More on Hybrid 12V (auxiliary) battery charge control

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.

 

Here is another:

 

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.

 

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.

 

Yet another. Why is the voltage spread (and thus the current spread) so different this time?

 

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.

 

It can be complicated.