Charging voltage creeps up. WHY?

[saltwater_gypsy]

My boat has a 24V system and has been laid up for about a year. The batteries are not in top condition but are still useable after charging with shorepower.
When I charge them with the engine, the charging voltage starts at 27.5V and then slowly increases to 31V over a few minutes. I haven't waited to see if it comes down again in case I cook the batteries and get an explosion.
Any previous experience or bright ideas out there ? Thanks.

 

[nathanlee]

It's the initial stage of the charge. That's normal.

It'll then keep the maximum voltage while slowly reducing the input current (absorption stage), then resort to float mode where it'll drop the voltage back down.

What charger are you using?

 

[saltwater_gypsy]

It's the initial stage of the charge. That's normal.
?

Everything looks fairly normal with the Victron charger. Its when I charge with the engine alternator that this voltage increase occurs.

 

[VicS]

31 volts is too high although short period at that sort of level once fully charged will do no harm to ordinary flooded batteries.

You should get your alternator checked, in particular the regulator, but before you do, check the sensing connection if it is a battery sensed one.

 

[Bilgediver]

It's the initial stage of the charge. That's normal.

It'll then keep the maximum voltage while slowly reducing the input current (absorption stage), then resort to float mode where it'll drop the voltage back down.

What charger are you using?

These comments will be correct if he has a posh 3 stage charger, If he has a Halfords special and the batteries are on the way out it could keep climbing.

 

[pampas]

Remove regulator, clean all connections,assemble and try again, could be slight corrosion at the point of termination, well thats what I would try before going any further.

 

[VicMallows]

Looking at the type of boat you have, you will almost certainly have some form of 'smart' regulator and the readings you quote appear absolutely reasonable. Depending on the state of charge of the batteries and size of the battery bank (which is probably large) it could well take several hours before the charge voltage drops back to a more usual float voltage around 28v

Vic

 

[dt4134]

The output from my Sterling alternator to battery charger peaks at 15.5V for a 12V system, so 31V for a 24V system sounds exactly equivalent.

Keep an eye on it when charging. See how hot your batteries get. You should also check the water level as the smart fast chargers do use up water.

 

[saltwater_gypsy]

Thanks for the comments.
I think the problem is corrosion on the battery sensing wire which makes the regulator think it is "looking" at a lower voltage than there actually is.
Full marks to Vic and Pampas.
Cheers