Battery Storage: keep connected or top up?

Question to 12V Guru:

I enjoy reading your column – it’s very informative. I have two 120AH AGM batteries in my caravan. Currently when it’s not in use and stored at home I recharge the batteries, through a Ctek 15amp charger, by running a 240v lead from the house approx once a month.

Two questions:

  • Is it better for the batteries to be continuously connected to a recharging source than topped up once a month?
  • If I were to charge them through my Projecta 25Amp DC/DC charger continuously through a solar panel, what size solar panel should I purchase?

Paul Manton

12V Guru answers:

Thanks for the question Paul, as it is always wise to get this right to ensure that you maximise the life of your investment in the batteries whilst your caravan is in storage. Mismanagement of lead acid batteries during this caravan down time can be a major cause of diminishing their lifetime capacity. It is not a simple matter of “set and forget” during an extended period of storage of between 2-6 months.

Battery Chemistry

Many will argue that simply leaving your batteries connected to a charger that has a “float” mode will keep the batteries fully maintained and ready for use.  This is true to a degree but there is also the possibility of gradual degradation of the batteries if constantly kept in this stage. 

Lead acid batteries contain lead plates that are surrounded by a liquid or electrolyte solution of diluted Sulphuric Acid and as the battery charges and discharges a chemical reaction happens between the sulphuric acid in the electrolyte and the lead plates.  The batteries are designed to be charged and recharged on a regular basis, so having them in a float charge mode at a constant voltage (normally between 12.2-13.4V) can cause the battery to become “dehydrated”.  When a battery enters this dehydrated state, it means that the level of electrolyte recedes below the level of the top of the lead plates.  This would cause irreparable damage to the plates and therefore damaging the battery.

Therefore, in summary, you are currently maintaining your battery to the ideal method.  No doubt you are already doing this but ensure it is just natural battery drain (open circuit) or a noticeably light load to not inadvertently over discharge during the storage of the caravan.

Charging from solar

To clarify in regard to your question on the solar panels – if you are inferring to use these panels during storage the advice above refers to charging from not only AC power but also DC input power such as solar. It is the DC power that is applied through the different charging stages to your batteries which is what is important.

If you were to use solar, the batteries would be going through a mini discharge and recharge every night as the sun disappears (as too consequently does the charging) and the battery will drop voltage overnight, recharging when the sun recommences generating power the following day.  Even during winter, you would receive some charge to the batteries.  However, would still recommend to open circuit your battery and leave for 2-4 weeks at a time before putting the solar panels on. 

Smarter battery storage

In both scenarios you could use an electronic timer for AC or DC power to let the battery drain and only come back on for say 2 days every 3 weeks. This would save having to remember every month.

Overall, I would suggest the preference would be to install 450W of solar panels, space permitting, as this would enable you to charge your panels whilst on the go and enable off grid camping and setup either a DC timer (battery operated) or a simple switch to turn off panels for a few weeks at a time whilst the caravan is in storage. Best of both worlds!

Safe travels,

12V Guru

The article was written for Caravan World Magazine.




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