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Battery "Fuel" Gauge circut help

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Hello All,
I am toying with the idea of building a battery fuel gauge for a NiMH powered dive light and have found a 10 segment LED "bar graph": 10 Segment LED Bar Graph and a LM3914 driver: LM3914 Driver

I am having some trouble determining if I can use the charge level of the batteries to provide the input signal or if I have to step the voltage down. Any thoughts or ideas?

Thanks for the help.

-Nate
 

MikeMl

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All the 3914 with LEDs will do is make the equivalent of a voltmeter.

The fundamental question you need to ask is:

Can I estimate the power remaining in a NiMh battery pack by measuring its voltage?

Everything I know about NiMh batteries suggests that this is not a very reliable way to estimate the power remaining. In other words, the relationship between voltage and remaining capacity is highly non-linear.

There are battery fuel gauge ICs that mathematically integrate discharge current vs. time to display Ah used/remaining. Assuming your dive light is not continually switched on/off, a simple timer is a better way to go. If your light is turned on/off during use, then a simple count-down timer which is started/stopped with the light will work better than the voltmeter idea.
 
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crutschow

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Mike is right. Use a fuel-gauge IC or a counter to determine the battery charge remaining. Voltage is not a reliable way to determine the charge state of a NiMH battery. Once the voltage starts to significantly drop you only have a small percentage of the battery charge left.
 

audioguru

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The electricity failed in a building under construction and it was very dark when I was at the top of a tall ladder. But I had my flashlight (called a torch in Britain). My flashlight was fine for a minute then dimmed for a few seconds until it was useless. The Ni-MH battery ran out of charge. It was dangerous to climb down in pitch blackness. It might be the same when diving.
 

crutschow

Well-Known Member
Most Helpful Member
A caution: The fuel gauge or counter will only work well with a freshly charged battery. After a few months, standard NiMH batteries lose much of their charge through self-discharge. The new "precharged" or low self-discharge NiMH batteries are much better in that respect and generally still maintain a high percentage of their charge, even after a year of storage.
 
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