Hybrid vehicle has not been used for several months due to COVID-19 pandemic. Vehicle has a 3 year old 12V AGM battery. Battery is not used as a true "starter" battery; it just boots the vehicle's computers so the hybrid batteries can start the ICE.

Vehicle is not located near any electrical outlets.

The 12V battery was not cheap, and I would prefer to continue to use it for cost and environmental reasons. The battery might still be under warranty.

With all that in mind, which option is best to get the vehicle up and running again?

  1. Buy a battery maintainer. Pull the battery from the vehicle and charge it for 1-3 days somewhere safe using the battery maintainer.

  2. Buy a portable jump starter. Jump the battery to get the vehicle's computers running, and start the vehicle's ICE. Drive around for an hour or so.

  3. Buy a new battery and recycle the current one.

Note that #2 is the easiest (no pulling a battery required) and that #3 is much more expensive than #1 or #2.

2 Answers 2


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):

  1. 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.
  2. 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.
  3. Whether it has temperature compensation for charging in very cold or hot conditions
  4. 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)
  5. 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
  6. 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.
  7. 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:

  1. 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.
  2. 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.
  3. 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)
  4. 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.


You will need an actual battery charger. Most battery maintainers will not charge a completely discharged battery.

  • Thank you. I wonder if I have my terminology wrong, or have read incorrect marketing materials. The "battery maintainers" I'm finding seem to be more like "battery chargers and maintainers". Can you provide a couple examples of what you think will work? Oct 2, 2020 at 22:35
  • I can't vouch for the quality of this device, the link is provided for reference only, however this is the type of device you will need. amazon.com/BLACK-DECKER-BC15BD-Battery-Alternator/dp/B00KNMKRU8/…
    – EᑎOT
    Oct 5, 2020 at 14:12
  • Thank you for the reference. Feedback: (1) I appreciate the reference! (2) I forgot that SE sneakily sends all clicks to Amazon to another website (likely to make more money) (3) if others want to see the product directly, just go to Amazon and search for 'B00KNMKRU8' (4) the review entitled 'Not good for agm batteries' is very concerning - but as you said, you just provided this as a reference, and nothing more! (5) do they make something similar that does not need to be plugged in to AC? Oct 5, 2020 at 22:23
  • I think this is incorrect. Some actual battery chargers may refuse to charge a completely discharged battery, some jump starters may refuse to jump a car with a completely discharged battery (zero volts), but some battery maintainers, especially cheap ones, WILL happily charge a completely discharged battery, but it could take a week to get the battery completely full if the maintainer is intended for only maintaining the battery charge.
    – juhist
    Dec 10, 2022 at 13:58

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