Bad or weak alternator ?

[Patrick_L]

Two fully charged deep cycle Interstate SRM-24 batteries. They’re about 5-6 years old. When testing with multimeter engine off, i get 12.26v on battery one and 12.52v on battery two

According to my Garmin readout whether it’s at 20 or 75% speed and with only stereo on (nothing else on), it reads between 11 and 11.4v. Rarely ever above 12v.

Using 5 channel Pioneer amp via thick 1 gauge cables powering four 6.5” speakers and a 12” sub. Not loud, 20% volume maybe.

Bilge or nav lights on, or both: 10.5-10.8

Testing with engine ON, i only get 12.65v on both batteries. Is this the case of BAD OR WEAK ALTERNATOR ?

 

[Chris1956]

Put a voltmeter on the battery cables at the battery and measure voltage with nav lights on. Rev engine a bit and see if it comes up to 12V+. If not, alternator is bad.

 

[alldodge]

Fully charged
A 12.26V the Bat needs to be replaced, the 12.52 is ok but on its way out

The ALT should be around 14.5V but can show lower if a battery is pulling it down. Disconnect the 12.2V Bat and see what the reading is.

What is the ALT capacity, thinking this is an OB maybe?

 

[Alumarine]

You can't rely on the voltage readings from your Garmin.
They are often .5 volts low.

As chris1956 said, use a voltmeter.

 

[achris]

Two fully charged deep cycle Interstate SRM-24 batteries. They’re about 5-6 years old...

Check the alternator output, but I suspect it's time for new batteries. 5 to 6 years is a good life from a flooded cell battery, especially a deep cycle in a starting environment. When you replace them, don't use deep cycles, get proper marine batteries.

Chris......

 

[Patrick_L]

Chris1956 with nav lights on using multimeter i get 12.73 on both batteries (touching cables and not posts) and pretty much same read out at half throttle. I guess alternator it is

 

[Chris1956]

Normal behavior is: With motor not running, and nav lights on, voltage at battery should be under 12VDC. When motor is running, voltage should be 12VDC+. Is that what you are seeing?

 

[achris]

What engine is this all happening on?

If it's a stern drive or inboard, then with the engine running at about 1,500rpm, the battery voltage should reach 13.8 to 14.4v... If it's a late model outboard, about the same, early outboard, all bets are off, an unregulated output could be anywhere, depends on the state of charge of the batteries.

Chris......

EDIT: Just did a search. A couple of years ago you reversed your batteries. Have you had this non-charging problem since then? If so, you fried your alternator when you reversed the batteries.

 

[Patrick_L]

Chris1956 yes engine off lights on and engine on lights on - same read out, no changes.

achris (yep never rush and have enough daylight :facepalm: )
It’s a 94 Mercruiser 4.3LX I/O and since i bought it i dont think i ever recall seeing my gauge showing anything over 12v . I never got stranded on the water so my guess is it is charging them but barely.

 

[alldodge]

With you voltages being read by a handheld meter, your ALT is barely working, its min output is 50amps.

 

[achris]

Are you doing these voltage reading with a proper digital multimeter, or the dash voltmeter?

 

[Patrick_L]

All good. Cleaned up and sanded all of the connectors including the orange positive post. 13.7v back again