So what does the charge voltage actually look like with a thermistor protected alt charging a deep cycle house bank? It looks something like this.


This alternator was 80A rated feeding a 400Ah bank. In just 13 minutes the alternator was surpassing 225F (Fluke 289 with temp probe). At that point voltage stopped climbing and became somewhat steady. Once voltage was held steady the amperage output started to decrease slightly thus allowing some cooling of the alternator. About 18 minutes later the voltage rose to just under 13.7V but the alt temp had increased again so it continued to bounce along somewhere between 13.64V and 13.69V. I data logged this voltage for 1.5 hours and it never exceeded 13.81V. Needless to say the batteries were getting chronically under charged.


Voltage is the pressure that allows current to flow into the batteries and without a sufficient charge voltage only so much current will flow into the battery based on its state of charge and terminal voltage. By reducing the terminal voltage you also reduce how much current the battery can accept.


The mooring sailed boat I created this graph from destroyed a set of AGM batteries, using a Hitachi internally regulated alternator, in just 18 months. Ouch!!!!!!!!!