LED Bargraph battery monitor - part 1 · 18 May 07
Hello everybody,
About 6 months ago I wrote an article here (Battery undervoltage warning). The idea was that I would be able to avoid my nemesis, unequalised batteries (explained here), and have early warning of bad connections like the one which caused this terminal failure in my car. Argh!
The battery monitor was fairly successful but only gave a crude qualitative measurement of voltage drop. I have been looking for something better. Something with which I can see the actual voltage each battery is at.
When I saw the battery monitor on Jim Fell’s Seicento I had to have one. It’s based on the robust LM3914 LED driver IC.
The circuit I found and adapted can be found here. National Semiconductors have kindly provided an example circuit and some very good explanation.
I adapted this circuit to show 10 lights for 6.37V and above and no lights for 5.75V and below, with the rest of lights showing about 0.6V intervals. Each interval loosely approximates to 10% DoD (Depth of Discharge). Here’s the schematic: 
Here’s a table which shows number of lights shown against voltage and State of charge (100- DoD):
1. 5.75V ~10%
2. 5.82V ~20%
3. 5.89V ~30%
4. 5.96V ~40%
5. 6.03V ~50%
6. 6.09V ~60%
7. 6.16V ~70%
8. 6.23V ~80%
9. 6.30V ~90%
10. 6.37V 100% upwards
The LM3914 has 2 modes: “bar” where it will show all of the lights up to a certain light and “dot” where it will only show one (or occasionally 2 lights at a time). I started off using bar mode but I found this too bright and distracting – it lit up the whole car. Lighting up all those LEDs also drew a higher current and generally read lower due to the associated voltage drop.
You’ll see that I have designed this circuit for 6V Flooded lead acid batteries. For those of you running 8 or 12V you want to replace Rx with 33K or 56K respectively. I tested this at 12V with 56K and it works just fine.
For each meter you’ll need these components:
LM3914
10 DIL bargraph display
10 uF electrolytic capacitor
100R resistor
1k resisitor
270R resistor
10K resistor
22K resistor (for the 6V version)
200R precision cermet trimmer
500R precision cermet trimmer
IN4001 silicon rectifier diode or similar.
a piece of stripboard 10 holes x 16 holes (strips run along the 16 hole side)
All resistors should ideally be 1% or 2% tolerance.
The electrolytic capacitor needs to be very small and short if you want to mount all the boards together as shown below.
Tools required:
Small soldering iron and solder
Adjustable power supply (5.5 – 6.5V for 6v batteries)
Digital multimeter
If you have made the circuits tidily you should be able to glue the LED modules together and mount the whole thing on another stripboard.
Now to put it in the car.
I have left out all of the relays I used in the original battery monitor and the unit is now permanently on. I measured current usage and found that if you are using “bar” mode with all the LEDs on, you can expect to draw about 100mA per meter at 6V, probably about half that at 12V. If you use “dot” mode then you’ll draw about 10mA per meter. Certainly for my battery pack this is a very small loss.
I am so much more confident in my battery pack now that I can see what each individual battery is doing. I am sure that it’ll make the batteries last longer because I’ll be able to know about a discharged battery before I cause a cell reversal or over gassing.
That’s all for now! Step by step stripboard instructions coming up in Part 2.
James May in the UK
James – nice work! I can imagine how useful it must be to see how the entire pack is doing at a glance.
This is something that will be of concern to us with the ForkenSwift because we’re using old batteries (which are bound to be more mis-matched than a good, new set).
I’ve already wrecked 2 batteries by discharging them too far :) – reversed cells. Nothing like learning by breaking stuff! Fortunately, free stuff.
I was a little bit late with this post. Did you see the link on this site last week to the PakTrakr device. I think it might just do what this does, and more. My meters would be good value if you don’t have too many batteries, or you could use one for the 12V lights and ignition battery as I do.
Great job, James! Thanks for the excellent article and circuit plans.
I don’t see any isolation or other safety feature, so I presume you plan on bringing full pack voltage to the dash?
Hi Peter, Yes, it does bring full pack voltage to the dash. It’s only 96V for me so not too dangerous as a shock hazard, still, it’s not ideal. Each wire is fused right next to the battery at 250mA so I’m not really worried about any short circuits.
A QUESTION FVERYTHING I HAVE SEEN SAYS 6V BATERIES LAST LONGER AND PERFORM BETTER. DO YOU AGREE
Hi Jack,
I haven’t used any other batteries so I can’t make a comparison. I think the 6V golf cart batteries should last well because of their thick plates and relatively large cells. And like you, I have also heard that they are particularly robust. The Trojans have a good reputation.Hi James –
I can’t find the Part 2 of this project. I must be missing something simple. Could you please post the link?
How has the monitor been working for you?
I have been monitoring a single battery with a DMM, like Dr Larry.
I monitor the one (of 8) which consistently shows the lowest voltage after charging/resting, on the assumption that it will be the weakest under load as well – though I’m not so sure there’s a direct correlation.
Having driven the ForkenSwift for a few charges, I can appreciate how nice it must be to know at a glance what’s going on with the pack.
Argh!
You caught me out! I haven’t written part 2. I actually thought when the PakTrakr came out that that device made mine rather obsolete.
