Voltmeter or Ampmeter

A lot of folks seem to think that a voltmeter can't be used to determine the true charge state of your battery. I say it can and here's a guide that will let you do it. These are the standards that battery manufacturers set for their batteries and the same standard that is used to calibrate the light bars like your trolling motor might have built in.

I tried a couple of times to format this as a table but it didn't work. I'll post the link to wikipedia that has the same info instead:

http://en.wikipedia.org/wiki/Car_battery

• Quiescent (open-circuit) voltage at full charge: 12.6 V (100% Charged)
• Unloading-end: 11.8 V (60% Charged - 0% usable charge remaining)
• Charge with 13.2?14.4 V
• Gassing voltage: 14.4 V
• Continuous-preservation charge with max. 13.2 V
• After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
• Wait at least 12 hours after charging to measure open circuit voltage, the resting time allows surface charge to dissipate and enables a more accurate reading.

A 0% charge by these standards is a 60% actual charge of the battery. It is harmful to discharge any lead acid battery below 60%, (this includes AGM's).

Re: Voltmeter or Ampmeter

Amp meters for those of you that are not old enough to remember when cars actually had them, told you two things. 1) The generator was either putting juice INTO the battery (a +xx charging reading) or 2) whether more juice was being taken FROM the battery than the generator was putting back (a -xx discharge reading). Depending on the generator output and the calibration of the gauge, the problem with them was that the needle rarely moved much off the "0" point on the scale so it was really not much help until nightime when you flipped on the headlights and you saw the needle well into discharge territory. A voltmeter is a better indicator of what's going on. Turn the key to RUN (not start) and look at the voltage. A reading below 12.6 indicates a discharged battery. Start the engine and watch the meter. If the voltage pops up to 13.5 - 14.5 you have a healthy charging system and the battery is charging. If it takes a bit of moderate throttle to bring the voltage up you have a battery that was fairly deeply discharged. But in either instance, the system is behaving as it should. If the meter shows something less than 12.6 static and does not change with the engine running, even at moderate throttle -- you are advised not to leave the dock as the charging system is not working. I vote for voltmeter. Any boat I come by that doesn't have one, gets one although that hasn't been a problem since my last three boats have been new and included it.

Re: Voltmeter or Ampmeter

Amp meters for those of you that are not old enough to remember when cars actually had them, told you two things (and neither of them told you the STATE of charge). 1) The generator was either putting juice INTO the battery (a +xx charging reading) in excess of the load or 2) whether more juice was being taken FROM the battery than the generator was putting back (a -xx discharge reading). Depending on the generator output and the calibration of the gauge, the problem with them was that the needle rarely moved much off the "0" point on the scale so it was really not much help until nightime when you flipped on the headlights and you saw the needle well into discharge territory. A voltmeter is a better indicator of what's going on. Turn the key to RUN (not start) and look at the voltage. A reading below 12.6 indicates a discharged battery. Start the engine and watch the meter. If the voltage pops up to 13.5 - 14.5 you have a healthy charging system and the battery is charging. If it takes a bit of moderate throttle to bring the voltage up you have a battery that was fairly deeply discharged. But in either instance, the system is behaving as it should. If the meter shows something less than 12.6 static and does not change with the engine running, even at moderate throttle -- you are advised not to leave the dock as the charging system is not working. I vote for voltmeter. Any boat I come by that doesn't have one, gets one although that hasn't been a problem since my last three boats have been new and included it.

Re: Voltmeter or Ampmeter

sschefer said:

A lot of folks seem to think that a voltmeter can't be used to determine the true charge state of your battery. I say it can and here's a guide that will let you do it. These are the standards that battery manufacturers set for their batteries and the same standard that is used to calibrate the light bars like your trolling motor might have built in.

I tried a couple of times to format this as a table but it didn't work. I'll post the link to wikipedia that has the same info instead:

http://en.wikipedia.org/wiki/Car_battery

• Quiescent (open-circuit) voltage at full charge: 12.6 V (100% Charged)
• Unloading-end: 11.8 V (60% Charged - 0% usable charge remaining)
• Charge with 13.2?14.4 V
• Gassing voltage: 14.4 V
• Continuous-preservation charge with max. 13.2 V
• After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
• Wait at least 12 hours after charging to measure open circuit voltage, the resting time allows surface charge to dissipate and enables a more accurate reading.

A 0% charge by these standards is a 60% actual charge of the battery. It is harmful to discharge any lead acid battery below 60%, (this includes AGM's).

Click to expand...

Fine...you have a battery that registers 12.6 volts (6 internal cells @2.1 volt potential each)...that is what a new battery can produce.

I deal with starting batteries all day long...and I can show you a 'pile' of them, that register over 12 volts. But they couldn't even start a Vespa...why?

