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Old 19-05-2011, 11:47   #1
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Basic Battery State Monitoring

I have a sailboat with 2 batteries, both of them are wet cell deep cycle, hooked up to a guest 1,2, all, off switch. I normally start the boat on all. My electrical usages are minimal. I have an alternator and a 15 watt solar panel.

I would like to have a way to monitor the % of batter life in amp hours left on each battery at a given time. I don't want to buy a fancy contraption. I have a two part question:

1. If I install a voltmeter, and visually judge the voltage between 11 and 14 volts, will that give me a good idea if reserve capacity, or does voltage drop accelerate as amp hours are depleted?

2. If I install a voltmeter, can I just hook it up to the output of the battery switch, so that it will read voltage of whatever circuit the switch is completing (i.e. 1, 2, all or none)? I found this Adding a voltmeter to sailboat at Easy Ac/Dc, Boat Wiring and Marine Electrical Questions, Answers and Comments online but it seems overly complicated since i already have a switch.

Thanks!!
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Old 19-05-2011, 11:56   #2
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Re: Basic battery state monitoring

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Originally Posted by sandycohen View Post
1. If I install a voltmeter, and visually judge the voltage between 11 and 14 volts, will that give me a good idea if reserve capacity, or does voltage drop accelerate as amp hours are depleted?
It won't tell you how many hours you can continue to burn your navigations lights, but it will be as accurate as the gas gauge on a car, giving you an idea if your batter bank is full, half full, or a quarter full. You will probably need to calibrate it to convert voltage levels to that kind of information.

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2. If I install a voltmeter, can I just hook it up to the output of the battery switch, so that it will read voltage of whatever circuit the switch is completing (i.e. 1, 2, all or none)?
Yes. One lead from the voltmeter goes to the output side of the battery switch. The other goes to the common ground (usually a negative battery terminal). You could add a switch between the voltmeter and the battery switch to avoid diverting power to the voltmeter when you're not reading it, but the amount of power consumed by the voltmeter is negligible.

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Old 19-05-2011, 13:38   #3
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Re: Basic battery state monitoring

I suggest you get a digital gauge that reads to 1/10 of a volt. I think both Newmar and Blue Sea Systems make them.
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Old 19-05-2011, 14:23   #4
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Re: Basic battery state monitoring

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If I install a voltmeter, and visually judge the voltage between 11 and 14 volts, will that give me a good idea if reserve capacity
As has been discussed on this forum many, many times, the relationship between voltage and state of charge (SOC) is only meaningful IF the battery being tested has neither been charged nor charged for 12 to 24 hours. Then, a small load is briefly connected to the battery to dissipate the surface charge, and the reading is taken and compared to a voltage/SOC chart.

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Old 19-05-2011, 16:08   #5
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Re: Basic Battery State Monitoring

Unless the battery has been idle for between 12 and 24 hours the volt reading is absolutely 100% meaningless, even in relation to itself (i.e. the voltage isnt correct in the absolute, but it does show relative fluctuation in life left)?

So how do I measure battery capacity remaining, on the fly?
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Old 19-05-2011, 16:11   #6
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Re: Basic Battery State Monitoring

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Originally Posted by sandycohen View Post
Unless the battery has been idle for between 12 and 24 hours the volt reading is absolutely 100% meaningless, even in relation to itself (i.e. the voltage isnt correct in the absolute, but it does show relative fluctuation in life left)?

So how do I measure battery capacity remaining, on the fly?
Fancy electronic thingy that the OP didn't want to buy, amps consumed meter such as Link 10.

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Old 19-05-2011, 16:15   #7
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A voltmeter is useless

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Old 19-05-2011, 16:23   #8
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Re: Basic Battery State Monitoring

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A voltmeter is useless
Disagree in this person's case. He has a simple system with relatively light battery loads. An accurate volt meter will give him a relative state of charge that is plenty useful. It's all I've ever used in 30+ years of cruising. If you want to go fancy, check out the Smart Gauge.

I should add that something like the Link could theoretically work if it was accurate, but from those I talk to and all the Internat jabbering about them it seems that they are always out of synch--the big question is by how much and how to get them reliably back into synch. I think a better investment in your electrical system would be to simply buy another battery or two with the money saved in not buying the Link. But, I'm sure we'll now hear from the people that love them.
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Old 19-05-2011, 16:41   #9
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Re: Basic Battery State Monitoring

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An accurate volt meter will give him a relative state of charge that is plenty useful.
An accurate voltmeter will give him a voltage. It will tell him if his battery is dead flat (<10.6VDC) or fully charged (>12.6VDC). Other than those data two points, it will not tell him the SOC either in % or amp-hrs remaining which is what he stated he was looking for in his OP.

As to the Link family of battery monitors, keeping them in sync is as easy as fully charging the battery or bank that they are monitoring. I have used one for almost 20 years and installed dozens.

Recently I have become a fan of the Victron BMV 600S (602S measures voltage on starting battery. Go or no go indication.) Less than $200, very easy to install and with more features than the Link family of monitors.

Hope this clarifies my earlier post.
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Old 19-05-2011, 17:24   #10
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Re: Basic Battery State Monitoring

Voltage, as an indicator of your batteries' state of charge, is meaningless in the context that it is used on a boat. One would have to measure the voltage after at LEAST 6 hours of "0" power going in or out. This is seldom the case on a boat. Even then... it is not very accurate. The average of the specific gravity of each cell is rather accurate, but VERY inconvenient!

Even with a simple system, your best bet is a battery monitor like a Link 10. It measures the amps going out at a given moment, how long you can run @ that rate, the line V, and the Ah down from full. If you top off your batteries regularly, as you should, it will remain accurate for years, and you will not be left without power.

They are money well spent!

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Old 19-05-2011, 17:29   #11
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Re: Basic Battery State Monitoring

The battery monitors give very useful info and can help u save electricty
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Old 19-05-2011, 17:40   #12
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Re: Basic Battery State Monitoring

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Originally Posted by sandycohen View Post

I would like to have a way to monitor the % of batter life in amp hours left on each battery at a given time. I don't want to buy a fancy contraption.
Get down the local car shop and buy a hydrometer. Or spend money on a posh meter.

Hydrometer - Wikipedia, the free encyclopedia

You can also test to see how dehydrated you are. So it says.
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Old 19-05-2011, 17:52   #13
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Re: Basic Battery State Monitoring

If you search the web you can find a bm at a good price The info it provides makes it well worth the bucks I would say if you are living aboard you need that device
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Old 19-05-2011, 17:52   #14
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Re: Basic Battery State Monitoring

Read this explanation of why amp-hour measurements are really worthless. Sure, it looks like you are getting useful information, but not really.

In practice, on most boats, you quickly learn about how much you use and how much you need to charge. My advice, spend your money on better charging systems (more solar in your case) and more battery capacity and you won't have to worry about making these measurements.
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Old 19-05-2011, 18:33   #15
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Re: Basic Battery State Monitoring

Voltmeters are a tool, one of many, it can give you an idea of the state of the battery, but it won't say the battery is XX% left by it's self, if all you have for a meter is a votlmeter and you pay attention to the readings on that perticular meter, as long as the numbers are repetable, then over time you get an idea what it means, if you turn something on you will see the volts drop, if it's something big it will drop alot, if small not as much, if you turn something on small and the volts drop big then your almost out.
As to how much capacity, AH, your battey has left, it keeps getting smaller as your battery wears out.
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