T&T: 3 Stage Chargers

[capteric36]

 What an interesting discussion!
 Just to throw another point into the thread, I'll 
put in my two cents about chargers.

 Having read the specs of a LOT of 3 stage 
chargers I have found that they do NOT all operate 
the same way. Oh yes they all have three stage 
charging, Bulk, absorption, and float. But there 
are differences.
Some chargers put out maximum current until the 
battery reaches a fixed preset voltage that is 
correct for only one type of battery (flooded lead 
acid). Others let you select the type of battery, 
flooded, gel, or AGM. Many do not have any AGM 
setting (Concord says it is OK to use the flooded 
setting on their AGM batteries). Some chargers 
offer a temperature sensor that modifies the 
voltage settings based on the temperature of the 
battery the sensor is connected to. I would not 
buy a charger that did not offer this, the 
temperature of the battery changes the proper 
voltages enough to KILL a battery that is at 100 
degrees F when the charger assumes 77 degrees F..
 Now after the battery reaches the set voltage the 
charger goes into the absorbtion stage. This 
voltage is the same as the set point for the end 
of the bulk mode, still adjusted by battery 
temperature (we hope). Here is where the biggest 
difference occurs. Some chargers stay in this mode 
until the output current drops to a level around 
10% of the maximum output of the charger. Other 
chargers stay in this mode for a period of TIME, 
sometimes YOU get to set that time period.
 Then the chargers go into float mode. Once again 
the float voltage gets adjusted by the battery 
temperature IF you have a charger with a 
temperature sensor.

 Here is the deal, NEVER connect ANY battery 
charger to batteries that are not located VERY 
close together! The temperature sensor can only be 
attached to ONE BATTERY so it needs to be 
representative of the temperature of ALL the 
batteries in the bank being charged. That is not 
possible when the batteries are physically 
separated.
 There are two good reasons for monitoring the 
battery temp. First if you only charge at the 
appropriate voltage for the type AND temperature 
of your batteries they will last the longest. 
Second the temperature sensor will detect an 
overheating battery and DISCONTINUE CHARGING. This 
helps prevent damaging your batteries, and maybe 
even your BOAT!

 Thus, in my opinion one should buy a separate 
charger for each and every battery/battery bank on 
one's boat. Each charger should be equipped with a 
temperature sensor. The sensor should be placed on 
the battery nearest the middle of the battery bank 
as damage will only occur when the temperature 
rises (charge voltage goes DOWN as the temperature 
rises) and the middle battery is most likely to 
accurately reflect the actual temperature of the 
other batteries in the bank.

 There are some inexpensive 3 stage chargers that 
offer temperature sensing. One brand even has that 
'pulse' feature, the Battery Minder. I use one of 
those (the 2-4-8 model, I think it is called) for 
the AGM battery for my electrically assisted 
bicycle. That battery fell into the S.F. Bay a 
bunch of months ago and the charger restored it to 
full operation after an unknown number of hours 
immersed in salt water. Not bad. For BOTH the 
battery AND the charger!!

 Still I say---NEVER use one charger for more than 
ONE battery or battery bank!

 OK. My rant is over now...

Eric Thompson
S/V Procrastinator
South San Francisco
capteric36 at sbcglobal.net