Because...it isn't the voltage that starts your engine...it is the cranking amperage. I can give you 12 volts out of a handful of "D" cell flashlight batteries...but not the required amperage. And this is the same with the pile of useless starting batteries, that I mentioned above.

In short...12+ volts across the terminals, even after a full charge, means nothing...until you measure the available cranking amps. This test, can only be done with a decent load tester...or with a Midtronics 550XL, like we use daily. If anyone reading this wonders about their battery...and doesn't have the proper tester...take it to any honest dealer, or store, and find out the bottom line!

Re: Voltmeter or Ampmeter

I agree with rockyroad. At 12.6 volts the battery is fully charged and ready for a load test. The load test determines the condition of the battery.

How many flashlight batterys to start my truck?

Re: Voltmeter or Ampmeter

BaileysBoat said:

How many flashlight batterys to start my truck?

Click to expand...

Depends...import?...or domestic?

I swear that the little Energizer Bunny, could start a tank...and keep on 'trucking'!

Re: Voltmeter or Ampmeter

You guy's are too funny. You really have to read the info on Wikipedia to get the info you need to understand it. First off, I was refering to using a voltmeter for montioring the status of you battery, perhaps I wasn't 100% perfectly clear on that.

You cannot check the physical condition of a battery with a voltmeter or an ampmeter, that can only be done by measuring the specific gravity of the electrolyte in each cell. You can monitor the charge status of a battery with a voltmeter if you have verified that the battery is up to mfg's standards. I, for one, don't take an old junk battery, throw it in my boat and head off for a day of fun and fishing. They make lousy anchors and won't hold you off the rocks when your engine won't start.

Re: Voltmeter or Ampmeter

Rocky_Road said:

.....In short...12+ volts across the terminals, even after a full charge, means nothing...

Click to expand...

Agreed. I have a 5 year old marine cranking/trolling that won't even take any charge, reads 12.6 on a meter, but will die after less than an hour of running just my sonar. It's just worn out, and will quit after a just a few closely interspersed stops-and-starts too. So I got a new one, which works fine. Both of them read 12.5+ volts on a meter

I don't have any gages in my boat, but my motor will run without a battery and I can pull-start it. So I have not considered gages. I do carry a multimeter however.

Re: Voltmeter or Ampmeter

Once upon a time, way back when 'electronic load tester' was non existent, a voltmeter and a pair of headlights or a 100w handheld spot light was all you needed to load test a battery.

A good analogue voltmeter with small graduations, read properly, is all you need.

Re: Voltmeter or Ampmeter

The correct term is ammeter.

SeaDog makes a cheap analog voltmeter specifically for marine batteries. It has the scale expanded so you see the 11-14v range you're interested in monitoring. It will show (of course) only the voltage, not the health of the battery. Assuming a healthy battery bank, it's a decent indicator of state of charge.

Personally, having a cruising sailboat with outboard, (with little charging capability) I like having an ammeter, as I can tell whether I am charging or discharging -- something a voltmeter cannot.

Re: Voltmeter or Ampmeter

I prefer an ammeter to determine charging/discharging states. The problem with most marine volt meters is that they are not marked or calibrated fine enough to determine the difference between 12 and 12.6 volts.

Re: Voltmeter or Ampmeter

Not really necessary to have the scale graduated that finely. Any reading under 13 volts is an indication you have a problem. Then there is the issue of gauge accuracy. Most of the off-the shelf gauges are about as accurate as a marine pitot speedometer. I have two locators on my boats and rely on the voltage display for accuracy.

Re: Voltmeter or Ampmeter

Silvertip makes a good point. Most off the shelf gauges aren't accurate enough to precisely determine the condition of your battery and they are barely adequate enough to determine if your battery is being charged or not. I actually think they were put in cars because people got offended by the term "Idiot Lights".

Many of the upper level nav systems and fishfinders have a decent voltmeter built in that will show you your voltage in a digital manner that can actually be used to some good.

If you've installed your nav/fishfinder/gps so that it gets its power from your auxillary battery(s) as most manufactures recommend then your normal voltmeter that monitors your starting battery won't be of any good to you. A secondary voltmeter is needed which is why these devices have them.

That said, any good battery that has settled after accepting a full charge should produce a sustainable voltage of 12.6 volts for a specific period of time at a specific level of current draw. Since the manufacturer recommended minumum voltage before recharge is 11.8 volts. You can use an accurate voltmeter to determine when it's time to switch batteries or stop using the current one before you cause battery damage.

With regards to those batteries that read 12 volts and are bad. If you were to put the battery under load you would see a significant drop in voltage, (Ohms Law).

As a side note: I had a Minn Kota Terrova with autopilot that would go whacky if the voltage dropped below 12v. Spinning your boat around in circles is not my idea of a good way to learn that your battery needs a charge!...LOL.

