ScanGauge II – Prius Gen 4 XGauges – Observations on State of Charge

The first few XGauges for the Gen 4 Prius are now being published. Previously, with the Gen2 and Gen3 models, the Hybrid Battery State of Charge (SOC) and Battery Current (BTA) were two XGauges that many PriusChat members found useful.

This post focuses on my initial observations using SOC on a Gen4 and to compare them with my previous experiences with the Gen3 and Gen2.

The XGauge I have tested works on my Gen4 in the UK. I can see no reason why it should not work elsewhere, but YMMV!:

TXD: 07DF0176
RXF: 444105762000
RXD: 2808
MTH: 006400FF0000
Name: SoC (a.k.a., SOC, ASC, can be anything you like)

NOTE: This XGauge is different from the one currently listed on the ScanGauge website. I have tried that one and it does not work for my car (producing no results). This alternative XGauge was provided to me for beta testing by Linear Logic and appears to work.

This XGauge displays a value in % that is communicated from the Battery Management System (BMS or Battery ECU). As with previous generations of the Prius, the dashboard display of 8 blue bars only represents the ‘usable’ portion of battery charge. On earlier models, this represented 40% of the battery capacity (ranging from 40% SOC to 80% SOC) and the BMS controls this to maximise battery life.

Here are my first observations which might be of some interest. Firstly, there appears to be a poorer and non-linear correlation between the displayed blue bars and the values returned on this XGauge on the Gen4. On earlier generations, the SOC values appeared to be evenly spaced out for each blue bar between values of 40% and 80% (approximately 5% for each bar and allowing for the limitation that there was less experience and testing at 1 bar and 8 bar). My initial observations on the Gen4 are that 4 bars and 5 bars represent bands of about 4% SOC each, while 6 bars on the display can range from 55% to 64% SOC. At the lower end, 2 bars also seems to represent a wider range of %SOC:

All testing performed in Eco mode

	Blue Bars on display​	SOC(%)​	SOC range(%)​
1	8​	not tested yet​	N/A​
2	7​	?65 and above*​	not tested yet​
3	6​	55 - 64​	9​
4	5​	52 - 56​	4​
5	4​	49 - 53​	4​
6	3​	44 - 50​	6​
7	2​	38 - 45​	7​
8	1​	?37 and below*​	N/A​

*I have not been able to properly test 1 bar or 8 bar as yet. The lowest indicated SOC I have seen is 38% at 2 bars before the ICE kicks in to start regeneration. At the upper end, I see 7 bars when above 65% but have not had the driving conditions to experience the transition to 8 bars.


The table shows overlapping SOC values because I have a new finding of what appears to be some ‘hysteresis’ in the SOC values compared to the blue bars. When charging the HV battery and rising through the bars, the SOC values show values about 1% higher at each transition point. Similarly, when depleting the battery and dropping down the bars, the values are about 1% lower. So for example, rising from 4 bars to 5 bars takes place at 53% whereas dropping from 5 bars to 4 bars takes place at 52%. The same applies for the next transitions above and below that point, at 55-56% and 49-50%, for 5 to 6 bars and 3 to 4 bars respectively.


This degree of hysteresis was not something I had previously noticed on earlier generations and wondered if others could post their experiences for comparison.


I also had not previously noticed SOC values below 40% on earlier generations, so this behaviour on the Gen4 is interesting, taking into account that I have not yet been able to produce a 1 bar on display as yet. At the other end of the scale, 7 bar and 8 bar displays must represent SOC ranges of 7 – 8% each, if 8 bars is going to represent 80% SOC at the top end, as on earlier Generation models. At this time, it is looking possible that the total usable SOC range on the Gen4 (with NiMH battery) might be set slightly lower than for earlier generations.


Finally, I also had not previously noticed this degree of non-linear representation of SOC on the battery bars display, notably the range covered by 6 bars set at twice that of 4 bars and 5 bars.


Apart from having a more detailed display of the traction battery State of Charge, observing the SOC% allows the driver to better understand and possibly predict the behaviour of the ECU. As with previous generations, it appears there is a programmed preference to keep the battery at or around 60% SOC (6 bars). Above that value and the car will spend more time in EV or EV assist. Below that value and the car will (slowly over a period of time) try to have more time spent with the ICE-on and regenerate, if the driving conditions do not provide enough regenerative braking.


On the Gen4 in Eco Mode, the ECU appears to be much more comfortable allowing the battery to range between 45% and 55% SOC (3 - 5bars) for long periods in dense urban driving at 30 mph (50 kph) and below and will not strive to push it up to 60% until there are more open roads and reaching speeds above 50 mph (80 kph). When falling below 45% in dense urban traffic, the ECU appears tolerate lower SOC than on previous generations, presumably to provide a greater chance of traffic conditions improving to avoid being forced to regenerate with the ICE.


I had one further interesting observation of behaviour in between those two sets of speed ranges (which will need confirmation), regarding a possible change in ECU behaviour on the use of cruise control (DRCC). I was driving on a UK urban ‘A road’, mixed single and dual carriageway, with speed limits of 40 – 50 mph (65 – 80kph), and the battery displays remained stably ranging between 45% and 55% SoC (3 - 5bars), taking the opportunity to regenerate when slowing down and driving EV as much as possible, keeping ICE use to a minimum. This went on for at least 10 minutes. As soon as I activated DRCC, the ECU responded by activating more ICE-on and pushed the SOC to 60% (6 bar) over the next 3 - 5 minutes and where it hovered for the remainder of the journey. It was almost as if the activation of DRCC was interpreted by the ECU to expect the prevailing driving conditions and intentions to be stable, and therefore be permitted it to strive to reach the preferred SOC goal of 60%.