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  How many volts should my charger be putting out?

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Author Topic:   How many volts should my charger be putting out?
Landlocked posted 03-30-2009 10:11 AM ET (US)   Profile for Landlocked   Send Email to Landlocked  
I assumed a 12 volt 6 amp charger should be putting out 12 volts, but last night, I stuck a volt meter on the output leads and measured 3.6 volts.

Is this normal or do I have a bad charger?


jimh posted 03-30-2009 10:56 AM ET (US)     Profile for jimh  Send Email to jimh     
A charger that puts out 3.6-volts is useless for charging a nominal 12-volt lead acid battery. IT has a serious malfunction.


A lead-acid storage battery stores electrical energy in the chemistry of its cells. In a lead-acid battery, there is a very handy property of the chemical reaction that allows it to be reversed so that energy can be put into the battery and the stored energy can be replenished. In order to reverse the chemical reaction you have to apply an external voltage to the battery terminals that is greater than the terminal voltage being generated by the chemical reaction in the cells themselves.

The terminal voltage of a lead acid battery is determined by many factors, but primarily by its state of charge. If the battery is fully charged, the terminal voltage will be around 12.9-volts. To put more charge into the battery, you have to exceed 12.9-volts. If the battery is discharged, it will have a lower terminal voltage, and it will begin to charge as soon as the external voltage exceeds the terminal voltage. A deeply discharged battery will have a terminal voltage around 11.0 volts. Any external voltage greater than that will begin to charge the battery.

To reach a fully charge state and to condition the battery elements, you typically have to apply about 14.3 volts to a lead acid battery. This higher voltage will help improve the chemistry of the battery and drive off any contaminants on the battery plates. Good chargers have precise voltage regulation which maintains the charging voltage at this level. Higher charging voltages begin to cause undesired chemical reactions in the battery, including the boiling of the electrolyte by release of hydrogen gas.

Landlocked posted 03-30-2009 11:02 AM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Excellent info. Thanks for the reply.


newt posted 03-30-2009 12:16 PM ET (US)     Profile for newt  Send Email to newt     
Before you toss the charger, hook it up to a battery and see what voltage it puts out. The output voltage may increase when the charger senses a load.
Landlocked posted 03-30-2009 12:50 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Showing my ignorance...

If I hook it to the battery would the 12 volts from the battery not be coming back across the leads from the charger even if the charger was only putting out ~3? How would you measure?

It is a "smart" Minkota charger so what you say is very well the case. Just wondering how to test.


kwik_wurk posted 03-30-2009 01:47 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
You need a load for the charger; most modern chargers will not do anything if no load is detected.

Test the battery with and without the charger. You should see a change in voltage. I am guessing ~12.5 volts without charger and ~14.3 with charger. (This all depends on battery type, state of original charge, and the charger type itself.)

If you are really concerned about the other electronics, disconnect everything (if you don't have an isolation switch).

Landlocked posted 03-30-2009 02:15 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
I do have an isolator switch but I was attempting to charge both batteries at the same time. Switch set to "Both", leads from charger connected to positive on one battery and negative on the other. (By the way, I checked the voltage across these two points and it is 12.)

I was concerned when the batteries charged for like 8 hours and never reached full charge. That's when I unhooked and checked my charger. the larger of the two batteries is brand new. My starter (Group 24) battery is about a year old.


jimh posted 03-30-2009 07:19 PM ET (US)     Profile for jimh  Send Email to jimh     
If you want to connect a load to the charger for testing it, use an automobile headlamp sealed beam replacement lamp. They are a 12-volt nominal load and will draw several amperes of current. At one time you could buy a headlamp replacement bulb for a few dollars.
Landlocked posted 03-30-2009 08:50 PM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Didn't think of that! I think I actually have one around here somewhere.


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