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[Shop-talk] A tale of two battery charger/maintainers

Subject: [Shop-talk] A tale of two battery charger/maintainers
From: Jimmie Mayfield <>
Date: Fri, 19 Mar 2021 17:48:18 -0400
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I have a pair of 12V battery charger/maintainers/desulfators:  a Battery 
Minder 12248 (2/4/8A) that I've had for maybe 15 years and a Ctek 
MXS-5.0 (~4A) that I bought last year so that I could keep both of our 
vehicles charged during last year's COVID lockdowns. Both are 
temperature-compensating (lower temps = higher charge voltage).

The Battery Minder's charge profile seems to be this: Bulk-charge at the 
preset current until voltages reaches around 14.5V.  Then it holds the 
voltage constant and continues charging until current drops to around 
10% of the initial current (so 0.4A if you started at 4A).  Then the 
charger switches to float mode at around 13.6V and the current tends to 
oscillate between 0.17A and 0.23A.

The Ctek is similar except that when it switches to float charge, its 
float voltage is 13.1V and 0.2A for the first 24 hours then it switches 
to around 13.6V, also around 0.2A (rock steady, no oscillation).

Both chargers seem to work based on my meters but on a whim I decided to 
try both chargers head-to-head and discovered that the battery behavior 
after disconnecting the charger is different.

My test involved connecting a charger to my truck battery until it 
switched to 13.6V float mode and then staying on float for 12-18 hours.  
Disconnecting the charger and driving 30 minutes including one engine 
stop/start along the way.  When I got home, I'd check the battery 
voltage with my meter.  Over the past couple months, I've repeated the 
test 4 times with each charger.  (Since the Ctek charger floats at 13.1V 
for the first 24 hours, this meant that I had to let the truck sit for a 
day and a half in order for it to float at 13.6V for half a day.)

For times when Battery Minder was used, the Vbat upon returning home 
ranged between 13.2 - 13.5V.  For the Ctek, the Vbat was never above 
13.1V and twice it was 12.7V.  It's worth noting at this point that when 
driving my truck's alternator varies between 13.2V and 13.8V according 
to my ODB2 scanner.  So it's not surprising that the battery would show 
 > 13V immediately after cutting the engine.

This behavior seems to be very repeatable.

If the alternator was going flaky, I wouldn't expect the weirdness to be 
limited to when the Ctek charger was used.  The main difference between 
the charging profiles is the Ctek floats at 13.1V for 24 hours before 
switching to 13.6V.  Since the battery floated at 13.6V for at least 
half a day using either charger, though, I'm a little surprised it made 
a difference.  But apparently so?

My question is: Why?  What's going on here?


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