battery charging question

[maulbeagle]

Hi,
a question for you battery charging experts.
on the weekend, I hooked up my projecta IC1500 smart charger to the main battery (I have a 2011 D22 with factory twin batteries plus isolator).
the isolator joined the batteries, and they charged for some time (an hour or more). after this time they were still showing charging in absorption/equalization mode, so I got curious.
did some testing, if I isolate the batteries, and charge either one individually, they pretty much stop charging straight away and show fully charged.
however, when joined together, the charger stays in absorption/equalization mode.

what would cause this ?

thanks

 

[sparra04]

at a guess
voltage drop/absorption across relay and associated wiring

 

[Old.Tony]

When you join two batteries together there are a couple of things happening.

First, there's a voltage drop across the cable between the two batteries. A pair of identical batteries fully charged and joined by cable together will see a lower voltage on the other end of the cable, a valid electrical circuit, and will feed power to the other end of the cable. The amount of difference is very small, but the two batteries will feed each other and because there's always a small amount of loss in the cables, they will eventually flatten each other (closely matched batteries will take a while to do this).

Placing a load on a battery - even in the above "identical" scenario - causes the battery's apparent voltage (often referred to as "surface voltage") to drop as it starts to deliver power. Grab a fully charged battery, connect a multimeter to it and read the voltage. Now connect a halogen driving light to it, note the voltage, then disconnect the light - the voltage will return to almost the previous voltage.

Now, when you connect to dissimilar batteries it gets interesting, complex and something to stay clear of. A smaller capacity battery will show a higher surface charge before a larger battery when charging - so a small and large battery parallel-charged will see the smaller one charge first and present the higher voltage to the charger - which will then stop charging, leaving the large battery wanting for more. Disconnect the charger, and the smaller battery will feed the larger one at a fairly high rate until the voltage fairly much equalises, with both batteries ending up far from fully charged.

The same thing happens when they're discharging. Smaller batteries lose surface voltage faster, so the larger battery will start feeding it (and yes the "load" reduces the surface voltage of the larger battery) - the smaller battery's surface voltage will rise and the two batteries will switch roles. Basically they'll eat each other - and if they were blondes you might not mind, but if they're keeping the beer cold ... time to get upset.

Leave the two batteries on the charger for some time and the charger should be able to get the apparent voltage up enough to consider them charged, but it's not an ideal situation. I have 3 auxiliary batteries in my combined rig and all three have their own individual chargers - to avoid any of those problems firstly, but also because the three batteries I have are so vastly different that the eating problem would be VERY evident (one is a flooded-cell 50Ah deep cycle, one is a Century 100Ah Gel and the last is a D31A Optima spiral-wound AGM).