Battery supply

To whom it may concern, I have a Davis Weather station.  The battery went out after a couple of years.  I replaced it with a rechargeable 3 volt battery.  I prechraged the battery. Then installed it.  The battery does well for eighteen hours. The next six hours it is dead until sunrise.  Would someone advise me on the best way in getting the battery to last the last six hours.  I am thinking of like a secondary battery to kick in when the first battery goes down.  Thanks for your help.      Sincerely, Doug Miller.

For starters, what was the original battery, how long did it last etc., and why if it worked can't you use the same system.
Putting batteries in parallel is OK, but they need to be the same type, and it is best to put them both in fully charged so they both contribute to double the ampere hours capacity. If you put a fully charged battery directly across a discharged one you will loose some capacity However, if they have the same charged capacity, it would work since you only need another 6 hours. That being said, if the discharged battery becomes faulty through continued deep discharges, it could over heat and cause some damage eventually, common practice would be to electrically isolate the discharged battery from the fully charged one.
To get your 3V, If you are using something like a series string of Ni-MHi cells, you really do need to electrically isolate the discharged batteries. My feeling is that if you can bump up the battery capacity of the existing setup by 30%, that would be the tidiest way to go about it, it should then run for the full 24 hours.
Cheers,
Richard

Thank you for the reply Richard. I was thinking of adding another battery in parallel. I will try that scenario when I get my first chance . Thanks for bothering your self and messaging me back. Thanks again. Best Regards, Doug

The original battery was actually an alkaline battery. A non rechargeable battery. I did not expect that by a long shot. With it having a solar panel on it, I thought non other than a rechargeable battery. I was wrong. Yes the batteries will be the same. When I first ordered them, they came in a pack of three. So I have two more of the exact kind and voltage . This batteries are replacing the CR 123 battery. Small short fat battery.. Thanks again for your help Richard

There are rechargeable alkaline batteries available, however, the CR123 style cell does include a Li-ion rechargable batery type.
A solar cell implies either a rechargable battery, or a switch over sytem. You need to check that whatever it is is working properly.
Cheers,
Richard

Update Richard, Friday night I took down the weather station. I went to Radio Shack to find comprable holder for a CR123. It came in the way of a single AA battery holder. All I had in my shack equip was D cell. Way too big. So I managed to fit my spare CR123 in the holder and with some modifications got it to put out a wopping 4.125 volts. I then wired it in Parallel with the battery that was in the station. Excited to say for the last two early mornings it has been up and running. Has not missed a beat. Thanks for your help Richard. Have a great week ahead. Your Friend Doug