EHU starter battery maintenance voltage — what's normal?

[RoamingHoming]

[related post -

Smart Alternator - Charging Voltage / Driving Behaviour]
​

Question for the forum: on EHU (shore power), what maintenance voltage do you see on your starter battery?

From the attached chart, my van's AC-DC charger — which I believe follows logic set by the "J608 Special Vehicles" control unit — is holding the starter battery at around 12.33V. That seems low to me; I'd expect it to rest somewhere in the 12.5–12.6V range.

I'm investigating this because, after having a new battery fitted and coded (a non-BEM Varta AGM replacement), I'm occasionally seeing low cranking voltage (9.35V) and a laboured start. Trying to work out whether the low EHU maintenance voltage could be a contributing factor.

Would be good to know what others are seeing on their systems for comparison.

 

[WelshGas]

[related post -

Smart Alternator - Charging Voltage / Driving Behaviour]​

​

Question for the forum: on EHU (shore power), what maintenance voltage do you see on your starter battery?

From the attached chart, my van's AC-DC charger — which I believe follows logic set by the "J608 Special Vehicles" control unit — is holding the starter battery at around 12.33V. That seems low to me; I'd expect it to rest somewhere in the 12.5–12.6V range.

I'm investigating this because, after having a new battery fitted and coded (a non-BEM Varta AGM replacement), I'm occasionally seeing low cranking voltage (9.35V) and a laboured start. Trying to work out whether the low EHU maintenance voltage could be a contributing factor.

Would be good to know what others are seeing on their systems for comparison.

View attachment 152147

The Smart Alternator keeps the engine battery at about 80% capacity 12.3/12.4 v unless switched to maximum charge via the max charge button/heated seats/ main headlights or connecting a trailer electrics. This is to allow storage capacity for regenerative braking.

 

[GordonH]

The EHU charger is a bit of a law unto itself and I’ve been unable to find specific documentation around its charge logic, We’ve been travelling in France and Spain over the last month and on hookup at all the campsites. The trace attached shows the starter voltage. Seems to hold in float (>13V) for much of the time when not going through a charge cycle but occasionally allows the starter to drop quite low. I asked Claude for an analysis of the data as its a bit all over the place depending on exactly what the van was doing at the time. Results -

 

[RoamingHoming]

Thanks @GordonH — really useful comparison. Comparing my graph against your numbers, it looks pretty clear these are two different charger behaviours. Yours shows a real float band with occasional drops (maybe from engine cranking?) pulling the average down.

My chart doesn't show that at all. Between engine-start events, my voltage decays from c.12.5-12.6V down to a 12.3V floor over 2-3 days. EHU doesn't look like it's actively floating the starter battery.

Not sure where to go from here, or if I can pull any meaningful data to analyse from VDCS but looks like this is worth looking into!

 

[GordonH]

Is your van pre 2025 model ? Post 2025 had early issues with the hookup charger which were resolved via firmware.

 

[RoamingHoming]

Is your van pre 2025 model ? Post 2025 had early issues with the hookup charger which were resolved via firmware.

Yes, pre '25

 

[GordonH]

As is mine so you would imagine they would show similar profiles. The EHU charger is a dumb system in the sense it’s not connected to the GCs canbus so the charge profile will be simply voltage driven and not related to the measurements taken by the shunt on the negative which the alternator uses.

 

[daniel_thomason]

Is your van pre 2025 model ? Post 2025 had early issues with the hookup charger which were resolved via firmware.

Hi @GordonH Do you know the reference number for the firmware update which fixed your hookup charger?

I've been back from a trip for a couple of weeks now, with the vehicle sat on the drive (on hookup) and the interior display is still showing a drop in main battery capacity (graphic) over time, but the voltage being the same as the leisure battery (at around 13.7v).

I had hoped that the last 'major software update' I had (P3VL) would have fixed this issue, bit it is still occurring. My last fix for this problem was to remove the battery management cable from the main battery, wait 30 seconds and reconnect it.

I wonder whether your update still needs to be applied to my vehicle.

Thanks.

 

[AndrewCook680]

Hi @GordonH Do you know the reference number for the firmware update which fixed your hookup charger?

I've been back from a trip for a couple of weeks now, with the vehicle sat on the drive (on hookup) and the interior display is still showing a drop in main battery capacity (graphic) over time, but the voltage being the same as the leisure battery (at around 13.7v).

I had hoped that the last 'major software update' I had (P3VL) would have fixed this issue, bit it is still occurring. My last fix for this problem was to remove the battery management cable from the main battery, wait 30 seconds and reconnect it.

I wonder whether your update still needs to be applied to my vehicle.

Thanks.

Here is some info @daniel_thomason

Starter battery is not charged via shore power – TPI 2078498/1

In vehicles from model year 2025 onwards, the starter battery may not be charged via the shore power as intended. Error pattern The battery remains without re

www.grandcali.com

 

[GordonH]

Hi @GordonH Do you know the reference number for the firmware update which fixed your hookup charger?

I've been back from a trip for a couple of weeks now, with the vehicle sat on the drive (on hookup) and the interior display is still showing a drop in main battery capacity (graphic) over time, but the voltage being the same as the leisure battery (at around 13.7v).

I had hoped that the last 'major software update' I had (P3VL) would have fixed this issue, bit it is still occurring. My last fix for this problem was to remove the battery management cable from the main battery, wait 30 seconds and reconnect it.

I wonder whether your update still needs to be applied to my vehicle.

Thanks.

See Andrews link above. Im afraid I can’t help any further as I have a 2022 GC and not up to speed on all the MY25 differences.

 

[RoamingHoming]

As is mine so you would imagine they would show similar profiles. The EHU charger is a dumb system in the sense it’s not connected to the GCs canbus so the charge profile will be simply voltage driven and not related to the measurements taken by the shunt on the negative which the alternator uses.

Thanks @GordonH - agreed - behaviour should be the same, which suggest the issue is something local to my van.

I'm going to start by tracing the supply to the starter battery - possible a loose connection, or a fuse (?). The inverter itself seems fine as the leisure battery charges no problem, so hopefully safe assumption is that issue lies on the starter branch following the inverter.

There's a connector right on the output cable near the inverter under the seat, so will start there following voltage drop. Will update once I've had a poke around.

If anyone's got a wiring diagram for the starter battery charge path on the Grand California that'd save me a lot of guessing before I bother VW with it.

 

[GordonH]

The charger under the passenger seat has two connections. The mains one, an IEC60320 C14 power inlet (kettle plug type shape), and a 4 pole connector. The 4 pole is the 12V. The 4 wires are +12V to starter, +12V to leisure, negative & a 12V/0V signal wire that indicates when 230Vac hookup is live. That signal goes to the J608 KFG under the passenger dash. First thing to check is the +12V feed connected to the starter and is it giving out a voltage from the EHU charger. May be worth having a peak inside the charger unit to make sure nothing obvious has come adrift/blown/charred etc.