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DougE's avatar
DougE
Explorer
Sep 19, 2013

Part Time AGM Battery Maintainer

I have a Sear Platinum Group 34 AGM battery I use as an auxiliary battery for my portable 12v electric refrigerator. It runs the fridge when parked overnight (or for a couple of days parked) to take that load off of the vehicle battery. In the month or two when it is not being used between trips I have purchased a CTEK Multi US 4.3 Charger battery maintainer. Is it really necessary to run the CTEK 24 hours a day? It would be a simple thing to put the maintainer on a timer and just run it a couple of hours a day.

27 Replies

  • Almot wrote:
    Though I doubt there is ay place in the US that can be that hot for that long.
    Phoenix, AZ averages 90+ days a year over 100F. I would imagine that most of the southwestern, low desert areas do also.
  • When it's not in use camping, our Optima Blue-Top is plugged in to a WalMart Schumacher float charger, under $20.00. Six years of year-round camping trips and very happy - would NEVER go back to weat lead-acid batteries, just too many advantages for our application with an AGM.
  • It's certainly not necessary to have a continuous charge on the AGM. As others have mentioned, self discharge rate is very low.
    I've had an AGM outside on a shelf for over 6 months and, when returned to service, it had almost full charge.
  • Almot wrote:
    My manual says that drop below 60% may result in irreversible loss of capacity.

    A month or two with loads disconnected should not be a problem for AGM. Just make sure you do disconnect them - no fridge running, no radio on standby, nothing. At 68F AGM have very low self-discharge current, typically 3% of initial charge a month. When it's really hot, like 104F day and night, 24/7, then self discharge increases to 10-12% a month so it will take 2 months to drop to 75% - still no need to maintain. Though I doubt there is ay place in the US that can be that hot for that long.


    Interesting! Whom is the manufacturer and what model?
  • Almot's avatar
    Almot
    Explorer III
    My manual says that drop below 60% may result in irreversible loss of capacity.

    A month or two with loads disconnected should not be a problem for AGM. Just make sure you do disconnect them - no fridge running, no radio on standby, nothing. At 68F AGM have very low self-discharge current, typically 3% of initial charge a month. When it's really hot, like 104F day and night, 24/7, then self discharge increases to 10-12% a month so it will take 2 months to drop to 75% - still no need to maintain. Though I doubt there is ay place in the US that can be that hot for that long.
  • Most AGM batteries only loose 1-2% of their charge per month while in storage, unlike wet cells that can loose as much as 20%. There is no need for a maintainer if the battery is fully charged when put in storage and an ocasional check on the state of charge, not allowing it to drop below 50%.
    Allowing any battery to drop below 50% state of charge is inviting sulfation. Much better/easier to prevent it than deal with the result.
  • DougE wrote:
    Is it really necessary to run the CTEK 24 hours a day? It would be a simple thing to put the maintainer on a timer and just run it a couple of hours a day.
    Depends on how smart the Ctek is. If it backs off when the batts are charged, set it and forget it.

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