Hybrid 12v (auxiliary) battery charge control

Nobody at the rav4 forum seems to know, so I am hoping that longer experience with the Prius has led someone to wonder, perhaps even to answer:

We know that the Toyota hybrid 12v (auxiliary) battery is charged from the DC-DC converter, and that charging is modulated (somehow) by the current sensed at the auxiliary battery negative terminal, maybe integrated with messages from other sensors.

We can see that the voltage delivered from the DC-DC converter drops at times from 14.2v (bulk charging mode) to 13.6v (absorption / float mode).

Can anyone provide detail on how this change is triggered, eg:
Charging current (A)?
Coulomb counting (Ah)?
Does it count when the car is off?
Does it correct for charging efficiency?
Is it adaptive?
Can it be programmed?
If so, which parameters are programmable?

It matters because the optimal charging profile or algorithm would vary with different battery chemistries, or potentially even with different battery sizes. I don't mind paying for TIS if the answer is there, but the material I have seen quoted from there is very general, which does not help to answer my specific questions.

 

Thanks all. What a shame Toyota tells interested owners so much less than GM.
Still, I guess that creates a market niche for players like LTEC. Thanks Elektroingenieur, I did not know anything about them.
Meeder, I don't think it is that simple; see measurements below.
FuelMiser, true, but no help. My rant is that an owner can set all the relevant parameters is a ‘smart’ battery charger costing $150. Yet the same owner is not permitted to know (let alone adjust) the same parameters in a hybrid car costing $40,000.
Bisco, the LFP replacements are apparently being done 'blind'. Some appear to measure nothing, and do not even seem to think about charging current at low SOC. An LFP battery bms should be a defence of last resort if the protections in the charging system fail. It should never be the first line of defence. A high-voltage "sulfation" treatment (as shown by kevinwhite in the ChevySpark document) would be very bad news for an LFP battery.

Here are a couple of my measurements (sorry not Prius but Toyota hybrid, so they are probably the same):



I do not understand why the drop from 14.2v to 13.6v occurred at a charging current >2.6A with LA, but ~1A with LFP. Could it be Coulomb counting with an algorithm tailored to the inefficient charging of LA? Or some other complexity as mentioned in the ChevySpark document from kevinwhite?

I have repeated the LFP measurements several times. As an aside: the charging current was dangerously high (>1C) at lower LFP battery SOC (I stopped charging from the $40,000 car and used the $150 wall charger that time)! My advice is never to let an LFP 'auxiliary battery' (placed against advice into a Toyota hybrid car) fall below 30% SOC. At least that remains so while we can not customize charging parameters in such cars.

With a fully-charged battery charging from the car dropped from 14.3v @ 0.7A to 13.6v @ 0.3A within 3min after starting.

But to understand if it might vary in a way harmful to an LFP battery, we need to know how the drop is triggered. Hence my original questions.

 

Thanks alanclarkeau. I did find Prof Kelly's email at Weber uni, and I asked him before I started whether he had any knowledge or experience relevant to use of a LFP auxilliary battery in a Toyota hybrid car. I also asked Renogy and Power-Sonic and Toyota (Japan). They all gave me the same answer (no response).

I understand of course that they are all busy, and for liability reasons they are unlikely to disagree with FuelMiser above, or the response received from Toyota USA: “Toyota does not recommend or assist with modifying our vehicles from the original factory specifications.” Fair enough, though I asked about my vehicle not theirs; and such advice ignores the millions of Toyotas out there with non-OEM modifications such as dual-battery systems, or even dual dashcams or battery chargers.

Perhaps compatibility with LFP auxiliary batteries will feature in original factory specifications of Toyota hybrids in the future. In the meantime' any insight into how the charging voltage drop is triggered (or more broadly, detail of how the 12v charging circuit is controlled) will be greatly appreciated.

 

For those interested, I have noticed that the (rav4) hybrid system voltage can drop below 12.7v, and remain below 13.5v for hours while driven with an auxiliary battery that is near-full. When the battery is below full, the system voltage stays at 13.6v-14.2v. So we can assume that Toyota uses some kind of LA-specific algorithm in an attempt to save fuel by reducing DC-DC converter output when the auxiliary battery is near-full (as described above by kevinwhite for the Chevy Spark). As usual, Toyota seems not to provide owners with any technical detail of this fuel-saving control, or whether it can be programmed.

As is, it is probably good for an LFP auxiliary battery (life will be improved without trickle charging above 13.5v), but it creates a problem of how to keep a dashcam running when the dashcam circuit is cut off at 13.00v to avoid the problem of excessive charge current into an LFP battery that falls below this level. A fully-charged LFP battery reaches around 13.5v, but ‘commensal drain’ in a parked rav4 brings it down to below 13.4v before the vehicle enters a ‘deep sleep’ mode with <100mW of power use. After several hours drive with variable system voltage of 12.7v-13.4v, the LFP battery voltage was 13.33v. Yes, system voltage when driving is sometimes lower than auxiliary battery voltage measured when the car is turned off.

At the moment, my best work-around is to set a narrow gap to a restore voltage of 13.20v, so the dashcam is powered ‘most of the time’. The Victron Smart BatteryProtect seems happy with such a small spread, and for 3mW to enable Bluetooth it provides an accurate 2-decimal voltmeter along with status of the protected circuit.

 

I use a Viofo A129 Plus Duo. As far as I know, Toyota sells no dual dashcams. But the model of dashcam is irrelevant to the posts above.

However (changing tack slightly in what may be an incorrect interpretation of your interest), the time a dashcam can be used without battery damage depends on power use and battery features. With all Toyota OEM parts, one would get many hours but not many days of continuous dashcam use while parked (even if Toyota dashcams could be set for continuous dashcam use while parked - from what I can see they only have a post-impact capability, which will not suit many people). Apart from that, they look like very capable, but expensive, dashcams.

The constraints mentioned in posts above accompany use of an LFP auxiliary battery (currently non-Toyota by definition) while exploring potential for greater deep-cycle ability (e.g. longer dashcam use while parked) in hybrid cars. Anyone interested in the full story can see Hybrid Cars.

 

Following up on another possibility from kevinwhite's useful attachment (above): it appears that Toyota hybrid 12v charging control does not have any 'headlamp mode'.

At least in my 2019 rav4 hybrid (with original Toyota LED head and tail lights and halogen fog lights), the low system voltages in automatic 'fuel economy mode' do not change with any position of headlight and fog light switches. So that can not be used as a work-around to keep the system voltage up.

With more testing (and a fully-charged LFP auxiliary battery above 13.3v): when cut-off is set at 13.00v to avoid any subsequent excessive charging current into the LFP battery, even a very narrow gap to a restore voltage of 13.10v leaves the dashcam off most of the time while driving. I guess another work-around might be to add a relay to power the dashcam when the ignition is on, but this also has drawbacks. It could all be solved easily if only Toyota allowed programming of DC-DC converter output voltages (as “customize parameters”).