volts

[mbb]

The volt meter tells you how much charge is in your battery, indirectly.. If voltage drops <10V while cranking, its time to charge the battery or replace it. I dont know what amps will tell you thats useful, besides calculating how long you can run accessories before you drain battery. You should know that anyway (2.5 amps VHF, 1.5 amps depth, etc, (or whatever the actual # is)

 

[JB]

Re: volts

Yo, mbb. I think you intended to post a response to eireton'd post and inadvertently opened a new topic. You are mostly correct for a boat with no charging system, and using a non-deep cycle battery.<br /><br />The only thing that can tell you the state of charge accurately is a hygrometer. Lead/acid batteries, and particu;arly deep cycle batteries do not drop in voltage, even under load, at a linear rate. A voltmeter will give you pretty much the same readings under the same load for a battery at 50% charge and one at 100% charge. You are correct that if a starting load drops the output voltage significantly below what it does when the battery is known to be fully charged you are in trouble. On a boat with no charging system a voltmeter is useful to tell you when you are "out" of charge. Not much use for anything else. If you have a charging system in the boat a voltmeter can tell you at what voltage you are charging the battery (usually 14 to 14.5 volts). If it stays at 12.6 or lower, the charging system isn't. An ammeter is useful on a boat with a charging system to tell you whether the various loads you are putting on the electrical system are fully supported by the charging system, and the extra is going to charge the battery, or your loads are using up all the current available from the charging system and drawing some from the battery, too.<br /><br />Though they seem to tell you similar things, each can tell you important info about your electrical system that the other can't. Knowing exactly what is going on with both voltage and current can forewarn you of trouble that you would otherwise not discover until it is paddling time. They are inexpensive enough to justify having both aboard.<br /><br />Good luck.<br />JB