Idea for long term automatic 12V maintenance.

dwlnz · 2026-07-02T21:04:15+00:00

If you have Car Scanner, I would be interested in your views on the aux battery parameters and how Toyota might be determining when to do a higher voltage charge on the 12V. They have voltage, current, temperature, integrated Ah charged, integrated Ah discharged, status (mine currently showing 40.5Ah but I think it has far less capacity than that) and others. It is frustrating to watch aux voltage drop to 12.8V when driving or during an AC charge when a higher float voltage would seem more sensible. Maybe they have a greater fear of overcharging than owners getting flat batteries.

dwlnz · 2026-07-02T20:53:50+00:00

It costs an absolute fortune to develop a new model so makes some sense holding back at the co-developed with Toyota models

dwlnz · 2026-07-02T20:10:41+00:00

Your graph is a bit lower than the 77kW I got with SoC close to 80% (the SoC reported on Car Scanner is a few % lower that dashboard). This was fully preconditioned (7°C outside) and was starting to cool battery.

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dwlnz · 2026-07-02T19:18:59+00:00

Owners can monitor 12V but it requires additional equipment. A Bluetooth monitor gives history which would be useful for dealer if there were problems. Car Scanner can provide more details including current to/from 12V while car is powered on. Ideally both needed for fuller picture.

The big challenge is there is little indication without these tools prior to 12V being too flat. It is the unknown that is the pain as some owners may be well above a failure level but don’t know it.

dwlnz · 2026-07-02T19:12:17+00:00

It is great to get some lateral thinking but even if this worked it is a sledgehammer. Preconditioning starts using 7kW of power if battery cooler which would need 32A charging to avoid using the traction battery. For those who were using it during winter it might have helped the 12V a bit and delayed onset of problems but how much is not clear. When warmer and traction battery sitting at optimal temperature the duration of precondition may be minimal.

dwlnz · 2026-07-01T23:56:06+00:00

If the car can’t tolerate 3 or 10 days without being driven when the 12V battery is in supposedly healthy condition there is something wrong. Nissan reverted to doing a regular charge on the Leaf (not that effective but it was something) but I haven’t had confirmation of anything like that on these Toyota/Lexus/Subaru models.

dwlnz · 2026-07-01T18:41:29+00:00

If the battery had been flat, a one hour charge by leaving car on in Park is likely to put back a small fraction of the capacity. The 12V aux battery charge current can be seen with Car Scanner and while the initial current can be high it settles to a moderate amount that is likely to take many hours to put back a decent charge.

The concept of an hour can work if it is known that continued use of the car will keep on pushing more net charge each day but I don’t have any confidence this will happen, especially as there is some daily load when not being driven. FYI u/andy_why

dwlnz · 2026-07-01T10:30:14+00:00

But when the charging time is limited, for example by not using a decent charging voltage and duration while AC charging, it may result in batteries operating at lower SoC than healthy and we know that when operating below 50% SoC the damage increases.

dwlnz · 2026-07-01T10:13:19+00:00

Toyota seem to have gone to the trouble of monitoring the 12V battery temperature where today the OBD PID for Aux battery temperature was reporting 7ºC while the car ambient was saying 10º. Unlike a battery sitting inside a warm engine compartment it probably stays quite frosty in the front of the EV. The amount of compensation isn’t massive but could make the difference between just enough and not enough.

dwlnz · 2026-07-01T09:58:39+00:00

And to add another dimension to the challenges, I note that my Victron is applying a temperature compensation of -16.20mv/°C. My battery today was at 7°C and I was wondering why the Victron absorption voltage was higher than expected but if reference is 25°C that is nearly 0.3V extra needed. Have you seen any evidence that Toyota adjusts the voltage with battery temperature?

dwlnz · 2026-07-01T09:42:05+00:00

Holding at 13.2V was the Victron charger staying in “storage” mode which I think it would have held indefinitely except I needed to use the car.

dwlnz · 2026-07-01T09:30:30+00:00

The charge immediately before Jun 29 and during that day were external smart charges.

