I just received the latest Waypoints e-letter about gel batteries, where you state they must be charged at no more than about 14.1 volts. We do not use gel batteries in our boat, but do in our camper van. As I understand, Trojan deep-cycle gels can be charged up to 14.4 volts and East Penn marine gel batteries up to 14.6 volts, so why are you recommending the lower voltage?
Alcyon, Panda 40
As we described in our recent test of AGM batteries (see PS, May 2015 online), to fully replace consumed amp hours in a marine battery, the charging cycle must go through three stages;
Bulk/Constant Current: This is the charging stage when the battery can accept a high rate of charge and has not yet attained the limiting or absorption voltage.
Absorption/Constant Voltage: This is the stage when the battery has achieved the voltage regulation limit and current tapers off as voltage is held steady and state of charge (SOC) rises.
Float Charging/Constant Voltage: This charging stage is another constant voltage limit though a reduction from absorption voltage. Ideally, your charger would transition from absorption to float at about 98 percent SOC, though chargers are rarely smart enough to do this based on SOC.
Until recently, makers of gel-cell batteries have specified lower charging voltages than those specified for lead-acid batteries. The typical specification, for over 20 years, called for 13.8 to 14.1 volts for absorption charging, and 13.4 to 13.8 volts for float charging. (These numbers varied slightly by manufacturer.)
In recent years, some gel-cell battery manufacturers raised the specified absorption voltages for gel-cell batteries to as much as 14.4 to 14.6 volts. From a marketing perspective, this is great news. It brings gel cells recharge rates in line with competing technologies like absorbed glass mat (AGM); and the new charge rates effectively make them a drop-in replacement for AGMs and most other marine battery types.
East Penn, maker of the extremely popular Deka gel cells (rebranded under various other names), is one of the manufacturers that has raised the voltage. According to East Penn, the change was made after tests indicated that bulk charging at the higher voltage not only accelerates charging, it actually helps the battery last longer. Although the statistical significance of the results was not enough to prompt East Penn to change the 1,000 cycle lab-rated lifespan, it was enough to show that higher charging voltages did not harm the battery in a lab setting.
We remain skeptical. Gel cells have a history of being cooked at voltages above 14.2 volts. Second, we spoke with marine battery distributors, other gel-cell manufacturers, and battery tech experts who still recommend lower charging rates-even for batteries that supposedly won't be harmed by the higher charge rate. Third, weve just seen too many gel batteries end their lives early (five years or sooner) after persistent charging under the higher voltage regimen.
Finally, although Sandia National Labs has published a thorough, independent study demonstrating exceptional gel life at the lower charge rates, we have not yet seen a similar independent test under the new regimen. We routinely defer to the manufacturers expertise on such matters; but here, we would rather err on the side of caution.
If you have gel batteries, we recommend that you limit the absorption charging voltage to 14.1 (or lower, according to the makers specs). Gel batteries can have a remarkably long life charged under the lower voltage guidance (temperature compensated). Our tester has one in a clients boat that is going on 16 years of service. Were not yet convinced that we can get that type of performance at the higher charge rate. Bear in mind that high temperature is what cooks batteries, so a charging system that incorporates a battery temperature-sensor is key to ensuring optimum charging and long battery life.