Yes, it does work, it shows me my weak battery and what it is doing. I think my weakest battery is also not so good under load. It sags the most. Two reasons for this might be:
1. It’s SOC is lower therefore the acid is weaker and hte internal resistance of the battery is therefore higher.
2. The internal resistance of this battery is higher for some other reason.
It usually doesn’t use up water like the other ones.
Something weird about my meter which I have never figured out is that it reads low on the end batteries even though they are fine. It isn’t the wires, I have replaced some of them with bigger wires instead of hte original CAT5.
I think I permanently aged my weakest battery by letting one of it’s cells reverse under load before I made my power meter. Yes, they are worth having and show you when you need to do an equalisation charge or charge an individual battery.
I am considering upgrading to a PakTrakr.
You are running 6V batteries?
If I upgrade you might get a present in the post.
“I actually thought when the PakTrakr came out that that device made mine rather obsolete.”
But does the PakTrakr show you the collective pack at a glance, under load, like yours does? I looked through its specs and didn’t see that. It seems to me you can only cycle through the pack looking @ individual batteries.
An EV owner from Ottawa has lent me a simple box with an 8 position switch and a single analog volt gauge (well suited to our car’s 8 6v batt pack), but I haven’t installed it yet, because it also won’t show the whole picture at once.
Maybe I’m unneccesarily fixating on the at-a-glance battery comparison, but that strikes me as the most potentially useful feature of a monitor.
Is this suitable for my 6v 4A SLA? Is the state of charge the same for SLAs?
Thank you, James.
Hi Lucky,
Yes, I believe it’s the same for SLAs. If the full / empty voltages are slightly different (read your documentation) then you just calibrate the bargraph accordingly.
Hi James…
Any news to report on the DIY monitor vs. PakTrakr comparison?
Another option I’ve been mulling over is an Arduino-based battery monitor. I’m not really a hardware guy, and barely a programmer, but from what I’ve read of this open source platform, it shouldn’t be horribly difficult to build a 6-battery monitor (the board has 6 analog inputs) that can send live data to a laptop: arduino.cc
I may just look into this as a winter project. The pre-assembled board is about $30 US.
Sounds like a good idea. I will look in to it as well. My PakTrakr hasn’t arrived yet. :( I’ll chase it up if it doesn’t come soon.
Has anyone seen the new “EVision” unit that Metric Mind have on offer? It looks pretty sweet and includes an odometer, tachometer and even fuel gage driver. Bit on the expensive side though! It looks like an all-in-one solution if you were going to build an EV from scratch. www.metricmind.com/evision.htm
Hi Nick.
Yes it’s beautiful. It’s out of my price range too. It looks like a better featured e-meter. I must admit, My e-meter is a marvellous thing, it seems to give a good idea of my SOC (State of Charge) which is just about the most important think in an EV. My favourite two features listed for the EVision are instantaneous kW and Wh/mile. These things will help with your ‘fuel ‘ consumption.
My 8×6V PakTrakr and 8×6V remote have just arrived at the postal depot. I will try to fit in time to collect it tomorrow. I’m a bit excited to have a new bit of EV kit because it’s been a while.
This winter, James sent me an 8-battery portion of his LED bargraph after he got his PakTrakr.
Yesterday Ivan and I installed it into the ForkenSwift.
It’s awesomeness can’t be understated! Up to now, we’ve only been monitoring a single battery in the pack – the one we thought was the weakest. Rule of thumb: avoid drawing the weakest 6v battery below 5.25v under load to avoid damaging it.
Well, oops! Within 15 feet of driving, the dancing equalizer-like LEDs showed us several things, not the least of which was that we had NOT been monitoring the weakest battery all this time. We had a true dud in the pack that fell on its face under the lightest load.
It’s very cool to watch, and immensely useful. Every EV should have one (esp. if you’ve got an older, less balanced pack).
Thanks, James!
No problem, glad you like it!
Just posted a YouTube video showing James’ LED battery gauge in action .
PS: don’t mind the messy “installation” (and I use that term loosely – there is duct tape involved after all).
We’re going to be tearing the car down next week to clean up & re-paint the motor compartment, battery racks and all the components before reinstalling everything properly.
Wow. What a great video Darin! I wish I had your presentation skills!
I think I am going to go digital with the next meter. Making a nice display for the Paktrakr out of a handheld device or an old laptop.
The Paktrakr is really nice and convenient, but it doesn’t have the ease of reading and nice quick response of the LED bargraph meters.
That LED bargraph Looks great!
Any way of getting one or posting Part 2 of how to construct it?
Looks like it is a fine piece of hardware to have. Just glance at it and you know immediately what each battery is doing. Nice work James!
Hi Jim
I have 8 × 6V modules left over at the moment. They are not as neat as Darin’s but they work. Yes, I really should post part 2 and give out the instructions. The individual Bargraphs are not hard to make if you get in a routine.
Update: since installing the LED pack monitor, I’ve swapped out four of the pack’s weakest batteries with four other, slightly less weak batteries from the “lead reserve”.
Usable range has gone up from roughtly 12 to 15 km. For the ForkenSwift, the ability to effectively use the LEDs as the display of 8 simultaneous load testers is incredibly useful!