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mora 11-19-2009 05:31 PM

PCB design for led bargraph battery monitor
Hello everybody.

I've been reading these forums a bit and I thought I could share something with you.

I noticed cool monitoring system in Forkenswift project and decided to build one. The original led bargraph battery monitor story was posted on in two parts. Link to first part here and second one here.

Original design is very simple and functioning. It was build on a stripboard which I didn't like much. I asked one of the commenters for a pcb he had designed and got it, but it was faulty. One of comments included design for 12V but there was no pcb design for it. Emailing others for design sounds a hassle. I made a pcb design and that's what I'm willing to share.

You can find it in .pdf format from
Index of /huski/paskoo/led_bargraph_battery_monitor_12V

Voltage range for this design is from 10.3V to 14.7V. Lowest led lights up at 10.3V and highest at 14.7V and stays lit if more voltage is applied. I'm not electrician or designer but could do pcb drawing. I made four of these myself and tried one straight out of car 12V battery. Voltage sagged to ~10.5V while igniting and jumped right to ~14.5V when motor started running. This design works at least.

I made also sleek design like the original was. I haven't built it yet but I will do so tomorrow. It is as tall as led bar so should be easy to glue led bars together. I'll post pictures and a video when done with testing.

I was going to post this as a reply in original page but didn't figure out how. Forums are easier, hehe.

mora 11-20-2009 05:08 PM

PCB version 1.1 confirmed as working one too. Now I need to make 9 of those for 120V system (10 x 12V)

Video available here:

atfab 12-06-2009 11:40 PM

DC Voltage Monitor Kit - VM-1 - Electronic Hobby kits from Electronic Rainbow, Danville Indiana

dcb 12-07-2009 12:17 AM

I do like that Mora has published a schematic, and has the part count is down to 9 components + low profile. The parts are prolly $5 so if you wanted to monitor your EV batteries in the pack independently then you can stack these pretty nicely.

Just not sure how you turn them all off when the key is off :) Might need a couple more components for an "enable" that gets pulled to ground or so.

Daox 12-07-2009 06:56 AM

Someone in the local EV club is working on a system to do that right now.

dcb 12-07-2009 09:24 AM

Worth noting that a $10 LCD 2x16 display can do bargraphs too:

could be arranged into 32 little graphs (8 bars hi) or 16 taller graphs

I like the 16 small graphs (or however many batteries you have) on top with text on the bottom for something like total pack amps and volts or something.

mora 12-07-2009 11:45 AM

Now that's a great meter, dcb. Wish I had some programming skills to do the same.

I didn't even think about shutting down all the meters until last week. Maybe one mainboard with in/out-connectors for each battery and simple transistor-switch for each circuit. 12V input for main board controls all the transistors at once.

I'm no electric designer so it might take a while to implement one. But I'll do one before our ev conversion project hits the road for first time.

dcb 12-08-2009 10:51 AM

I had been toying with that idea for a while, but that was skelly who actually made that graph

But still challenges remain for a whole pack battery gauge for it also :)

Tweety 12-08-2009 06:52 PM

First of I'm a newbie on this forum, so I really don't want to step on any toes, I kind of plan to pick your brains on EV's as much as possible... Buuut... Since I'm kind of good with designing circuits, I kind of think I can add something... And the circuit here has a little design flaw... Not something that makes it not work in normal operation though...

Mora... A few questions on the design... The diode D1, what is it supposed to do really? Cuz right now it's not really doing anything a normal wire couldn't do...

My guess the intention is to protect the circuit? Ie swap ground and +12V and nothing goes poof?

Then it really should be connected to the ground wire... Facing so that the ground flows out of the circuit is open, but nothing goes in that way, since that is what makes things go poof... Connecting ground to +12V won't make anything go poof... It just won't do anything hooked up that way, no lights, no magic smoke...

Another thing to remember when doing protection... In this instance it's kind of an moot point as the protection doesn't really do anything as it is anyway, but right now R1 is connected to the "outside" leg of the diode... Ie even if the diode in theory would protect something the R1 would still give a path to destroying stuff when connecting things up the wrong way...

And unfortunately two wrong does not make one right...;)

Move the diode to the ground connector and have it block the path into the circuit and it gives protection... As it sits now you can just replace it with a wire and the circuit will work the same...

Tweety 12-08-2009 06:57 PM

Oh, for a shut off circuit I'd say the idea of a "backplane" with a simple switch operating all the signals is a good one... It won't really by many components...

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