Hybrid Battery Temperature Control

I have not looked much at the power meter. Be careful about interpreting instantaneous readings: I see many surprises from logging, which is of course also safer when driving. I think TechStream can log, but it is not very convenient and there is a danger in anything that can also send control codes to the car while driving.

It would be interesting to look into the effect of hybrid battery temperature on the ratio of ICE to electric drive power. It may be possible using some of the Toyota PIDs for engine and motor power or torque, but I have not done it. Intuitively, there should be an effect; but it may be a very complex algorithm with many other input parameters.

Just because the ICE runs, it does not mean that the rav4 'kills the use of the HV battery'. Mostly they work together. A rav4 HV is not designed to be the same as any gen Prius, and they may not use the same algorithms. As owners, we do not know any of the algorithms. There will be a gradual change as the traction battery ages. Eventually it will drop below some design parameter and cause an error code. But if you have seen a sudden change in the way your rav4 behaves it could be a sign of a problem ... or it could be something that was before unnoticed. If you have tracked fuel use under similar driving conditions, that could be a clue.

I can not think of anything (other than fan life, maybe battery life if the ICE is off, fuel use, and your time) that would suffer by manually controlling the fan to keep the HV below 41ºC. Of course, these things are also important to some people. The OEM algorithm is their best attempt at a compromise between competing demands. So far I am impressed by it.

I do not know how any owner could convince a Toyota service technician, without logs or codes or some kind of history to show there was a problem. At a guaranteed $210 per service they are not motivated to spend much time with us.

 

According to Shurick (in a gen2Prius forum thread):

"For your question there is an answer from Hybrid Assistant: The same as every hsd: up to 36°C is ok, from 36 to 46 the fan is activated by the car and the recharge/ discharge threshold are progressively decreased. Above 46 the recharge is very very low until disabling of the battery usage."

Available information seems to be anecdotal. EV is supposedly unavailable at temperatures below use in HV. The thresholds probably vary between models (surely they must vary between battery chemistries).

Hybrid battery current (and rate of SoC change) are probably designed to respond to many variables including temperature: it will likely be hard to separate these out. In general I trust the Toyota algorithms in such cases, unless I see evidence of faults there. Nevertheless there are some interesting PID names {including value of the Hv/Ev system indicator (%), under combination gauge}. I aim to collect logs on a future drive.

Personally I would not recommend that you spend more money on Scangauge without further consideration of OBDLink. Possible advantages are ability to display multiple gauges, access many Toyota PIDS, store logs, and use less power in sleep mode. It is secure (requires physical button press before bluetooth pairing) and claimed to auto-sleep and auto-reconnect if you use their software and hardware (this is not my experience: I pull out the adapter and replace it then press 'connect' when I want to display or log, but I have never had to re-pair: YMMV). Price is lower, even if you factor in an old /dedicated Android tablet, which you can mount wherever you like.

 

Okay, I took the gen5 rav4 hybrid for a drive to Lowood (flat) over Mt Glorius (Up) and back to Brisbane (Down), while logging.

Some PIDs were not what I guessed from their names, so I did not plot them. In case anyone is interested the parameters on the left Y axes are Auxiliary Battery Voltage, (System?) BATT Voltage, all Temperatures (ºC), Hybrid Battery Cooling Fan 1 Drive Request, Engine RPM and HV System Indicator (which is named as a %, but clearly is not: the unknown units do not really matter). All the others are on the right Y axes of course, except Hybrid Battery Cooling Fan 1 Drive Status which gets its own labels. Note that some parameters are scaled as specified, to be revealing in the plots.

The point from all this that is relevant to SmellyTofu’s question is that the hybrid battery is indeed ‘shut down’ at times (hybrid battery current near 0 and HV system indicator near -150 in the ‘centre' graphs) but this does not depend on Hybrid Battery Temperature alone. It happened at times on the flat (when the battery was still cool) as well as up- and down-hill.

Personally I am happy to leave the complex algorithm behind the HV battery to the engineers who designed it, but the graphs may help as a reference set for SmellyTofu or others. Here they are:

Flat


Up


Down

 

46ºC on H2 in a gen5 rav4 HV is not a problem (unless you have a blocked fan filter or other fault that stops it cooling from there as designed when driving) . Using a dongle to force the fan higher in order to drop the temperature faster below 46ºC is like pissing in a dark suit ... it may give you a warm feeling, but nobody else will notice.

 

This must be a trick question. OK, I will bite. ‘Anyone’ could turn on the car briefly at intervals and read the HB temperatures.?. I have never seen a problem (to me that would be >50ºC), but if anyone does it would be interesting to hear.

I am not a great fan of the Dr Prius app, but the latest version (5.29) reads everything it offers (including HB current) correctly from the 2019 rav4 HV. The app developer does advertise this capability on Amazon, but for some reason not in the Play Store. For those who are sure that Toyota’s design does not suit them, the app has a capability to override the default fan thresholds. It is not necessary to go bonkers, a slight change is possible; but it is probably necessary to keep the app connected for a permanent effect. Others have hot-wired such an override, but personally I don’t recommend it.