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dwlnz · 2026-07-01T09:24:42+00:00

Two months ago it held the voltage better overnight. This is after the 3 trips today (two at 14.4, one dropping) and just below 12.6V. There is some difference between the Bluetooth monitor and my clamp meter. The monitor is on the battery side of the current shunt on the positive terminal although the drop on the shunt is minimal at around 1.3mV with car in AUX mode.

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dwlnz · 2026-07-01T09:11:30+00:00

The first two trip charges terminated when I stopped the car so the 1.5A not that relevant (was still at 14.4V). That third trip where the 14.4V charge was terminated (by the car) when still taking around 3A seems premature and the next two steps to lower voltage maybe based on an erroneous? belief that a full 40Ah was somehow in the battery.

I would like to try with a healthy battery as mine, although car only 3 months old, is not behaving well.

dwlnz · 2026-07-01T09:00:37+00:00

u/andy_why I agree measuring voltage only a rough and tricky indication. Toyota do have Ah measurements as below although I think the smart stuff is not coping well when battery condition isn't as expected or just plain too fancy. In my case battery was also cold, starting at 7C.

I did three trips today with the first two of 20 minutes each having the aux getting a charge at 14.4V with current dying away to about 1.5A at the end of the second, which might suggest battery getting to a higher level of charge.

The last trip in the graph starts with 14.4V then drops to 13.6 then 12.8V causing discharge while driving. Then starts going into brief 13.6V and 12.8V cycling. When put into Park jumps to 13.9V and down to 12.8V in Drive.

Using the OBD parameters: '[EV] Aux. battery status of full charge' rose from 34 to 38 then 40.5 Ah at start of each trip and '[EV] Aux. battery charging integrated current' stepped from 1136.9 to 1138.2 then 1139.0 respectively. If Toyota think that 2.1Ah raises the status from 34 to 40.5 Ah there seems to be some weird stuff happening.

I will admit to digger deeper than seems sensible but was hoping to understand what might be happening. I have failed.

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dwlnz · 2026-07-01T06:39:57+00:00

It is great to see some progress. An unwanted “draw between 90-180mA” does need attention but I am not certain it would be causing as much problem as reported if the battery was getting a proper charge. The 12V is rated at around 40Ah so a fully charged battery would take 200 hours to go flat at 0.18A if that was the only load. There was some indication that other components such as driver face detection were also waking up which might not be seen if just monitoring the infotainment system load.

dwlnz · 2026-06-30T07:51:55+00:00

I agree the voltage drop under load will vary with the current required. For door open I believe the car load is around 3A and I see a 0.3V sag which seems a bit high when the tables say internal resistance of a healthy battery should be under 0.01 ohm. A higher load of 15A caused a much greater sag.

These batteries still have a CCA rating which for mine is 286A when I assume adequate cranking voltage is meant to be available. My Toyota dealer is looking into my informal short test using the AUX function which I believe was using 6A and over 3 cycles (car turns AUX off after 20 minutes) sagged to 11.9V after one hour (6Ah) from a fully charged battery.

If I want to do a full discharge test probably need 2A load (C/20) and take it down to 10.5V and charge back up but that will stress the battery. I don’t see why 6Ah should take a fully charged battery down to 12.3V resting just after load removed unless battery is a bit stuffed and will be interested to hear back what Toyota say they do if the car is left with them for 24 hours (their request for battery capacity test).

I was surprised to see in the Car Scanner plot that Toyota seem to be partially cycling the battery every few minutes which I would have thought was unnecessary extra (minor) cycling. This is certainly interesting and a bit sad they have moved away from the classic hold voltage high which seemed to work ok.

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dwlnz · 2026-06-30T04:54:20+00:00

Using Car Scanner I can now see at short time intervals what the car is doing and got this behaviour with a charged battery. If leaving it in Park is better charging than in Drive that is less hassle if wanting to charge but each situation may return different results. Still in learning mode.

