Power Braking

[BigRedSwitch]

When I first got my hybrid I used the B position on the transmission lever to help braking and to save wear and tear on discs and pads but then I read somewhere that this could be a false economy as it could have a negative impact on the transmission itself.  What do you guys regard as the truth of the matter?

[TonyHSD]

Hi,

“B” mode is only for use when going downhill to minimise brakes applied and recharge the Battery, like going into a lower gear on manual or overdrive on older automatics, however should not be used all the times, and it’s up to a speed of 65mph if I remember right. I only use it sometimes when going downhill for few miles then back to D. You want save much more of your brakes when in B as basically they work very little and most of the braking power comes from the motor generator even when driving in normal D mode. 

Regards 

[Catlover]

Agree with Tony, re hill braking.  Last week we were on holiday based in Gloucester.  Went to the west and toured Forest of Dean, then to the east the Cotswolds. Both areas had steep hills and as there is virtually no engine braking effect in a Hybrid the B mode came into its own. Depending on the severity of the hill you cannot rly on B alone, but it certainly takes a lot of braking out of the equation, therefore reducing heat and potential brake fade if using brakes all the time.

[BigRedSwitch]

Thanks Tony and Joe.  I'll start using "B" again but only on hills and under 65mph.  👍

[Saxmaniac]

I understood B mode is to help prevent overcharging Battery on long downhills by allowing the engine to spin to absorb energy instead of generating unwanted power. This seems to be the opposite previous posts. Did I misunderstand?

[PeteB]

Not exactly.  B mode is primarily to give additional control, as Tony said, like down-changing with a conventional gearbox.  It's another 'tool in the box' for choosing how to use the car.

The software makes sure the Battery cannot overcharge anyway.

However, one myth that prevailed when the original Prius came out almost 20 years ago was that B mode increases economy, whereas in fact it does the opposite.  This wasn't helped by the manual using a description something like "more aggressive regeneration".

Selecting B mode forces the petrol engine to run but without injecting any fuel, so that the pumping compression in the cylinders gives addition braking effect.  That means the Motor/Generator has less to do, so puts a little less energy back into the Battery.  Normally, depending on the speed, the engine will be stopped anyway, or at higher speeds turning but the the timing heavily altered to minimise compression.

The one time it makes no difference to economy is when the HV Battery is so full (aka 'maxed out') it cannot accept any more regenerated energy.   Most drivers will rarely, if ever, encounter that condition.  You'd need a very long, steep hill, and it happens quite a while after all bars are lit on the battery gauge.  When the battery is 'maxed out', in reality it is only about 80% full (conversely if the gauge ever showed empty it;s about 40% full).  This, along with charge rate and cell temperature management are part of the reason the Hybrid batteries have been so much more reliable than the pundits predicted in the early Prius days.

It's been reported that since the 3rd Generation Prius with the 1.8 litre engine came out in 2009 that the software had been updated to effectively operate B mode whenever the battery had 'maxed out'.  I've not had a full battery to check that since I got my 3rd and 4th Generation Prius (over some 90,000 miles).

B mode might help avoid using the friction bakes on a very steep hill, but you get the most out of the regenerative system just using the normal brake pedal as long as the Hybrid System Indicator is within the CHG area.  At the very end of the CHG area you're getting maximum regen, press the pedal any harder then you get extra friction braking as well.

My only experience of a truly maxed out Hybrid battery (and that's in over 320,000 miles in all four generations of Prius) was on a Gen 1 Prius (the least powerful) on a couple of holidays in Scotland over 10 years ago.  Even on 20% gradient hills lasting some 5-6 miles the car could climb at 60 mph on Cruise Control without assistance from the battery or particularly high revs, and going down it took at least 4 miles to max out the battery.  Even on these hills, the service brakes were sufficient to control speed using regenerative braking alone until the maxed out state was reached.  At that point, the car only used friction brakes and B mode helped reduce the wear and heat on them, although there was no sign of overheating.  The car felt subtly different once the battery maxed out, but the software managed the braking such that no extra pressure was needed on the brake pedal.

[Catlover]

1 hour ago, Saxmaniac said:

I understood B mode is to help prevent overcharging battery on long downhills by allowing the engine to spin to absorb energy instead of generating unwanted power. This seems to be the opposite previous posts. Did I misunderstand?

My wife has a 2010 Auris Hybrid, in the Auris Owners Manual, page177 it says    “B. Applying moderate engine braking when driving down hills or on steep slopes”.

For my Prius Gen 4 the Prius Users Manual says “B. Applying engine braking or strong braking when the accelerator pedal has been released on steep downward slopes etc”.

Look in your Auris manual and I strongly suspect it will say something similar.

[Saxmaniac]

I had all bars full going down hill in the Cotswolds (Frosty will know the one from the Air  Balloon!). Put it into B mode and although I know software probably protects engine, the revs rose that high that I'm more inclined to use friction brakes in future. I don't suppose the pads and discs will lose much life as a result

[Catlover]

Not so much loss of brake life, its the fact discs/pads can overheat, that heat goes into the hydraulic line and can cause loss of brakes in extreme situations.

Excessive engine noise - could it be that the downward hill needed less road speed? Surely the engine speed be the same whether your foot is not on the throttle, or B is engaged. Just a thought.

[Saxmaniac]

If I remember right, B mod caused engine to provide braking, but after a short while I'm guessing that the Battery didn't want any more charge and the electronics stopped any more charging and made the engine spin much higher to produce braking effect. I really didn't like it screaming away like that!