How full is my battery?

[thurps]

After getting home from an unsuccessful day of trolling for salmon I noticed that the voltage was down on my battery. Of course I instantly threw the charger on it and thought oh ****, charging problems. Well once the battery was fully charged I couldn't check the engine for charging so the next day I again unsuccessfully trolled for salmon but with the voltage monitor on my fish finder on. As the hours went on the voltage on the battery slowly went down to about 11.5V. I decided to head in and because the ocean was still flat enough to run at a fair pace, off to the marina I went. To my surprise by the time I got in the battery was at 13.5V.
Ok I have an 85' 40 HP Johnson. At what RPM does it start to charge and how low can I let my battery get and still start the engine? I have a massive deep cycle battery.

 

[Silvertip]

Re: How full is my battery?

At idle speed the alternator output will be quite low and voltage will be 12.6 or slightly less depending on what accessories you have operting at the time. With the engine running at 1500 RPM or so, voltage should measure something above 13.5 volts. BUT -- you didn't say how old the battery is and what accessories you have in operation. Using your term, a "massive" deep cycle with just a moderate load, such as a locator, radio, perhaps a live well pump, should not run down. A battery that quickly runs down with just a moderate load is telling you it is headed for battery heaven. Have it load tested. I'm betting it has one weak cell and your charging system is functioning normally.

 

[tashasdaddy]

Re: How full is my battery?

look on the battery for cranking amps. you are really better of using 2 batteries, one cranking, one deepcycle for the troller, and electronics. you have to be above 2500 rpms to get any charging. the motor is only equipped to maintain the starting battery. and charge the deep cycle at home. that way you never get stranded. but with a 40 it can usually be rope started pretty easily. practice rope starting it on muffs in the driveway, so you will be prepared if necessary. Silvertip and i were typing at the same time and he made some very good points, also that i missed.

 

[thurps]

Re: How full is my battery?

Thanks gentelmen.

 

[jtexas]

Re: How full is my battery?

11.5V is pretty darn low. was I you, I'd experiment in the driveway to find out just how low you can go...I wouldn't a thought 11.5V would be enough. On the other hand, battery voltage will drop with a sustained heavy load like a trolling motor, then recover somewhat. So possibly the 11.5 was just a low measurement.

Also note that immediately after charging your voltage measurement will be high due to "surface charge" which will dissipate over a few hours (or quicker with a load on it).

Separate trolling motor battery is still a good idea, IMHO. I also carry jumper cables so I can jump start myself if it comes to that.