Regenerative Braking while Towing: Is the BYD Shark 6 really safer downhill?
How does regenerative braking help or hinder when towing downhill?
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QUESTION
Hi John,
I recently came down Thunderbolts Way in my 150 Series Prado (2 people plus dog, 210Kg) with a caravan in tow (2600kg). It has a 12 per cent gradient over 6km into Barrington Tops. The speed limit is 40km/h for trucks and 80km/h for cars. I attempted 40km/h in 1st gear and when the tacho redlined, I braked firmly back to about 30km/h. By the time I reached the bottom, it felt like the caravan was doing most of the braking.
Can the BYD Shark 6’s regenerative braking perform better than this or would it rely on the brakes all the way.
Would discharging the battery on the way up from Kurrajong give me a sufficiently empty battery to make regenerative braking useful to get down at Lithgow?
Also, can you use low range on the highway? In low range I think I could have made it down without the brakes but I couldn't have got out of the way if a truck came along.
There is a nice spot to pull over about 2/3 the way down, but it was too late to use when I saw it and its on the right with blind corners. Nice photo op if you’re going up.
I usually use the Bells Line of Road (a 60-km detour for the Blue Mountains to avoid these hills). But I wonder if a BYD Shark might be a suitable upgrade for the Prado in this situation.
Regards,
-Anonymous-
ANSWER
There are a couple of different issues tangled together here.
Regenerative braking can certainly be useful when towing downhill, but that isn't fundamentally what regen is for. Its basic job is energy management: instead of converting the vehicle's kinetic energy into heat in the brake discs and pads, the electric motors operate as generators and convert some of that energy back into electrical energy, which is stored chemically in the battery.
That can provide substantial retardation, but it isn't unlimited.
The amount of regenerative braking available depends on things such as battery state of charge, how much charging power the battery will accept, battery temperature, motor/inverter limits and the vehicle's software strategy. If the battery is full, or simply cannot accept energy at the rate being generated, regen has to reduce and the friction brakes take over.
This becomes especially important with a caravan because you're not merely slowing the tow vehicle. You're controlling the gravitational energy of perhaps four-to-five tonnes of combined vehicle and trailer continuously down the hill.
So I wouldn't regard regen as the primary safety strategy for a descent like Thunderbolts Way. The primary safe descent strategy is low speed.
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This is why when you were a kid, you should have paid more attention in science: Let’s say I run two experiments: equivalent descents involving your Prado and van. Same hill. Same vehicle. Same van.
Experiment #1: Travel speed is maintained at 40km/h using the brakes.
Experiment #2: Travel speed is maintained at 80 using the brakes.
Isaac Newton and Sadi Carnot are very clear on this: In both experiments the brakes have to deal with the same amount of energy (mass x g x height) but they have to do it twice as fast in Experiment #2 (ie in half the time), so it’s much more likely the brakes overheat in Experiment #2. (Obviously this is why truck speed limits on downhill grades are so low.)
Slower speed = less chance of overheating the brakes. It’s also a mistake to ping the engine off the limiter to control the descent. Stop doing that, because it can lead to catastrophic valvetrain damage. It’s not a truck; the valvetrain is not designed to slow the vehicle down in that way, especially when towing a heavy van. Moderate revs in a low-ish gear will be fine. (2/3rds of the way to the redline.) It’ll help take a bit of load off the friction brakes. This redlining business: Mistake.
The lower your speed, the easier the entire physics problem becomes. The higher the speed, the higher the chance of overheating the brakes. Select a moderately low gear. Use the friction brakes as needed to constrain your speed - it’s what they’re for. Aim for a constant conservative speed. And pull over if you’re holding the world up. No need to be an arsehole (ie - typical caravaner) to following motorists.
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A suitably configured BYD or other electrified tow vehicle might provide much stronger retardation through regen and make the descent considerably easier. But I would absolutely not assume that it could regenerate continuously for six kilometres with a 2.6-tonne caravan without eventually requiring the friction brakes. That depends on how much battery capacity is available and, crucially, how much regenerative power the system will accept.
Deliberately arriving at the top of Bells Line of Road with some battery capacity available would certainly give the vehicle somewhere to put recovered energy on the descent into Lithgow. In principle that's sensible. But I wouldn't attempt to calculate it as, "If I discharge X per cent going up, I therefore have enough regen to get down." The vehicle may reach its charge-power limit long before it reaches its energy-capacity limit.
In other words, you can have plenty of empty space in the battery but still be coming downhill faster than the battery is capable of accepting the energy.
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As for low range: I wouldn't treat that as a solution for highway descents. Whether low range can safely be used on a sealed road depends very much on the particular transfer-case design and whether using low range also locks the centre differential or otherwise prohibits use on a high-traction surface. The owner's manual is the authority there. Bottom line: Low range is completely unnecessary for highway descents.
And, as you've identified, very low gearing can create another problem: you may have almost no useful road-speed capability if you suddenly need to accelerate out of trouble.
For me the hierarchy would therefore be:
Enter the descent slowly.
Select the lowest sensible gear before speed builds.
Use whatever engine braking or regenerative braking the vehicle provides, without flogging the engine near the redline.
Allow the trailer brakes to contribute as designed.
Use the friction brakes as necessary and try not to drag them continuously if possible.
Never rely on regen being available indefinitely.
Regen is an excellent tool in this situation, but I'd regard it as additional braking capacity and energy recovery rather than the thing that makes an otherwise marginal descent safe.
Regards,
John
