I would try purchasing either a battery charger or jump starter.
When purchasing a battery charger, the most important features to look for are (apart from the voltage which should be 12V for most cars):
- Whether it has a mode to forcibly supply current to a battery at zero volts. A modern intelligent charger can simply refuse to charge something that's at 0 volts. Thus, you may be out of luck. This is the most important feature.
- Is the charger suitable for use in wet environments? Some are "indoor only", it's extremely likely a high-amperage (25A) charger that's very cheap is "indoor only" because it needs forced air cooling with a fan for such a high current done such cheaply.
- Whether it has temperature compensation for charging in very cold or hot conditions
- Whether it has a mode for batteries that requires slightly higher voltage than others (usually batteries are charged with 14.4 V but some batteries like some AGMs might require 14.7 V)
- Whether it has a mode to supply voltage even when the battery is disconnected -- this is useful for maintaining the radio settings when swapping a battery that's dying but not fully dead
- How high maximum current can the charger provide? Note lead-acid batteries in cars are usually around 40-50 amp hours, so 4 amp charger takes half a day to charge. If the only outlet you have is a timed "2 hour" outlet mainly intended for running block heaters, you need to turn the 2 hour clock quite many times to get acceptable charge with a 4 amp charger.
- Are the cables made of good quality rubber as opposed to PVC? (PVC doesn't bend in freezing temperatures well)
Battery charger would be my recommendation if there's an outlet nearby, since rescuing a dead battery could take a lot of time and so much energy that a jump starter doesn't have such high energy amounts in the battery. If no outlet nearby, then you can only try a jump starter and hope for the best.
When purchasing a jump starter, the most important features to look for are:
- The joules rating, how many joules it can provide in 3 seconds. Some jump starters claim high peak currents but are missing the important number, how many joules it provides for a 3-second start. 800 amperes doesn't mean anything if it's maintained only for a fraction of a second.
- Is there a mode to forcibly supply current for a battery that's at 0 volts. This forcible supply mode can also be useful to save radio settings when swapping a battery, if the jump starter doesn't have a short "timeout" to automatically turn off.
- Can you USB charge it? (If it has non-USB charging, you may lose the charger and the jump starter becomes a very expensive brick)
- Is the cable span long enough to use it in your car? Some hybrid cars have the battery in the trunk (and you can't access the trunk with flat battery if it has an electric trunk door), and under the hood there's only a single positive terminal. You may need to find negative grounded object with good ground very close to the positive terminal to be able to use the jump starter.
Note that lead acid batteries don't like deep discharge. If it has been flat for a month, it's likely some life have been stolen away from the battery. If you can rescue it, it's very likely you have something like 10-20% of the capacity left that it used to have. If it has been flat for only a day due to lights forgotten on, it may still have more than 50% of its capacity left.
Also completely flat batteries can have frozen electrolyte (fully charged batteries won't freeze at reasonable temperatures but fully depleted batteries will). Don't try charging a frozen battery.
