Equalization and sulfated AGM batteries

Contributing Editor Steve D’Antonio’s recent article on batteries (Battery know-how Issue 135, Jan./Feb. 2004) was excellent, but I was sorry that he didn’t address the issue of battery equalization.

I installed flooded-cell deep-cycle batteries with a total capacity of 1,100 amp-hours on my new Sundeer 60 in 1997. As I knew nothing about batteries at that time, I managed to abuse them terribly, and I had to replace them in 2002. I was seduced into buying the new generation of zero-maintenance AGMs and installed four of them, totaling more than 1,000 amp-hours in capacity. I have been extremely disappointed, as after only six months of cruising, we were getting the equivalent of only 550 amp-hours from them.

The procedure I use to measure the capacity is this: I have a battery monitor attached directly to the house bank to avoid voltage drop, and on it I can monitor amp-hours used and voltage to two decimal places (and amps, too). After a period of prolonged shore-power or engine use, when the batteries are around 90 to 100 percent, I monitor the voltage and amp-hours used on a regular basis, and after maybe a day or two at anchor, I interpolate the amp-hours used when the voltage reaches 12.20 (resting voltage with zero amps drawn for at least 15 minutes). This is the value where my research tells me the batteries are 50 percent full and where recharging becomes necessary. The AGMs after six months produced about 250 amp-hours during this period, which (assuming a 45 percent drop) gives a total capacity of only 550 amp-hours!

The installers had told me not to equalize the AGMs, (which I did at regular intervals with the flooded cells when shore power was available to reach the necessary 15 volts, after which the performance definitely improved). After contacting the manufacturer, who advised me to try equalization, I did attempt it. However, my charger cut out after a few seconds at 15 volts, with an LED signal saying “voltage too high.” Repeated calls to the charger manufacturer from a knowledgeable technician resulted in no help at all.

Another wrinkle with AGMs — with my 275-amp alternator, I can charge the batteries at around 200 amps for only half an hour or so, using a regulator set for AGMs. After that time, the amperage going in drops quickly to around 50, so we have resorted to charging for several short periods a day instead of for extended periods. However, I reckon that that increases the number of available battery cycles and decreases the life of the batteries.

To sum up, I have been extremely disappointed with AGMs, and now wish I’d installed new flooded-cell batteries instead. And I don’t know why I can’t equalize them to try to improve their performance and extend their useful lives. I’d be interested to know if other readers have had any similar experiences.

John Thorpe is based in Colorado and has sailed his Sundeer 60, Sea Fever, more than 26,000 miles. His son and family plan to take the boat on a circumnavigation in 2005.

Steve D’Antonio responds:

I, too, regret excluding a section on equalization, as it is often fraught with misunderstanding by the boating public. Unfortunately, I am limited in the space allotted for any article, and thus, some topics must fall by the wayside.

I’m sorry to hear of the difficulties you’ve had with your new AGM batteries. You shouldn’t, however, feel as if you were seduced into this purchase. Many boats, new and old, are now equipped with AGMs and they do, on the whole, perform very well. In my opinion, AGM batteries possess two strong attributes: They do not require watering or maintenance, and they are capable of accepting a charge quickly, which equates to shorter engine or genset run times. Beyond that, they are not as long-lived or durable as some lead-acid batteries.

Without seeing your electrical or charging systems, it is difficult for me to know where the fault lies in your particular circumstances. Based on your measurements, it does sound as if your batteries have sulfated. In my experience, AGM batteries are particularly susceptible to this phenomenon if they are allowed to go nearly or completely dead, i.e., under about 10.5 volts. In this case, equalization, following the manufacturer’s guidelines, will usually resuscitate them to their original or nearly original capacity.

The difficulty you are facing with your inverter/charger may be a result of an AGM/gel safety lock-out. Some chargers are designed to cap voltage output, even during equalization, if the charger’s profile is set to AGM or Gel. In my experience, proper equalization cannot be performed unless the charger’s profile is set to lead-acid/flooded. This will, however, violate the operating instructions for these chargers. Additionally, AGM equalization is not a set-and-forget procedure, the voltage and battery temperature must be monitored continuously and maintained within the manufacturer’s recommended parameters.

If your inverter-charger will not satisfactorily perform an AGM equalization, you can use a constant-voltage portable charger, or take Sea Fever to a yard that employs one. This charger will enable you to carry out an equalization with controlled voltage output.

The fact that your alternator regulator is dropping to a diminished output so quickly may indicate that your batteries are sulfated and in need of equalization. Some alternator regulators will perform an equalization. You may wish to consult the operating instructions for your model to ascertain whether it possesses this capability.

By Ocean Navigator