kilowatts as the typical upper limit of onboard chargers, as that generally provides about 50 kilometers of range per hour of charging, and much above 12 kilowatts isn't easy for most homes to add to their existing electrical service. |
| 01:06 | Almost all onboard chargers are water-cooled, as the heat generated by the charger is significant. |
| 01:11 | For context, with a charger that has a 93% efficiency, charging at 12 kilowatts produces 840 watts of heat, or about as much as a small space heater. |
| 01:21 | Most chargers are around 92 to 94% efficient at converting AC power to DC, but the efficiency drops significantly at low currents or when using low voltage, such as with 110 volts here in North America. |
| 01:32 | For those of us working on EV conversions, it's generally wise design practice to use this heat to feed into the battery. |
| 01:38 | This way, the charger can warm the batteries as it starts charging, and a radiator fan can cool that loop down if needed once the system is up to temperature. |
| 01:46 | Charging is one of the most dangerous aspects in terms of battery damage, and that's why it is so critical to have a proper VCU and or BMS in the system. |
| 01:56 | Many chargers receive a CAN message commanding a specific voltage and a current target. |
| 02:01 | The charger will output at that current target until the battery voltage increases to the set voltage target. |
| 02:07 | At that point, the charger will maintain the specified voltage, and the current will slowly taper off as the battery continues to top up. |
| 02:14 | This is called constant current, constant voltage charging. |
| 02:17 | Eventually, the current going into the battery will be so little that it doesn't make sense to continue charging, so the charge will be terminated either by the VCU or the BMS, depending on the system design. |
| 02:28 | The danger comes when the charger is allowed to overcharge a battery, exceeding its safe maximum voltage, or by charging the battery when it is too cold or hot to safely receive a charge. |
| 02:37 | Either of these events can cause the battery to be permanently damaged, or in the worst case, go into thermal runaway, which is when the temperature inside the battery gets high enough that it causes a chemical reaction, which then pushes temperatures even higher, starting a dangerous chain reaction. |
| 02:52 | Thankfully, the VCU or BMS should deal with this risk through a multi-pronged approach. |
| 02:57 | The first simple step is to make sure that the charger is correctly configured to never target a voltage that exceeds the safe pack voltage, which is typically around 4.2 volts multiplied by the number of cells in series. |
| 03:09 | A clever VCU will also adjust the requested charge current based on the battery temperature and state of charge. |
| 03:14 | For example, it might only lightly charge the cold battery and then wait for the heat produced by the charger to warm up the battery before increasing the charge current. |
| 03:22 | The next safety in place are limits relating to the battery, dictating the maximum power that the battery is allowed to receive. |
| 03:28 | This can be determined by the BMS or the VCU. |
| 03:31 | Whichever is controlled the charger will set a current limit that doesn't exceed the battery's power limit. |
| 03:35 | The final emergency safety that must be in place is a hard limit in the BMS or VCU that triggers a fault if an individual battery cell exceeds a predetermined limit, say for example 4.22 volts or a temperature of 65 degrees Celsius. |
| 03:48 | If this voltage or temperature is exceeded, the system will fault out, command everything to shut down, and pop the contactors. |
| 03:55 | Running a charger on an EV with no safeties and no upper voltage limit is like running a 300 shot of nitrous on 87 octane pump fuel with a 13 to 1 compression engine. |
| 04:04 | You just don't do it unless you're purposely looking for trouble. |
| 04:07 | Let's quickly sum up this module before moving on to the next section of the course. |
| 04:11 | Besides dedicated race cars, all EVs have onboard chargers. |
| 04:15 | These chargers convert external AC power from your building or home into DC to be fed into the main battery system and produce substantial amounts of heat in the process. |
| 04:24 | Onboard chargers need to be carefully managed either by the BMS or VCU to ensure that charging stays within safe limits. |
| 04:31 | Failure to implement this management properly can result in a damaged battery system or worse, but generally a number of omissions need to be made as a properly designed system will have multiple safeties in place to prevent overcharging from being able to occur in the first place. |
