Alternator charging behaviour

[Escapade]

Today I had to move the boat from the marina to a nearby boat yard, some 3 miles away, for annual maintenance.
After starting the engine I had an eye on my battery monitor (BM-2 NASA) and noticed that the bar on the right showed 90% charge, while voltage was up to 14.2 V and Amps input some 5A, hence covering the instrumentation and Vhf consumption and giving a slight charge to house battery. All looked normal.
After motoring for about 40 minutes before switching off the engine I had a look again at the battery monitor to check the charge level.
To my surprise the bar was between 60 and 80 hence battery had discharged.
The V where steady at 14.2. Amps input was fixed at 1 Amp irrespective of engine rpm.
What shall I check to verify if the alternator is charging?
Thanks
Oliver

[Craig]

Use multimeter to check voltage at battery terminals.

If batteries have not had charge put in or current out in last hour, reading should be around 12.8 or 12.9 if fully charged. Turn on engine, reading should increase to 13.5 or maybe 14.2 depending on charge state of batteries.

If voltage increases, then alternator putting  in charge.

Craig
"Shirley Valentine"
Gold Coast
Australia

[Jeff Jones]

Hiya, first thing to understand is that battery monitors try count Amp/hr in and Amp/Hr out. They then use an equation based on Voltage and amps to calculate the State of Charge (SOC) of what it thinks is in the battery - they are a indicator...not definitive or accurate.

You had 14.2vdc whilst motoring so the alternator is producing charge voltage, but if the batteries are full then the charge current would be low (as you seen 1amp).

Having the engine running is confusing what is going on.

If you first check the battery voltage with all loads disconnected - see what happens to the battery voltage...what voltage does it stabilise at after say 1hr after running engine or disconnected from shore power..the stable voltage will give an indication of the SOC for your type of battery (You can google a voltage/SOC table). Agm's / Wet Lead acid batteries etc have slightly different SOC voltages for a given % of charge.

If the battery is stable at 12.8v its deemed to be nearly full.

After this if you turn on a known load (deck halogen flood light) to use power whilst checking the current shown on the BM2 is what you expect to see for that load. if you continue to monitor the battery voltage. if it drop quickly you may have a battery issue to check further.
Alternatively you could use battery power for say 1/2day and don't let the battery voltage drop below 12.2v. then turn the engine on again and you should see a relevantly high charge current.

To help further we will need to know the engine type / alternator size, the house battery size and the type of batteries.

Having said the above - seeing the BM2 dropping to 50-60% whilst at battery voltage of 14.2vdc just seem strange..i guess if you turn off all loads after giving the battery a good charge the BM2 would sort its self out.

PS, you can reset and zero the BM2 Amp/hr counter by pressing and holding one of the buttons (sorry cant remember which)

[Escapade]

Thank you for the very detailed information,
I will first measure the battery terminals this morning, as the boat has been hauled out and there has been no charge on the batteries since yesterday.
Further the other checks as suggested and see what happens.
Incidentally the engine is MD 2030 B vintage 1997, alternator 60A.
Cheers
Oliver

[tiger79]

The "state of charge" bar graph on Nasa monitors is notoriously vague, I'd ignore it and keep an eye on volts and amps instead.

[Escapade]

Tiger79 I agree with you . What  surprised me is the bar was consistent with V and A values in the last few months. Hence something may have happened, including a random 6behaviour of the BM-2.

[Escapade]

Just to close this thread,
 I have measured batteries terminals and both showed 12.9-13V at no load.
Applying a constant charge of approx 3A for a few hours was impacting marginally on the house battery state of charge. As expected having a 180A battery.
Even the charge bar level has come back to 100% after motoring back to the marina.
Still I did not understand what caused the false readings a few days back, but I wish to thank you all for the useful suggestions which helped me to double check the real status.
Cheers
Oliver