Piston Slap: Tender loving care for battery tenders?


Max writes:

I have two cars that I don’t drive very often (a BMW and a Porsche). Now that the weather has turned cooler, I encountered some battery issues. I purchased two battery tenders so I can avoid problems in the future.

Is there anything I need to know or be careful about as I use these tenders?

Sajeev answers:

Kudos to you for buying tenders for your two Germanic toys; they will earn their keep in a matter of weeks. Or months. Or immediately, because they provide peace of mind that is unquantifiable. But I digress …

I’ve seen a fair bit of expected and unexpected fail points with these charging apparatus (apparatuses?) over the years. The following issues are not specific to an individual brand of battery, battery tender, or vehicle. These are just the messes I’ve seen over time with a fleet of collectible vehicles. So, without any further ado, be aware of these potential problems:

  1. The battery stops charging or won’t charge at all (usually pertaining to discharged AGM cells)
  2. Tenders don’t solidly attach to side-post battery terminals, especially when the cables are disconnected from the battery. (Which is necessary when there is excessive battery draw from a misbehaving vehicle.)
  3. Tenders with big clamps aren’t terribly thrilled to clip onto battery terminals in tight spaces (a la C4 Corvette).
  4. The battery explodes days/weeks/months into tendering, probably due to age and low water level.

This list isn’t intended to dissuade anyone from using battery tenders, so below are my resolutions for the above problems.

  1. Have a multimeter handy in your tool box and check the battery voltage periodically. Car batteries needs to be above 12 volts to assume the tender is doing a good job.
  2. Wiggle the charging cables to ensure the tender has a good grip on the battery. Perform step #1 if you have any doubts.
  3. Remove the battery to charge on a workbench, or perform #2 on a regular basis.
  4. This is unlikely with a quality battery, a “smart” battery tender, periodic monitoring of the water level, and keeping it topped off with pure water (not tap water).

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