charging system on a 2004 volvo grader 2- 12 volt bats in series 1@12v 1@15.54

[jimmie]

with new alt. and 2 new batteries it works good for about 2 days then i'm back to boosting again every morning. alt is putting out 27.2 volts yet one battery is at 12.34 and the other is at 15.54 running or shut off. I turn the master off every nite. this is the third set of batteries and the second alt. can anyone help me out

[Ed.Kloonk]

Is there a 12v appliance connected that is 'pulling down' one of the batteries? Are both batteries the same brand and age?

[jimmie]

as far as i can tell there is no draw on the batteries when the master is turned off, the batteries were both replaced as well as the alt. 4 days ago. cables are in good condition bat posts are clean. I don't have to wait for the bat to charge off the truck just hook up the cables up hit the starter, it run right off the hop

[ConKbot]

Are they in series with just a simple link from + to - ? It definitely seems like a load on one battery is drawing it down. If it's a simple link, then obviously this isn't the case, and its as weird as it seems. If there is anything hooked to the mid point, be suspicious of it. Especially since the master switch won't be disconnecting that.

[edpalmer42]

If one battery is 12.34V whether the grader is running or not, it sounds like it's not being charged or discharged.  i.e. it's not connected.  The other battery at 15.54V is being horribly overcharged.  The alternator output of 27.2V seems too low to charge two batteries in series.  I'd expect something over 28V.  Even adding the two battery voltages together gives 27.88 rather than 27.2 so none of your measurements match up with each other.

I don't know your history with this machine, but if it's new to you don't assume that the previous guy knew what he was doing.  Go back to the beginning and trace every wire and connection.  Check them all for tightness and corrosion.  Check the voltage drop across each cable in case one is going bad.  Ask all the dumb questions that are so obvious that they don't need to be asked.  I'm talking about things as dumb as "Did I plug in the power cord?".  Yes, I realize that doesn't apply here, but you get the idea.

Ed

[Gregg]

If the lower voltage battery has a shorted cell, the alternator will over charge the other battery.
If the batteries are not sealed, use an old fashioned hydrometer and check each cell.  Another old trick with flooded batteries that you can access the electrolyte is to dip a lead electrode into the electrolyte of adjacent cells and measure the voltage difference with a digital meter.  Using copper to dip into the electrolyte can cause copper ions to contaminate the electrolyte solution.
Alternately you could try putting a 12 volt charger on the lower voltage battery and attempt to charge it; but be careful not to let it get too hot; a shorted cell will cause the other 5 cells to overcharge and get hot.

[jmelson]

with new alt. and 2 new batteries it works good for about 2 days then i'm back to boosting again every morning. alt is putting out 27.2 volts yet one battery is at 12.34 and the other is at 15.54 running or shut off. I turn the master off every nite. this is the third set of batteries and the second alt. can anyone help me out

27.2 is low for charging.  I'd expect more like 28.4 minimum  up to about 28.8 or so maximum. 

The battery at 12.34 V even when charging is very bad, that battery is either losing charge or hasn't been charged.  I'd get a charger and put it on the 12.34 V battery overnight and see what happens.  After taking it off the charger for a while, if it continues to run down, then that battery needs to be replaced.

But, you need to have the charging voltage up to at least 28.4 to properly charge the 2 batteries.

Jon

[Tom45]

Is the low battery the one connected to ground? If so, I would suspect some 12 volt circuit problem. Is the 24 volts used for anything else besides starting?

[Gregg]

If you have two 12V batteries connected in series, you basically have 12 individual cells in series.
The charging system is only regulating the total voltage across the 12 cells; no cell balancing like many lithium packs have.
The symptom is just like one cell is missing, it wouldn't matter where in a series connected system; the bad cell may even have been faulty from the day of purchase; it just wasn't evident until it was in service.

[Tom45]

The symptom is just like one cell is missing  ...

But he said the batteries and alternator had just been replaced. Either he is really unlucky, or it isn't a matter of a dead cell.

An interesting experiment would be to swap the batteries. Then see if the low voltage (12 volts) battery stays low in its new position, or does it rise to 15 volts and the previous 15 volt battery drops to 12 volts.

[Halcyon]

The other battery at 15.54V is being horribly overcharged.

Measuring 15 volts (or more) is quite normal for batteries which have just recently been connected to a charger particularly during the absorption or equalisation stages. I have a smart charger which does exactly this. After the cycle completes, the voltage will gradually fall slightly and the charger stays in constant voltage mode and floats the battery at 13.5v (or there abouts). That being said, I charge my batteries where the ambient temperature is usually 20 degrees C or below.