In a message dated 11/7/03 9:36:56 PM, [EMAIL PROTECTED] writes: >re there any electrical engineers out there listening:
I am not an Electrical Engineer, but I will speak to what I know: >I was told today,at an large alternate energy store that if you use normal >gel cell, also called AGM, or standard deep cycle liquid acid batteries >between 85 amps and 150 amps (size) you do not need a charge control on >the solar panels if the solar panels are the low voltage type at 16.8 volts >closed circuit or less. First of all, several have confused Gel cell type batteries with AGM batteries. They are not the same, and should not be lumped together for purposes of charging. While both are of the sealed, recombinant variety, there is a very important difference. Gel Batteries employ a viscous thixotropic gel between the positive and negative plates. AGM's on the other hand, employ an absorbent glass mat between the positive and negative plates. This mat material is saturated with a small amount of liquid (not gel) electrolyte. Why does this matter? Well, the Gel Cells are extremely sensitive to overcharging VOLTAGES. As a general rule, any sustained charging above 14.1 volts can cause the forming of permanent air bubbles or voids in the gel electrolyte. The plate areas in contact with these bubbles/voids become 'dead-zones', which is to say that they become permanently useless portions of the battery. It does not take long to lose significant quantities of battery capacity as a result of overvoltage charging. An AGM (Absorbed Glass Mat) battery, on the other hand is much more tolerant of overcharging because any bubbles formed in the liquid electrolyte will be replaced by flowing liquid electrolyte. That is not to say that you cannot overcharge an AGM--you can--but they are typically tolerant of much higher voltages, and for much longer periods. It used to be that Gel's had an appreciable capacity difference over the early generation AGM's. The gap is closing, and many are now touting the AGM as the ultimate battery. The AGM has lower internal resistance, enjoys a very low self-discharge rate, and tolerates deep discharges. I have switched to AGM's exclusively in my cars/trucks, boats, and in PV systems. By the way, someone called the Optima spiral-type batteries a Gel Cell, but it is technically an AGM. As for unregulated charging, the rule I constantly hear tossed around is the 2% rule--that is, an unregulated panel should not produce more than 2% of the battery AH capacity. For purposes of my M15 with its group U-1 AGM batteries (32AH each) should not be charged at more than .04A. My flexible Unisolar USF-11 panel (fits nicely on the sliding companionway hatch) is rated at 10.3W/0.85A @12V, and as such should probably be regulated. On the other hand, if panel voltage stays at or below 14.3 volts (for an AGM, not GEL), I am told I should be OK without regulation. I will defer to the engineers on this latter point. Scott, M15 #478 'bebe' _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats
