PDM Installation & Configuration: Isolation
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Isolation
07.29
| 00:00 | - When building the electrical system for a performance car and in particular a race car, it's common to have some sort of battery isolator fitted. |
| 00:08 | This is simply a way of disconnecting the battery from the rest of the system, ensuring the rest of the system is therefore powered down and safe. |
| 00:15 | There are two reasons for including one of these in your design, the first being safety. |
| 00:18 | In the event of a crash or a major electrical system malfunction, the isolator can be tripped to disconnect the battery, ensuring any damaged wiring will not be able to create a short circuit and eliminate the risk of an electrical fire. |
| 00:32 | The second reason is far more mundane and comes down to vehicle storage. |
| 00:37 | The electronics in a car are constantly drawing small amounts of current even when they're switched off. |
| 00:43 | Over time, this will flatten the battery in the car. |
| 00:46 | Particularly with race cars that might see long periods of storage between use, being able to easily isolate the battery is a good way to know that it's not going to be dead flat next time you get the car out for an event. |
| 00:57 | There are two main styles of isolator used, the first being a manual keyed switched and the second being a solid state device. |
| 01:05 | The way they're wired up and configured is a little different so we'll have a look at these differences now. |
| 01:11 | Starting with the manual keyed switch, you've likely seen these fitted to racecars as they're pretty common. |
| 01:16 | They're a simple switch that's actuated with a key which closes the contact between two large threaded stud terminals. |
| 01:23 | These are fitted as the first element after the battery positive terminal in the circuit. |
| 01:28 | So opening the switch will disconnect the battery from the rest of the system, isolating it. |
| 01:33 | Now I've said these are fitted as the first element after the positive battery terminal but they can just as easily be fitted to the negative battery terminal connection because in this position they'll still isolate the battery when they're switched. |
| 01:45 | There's a tricky point here though, when the vehicle is running, it's actually the alternator that supplies the electrical energy to the system, with the battery being there just to smooth out that supply. |
| 01:56 | If the isolator switch is thrown while the engine is running, isolating the battery from the rest of the system but not the alternator, the alternator will continue to supply the electronics and the vehicle will likely continue to run. |
| 02:09 | This is actually a really dangerous situation. |
| 02:12 | We were expecting flipping the isolator switch to kill the engine and the electronics in the vehicle. |
| 02:17 | Most likely because we're in some sort of emergency situation. |
| 02:20 | But that hasn't happened. |
| 02:22 | Additionally we've just removed the battery from the circuit so there's no longer any element smoothing out the supply from the alternator. |
| 02:29 | This is known as a load dump situation as the alternator was likely supplying some current to the battery, recharging it after it was used to start the vehicle. |
| 02:37 | When we suddenly remove the battery from the circuit, that current is still being pumped out by the alternator and this will cause a massive voltage spike, very likely damaging the connected electronics. |
| 02:49 | One solution to this is to wire the isolator as the first element after the positive battery terminal but connect the alternator to the battery side of the isolator. |
| 02:58 | This means that if the isolator is switched the alternator will also be disconnected from the rest of the electronics. |
| 03:05 | The complication with this method is that some racing classes specifically require that the isolator switch disconnects the battery from all other elements including the alternator. |
| 03:14 | A better solution to this is to use a manual isolator switch that actually has multiple switching circuits within the one casing, all of which are operated at the same time by the same red key. |
| 03:25 | The main threaded studs on these units will isolate the main battery but when the switch is thrown, they're wired in such a way as to switch the alternator output from being connected to the battery to being connected to ground via a supplied load resistor. |
| 03:41 | This allows any residual electrical energy generated as the engine spins down to be safely dissipated in that resistor. |
| 03:48 | With manual isolators covered, let's have a look at solid state isolators. |
| 03:52 | Much like how PMUs use solid state electronics to replace the physical switching of relays, these isolators use solid state switching to replace the mechanical switch of that manual isolator. |
| 04:04 | This solid state element is a high power MOSFET or possible multiple high power MOSFETs all switching together in parallel. |
| 04:12 | Commonly these solid state isolators will also have a micro controller built into them that can control the safe isolation of the battery and prevent load dumps from the alternator in a more sophisticated manner. |
| 04:22 | This can sound a little complex but more often actually makes them easier to install than a manual isolator. |
| 04:30 | These isolators usually have two physical switches wired to them, an internal kill switch mounted within easy reach of the driver and an external kill switch, often located at the base of the windshield, signalled so that a race marshall can easily find it in an emergency situation. |
| 04:46 | The micro controller in the isolator reads the state of these switches and if isolation is required it can spring into action. |
| 04:53 | The different solid state isolators on the market do things in different ways but the general isolation strategy is to first shut down the engine via removing power to the ECU or ignition system. |
| 05:05 | And then after a set delay time, switch off those large power MOSFETs isolating the battery. |
| 05:11 | This delay allows time for the engine to stall, guaranteeing that there won't be an alternator load dump. |
| 05:17 | As the internal and external kill switches are now only being read by a micro controller, they don't need to handle the switching of any large currents so they and the wiring going to them can be very small. |
| 05:29 | This is a major advantage over a manual isolator because the driver would need to be able to reach that manual isolator while belted and tightened into his seat and the large main power cables often have to be routed in inconvenient ways to ensure this can happen. |
| 05:45 | With the alternator output now handled in a much more sophisticated manner, isolating the battery on the negative terminal side is now also a possibility. |
| 05:52 | In fact there are solid state isolators out there that are specifically designed for this. |
| 05:56 | They'll only have one connection stud and the other connection point being the metal case of the isolator itself. |
| 06:04 | These isolators must be solidly mounted to a metal part of the vehicle body. |
| 06:09 | When they're tripped, they disconnect the vehicle body from the negative terminal isolating the battery. |
| 06:14 | Depending on the model, a solid state isolator might be able to receive isolation commands from other sources like a PMU, ECU or dash display via CAN. |
| 06:24 | Really opening up the possibilities and keeping the hole system really flexible. |
| 06:29 | The isolator may be able to send out really valuable data like the direction and amount of current that's passing through it. |
| 06:35 | This can be important in detecting an alternator failure that's in progress or issues with the grounding of the rest of the electrical system in the vehicle. |
| 06:43 | In this course module we've looked at the reasons why we might need to isolate the battery from the rest of the electrical systems in a race car, along with discussing the most common methods of doing this. |
| 06:53 | Manual switch isolators can be used but thought must be given to the alternator output wiring to ensure the isolator works as expected and there isn't a chance of a load dump from the alternator into the rest of the electronics. |
| 07:05 | Solid state isolators perform the same task but have programming in them to safely shut the system down and isolate the battery. |
| 07:13 | They're usually easier to install as they can be placed more conveniently and often have communications allowing them to receive kill commands from multiple sources and communicated back important information to the rest of the system. |
