10,8 volt on aux batterie

If it works fine, I'm wondering how you measured the voltage? That is, did you get a reading off the screen itself? What brand of DMM did you use? Is the DMM calibration current or is it in need of calibration?

Did you measure the voltage directly at the battery? If you measure at the underhood jump point, even a teeny bit of corrosion will give a falsely low reading

For example, if I check the voltage under the hood of my Prius at the jump point, I got 0 the first try. I used an emory cloth to brighten up the jump bolt and a bare bolt nearby. Then I got 13.24 vdc, which is correct as I still had the VDC Battery Minder plugged in

If the battery really was 10.8 vdc it would be unable to power the relays

 

What brand of DMM are you using to check the voltage? Many cheaper brand DMM's have very poor calibration.

Also, check the probes on the DMM. A poor connection on one of the probes can cause a false reading. A low battery can also cause false readings even before the Replace Battery indication is given by the DMM

 

Something is not right then. If the voltage truly was that low, by rights the car shouldn't even boot up

A commonly overlooked trouble point is the teeny wire that goes from the neg post to the unitbody. There is a small bolt that secures the ring terminal to the unitbody

I checked that bolt about a year after I got my 2004 model, when I had my battery out, and the bolt had corrosion on the threads. If the corrosion gets bad enough, that could cause a poor connection

 

The unitbody is the metal shell of the car. I found this photo, but apologies to the person who took it as I've forgotten who it was

This photo was taken from in front of the battery looking rearwards. The red plastic cover over the positive post has been removed. You can see the little ground wire, the one with the rubber band on it. The bolt that secures the ring terminal to the unit body has a tendancy to corrode.

You can't see that corrosion unless you loosen up the bolt to expose the threads. Once you loosen up the bolt, snug it back down. That alone will break the corrosion

If all the driving is very short trips, see if there is any sort of maintenance or "float" charger available in the EU. The one I use is designed to work on 120 v, 60 Hz, and will not work on EU mains

 

At least in the EV circles I've encountered, its highly discouraged to put flooded/wet batteries inside the vehicle. Under various circumstances the battery can vent Hydrogen gas in to the vehicle, and in an accident you have the danger of hot sulphuric acid splashing around.

Rob

 

Yes, the cable with the rubber band on it is the negative cable. Corrosion on the bolt that holds the ring terminal to the unitbody can cause a poor connection.

The Prius 12 vdc battery has a vent tube so if there are H2 gasses produced, they are vented outside the passenger compartment. So hopefully no kaboom.

There is another thread on this forum about a replacement Optima battery having a vent tube provision

 

There is a vent tube on the Prius 12 vdc battery. I had a hard time fiddling with it due to my ape arms

The 12 vdc battery in my Prius clearly has free electrolyte in it. I've had it out a couple of times and if you shake the battery, you can see the electrolyte sloshing around

 

That happened to me once. I was trying to jumpstart a co-workers truck at -36 C. Apparently the battery had froze. I made the last connection - the neg cable - to my vehicle on the alternator frame and BANG his battery exploded

I think I needed new underpants after that ...

So perhaps just the short trips are catching up to that battery. I'm surprised it will even power on with the voltage that low.

Is there any chance of you getting a battery maintenance charger? The ones I recommend only work on North American 120 vac 60 Hz mains, not the usual 230 vac 50 Hz mains in the EU

Whatever charger you do happen to purchase, make sure it is rated for indefinite use, that it is a "float" charger, not a "trickle" charger.

 

No, only a sealed battery equipped with a vent port to hook up to the vent tube. As far as I know, one Optima model - and the standard battery that is in a Mazda Miata - will work in a Prius. The Optima has more capacity than the standard battery

From this information I gather you must park outside all the time? A 2w solar panel may or may not work, but it's essential the solar panel have some sort of regulation built in

My Prius standby power drain is usually around 28 mA steady-state. I'm not sure what the spikes are when the brake boost pump runs.

For those wondering what happens if a Battery Minder is plugged in and the power goes off, they typically draw around 12 mA off the battery

 

This is strange

Oh, a EU market car doesn't have the thermos or the coolant transfer pump. Maybe that lack of extra load is what is keeping the car going?