2 FM80 but only one will do an absorb charge

[TBsolar]

I am running dual FM80 with about 1.6KW of panels on each. Both chargers will bulk with no problem, when it comes time to absorb one will switch slightly earlier than the other and when the other switches to absorb slightly later it will cause the first one to go to 0 amps for its output. This is causing a problem with fully charging my batteries. Is there a way to make sure that both FM80's output an absorb current during the whole absorb cycle. I have already adjusted the VBATT so that they both read the same voltage most of the time but obviously there is still a bit of a difference. any help would be appreciated.
Cheeers

[raysun]

VBATT is useful for getting the controllers into the same voltage ballpark, but to use multiple chargers with less than 0.1V precision, they must be trained to Olympic Synchronized Swimming Gold Metal standards.
Here's how:
viewtopic.php?t=15400

[TBsolar]

Thanks for the reply, I'll adjust the voltage readings next time I'm at camp.
The major problem is that one of the FM80's outputs no current to my batteries, while the second is working properly, and this is leading to not getting the proper absorb time and issues with my new batteries already.
Will having the battery voltage properly calibrated allow both FM80 to do a normal absorb or is there another issue, perhaps an incompatibility between firmware of the 2 controllers, one is 8 years old the other is 3.
Cheers

[raysun]

Thanks for the reply, I'll adjust the voltage readings next time I'm at camp.
The major problem is that one of the FM80's outputs no current to my batteries, while the second is working properly, and this is leading to not getting the proper absorb time and issues with my new batteries already.
Will having the battery voltage properly calibrated allow both FM80 to do a normal absorb or is there another issue, perhaps an incompatibility between firmware of the 2 controllers, one is 8 years old the other is 3.
Cheers

The controllers are driven by battery voltage. If the two controllers are reading a different voltage, due to miscalibration, then the battery voltage will climb above the Absorb Voltage of the higher-reading charge controller, which will cause that controller to go idle, thinking it has nothing to do.
Exact calibration is essential for both controllers to participate in Constant Voltage (Absorb) charging.

The trim pots that will be adjusted are small. I use a ceramic (non-conductive) straight bladed "jeweler's" screwdriver. Metal shaft ones work OK too, as long as you're steady handed (to not short against the adjacent metal shield.) Be sure to disconnect the PV array prior to calibrating, or do the calibration when the sun is down.

Along with calibrating, check the programming of both controllers to make sure the Battery Charging parameters are identical.