Re: Voltmeter or Ampmeter

There are several drawbacks to just using an ammeter. Calibration and sensitivity have already been mentioned. Additonally, unless you are using an ammeter with a shunt, you have to run heavier wires to the meter. If you are not running heavy loads or charging from 50% all the time, a voltmeter is all you really need.

Today, digital voltmeters are cheap and easily obtainable. Even the Chinese versions that plug into your lighter read in tenths and are accurate enough for what we use them for. If you need to trouble shoot, then you get out a Fluke meter.

For wet cells, my suggestion for longest life and reliablity is to do the following.

If you know your charging system is working fine, @14v, check water level and load test monthly. (100 amp load testers can be bought from Harbor Freight for $30 on sale. If you have a rectifier and not a regulator, and it allows your voltage to go to 16v, check water every trip.

Buy an on board tender that keeps the battery @13.5v and plug it in when ever possible.

Do not discharge battery below 80%, unless it is a DC and then not below 50%.

Remove the batteries in the off season and store them on a tender in your basement or garage. I still check the water monthly, just to stay on the same routine. You can have a longer interval if you wish.

This may seem like alot of work but I keep my batteries 5 to 6 years and spend my savings on fishing gadgets.

This advice is worth exactly what you paid for it.

John

Re: Voltmeter or Ampmeter

Ecellent advice John, seems you understand this theory very well. At 11.8 volts under load, the average deep cycle battery should (depending on quality and age) be at about 60% discharge. All of my batteries info stickers state not to discharge below 60% so that's the mark I try to stay with.

I treat my starting batteries as if they were God's. Probably because, "Oh God" are the first words out of my mouth when one fails on the lake. I only had that happen once but that's all it took for me to learn a valuable lesson.

Testing under load is one thing that a lot of folks don't understand. So many think that if a cheap voltmeter reads 12 volts on a open circuit it's good to go. We both know and hopefully the readers and contributors of/to this post now see it also.

One thing you touched on and I'd like to take a bit furher is that when tending or maintaining a battery it is important to use a hydrometer to measure the specific gravity of the electrolyte. You can get them at most auto parts stores and they can be an invaluable tool when testing a batteries physical condition. Adding water dilutes the electrolyte and keeps the plates fully immersed but it does not regenerate it like some often think.

Finally, the importance of a proper charging system cannot be over emphasized. Anything above 14.4 volts and you will begin to gas the battery. Gassing is the separation of the hydrogen molecules from water (h20) and you will create dangerously explosive hydrogen gas with an ample supply of oxygen to create the fire triangle, (fuel, heat, oxygen). You can also blow up a battery from the internal pressure that's created when that happens.

Re: Voltmeter or Ampmeter

"Adding water dilutes the electrolyte and keeps the plates fully immersed but it does not regenerate it like some often think."

No, adding distilled water replaces the water that gassed off, maintaining the electrolyte -- not diluting it. The off-gassing increases the electrolyte's acidity.

Minor point, but worth understanding the chemistry. That's one reason why one waits several hours after adding water before taking a specific gravity reading with the hydrometer.

Re: Voltmeter or Ampmeter

I am amp meter guy, but I don't use my amp meter for the battery, but for system charging. If the system is working good and you'll turn on the lights, you should see a quick draw and then a recovery to zero. If you run your battery down then you should see the system charging it. If turn off your engine and you leave your lights or aux. gear on you will see dip in the gauge. Amp meters are a great system tool.

Re: Voltmeter or Ampmeter

Ampmeters are a great tool for monitoring your charging system. Combine them with a digital voltmeter and you can be very effective at monitoring your entire electrical systems state.

Re: Voltmeter or Ampmeter

without reading through all this tech stuff I'm going to just say the voltmeter in my CAR is one of the best guages I have, I know right away if something isn't right, I left my park lights on one day for a few hrs and the low reading on the volt meter caught my eye as soon as I turned the key to on, i got a jump and could see the volts go up by reving the motor so I knew it was charging, eventually the volts went to normal at idle and all was good, my boat also has a volt meter and it's the first guage I look at when I turn the key on it does drop a little when the blower is on engine not running.

Re: Voltmeter or Ampmeter

a70eliminator said:

without reading through all this tech stuff I'm going to just say the voltmeter in my CAR is one of the best guages I have, I know right away if something isn't right, I left my park lights on one day for a few hrs and the low reading on the volt meter caught my eye as soon as I turned the key to on, i got a jump and could see the volts go up by reving the motor so I knew it was charging, eventually the volts went to normal at idle and all was good, my boat also has a volt meter and it's the first guage I look at when I turn the key on it does drop a little when the blower is on engine not running.

Click to expand...

In your car or on your boat....same same. Volt meters and Amp meter give you a good indication of the electrical system.