Dick wrote: "My question arose because I came across information last year that seemed to explain why my inverter/charger would charge hard for a while after a full day of cruising and then plugging into shore power. (Comment 1) I have a 100 amp alternator that jumps up to 60 amps immediately after starting and my ammeter generally shows 10-20 amps while cruising but my batteries were obviously not fully charged at the end of the day. (Comment 2) The answer seemed to be as marine consultant Steve D'Antonio puts it: "stock alternators are" internally regulated and therefore not equipped to deliver the multi-stage charge required by deep cycle house batteries. (Comment 3) Second, with few exceptions they are not designed to deliver their full output except for short periods of time. (Comment 4) When called upon to do so, by retrofitting one of these alternators with an external regulator, "they often expire prematurely as a result of overheating." (Comment 5) Comment 1: That's probably normal operation, even for a fully charged battery. When first powered up, the charger goes into BULK mode long enough to figure out what the battery state-of-charge actually is, then falls into float. It would be normal for the charger to stay in BULK for 30 minutes or so. Comment 2: I always recommend a Coulomb Counting battery monitor, properly configured for the amp hour capacity of the battery/battery bank being monitored. A battery monitor is the only reliable way to know if the batteries are fully charged when there are questions like this. Comment 3 & Comment 4: There are two different concepts here that Steve is talking about. One is, OEM internally-regulated voltage regulators do not generally provide three-stage charging. That is true, but not a problem. That's how million of car charging systems work. The alternator puts out a steady-state 14.6V, which definitely charges the batteries. The second concept is, how much power is demanded from the alternator and over how long a period of time? Starting an engine takes very little power, and is easily replaced by the alternator, so the alternator is almost never operating anywhere near its full rated capacity. Car batteries are start-service batteries that charge quickly because the charge they need is tiny. Car alternators are in the air stream of the car, so they are "super-cooled" even when demand is high. Most car alternator power goes to running the electrical components of the car. Deep cycle batteries require full output from the alternator for a very prolonged period of time, so the alternator very quickly overheats. There is no free flow of cool air, so cooling the windings is a significant engineering challenge. When the alternator gets hot, the VR cuts back the output to reduce the heat dissipation load to save the machine from self-damaging overheating. In most cases, an OEM single-stage alternator will never cool off enough to get back into full load criteria. Comment 5: This depends on the voltage regulator. The Balmar regulators monitor alternator case temperature via a thermistor mechanically bolted to the alternator. As the temperature of the frame rises, the VR reduces the field drive to keep that case temperature under the criteria (103ºC). Dumb internal VRs can't/don't have that level of sophistication. So, it's quite common for small frame alternators to "burn out" due to self-damaging overheating. Hope this helps, Jim Peg and Jim Healy, living aboard Sanctuary http://gilwellbear.wordpress.com Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436