I have a car battery hooked up to an alternator designed for a 14V system and would like to lower the voltage a bit.
Does anyone here know how to reduce the output of the alternator from about 15,5 to 13,8 Volt without having to lower the rpm?
You dont. I assume its a wet lead acid cell battery?
They want charging at about 14.4 - 14.8 & even up to 15+ occasionaly to stop sulphation & to equalise the cells. Also the battery will hold the voltage down whilst charging till the battery is nearly fully charged then the alt reg will control the volts.
14.7 is normal, 15.5 indicates a battery pulling in a heavy charge, possibly after a heavy load from starting the engine.
I take it you are using a calibrated meter such as a Fluke and not a £10 special from some bargin shop? Cheapy meters are a no no for this type of adjustment as they give an indication only, not a good accurate measurement.
It is a wet lead acid battery indeed, but in the Netherlands they recommend charging between 13,8V and 14,1V. I heard that you run the risk of boiling the battery when charging at 14,4V or more. I measured 15,5 at the battery with the engine running idle, which is over the safety limit they recommend here and I'd hate to see it explode or whatever it may do :-(
15.5 is high. But what state is the battery in? This will affect the charging.
'Charging' at 13.8 is not charging. You have to have a level above the batteries open circuit voltage. If you have two voltages the same there is no current flow.
I have an old XS650 with adjustable regulator, plus i used to set the charging on light aircraft. 14.5 is normal, any lower and you run a real risk of discharging the battery. As I say you need a good meter to do these adjustments as its easy to fry the battery
Don't confuse service charging with what the vehicle's system is doing, the vehicle will place various loads on the system so charges at a higher level, a battery charger, if it is the constant voltage type will sit at a lower value.
What is the voltage at the rpm you drive at ?
The temperature is also part of the equation. http://www.powerstream.com/SLA.htm
Some alternators put out a decent voltage at idle, but go high voltage at highway rpm.
I replaced a regulator that put out 12.8 at idle and 17 at highway speed.
The battery got hot, smelled like rotten eggs and boiled the fluid out.
Robert
13.8 will charge fine. Many alternators will drop down to about 13.2 after 20 minutes as to not boil the battery dry. 14.4 will meet the max recommended current for charging, but will boil dry if left in that state.
I built chargers for my bikes and mower that maintain 13.2v. I can keep them on the charger all winter with no problems.
BTW, the only way to change the output is to replace the regulator.
Some rectifiers had a trim pot, not many these days.
I would say if their is no charging fault the meter is off.
At 14.40 volts the battery will start to gas, so if its losing alot of electrolite there is a problem.
If you ran at 13,2 volts you'd have a flat battery in no time. Been there and done it. Thought by trimming the regulator back it would give the battery and generator an easy life. Set it to 13.8 and the battery was dead after a day's running with the headlamp on.
So if you like chatting to breakdown drivers or pushing bikes home, adjust the regulator to 13.2 volts.