Fuel Consumption Q - please help my friend

[RaymondR]

When lead acid batteries are charged by an engine driven alternator the charge current decreases as the battery voltage increases. It might help reduce fuel consumption if your friends pull the batteries down towards the bottom end of the battery operating voltage and only charge them to say half charged more often.

[JPA Cate]

^^^^The risk with the above plan is trashing the batteries.

Ime, and ho, the wind will come again, and the problem will go away. They can lie ahull, if needs be, for 2 or 3 days, and occupy themselves with trying to get the boat to move via sail power. Again, ime, movement in any direction at all at .5k can be heartening, and really doesn't do you much harm if it's not the preferred direction. The wind will come back, and they'll be able to get on course. And, as long as there's water, if food gets short, they'll ration it, and shall have had an adventure, and a learning experience. There's a reason why experienced cruisers provision for 50% more days at sea than the planned passage.

Ann

[RaymondR]

11.7V is flat and 12.6 fully charged so cycling them around about half charged, say 12.1V for the short term is not going to trash them. Being mindful that these are rested voltage. The point is that trying to fully charge them when the voltage from the alternator has wound back is going to waste fuel.

I have used diodes in the voltage sensing line to the regulator to fool it re battery voltage whilst monitoring the battery voltage to charge them faster without damaging them.

[Pizzazz]

Most cruisers accept that charging batteries via the alternator makes sense only up to 80-85% and discharging to 45-50% is healthy, down to 15-20% is not healthy. You will not trash the batteries this way, just reduce their life span in half. Charging with solar or an inverter generator is more flexible and will allow you to go to 100% provided you have sufficient solar and enough sun. For most boats it is not practical if not ugly to install sufficient solar but some people do it.