While exploring LFP auxiliary batteries (for greater deep-cycle ability as needed for dashcam use while parked), I have logged the output from some sensors in the 2019 rav4 hybrid. The results vary somewhat from day to day, particularly in the timing and depth of ‘power save’ voltages, for reasons I do not understand, but the mild example shown may interest some forum users. If my interpretation is sound: ‘BATT Voltage’ and ‘Auxiliary Battery Voltage’ seem to be coming from separate sensors. It is possible that ‘BATT Voltage’ is really ‘System Voltage’ (from the DC-DC converter) with some low-side limit from auxiliary battery input to the circuit. If so, the LFP auxiliary battery appears to discharge into the system when system voltage drops below 13.3V; and be charged by system voltages above 13.3V. At low SOC, the LFP charge current can become dangerously high (> 1C) when system voltage jumps to 14.4V. Because system voltage is set to drop below LFP voltage in ‘power save’ mode; this can gradually lower the LFP voltage over a series of ‘power saves’ (which occur in about 1 minute cycles). It is poor implementation by Toyota to let system voltage when driving fall below the open circuit voltage of the auxiliary battery. As detailed by kevinwhite, GM Spark EV got this right back in 2014! The consequence in the rav4 is that there are periodic ‘wild swings’ in current to and from the LFP auxiliary battery as the charging algorithm intervenes to bring it back to full charge, then lets it fall too low again. This is probably not great for LFP battery life. Programming ‘power save’ voltage to match auxiliary battery voltage (or at least stay above 13.3V) would solve all this. Programming maximum charge voltage around 14V might work to keep charge current below 1C (Ohm’s law). If so, an LFP auxiliary battery user could set dashcam cut-off lower (~12.7V rather than 13V) to get the most out of LFP deep-cycle ability while parked, without risking excessive (>1C) charging current. But a car owner can not make these adjustments without help from the manufacturer. Currently this is withheld by Toyota. Sorry, short rant. But any ideas will be gratefully received.
I have run the maths (Ohm's law). Programming maximum charge voltage around 14V would not help much. In fact, no voltage suitable for LFP charging would keep charge current at low SOC below 1C. Then there is the problem that battery resistance (hence charge current unless it is controlled) varies not only with SOC but also with temperature and battery age. So an LFP auxiliary battery user aiming to get the most out of LFP deep-cycle ability while parked (by setting dashcam cut-off to around 12.7V rather than 13V) is still stuck with finding a way to directly control charge current so that it stays below 1C. I don't think this can be done without help from the car manufacturer. It may not be simple even for them, as higher currents are needed at times in the car electrical system (eg when driving on cold, wet nights). Programming ‘power save’ voltage to match auxiliary battery voltage (or at least stay above 13.3V) is still important, and this should be simple for Toyota.
After logging a few long drives, I think those 'wild swings' occur when the car is put in 'park, and/or at the start of the next drive. Noting the correction here because the consequence is about the same, but the fix might be slightly different. I don't know any better way to get sensible improvements onto Toyota radar than for (hopefully multiple) owners / potential buyers to make sensible suggestions. Maybe I am 'Robinson Crusoe' in this. But I don't believe we should we all just roll over and say 'it is all too hard'.
Surely all who admire Toyota hybrid cars (or at least all the wise ones) read (or at least glance at) priuschat (eventually).
Here is another log for those interested in Toyota hybrid auxiliary battery charging control. As for those who seem to think they have all the answers (Charging 12V step change - 14.0 to 13.5 volts. | Page 4 | PriusChat) - I am still frequently amazed, after looking at many data logs.
