Join Date: Aug 2006
Location: Arlington, VA
Thanked 11 Times in 9 Posts
Rep Power: 11
Re: AGM Battery Upgrade and Charger
The recommended charging voltages for all lead-acid batteries -- of which AGMs are just one type -- vary considerably from one manufacturer to another. In part, this is because of slightly different construction practices. In part, it's because the recommendations tend to be very conservative -- sometimes because of liability concerns. And, in part, it's because the recommendations are sometimes developed by sales and marketing personnel and not the engineers who designed the batteries.
Mastervolt recommends a maximum of 14.3 VDC for absorption charging of their 6V AGMs in series configurations. That's a very conservative rating.
Other manufacturers of 6V AGMs (in series to give 12VDC) recommend higher maximum voltages, viz:
Surette AGM 14.9V
Concorde Lifeline 14.6V
In collaboration with a major manufacturer of AGMs and a well-know expert in the field, I've been doing some research on AGM charging for the past few months. We've found that charging AGMs at higher than "recommended" voltages can have several benefits, including cost and longevity.
We have already proven this, and Nigel Calder gave a presentation to that effect at the SSCA Gam in Annapolis just this past weekend. Concorde is right now running a series of tests for us using 14.8VDC absorption voltage and several different maximum charging rates (to simulate what happens with different size chargers and alternators).
By the way, many AGM batteries can take WAY WAY more charging current than Mastervolt specifies. For example, Lifeline AGMs at 50% state of charge (SOC) can take 1.3X their rated 20-hour capacity initially, tapering off quickly thereafter. So, a 100AH AGM which is 50% discharged can take 130 amps initially. If they are more deeply discharged, they can take 5X their rating, i.e., a 100AH battery could accept 500 amps for a short time.
TPPL batteries -- a variant of AGMs -- can take even more charging current that this, reaching truly stupendous levels.
Moreover, these elevated charging currents are good for the batteries! They last longer, and deliver more cycles. In the traction field (forklifts, etc.) a new charging technique called "opportunity charging" has been instituted which involves frequent high-amperage charging whenever possible.
Bottom line for boaters with AGM batteries, however, is to fit large alternators and battery chargers whenever possible and -- most important -- make sure your AGMs receive a full charge frequently. Big chargers and alternators help to cut charging times, but the time required to reach a full charge is still quite long, normally much longer than you'd want to run your engine or generator for. Solar panels, wind generators or shorepower are the only practical solutions to ensure your AGMs reach a true full charge frequently.
Last edited by btrayfors; 09-30-2012 at 09:40 PM.