Hybrid Cars: Dashcams, Batteries & OBD-PIDs

Hybrid cars that use high-voltage electric traction motors as well as an internal combustion engine still use a 12 volt system for most vehicle components, and they are generally supplied with a 12 volt lead-acid 'auxiliary' battery. Typically this is a low capacity (45-60 Ah) 'cranking' battery, even though it is not required to crank an engine. Some owners, for example those who use accessories such as dashcams while parked, wonder whether it is feasible to replace this cranking battery with a deep-cycle battery. Among current technologies, LiFePO4 (LFP) chemistry provides the best combination of safety and deep-cycle performance for use in cars. But LFP batteries need an appropriate charging profile for safety and optimal performance. This site explores whether it is feasible to use LFP 'auxiliary' batteries in hybrid vehicles, and some related issues.

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Front and Rear Dashcam Installation in a Rav4 Hybrid

Summary: This article: (1) describes how to install a dual dashcam taking the Viofo A129 plus duo into a Toyota rav4 hybrid as an example; (2) recommends against long use of dashcam parking mode from the small auxiliary battery usually installed in a hybrid car; (3) gives some limitations of power banks to run a dashcam; (4) describes a better way to achieve automatic activation of parking mode in a Viofo dashcam; (5) considers power use of the dashcam in relation to available battery capacity; and (6) concludes that for use of a dashcam in parking mode beyond a few days, there is no practical alternative to installation of a deep-cycle vehicle battery of suitable capacity.

Reference & Link: Birch RG (2021) Front and Rear Dashcam Installation in a Rav4 Hybrid. https://scithings.id.au/Dashcam.pdf

Is It Feasible to Use an LFP Auxiliary Battery in a Hybrid Car for Improved Deep-Cycle Performance?

Summary: The use of a LiFePO4 (LFP) 12v battery is only worth considering by a few hybrid car owners: those who drive in mild climates, use power-consuming accessories such as dashcams while parked, and are willing to ensure that battery state of charge remains high enough for safe recharge currents. The capability might be improved by interposing an LFP-optimised charger and return power diode in the 12v circuit, but this would be expensive, and a job for a professional automotive electrician. For most current hybrid car owners, feasibility requires some help from the manufacturer, in the absence of which it is best to stay with the OEM (lead-acid) auxiliary battery. Those who drive at ambient temperatures beyond the 0-40℃ range, and/or do not want deep-cycle ability from a 12v battery, should certainly stay with lead acid.

Reference & Link: Birch RG (2021) Is It Feasible to Use an LFP Auxiliary Battery in a Hybrid Car for Improved Deep-Cycle Performance? https://scithings.id.au/LFP.pdf

Why You Probably Can Not Make ‘Custom OBD PIDs’ Work for a Recent Car, and What You Can Do About It

Summary: Some manufacturers of recent car models use very effective methods to prevent owners from accessing information about their own cars, even though that information is present in the car network connected to the On Board Diagnostics port. This probably also helps to prevent accidental or malicious damage to the vehicle. Some manufacturers licence the information needed to access the data, to companies that sell OBD tools. This can provide very convenient and inexpensive access for owners, while remaining secure if the tool includes features such as Bluetooth pairing only for a short time after pressing a physical button on the tool. This article describes what is possible, using as an example the OBDLink Android app and MX+ interface to the OBD port in a 2019 (gen5) Toyota rav4 hybrid.

Reference & Link: Birch RG (2021) Why You Probably Can Not Make ‘Custom OBD PIDs’ Work for a Recent Car, and What You Can Do About It https://scithings.id.au/OBD_PID.pdf

Hybrid Battery Cooling

Summary: All hybrid vehicles need some form of cooling of the traction battery. In the case of cars with a NiMH battery pack under the rear seat, this is typically accomplished by control of a fan that blows air from the cabin over and through the battery pack. This involves some compromises between fan noise in the cabin and battery cooling. This article uses logs from sensors in a 2019 (gen5) Rav4 hybrid to show some features of the hybrid battery temperature control system.

Reference & Link: Birch RG (2021) Hybrid Battery Cooling https://scithings.id.au/Bat_Cool.pdf

Hybrid Synergy Drive (HSD) and Hybrid System Indicator (HSI)

Summary: The gen5 Toyota rav4 hybrid AWD car (AXAH54) delivers force to the wheels from: an A25A-FXS (2.5 l, naturally aspirated, simulated Atkinson-cycle) internal combustion engine (ICE: 131kW, 221N·m), a front electric motor (MG2: 88kW, 202N·m) and a rear electric motor (MGR: 40kW, 120N·m). These motors can also contribute ‘regenerative’ charge to the traction battery (HB: 6.5Ah NiMH). Further electric power to charge HB comes from ICE via a generator (MG1: 31kW, 54N·m) which can also act as a motor to keep ICE rpm in an efficient range. The 245V DC from HB is converted into (=650V) 3-phase AC for the motors, by voltage boosters and inverters. These control AC voltage, frequency and current; thus power, speed and torque of the wired-stator, permanent-magnet, synchronous motors. Current can flow between MG1 and MG2 via the inverters. All of this is managed by a computer control system, and at the front axle a planetary gear set that integrates forces to and from ICE, MG1 and MG2. This transmission system is referred to as the (gen 4) ‘Hybrid Synergy Drive’ (HSD). It is sometimes given the generic name ‘Toyota Hybrid System’ (THS-II, of which there are many sub-variants). The car has several options to display the direction of power flows between ICE, battery, motors and wheels (data sources behind such graphics are unspecified). In addition, the magnitude of power flows is presumably used in the Hybrid System Indicator (HSI) display. This article presents some real-world logs, and attempts to explain what can be discovered by owners about these systems.

Reference & Link: Birch RG (2021) Hybrid Synergy Drive (HSD) and Hybrid System Indicator (HSI) https://scithings.id.au/HSD_HSI.pdf

Data Refresh Rates from OBD-II over Bluetooth or USB

Summary: Manufacturers of On Board Diagnostic (OBD) devices for cars, and the associated software, may advertise rates of data refresh that can never be achieved by users in practice. The rate-limiting steps in the fastest OBD systems appear to be (1) gateway modules in modern cars and (2) matching in unregulated features of the SPP profile used in communication between a Bluetooth OBD interface (which is plugged into the car OBD port) and the software (which is installed in a host / display computer). For example, the OBDLink MX+ system is advertised at a maximum of ~100 PIDs/sec in Windows 10 and Android host devices, but in reality a modern-car owner will be lucky to get 30 OEM PIDs/sec; and 7-12 PIDs/sec is more common. SAE PIDs may be a little faster. Rates vary between cars, depending on data bus speed and limitations interposed in any gateway module. This article lists some of the factors affecting OBD data rates, gives examples of real rates achieved, warns about different methods used to state refresh rates, explores the factors likely to be rate-limiting in the process, and explains why consumers can not yet predict the rates they will achieve before system purchase. USB devices are an alternative for those who want faster OBD data refresh rates without ‘luck of the draw’ Bluetooth SPP profile matching; but there are other inconveniences, and data refresh rates are still typically far below those advertised. Mention is also made of methods that avoid limitations of OBD designs, to achieve much higher data rates. Unfortunately, at present these newer systems are also more expensive.

Reference & Link: Birch RG (2022) Data Refresh Rates from OBD-II over Bluetooth or USB https://scithings.id.au/OBD_rate.pdf

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