I made a few errors in the post above. The main one is that ‘Hybrid Battery Cooling Fan Drive Status’ should be 4-3-4 not 3-2-3 in the ‘Up’ series. Here are a few added insights from the logs:

In the time ‘between’ graphs (in those above the car was parked in the sun for about 50 min), and depending on previous driving conditions, HB temperature may increase by a few degrees after the car is turned off (which turns the cooling fan off), then gradually drop to cabin temperature. When parked in the sun, cabin air temperature will rise faster, but given enough time HB temperature will equilibrate with it.

I noticed on the MID that 4WD was used often and seamlessly on the Mt Glorious ascent (a good bitumen road suitable for 2WD). Rear wheel drive in the rav4 HV is entirely electric. As can be seen in the graphs, this did reduce HB SOC at times, but the car evidently took opportunities to recharge as they arose. In this model, ‘Hybrid Battery Cooling Fan Drive Status’ seems to be programmed with the aim of keeping HB2 temperature between 40ºC and 46ºC, though even 46.5ºC did not trigger Fan Drive Status 5. Some say this requires >50ºC, which would be a bit hot for my liking.

The rav4 HV is probably not a vehicle for prolonged, low speed, uphill, off-road driving. I have previously tested it in ‘Trail Mode’ (without OBD logging) on a very steep, rough gravel, 4WD-only track and it performed admirably (considering the stock tyres, limited clearance and limited wheel articulation). But it may overheat something if this is kept up for too long (which should trigger a warning on the dash). I vividly recall that coming back down was more hair-raising. The car has no descent control function and no engine braking, even when low gear is selected; so brake early and continuously to descend very slowly and securely. Of course the rav4 is not designed to be a serious ‘off-roader’; it is a ‘soft-roader’ (even if it is marketed as an ‘adventure vehicle’).

 

Hi burrito. I don't know about Prii, but there is no 'hill descent control' switch in a 2019 rav4 HV. Also the e-CVT does not provide any substantial engine braking even in Sport or Normal mode with manually-selected low gear. I tried the usual 4WD advice: "select low range and don't touch the brake pedal". Then just before the car (or was it the mountain) was ready to kill me, I resorted to the brakes. After getting my heart back in my chest and stopping the downhill skidding towards the cliff edge, the remainder of the descent with the brakes applied was fairly civilized.

To avoid any misunderstanding, I should reinforce that these paragraphs relate to very steep and rough gravel tracks. You would not sensibly , and could not physically, drive a Prius there.

On steep bitumen roads, I suspect that these models have similar engine braking capability, though it is accessed differently.

 

Here is another interesting (and unexpected to me) effect. When highway cruising, the car easily keeps the hybrid battery cool, but the interesting thing is that with the aircon set to Auto (in Eco mode) it automatically reduces the cabin temperature while it is cooling the hybrid battery. Maybe you thought 'çlimate control aircon' meant that it controlled constant temperature for passenger comfort; but it is regulating cabin temperature according to feedback from the HB temperature sensor. I promise, nobody touched the aircon controls. The stop in the middle was at roadworks. Now you know why you sometimes feel cooler than at other times on a highway drive. (Contrary to my wife's opinion, it is not just the hot chick beside me).

 

Some RC users comment that voltage drop limits useful current flow from their NiMH batteries. At the appropriate scale, voltage drop is also evident during high current drain from the rav4 NiMH traction battery (down to 220V from nominal 244.8V but peaking at 336V, with HB SOC ranging from 35-80%). In the graph below, results are scaled to be revealing. Everything except current is on the right Y axis. Strangely, hybrid battery current specified as + for discharge, opposite to auxiliary battery current. Assuming the Toyota hybrid current PID is specified at traction battery voltage, the observed range of almost +/- 130A approaches +/- 20C. So the traction battery must be designed very differently from the OEM ‘auxiliary’ battery (or LFP replacement auxiliary battery for those who are adventurous/informed/careful/foolish enough), which rarely exceeds 1C while driving. This graph corresponds to the 'flat' series in post #25.

 

Hiアレン・スピクタロン, I understand that the Prius C has a NIMH traction battery. I agree with your observations that (i) the Toyota algorithm does not activate the traction battery cooling fan until about 37ºC (it is 36ºC on H2 for my rav4), and (ii) the lowest fan status does not provide enough cooling to stop the temperature from rising more around town. Higher fan speeds are activated at higher battery temperatures, and also at higher speeds.

Can you say what temperature you consider too hot for this traction battery, and why you think this temperature will shorten the lifespan of the battery? Without much direct evidence about Toyota’s design, I would personally not like to see the NiMH traction battery above 50ºC; but I have never seen it reach that level.

I don’t think the OBDLink App can increase fan speed (at least not in my rav4), so I guess you use some other App like Dr Prius through the OBDLink LX interface? Personally, I have never found it necessary to intervene (but I would consider it above 50ºC).

 

Bernie, from memory Dr Prius can do it. But I would not go there first. Can you log the relevant OBD PIDs and use the resulting plot to show your local Toyota service people the problem / ask their advice first?

I have never had the problem, but at least one other person (plus SmellyTofu's experience above) has told me they experienced the problem, so maybe it is more widespread.