I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

[CharlieZulu]

I decided to install a Voltmeter (digital) in my boat. At rest I am reading 13V or so, but in cruise I am getting 17.5V (all throttle settings).

This seems way too high to me..... am I cooking my battery? What is the cause? (I know zip about the charging system and how it works)

Thanx in advance.

 

[Don S]

Re: I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

I'm guessing this Merc 90 is an outboard, so will move this over to the Mercury outboard forum for you.

 

[Faztbullet]

Re: I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

Check your connections at rectifier and have battery tested . A battery with a weak cell can cause this...

 

[canoemang]

Re: I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

I know the older merc charging systems dont play well with Maintenance free batteries.

I used to get those high numbers if i charged my start/acc battery after every run (with a batt charger), i dont charge it at all now, accept with what the charging system and my volt's sit with in spec.

 

[PHE]

Re: I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

The charging system on old mercs do not have a voltage regulator. It relies on the battery to regulate the voltage. As canoemang mentioned, Maintenance free car batteries don't work well in this application. I use a deep cycle for my starting battery, add water occasionally, and usually have my fish finder, VHF radio and stereo on. The current battery is 6 years old. I also never charge that battery during boating season. It gets overcharged plenty since I usually take it out at least once a week.

 

[CharlieZulu]

Re: I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

Thanks ... my battery is a Marine starting battery, I think about 3 years old, 525 cranking amps, and the electrolyte levels seems OK. Here's the funny thing:

When I first start the boat, the volmeter reads 13.5V and then begins slowly climbing up as I watch it. (I hadn't noticed this before). I just used another Voltmeter connected directly across the battery and went for a spin and my meters track - so it's not the meter.

The voltgae goes from 13.5V to 17.5V after about a half-hour of crusing....... I can't understand how a bad rectifier would cause this, it's just some diodes (rght?) and either works or doesn't.

Time for a new battery? Am I doing any harm in running it at 17.5??

 

[j_martin]

Re: I'm getting 17.5V across my battery.... (Merc 90, circa 1988)

Definitely not a bad rectifier. Battery may be approaching end of life. The ignition doesn't care about battery voltage, but an oil alert module might. Depth finders, radios, and running lights can get stressed with too high a voltage. Most like 16 or under.