simple voltmeter for newbie

Hi all,

First of all let me explain I am almost totally self taught and have until now confined most of my activity in electronics to kit building.

I have the need for a simple moving dot voltmeter employing something like an LM3914 to monitor the charge state of an Optima deep cycle auxilliary battery installed in an RV to power a refrigerator and other accessories.

Optima has advised me that fully charged this battery will read 13.18 volts and when it has been discharged to 70% it will read 11.89 volts. Also I understand any attempt to discharge it further than 70% will result in a drastically shortened battery life.

Ideally I would like to use an amp hour meter but these are quite expensive so I will have to be happy with a volt meter.

The problem is all of the circuits I have seen monitor between approx 12.5 and 10.5 volts. eg one circuit I tracked down supposedly indicated 100% - 70% charge remaining. The problem is the 100% LED extinguished at a voltage of 12.47. At this voltage the optima is already approaching 40% discharged and if I were to continue using the battery until the "70%" LED lit I would have a very flat battery indeed.

I think the LM3914 may suit my application but I would have to adjust it to monitor a range of say 13.2 - 12 volts if this possible.

Can someone please tell me if this is possible and how to do it. I looked in the data sheet but couldn't find anything on how to accomplish this.

Any assistance you can give me with this would be greatly appreciated

Ozzie

You can adjust the LM3914 circuit to have any low and high voltage reading that you want. It has a built-in adjustable voltage reference with an output at pin 7.
The voltage at pin 4 determines the input voltage that turns on the lowest LED and the voltage at pin 6 determines the input voltage that turns on the highest LED.

The battery voltage is affected by its load current.

I took a look at some detailed information on batteries a few years ago and concluded that reliably determining the percent charge was actually quite challenging. I viewed charts that showed no load battery voltage at various temperatures (internal to the battery). Add that and other error such as in measurement and it seemed that unless you adjust for some other variables it would be difficult to rely on voltage as anything but the most crude of indicators.

In my experience with consumer electronics I find that the little battery indicator looks like it provides details on capacity but in actual practice it's unreliable. A half-full indication may not even closely represent what's really left.

Remember carbon-zinc batteries? Their voltage would drop very quickly as they were used.