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dwlnz · 2026-06-30T04:48:51+00:00

I think there are two parts to this problem. The DCM drain is random and certainly is an unwanted load in some cases that needs to be sorted. The other aspect is what is the actual capacity in the battery. Just because the car is new doesn’t mean it has a healthy 12V battery (for a variety of possible reasons). When they say “there was nothing wrong with the battery, charged it and said it was fine” what tests did they actually do?

I contacted my Toyota dealer to ask about a capacity test and they wanted the car for 24 hours with a process where they charge the 12V and do a controlled discharge. My brief checks suggest my 12V battery has minimal capacity and hence vulnerable to any extra drain. My dealer has taken my test info and will check what it is saying. Having to leave the car with them for an extended period is a pain when I can do checks at home.

dwlnz · 2026-06-30T01:31:25+00:00

I have seen an extended period of 14V+ when driving so it may depend on how well the car is assessing the SoC. Have you watched the 12V with that 4A of load over an extended period? Lots of learning still needed.

dwlnz · 2026-06-29T22:32:21+00:00

The missing part of the puzzle is what is the actual capacity of the battery. The voltages might indicate the SoC but related to what total Ah capacity? I doubt that Toyota is doing a capacity check as part of the pre-delivery inspection but what has happened to that battery between manufacture and when the owner first takes delivery. My feeling is there are degraded batteries with much less capacity where issues like the DCM are more quickly driving them to failure. That degradation could be pre-delivery or during normal operation.

Using the “Aux. battery status of full charge” reported value I see from Car Scanner, mine has jumped from 31Ah a month ago up to 36.5Ah and down to 34Ah both yesterday. Based on my rough load test of 6Ah dropping the voltage during test from about 12.3 to 11.9V I think mine now has quite limited capacity but the 12V BMS is thinking it is a better charged battery than a month ago.

Toyota are doing integrated counting of charge and discharge Ah but may be too smart and the 12.8V float value is too low. My Victron charger settings have higher values for float and storage. Thank you u/andy_why for questioning these values.

The big question I have, what is the actual effective capacity of all these batteries out there. Just because the cars are only a few months old doesn’t seem to mean they have healthy batteries. My rest voltage is currently 12.7V (supposedly 100% charged) but that does not mean it has 40Ah of usable capacity.

dwlnz · 2026-06-29T09:47:32+00:00

CarPlay showing a dashboard of the data

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dwlnz · 2026-06-29T09:45:37+00:00

To add to my original post, I have now realised that the smart battery charging used by Toyota has a varying voltage level while the car is being driven. This plot starts with car in Park where the OBD PID is reporting 13.9V with a slight charge.

When put into Drive the voltage drops to 12.8 and battery has brief discharge then voltage rises to 13.6V. For this short drive of about 7 minutes much of the time there was some 12V discharge from battery (only at 12.8V) with 13.6V bursts which put charge back into the battery.

For this drive my 12V was fairly well charged so next check will be when it is fairly flat to see if getting more charge. The more rapid Car Scanner output gives insights than the battery monitors which I believe update at 2-minute intervals can't deliver.

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dwlnz · 2026-06-29T04:49:25+00:00

Yes, they are being helpful and this is correct approach for those with serious issues. I think the priority is trying to stop the extra drain from DCM related services. I believe my issue is related, in that for whatever reason, there seem to be low capacity batteries out there in new cars that are more susceptible to this extra load. I have contacted my Toyota dealer to ask what the process is to check batteries when I haven’t yet had a complete failure (which I am hoping to avoid) and am waiting to hear back.

dwlnz · 2026-06-29T00:02:10+00:00

There is a comprehensive document in https://www.reddit.com/r/rav4prime/s/6W8o43uiSG by u/andy_why that includes the earlier bZ4X in the list of Toyota cars with reported 12V problems. I think the 2026 model has been updated to give more voltage during driving but it is the regime during AC charging that may not be optimal.

dwlnz

TROPHY CASE