Observations on battery current (BTA/amp) – Gen 2 and Gen 3 compared

Discussion in 'Gen 3 Prius Technical Discussion' started by GreenJuice, Nov 25, 2011.

  1. GreenJuice

    GreenJuice Active Member

    Sep 3, 2008
    London, UK
    2016 Prius
    In 2009, I posted some observations on measuring BTA/Bta using Scangauge II in a Gen 2 Prius which led to a conclusion that there was a small advantage in having the car in Neutral and applying the parking brake when stationary, e.g. at lights, rather than holding the car in D with the footbrake.

    This was confirmed as a technique used by Japanese hypermilers in a post that followed.

    I upgraded to a Gen 3 recently and have noticed differences which appear to suggest even less or no advantage in doing this with a Gen 3.

    As before, all observations were made on flat roads with lights, heating, A/C, interior fan off. Gen 3 was in ECO mode, external temperatures 5 – 15 C and my Gen 3 has LED lights which were not on the Gen 2. Using respective xgauges ‘BTA/Bta/amp’ on SGII from the spreadsheet by Adrian Black.

    The observations:
    Column 1 Column 2 Column 3 Column 4
    0 Gen 2 Gen 3
    1 Stationary in D light foot on brake 2 – 3 Amps 1.4 A
    2 Stationary in D firm on brake
    1.5 A​
    1.4 A
    3 Lift foot off brake in D (creeps forward)
    2 – 3 A​
    2.9 A
    4 Stationary in N or P foot on brake
    1.5 A​
    1.4 A
    5 Stationary in N or P foot off brake 0.9 – 1.1 A 1.4 A

    Gen 3: turn on LED headlights –> add 0.5 A
    Gen 3: turn on interior fan at lowest level –> add 0.1 A

    Other thoughts and observations:
    1. When slowing to a stop, regenerative braking appears to continue to a lower speed in Gen 3 compared to Gen2, before friction braking takes over. So slipping into N just before stopping might lose the potential for a final little bit of regeneration in a Gen 3.

    2. After stopping, light pressure on the foot brake works well on the Gen 3, compared to the Gen 2 which has a greater tendency to creep forward unless held by firm pressure on the brake. This seems to have been fixed on the Gen 3.

    3. Use of foot brake on Gen 3 does not show a measurable result in current draw, apart from a brief (1 second) surge of a couple of amps (this surge is also present on Gen 2).

    4. There may be other reasons to hold a Gen 3 in ‘Neutral with parking brake’ when stationary at lights, but fuel efficiency gains are immeasurably low. Measurably low in Gen 2!

    5. BTA baseline values may be useful as a guide for how much traction battery assist is taking place in a glide.

    Is there a take home message?
    Well, so far as a newbie to Gen 3, in open roads I can't seem to do better with fuel efficiency against the cruise control. In tight urban roads, ECO mode makes light of all the P&G tricks I used try in the Gen 2. The mpg gap between what the Mrs used to get against me has reduced. I guess the message must be - 'just drive it'!
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  2. adrianblack

    adrianblack Member

    Sep 26, 2009
    Los Angeles
    2010 Prius
    Yep, on Gen 3 cars -- the ECU only sends power to MG2 to move the car when you are almost completely off the brake pedal. Of course, a Gen 2 car you can just push the brake a bit harder and it will not do this... but on a Gen 3 car, you have to be off the brake so much the car will start to creep as soon as MG2 is getting power.
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