Alternator, big charge,low charge?

[vic008]

I run the diesel once or twice a week to keep the battery up. Sometimes she charges at 30 or so amp, other times will start and finish at 3-5amps. How do I tell when she will be charging big time or the low amp charge?
Have a Nextstep smart regulator, 300w solar, and NASA meter (volts,amps,soc, amphr)

 

[RichardS]

Is the battery connected to something which is draining it during the period that the engine is not running?

Usually the voltage will give a good indication of battery state of charge which will determine the charging current but the voltage is dependent upon whether the battery is fully rested or not.

Richard

 

[Tranona]

Using the diesel to charge batteries like that is not a good idea as it puts no load on the engine. Batteries in good condition should not need charging as often as that anyway. From what you say your batteries are not fully charged to start with, unless you have something that is draining them when you are not there. 30A on start up is quite normal as the alternator is putting back what was used by the starter. It will quickly drop to a low level if the batteries will still take a charge - that is not fully charged. The last bit of charging takes a long time because of the low acceptance rate. Achieving full charge is difficult just using solar unless you have used a mains charger to get full charge first. Then your solar will be able to deal with the small amount of self discharge. Self discharge on good batteries is low, or very low if you have AGMs. My AGMs only need topping up a couple of times over winter layup.

You have a basic battery monitor, so use it to get some data on how your batteries are performing. Switch everything off and leave for a day then check the voltage - it should be over 13V and there should be a small charge coming in from the solar and this should be the same after a further week if you have not used the boat. If there is any discharge the solar cannot keep up with, this will show up on your monitor as a net discharge so you need to investigate what is drawing power when the isolator is off.

You have posted before on this subject and been given similar advice. Unless your cruising style means heavy 12v usage such as fridge, nav gear, autopilot, running lights for night sailing and minimal use of engine the standard set up such as you have with the addition of solar plus a reasonable size battery bank with batteries in good condition is usually adequate.

 

[TernVI]

I run the diesel once or twice a week to keep the battery up. Sometimes she charges at 30 or so amp, other times will start and finish at 3-5amps. How do I tell when she will be charging big time or the low amp charge?
Have a Nextstep smart regulator, 300w solar, and NASA meter (volts,amps,soc, amphr)

What does the voltmeter say before you run the engine? And while the engine is running?
Are the voltages consistent with the charging currents? I.e. do you get more current when the battery is lower before charging?

IMHO it's good to run an engine once a week or so, but you want to give it some work to do and let it get to proper temperature.
A trip around the bay perhaps.