Being concise ... The setup on the older C&Cs is fairly bad for bulk charging of a larger house battery bank.
1) Wiring The path had the alternator output running the distance back to an ammeter on the instrument panel, then back usually to the starter post, then possibly through a switch and to the battery. The wire gauge is sufficient for topping off a battery at 10 - 15 amps, but considering that accurate bulk charging requires a voltage accuracy at the battery terminals of usually +/- 0.1 volt there is too much voltage drop. Workarounds - rewire and remove the ammeter, upgrade wire size, add a remote sensing regulator. 2) Alternator Most are designed for short bulk charges and then topping off a battery. There is too little airflow to provide proper cooling at sustained high output. Even the belt and bearing load becomes a problem when up sizing. Workarounds - add a second purpose built alternator on a flat belt, use a regulator that has a remote temp sensor for the alternator. 3) Mixed batteries There is no single charging profile that works when you are trying to charge a barely discharged smaller starting battery and a large fully discharged house battery in parallel. Workaround - see solutions below. Note that a Voltage Sensitive Relay or Echo Charger can help a bit but is not a solution. 4) Charge status If your are running a load on the boat and using a smart regulator at the same time the regulator has a difficult time figuring out what is going on. Consider a device that holds the voltage slightly higher for the absorption stage, which is based on a set percentage of house battery capacity in amp hours. Maybe with a 120 amp hour battery the regulator wants to see less than 8 amps ( all fictitious numbers ) before it switches from the absorption stage. If the boat load is 6 amps the regulator is effectively going to stay in the absorption stage too long, which may shorten the battery life. Possible solution I feel that 50 amps continuous from a alternator rated at 100 amps will match my needs and not overload the belt system. So the max bulk charge for me is about 40 amps, which fits a newer AGM 120 amp hour battery. The key to the setup is to use a DC - DC smart charger. This is an example of that genre of product: http://promariner.com/products/marine-dc-to-dc-charging-systems/ What it does is allow a normal alternator to be set at a voltage which is safe long term for topping off a battery. That same voltage would take way too long to fully recharge a large house bank. The DC - DC charger accepts the alternator voltage ( maybe 13.4 volts ) and runs it through an DC - DC inverter to raise the voltage up to 14.4 ( AGM ) 14.6 ( flooded ) and up to 40 amps. This reduces the bulk charging time significantly and is under control of a smart charger. The starting battery will be left at 13.4 volts to be topped off safely and not overcharged. Note - the two battery banks ( or more - you could run two DC -DC charges at the same time if the alternator was up to it ) need to be isolated, ie not on "both" on a battery switch and the house load if any should not be on the battery bank being charged. With the engine running the load could be switched to the starter battery. Even at 40 amps the charger supplies temp sensing inputs for both the battery and alternator. No affiliation with the manufacture, and not trying to recommend a product. Just using it as an example of a smart DC - DC charging system that can be used underway. For more information there is a link to the manual from the same URL. Online pricing: http://www.defender.com/product.jsp?path=-1|328|2289948|2289953&id=764855 At least one solution to look at. Michael Brown Windburn C&C 30-1
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