Originally Posted by skidoonut
also the small solar panel you have on it will likely not have a blocking diode in it. so every night the solar panel is discharging the battery not charging it. it will discharge more than it can charge all day. i have a feeling that the solar panel is your problem. if left unplugged i would turn off the battery disconnect to prevent the propane detector from drawing on the battery. the co detector is run off a 9volt battery.
This is an excellent answer.
Additionally, if the battery is fully charged at the beginning (WILD assumption in this scenario), a fully charged OEM 12 volt battery contains about 70 amps hours of battery life when discharged at a constant 5 amps. That life varies greatly by the actual discharge rate. Less amps; longer life. More amps; shorter life.
The graph below shows a new, fully charged, off the shelf, never discharged 100 amp-hour RATED battery's expected life at various discharge rates.
NOTE: Even though "rated" at 100AH at 20 amps, the ability to deliver that charge changes as the battery voltage drops during use.
So, to sustain a 100 AH total life (capacity), the draw must be about 5 amps. 100 hours is a bit over 4 days. A 70 AH OEM battery would last 70% of 4 days (or 2.8 days) for a fully charged battery.
To estimate a 70 AH battery's curve adjust the left axis to read "percent" of the total new AH of your battery or bank.
Note that LESS than 5 amps continuous draw will give additional battery life, but not a lot more. Getting a week (without your panel) is about "normal" for a fully charged battery; WAY less putting it away with something less than full charge.
For solar panel planning, a blocking diode is critical because anytime the sun is not charging the battery, the panel will consume power from the battery. Cheaper panels (without controllers) do not have them.
Completely discharging a battery (called a complete cycle in deep discharge batteries) damages the plates every time it happens resulting in reduced capacity due to the inability to hold as much charge as when new. Battery manufacturers design the plates based on the number of cycles the battery is planned to have before it needs to be replaced.
See the second attachment (text) to see how depth of discharge effects battery cycles. (and yes Virginia, there IS a difference between manufacturers and it shows up in the battery price).
This link has a wealth of information on this topic.
Battery Life and How To Improve It