Battery Monitor Inaccurate % when Using a Propshaft Alternator

[Fuss]

I have a Xantrex XBM battery monitor.
My propshaft alternator needs 3a at 24v for the field coils
My propshaft alternator begins producing power at just under 5 knots
So at around 5 knots the XBM monitor fluctuates between plus amps and negative amps, sometimes every second or so.
If this goes on for a long time then the monitor might increase the % of battery capacity from lets say 70% at the start to 75% by the time I have anchored 8 hours later.
The battery voltage at the time the propshaft alt is switched off is 28.4v
So it seems that the batteries are fully charged but somehow the %age increase was not increased.
I can verify that the batteries are full by switching on the generator and seeing the battery acceptance is down at around 10 amps and the voltage goes quickly to 28.4v.
When charging stabily using the engine alternator or the battery charger then the XBM monitor does increase the %age of battery capacity.
Does anyone have any ideas on this.
I think that Rick designs battery monitors so maybe he would know?
Thanks john

[GordMay]

Originally Posted by john Fussell

... I can verify that the batteries are full by switching on the generator and seeing the battery acceptance is down at around 10 amps and the voltage goes quickly to 28.4v.

... I think that Rick designs battery monitors so maybe he would know?

I wouldn't think that a battery bank accepting 10 Amps of charge is "full".

Rick may have definative advice.

[Fuss]

Originally Posted by GordMay

I wouldn't think that a battery bank accepting 10 Amps of charge is "full".

Rick may have definative advice.

Yes thats true, its not actually full, but still nearly full as the batteries are 24v 700amps.
I also should add, that if i leave the charger on for around half an hour the amps drop to something like 3.
Thanks for the reply

[chris_gee]

The last 20% charge will go in at around 3 amps or so. The ten amps on the generator will still be bringing it up to that float point. The 28.4 v is pretty meaningless as that shows a surface charge which will fall off if the batteries stand for a day or so. Do you know what current your shaft alternator is putting out? It sounds like it is pretty marginal, and one would expect your main alternator to be putting out a sufficiently high current to show up if you are trying to charge 700 amp hrs.

[CharlieJ]

Your prop shaft alternator make be making DC power with a significant amount of AC ripple on it. An oscilloscope would be the best diagnostic tool but a DMM on the AC scale may show you that you have some noise or hash coming out of the alternator.

Hope this helps.