ASK THE EXPERTS: Battery Queries

Trojan 24-AGM Battery
Trojan 24-AGM battery.
Trojan 24-AGM Battery

The charge controller for my off-grid system is a pulse-width-modulated (PWM) taper-charge CC120E from Heliotrope General. When I open the cover, I find a DIP switch for changing the output voltage, which I assume is the absorption charge. There is no mention of the float-charge setting. 

Trojan’s data sheet for the 24-AGM battery recommends an absorption charge between 14.1 and 14.7 V. I can make this change with the DIP switch. However, the original setting is 14.3 V, well within the recommended range for the AGMs. 

The data sheet also recommends the float charge at 13.5 V. I am not sure how to change the float setting, or if it even is necessary. Also, Trojan recommends limiting the maximum charging rate to 15 A. My PV system will provide as much as 80 A. How do I control this?

I had planned to start with a single AGM battery and if it went well, add additional ones in parallel later. I’ve heard that it’s a bad idea to do this, and I understand the reasoning. But what do I do if one of the original batteries goes bad? Do I then replace all of them? This could be very expensive.

Charlie White • via e-mail

The manufacturer of your specific controller closed its doors almost 12 years ago, so I would strongly recommend buying a modern PWM solar charge controller. They are not that expensive and feature a lot of bells and whistles that would greatly help maintain your battery bank properly. To answer your points:

  • The “absorption” voltage setting of Trojan deep-cycle lead-acid batteries is given as a range because there are various renewable energy (RE) applications they can be used for. Typically, the higher value is used for an adequately sized stand-alone PV-only system, since there is a limited time of solar energy availability.
  • An “absorption” voltage setting for the VRLA AGM cannot be lower than 14.1 V for a 12 V battery bank configuration, otherwise it would not charge the battery properly. This value should be temperature-compensated to account for the battery bank’s temperature, which requires a temperature probe within the battery bank, and connected to the controller.The float-charge is key to the battery’s health, helping to finish the charging. Without it, the battery will eventually face sulfation. Please check the charge controller’s manual to find out if there is an additional DIP switch related to the “float” charge phase. If there is no other DIP switch, perhaps the float-charge default set point is meant to be the same as that of the flooded lead-acid (FLA) batteries. The FLA voltage setting for “float” is 2.20 volts per cell (13.2 V for a 12 V nominal battery bank). For AGM  batteries, it is 2.25 VPC (13.5 V). The lower 13.2 V setting is OK for AGM batteries, provided that the system regularly gets to float voltage for a few hours each day.  Buying a modern PWM controller will put this concern to rest, since the AGM’s set point will address both the “absorption-regulation” and the “float” voltage settings at once.
  • During “bulk” charge, the battery will accept any amount of current from the PV array until reaching the “absorption-regulation” voltage set point. After that, the controller regulates the charging amperage to maintain that voltage.
  • Starting with a single AGM battery and adding additional ones in parallel later is a very bad idea, and will result in damaged batteries. It is best is to size the initial battery bank and PV array adequately, to use a modern PV controller, and then to commission the system with appropriate voltage settings.

John F. DeBoever • Trojan Battery Company

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