Charge auxiliary battery

Due to not using my car during this lat few weeks, the six (+12v) battery went flat. This meant I was not even able to open the car. You need to use the emergency lock to open, then open the boot and put the battery to charge. I did this until the battery had enough juice to open the car as normal. Then I left the car charging over night, on the assumption that the main battery will charge the aux battery. For a couple of days this looked true, however I left the car in the garage for about a week and the problem re-ocurred. Is there any proper way of charging the 12v battery? . How do you maintain it if you are not using your car? On an ICE, all you need to do is start the car every so often to charg it. How does the aux battery get charge normally?

Thanks

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the 12V battery charges when the car is turned on.

if you’re flattening your 12V battery then either you’ve got a fault or you’re not turning the car off properly when you lock it up.

What do you mean i am not turning off correctly the car?
You just push the start up button to off, then you lock the car.
Is there any other way?

I park mine on the drive and use a solar panel (12v) to keep mine topped up.

If you keep your car in the garage then just hook up a battery trickle charger.

That seems a good idea, what solar panel do you have? Do you connect it directly or do you need to disconnect the battery from the car?
I ask, because I have a basic charger and if I start charging with both terminals connected it blows the fuse of the charger- I guess my charger cannot supply the required amperage
Thanks

Get yourself a trickle charger to keep the battery topped up.

You don’t need a trickle charger or anything else, just plug in the granny charger, that charges the 12v battery as well as the main traction battery. If you don’t believe me then just connect an ammeter to the 12v battery lead and then plug in the granny charger, it goes up straight up to +10a charging.

Thanks for the advice
I do have a trickle charger, but its a pain to use it, as i need to disconnect one of the terminals to charge the 12v battery. You are right: If you put the car to charge, it does charge the main and the aux batteries, but the consumption is much higher: between 1.5-3 Kw , depending on the charger used. I think the cheapest and better method , as already suggested, is to use a solar panel charger - specially now with the good weather we are having. I just waiting to receive my one yo test it

You shouldn’t have to disconnect the terminals to top up the 12 V, just don’t try and connect the granny charger too. Does the 12V charge from a standard type 2 charger too?

Good point @mikeRES.

I have solar panel connected using clips, and mounted on the dash.