Car batteries in // with charge controller

[wingnut]

Hi all
Please help me because I feel I am slipping off into insanity.
I have solar panels which charge one car battery through a charge controller which keeps this battery at 13V.
I have a second car battery which is just as new or better which I wanted to trickle charge from the first battery.
So I wired the two batteries up in parallel, with one 1000 ohm resistor between the two + terminals and a conductor linking the -ve terminals.

But here is what is driving me nuts - the voltage on the second battery fell slowly from an original 12.76 V to 12.68 and falling.
How is this possible?

 

[Martaine2005]

Hi Wingnut
Can you put a diode between the batteries? Or will the 0.7v drop be too significant?

Martin

 

[duke37]

Say you have a voltage difference between the batteries and a 1k resistance, that means 1mA of current which will be less than the self discharge of the battery so the voltage drops.

You could connect the two batteries in parallel, depending of what you want.

An inverter to add a couple of volts would enable a regulator to control the voltage to 13V.
In the UK, car batteries are charged between 13.8 and 14.4V. I would go for 14V.

 

[wingnut]

Thanks for the advice. I definitely need to reduce the resistance, perhaps to 70 Ohms or just enough to not blow a schottky diode. For now I am just jumpering both batteries together in // for a few minutes every few days.

 

[GPG]

For now I am just jumpering both batteries together in // for a few minutes every few days.

No problem if they are the same chemistries. Think that yours are flooded lead/acid?

 

[wingnut]

No problem if they are the same chemistries. Think that yours are flooded lead/acid?

Yes they are regular lead/sulphuric acid sealed car batteries, in approximately the same condition. I hesitate to leave them permanently connected together in // (because the weaker one will draw from the stronger one at night, and weaken it too). I suppose charge controllers are meant to be connected to banks of batteries connected in //, so that should not be a problem.