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

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

Also,
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'!