It would be great to pin this down somehow. What are you using to do the datalogging? Techstream or some other app? Did you have to add the PIDs for these values you are reading and, if so, where did you get them? It looks like a 2019 hybrid RAV4 does have a battery state sensor assembly on the battery, like the 4th gen Prius has. I was recently speculating over here (though that is a thread for Gen 3 Prius, which doesn't have the battery state sensor) that maybe the power management control ECU takes a snapshot of aux battery condition right at power-on time, and that its later converter output adjustments while driving are based on that in some kind of fixed heuristic scheme. Your logs show an interesting other possibility, that during driving, the ECU may tell the converter to lower output to 13.3 once a minute or so, and take another quick glance at how the battery voltage and current hold up, and then return to a charging voltage. I am now curious for a similar datalog on a Gen 1, Gen 2, and Gen 3. All of those predate the battery state sensor, but Gen 1 and Gen 2 do have a remote voltage sense lead that runs all the way to the battery. Gen 3 doesn't, but it does have a temperature sensor in the air above the battery. Clearly they have thought of various different ways over the years to try to control the charging. I'm curious to see if any of the earlier generations do this "lower voltage and peek" every minute thing. I have a home heating furnace that does a similar thing; the blower motor controller every so often cuts power to the spinning blower for an instant and watches how fast the rpm drops, and uses that to calculate how many cfm of air it is moving. It's quick enough you can't tell by listening.
@Mark in NY and @Hybrid Hound, perhaps others, have replaced their batteries with this LiFePO4 one designed for the Prius: 12V Lithium Battery for Toyota Prius | ohmmu.com If they were to log this data we would see what it looked like on a very different battery. Apparently a lot of Tesla users are using the Ohmmu. Presumably someone has already logged such data for a Tesla.
ChapmanF: I am using OBDLink (= OBD Fusion) with an OBD MX+ (Charging 12V step change - 14.0 to 13.5 volts. | Page 3 | PriusChat). It can read Toyota data. I find it more convenient than Techstream. Neither reveals the PID #s (just names). Yes, the Rav4HV has a current (and temperature) sensor on the negative terminal of the 12v battery. Voltage is evidently sensed in multiple places ... but I do not know exactly where. The ''power save' cycle is about 30 seconds. I initially did not notice that the logged data X (time) values were going as categories instead of real times in XL. Since the initial post I have used a workaround (paste special > multiply x1) to help XL see the logged time values as numbers. Like you, I think we could learn more from other people's logs (easier to understand if Toyota revealed the charging control algorithms of course). Primefan: I am using a 12v LIFePO4 battery: a Renogy 50Ah (197L*166W*171H mm, which fits easily in the rav4 battery holder LN2 = 242L*175W*190H mm). The full story is at: Hybrid Cars (scithings.id.au). It would be good to see logs from the OEM lead (calcium) - acid battery, but personally I am not in a hurry to put it back. Maybe someone else can chip in with such data.
@ChapmanF I wanted to thank you for all your patience with me. I’ve now found these and apologize for all the questions which could have been answered there. https://www.offgridhappy.com/Downloads/The%20basics%20-%20your%20on-board%20energy%20system.pdf https://www.offgridhappy.com/Downloads/How_long_does_it_take_to_charge_a_battery.pdf https://www.offgridhappy.com/Downloads/The%20Victron%20EasySolar%20All-In-One%20Solution.pdf
Here is another. I have no idea why it behaved this way. Perhaps as ChapmanF suggests, some kind of snapshot of aux battery condition at power-on time alters charging behaviour in ways we do not understand.
#14 looks almost like a programmed-in occasional balancing cycle. I occasionally caught my Gen 1 doing something similar with the traction battery.
I have no idea what the rav4 ECU is trying to achieve in these instances (given that the charge control algorithms are presumably designed for the Lead-acid OEM battery). The Chevy volt EV info posted by kevinwhite is still my best clue. Increased LFP resistance with rising SOC, perhaps assisted by the Renogy BMS (which does cell balancing but not charge current modulation), seems to take care of it provided that a human keeps an eye on minimum SOC (to keep charge current at or below 1C). But it is too early to say anything about battery life. It happens all too frequently for my liking.
