12V Guru – Battery Storage Part II

12V Guru – Battery Storage Part II

Hi 12V Guru,

I just read that I shouldn’t leave my 2 x 120 AMP AGM batteries continually connected to a smart charger, a CTEK 25 AMP, if I want to maximise the life of the batteries. The article says the batteries need to be worked i.e., used and recharged or when in storage leave fully charged, turn the charger off then top up periodically or as necessary.

Am I doing the wrong thing by leaving the smart charger in maintenance mode permanently when the van is not in use?



12V Guru Says..

Hi Jeff,

Thanks for the follow-up question to a previous article

The advice to “exercise your battery” is general advice due to the wide variety of chargers and combination setups that we see installed in RVs worldwide.  Regularly, after extended periods of charging batteries whilst in storage over winter or similar, people return to their van and find that their battery is swollen. Many chargers promote themselves as smart chargers, however, often the software used to monitor and control the charging algorithm does not consider the temperature of the battery, exact chemistry such as GEL, Wet, AGM etc. and has limited protection mechanisms when there is a failure inside the battery. Charging batteries is done in 3 stages: constant current (boost), constant voltage (absorption) and float charge. Different chargers use a range of different voltage set points through these stages to manage smart charging. 

As such, we recommend every 3-4 months to check and maintain the effective charge state of your batteries.  Better quality systems will have a range of protection mechanisms that can be relied upon to allow a quality battery to remain on float charge indefinitely. The recommendation to not leave your charger permanently connected to the battery depends upon individual circumstances but can encourage a proper inspection of the entire power management setup. Also, when combined with allowing the battery to be “exercised” once or twice over a long winter or storage period.

The Checklist to Battery Storage

Check the capacity of your system – is my battery holding charge?  Maintaining the battery at float charge creates a potentially artificial environment whereby it may appear your battery is fully charged but when the battery drifts below the float voltage and forces the charger to go back into absorption or trickle charge mode at a constant voltage.  There is no way to know if your battery is holding capacity. Remember voltage is only an indicator of your battery’s potential to supply energy. If your battery is already aged, then this will potentially damage it further. When the charger is removed if the voltage on the battery decreases faster than expected it is likely your battery is on the way out.  Better to know this well before you are about to leave on the next trip.

Check system is safe – a visual check to observe corrosion, cable integrity, browning of terminals (increased resistance due to poor connections), battery has not vented and has maintained its structural integrity. If in doubt, check with a suitably qualified auto electrician.

Allowing the battery to be placed under load down to 75-80% of its capacity lets you see the charger or power management systems operating effectively.  If something has malfunctioned, the battery may have been accidentally allowed to enter into a discharged condition, leading to battery sulphation and a shorter life span. 

Temperature also has a big effect on appropriate charging and the capacity of your battery.  Most power management systems and chargers do not allow for temperature compensation.  Some may at the actual charger however don’t measure the temperature of the actual battery.  Batteries heat up during charging and if compensation for this is not allowed for over-charging can occur. Higher temperature increases the aging of a battery. At higher temperatures, it is well proven that the life of a battery is greatly reduced.  Charging and discharging your battery does not help prevent this but something to be aware of as your battery ages.

In summary Jeff, your charger may well be capable of maintaining your battery setup at float charge for an extended period and therefore the advice may not be relevant to your circumstances, however, there is no extended harm to your battery in letting allowing it to drop back in charge level to 75-80% of its capacity to check on the aforementioned and maximize the battery life.

Safe travels,

12V Guru

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1 thought on “12V Guru – Battery Storage Part II”

  1. Hi Guru
    I have a 2017 Jayco Silverline with a J35 model C battery management system and 3 x 100 AH AGM batteries.
    My question is, can change to a lithium battery using the J35.

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