Charge level indicator?

I need a nice, simple charge indicator. I have a battery pack that holds 4 AA NiMH batteries. Normally they charge up to 1.3V per cell or 4X1.3=5.2V total. I have a solar trickle charger on there too and I'd like to have a little charge indicator.  
Requirements are:
-low part count
-under $10
-small (it's on my bike helmet)

I was thinking something with a pushbutton an LED or two and a zener diode. Something that turns on the LED at  greater than 5.2 volts. But I don't have experience with zener diodes.

Has anyone seen a system like this around? Can you think one up?


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AnaG794 months ago

yeah even i need one but i need for 8 battery's of NI-MH so if anyone can help hat would be nice

I have heard good things about the LM3914 LED bargraph driver http://www.national.com/mpf/LM/LM3914.html#Overview
Data Sheet:

and I have even seen it mentioned somewhere here on Instructables before.

Page 7 of the data sheet linked above shows a circuit diagram that kinda explains how it works.  It has internal 1.25 V reference wired across a big voltage divider, and whole bunch (10 I think) of comparators that compare different levels on the divider to your input signal. 

I am guessing that the trick to making this thing work as a battery voltage indicator is to just put a voltage divider of your own (two resistors) across the battery voltage, so that the output of that divider is in the range from 1.25 to 0.  For example a two-resistor voltage-divider with R2=4*R1 that gives 1/5 the battery voltage, would take 5.2V in, and give 1.05 out, putting it approximately in the range where you want it to be.

Anyway, I have never actually used that IC, but if it works as promised it should be easy.  Much easiser that messing around with zener diodes and transistors.  Also I expect you could wire it to turn on only while a momentary button is pressed,  to conserve those precious milliampere*hours.

Final note.  There are other related versions of the same IC named 3915 and 3916, but I think those are for log-scale bargraphs, like what you'd want for measuring sound power.  I am pretty sure the 3914 is the one with a linear-scale, which is what you want for a battery voltage level indicator.
bobdole56566 years ago
well use somthing like resestors like a 2 colerd led each coler run an one pron and a sepret one one of the colers is domanet put a resteron it before find the voltag when you think the batrie is half charged get aresister that will let voltag out when it get over that mark like a 5v restor and half is 10 so the red led will be on till the votag reached 5v so u know when its at least half charged and ucould put the restor likeo 9v so it getsfully charced ot 10 to pervent over charging now i put it t 5 v so it woul tell you that u have not long left oredo thist twa oneice like a 9v rstor take a 2 colerd one put it on th domant coler of the two coler one and then a one coler onetht wold be 5v restor now that set up would giv u a fully charged led and when its halh chrged one ore get a charger that has a led in it and if your charging somthing thatcharges with A 5v volt limet make shure your chger puts out 4-5 better4 but it charges sower but no over heating repliy to this and ill like halp u build it ore buld one withyou
i know i suck at writing so if your confused contaced me
o ya and i did not put your voltages on it so if u need help figring it out cotact me