Posted: Sat Aug 25, 2012 4:34pm

hello! I recently had my alternator reconditioned as it wasnt charging well. Got it back and still now charging well! only at maybe 13v with new batteries! 2 strange things happened: 1 is that if i start the engine on 1 (on my 1/2/both ) which is my starter battery, it charges at 14v and i can lower the revs and it stays at 14v...but when i switch it then over to both, the engine lowers its revs and the voltage on goes down to maybe 13ish!?? Another weird thing is that on the back of my alternator I have a small rubber black box connected to the alternator, I looked this up and apparently it doesnt do much. But If i unplug it i get better voltage and if its plugged in!? Im really confussed! all the wiring is the same as when the alternator worked before it broke. I have recently added a nasa battery monitor, I also have a adverc hooked up. any help at all would be great! WHAT IS THIS WITCHCRAFT...

Posted: Sat Aug 25, 2012 6:45pm

1. If the small box is fixed to the back of the alternator and has a single lead connecting to a terminal on the alternator then I think it is a radio suppressor but I have no idea why removing it increases the voltage. 2. Unless correctly set up and regularly calibrated the NASA monitor is likely to be telling lies. It is almost certain it is telling you the batteries are better charged than they actually are. 3. Your Off-1-2-both switch should make before break. If for some reason it is breaking before making or intermittently breaking the circuit then it may have damaged the alternator. I assume that the alternator still has the special Adverc connecting wire fitted. When an alternator's output is approaching the designed maximum it automatically reduces the charging voltage to protect itself from burning out. The voltage can be reduced to about 13 volts but that is rather lower than one often sees. Engine batteries are normally more or less fully charged so the alternator output into the engine battery would be fairly low with the voltage being high. Despite what the monitor may be saying I think your domestic batteries are either very flat or faulty so they demand a high charging current - hence the low voltage. All this is normal. If the batteries are sealed please measure their terminal voltage after standing overnight. If it is less than about 12.2 volts the battery is either flat or faulty. If they are wet open cell batteries get a hydrometer and learn how to read it to assess the individual cell condition and state of charge. I suspect if you ran the engine for several hours the charging voltage would gradually creep up and when it got to about 14 volts the Adverc would start pulsing up to 14.4 volts. However you may be suffering from voltdrop on the charging and battery supply circuits and this may well give a false low reading on any meter that is not connected almost directly to the batteries. Tony Brooks

Posted: Sun Aug 26, 2012 2:19pm

thanks very much for your reply! well i know the batteries are fine because they are new and i read the batteries with a multimeter from the terminals and also from the neg battery to the + alternator and voltage still now, but yes i do have voltage drop but not massive really. the batteries are new, but used so maybe around 12.3 or something, The only thing that has changed is the nasa monitor and I added 2 new batteries. Maybe ill take out the nasa and take out the new batteries and see what happens? My 1/2/both is make before break but could be broken ( maybe that explains the voltage drop when i change it from engine to both when charging?) Thank you so much for your help! Learning sooo much about doing up my boat from here!

Posted: Sun Aug 26, 2012 2:39pm

If you have got hydrometer and multimeter readings why do you not give them to me? I also need to know the conditions under which you did the tests. For instance if your 12.3V at the battery was son after charging for even a short period of time it tells me that you have well discharged batteries. If it was taken after standing overnight then the bank is only half charged and is a far discharged as you should go if you want a long battery life. To be honest this just reinforces my opinion that your domestic battery bank is discharged and simply requires a good charge. Preferably on a mains charger. Buy adding two more batteries you will have increased the charging current the bank will accept. Putting two somewhat discharged batteries with two new ones will result in all the batteries ending up discharged to some extent. As I explained the higher the charging current the lower the charging voltage so that is consistent with your voltage reading and increased battery bank . If you care to give me the hydrometer readings (6 per battery - one per cell) I will see if it tells me something. Do not disconnect anything at present, get a mains charger on the bank OR take the batteries out and charge them individually. I wish I was given a pound for every time I have been told batteries are not the problem because they are new or that they have been tested but no readings are provided to back this assertion up. Until we can rule out a high charging current depressing the charging voltage we can not start to look at other things. Tony Brooks

Posted: Sun Aug 26, 2012 2:42pm

PS - the engine note altering when you switch to the domestic bank simply shows that it has gone on load - what is there to load it if all you did was move the switch. Answer: the alternator output going up for some reason and the most likely reason is flat batteries.

Posted: Mon Aug 27, 2012 11:24am

ahh, i get you, thank you so much for your help!! I normally live on my boat but am back in Dorset ( for the dorset steam fair) for a week. So when im on the boat again ill get a hydrometer and check the batteries. And get them charged fully. what you say makes perfect sense in the fact that it is 14 on starter ( because full) and 13ish on all ( because discharged ) for some reason I was under the impression that batteries always charged at 14v but it was the amps that changed when getting full. thank you so much! Im afraid I have allot to learn!

Posted: Mon Aug 27, 2012 5:06pm

No worries, that is what we are here for. At least you ask rather than bumble on. Just for clarity and mainly for other readers because you seem to have grasped it. Alternators protect themselves when delivering close to maximum output by automatically reducing their charging voltage. This prevents them burning out. The closer to the maximum current the lower the charging voltage. It is only once the battery is well enough charged to reduce the charging current that the voltage regulator starts to clamp the voltage at a pre-determined figure. Typically between 14 and 14.4 volts nowadays by on old systems it could be a s low as 13.8 volts. Tony Brooks