[Shop-talk] Float chargers

[DAVID MASSEY]

 The way these multi-phase chargers work is they charge at constant-current (Bulk Mode) until a voltage threshold (~14.25V) is reached at which point it switches to constant voltage (Absorption Mode) until the current drops below a threshold where it switches to Float Mode.  The data sheet doesn't talk much about this (or at all) but it is possible that this charger is calibrated for a 40 A-H battery more typical of what is found in private vehicles.  It is possible that the absorption current on a much larger battery (100 A-H) never gets below this threshold so the charger never switches to Float.  

But this is just speculation.
OTOH, I had a 5-year old Optima in my TR6 that displayed the same symptom.  But on my car I can connect the charger in such a manner that I can monitor the charge current on the in-dash ammeter which never dropped below a few amps.  In fact I first noticed the problem while driving as the ammeter was centered indicating zero amps early in the drive but when approaching the house the current was definitely in showing a few amps of charge which is not at all typical.  

I took it to the shop specializing in batteries and they rolled out their tester and pronounced it "good."  In my estimation, their test is not comprehensive and only checked for open and/or shorted cells and for current capacity.  They were more than happy to take my money when I insisted that the battery was not going to last and I was happy with replacing the battery since it was at least five years old and had been abused (completely discharged at least once).

So, I guess the short answer is what is the current into the battery when the charger hangs in the Absorption Mode?
 
Dave 

 
 
-----Original Message-----
From: Jimmie Mayfield via Shop-talk <shop-talk at autox.team.net>
To: shop-talk <shop-talk at autox.team.net>
Sent: Tue, Apr 7, 2020 9:34 am
Subject: [Shop-talk] Float chargers

The recent discussion about solar-powered float chargers has reminded me 
of an issue that I've yet to understand or resolve...

I have a Battery Minder 12248 (12V 2/4/8A) charger/de-sulfator with 
temperature compensation sensor.  While I've never been able to 
appreciably restore a dead UPS battery using the de-sulfation mode, it 
seems to work fine as a slow charger.  It has several modes -- gel-cell, 
AGM or flooded -- which, near as I can tell, adjusts the threshold 
voltage at which the charger switches from bulk-charge mode to 
float-charge/de-sulfation mode.

Every 12V battery that I've ever connected to the charger, even the old 
AGM batteries taken out of my UPS units, eventually reached the 
threshold voltage and switched over to float mode (usually within 30-45 
minutes for a nearly-fully-charged battery).  Except one.

About 2 years ago I installed an Interstate battery from Costco (size 
27F, 890CA, 710CCA) in my truck.  It's a flooded battery and if I 
understand Interstate's spec correctly, its capacity works out to around 
100Ah.  The battery has a resting voltage of 12.6V so it doesn't appear 
to have any shorted cells and has no trouble starting my truck.  So it 
seems to be a normally healthy battery, right?

Thing is...I've never seen my charger go into float mode when connected 
to this battery even when it was nearly new.  With the recent pandemic 
stay-at-home orders, yesterday I decided to connect my truck to the 
charger to stave off parasitic drain (measured to be 20-30mA) that may 
have accumulated over the past week of inactivity.  I'm currently at 16 
hours (@4A) and it still seems to be in bulk-charging mode.  I have 
difficulty believing that the battery needed 50+ Ah.

So what's going on here?  This is the only battery that I've come across 
that behaves like this.


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