Super Simple Cheap Battery Gauge

I Know that Jeremy posted a nice simple battery meter project a while back but thought I would show you an even more simple one that works really well, is totally analogue and bullet proof and is dirt cheap 8)

On my battery my pack capacity drops in direct proportion to the displayed voltage as with most chemistries of course but with Lipo this can be seen clearly, I have a watts up meter in the rack on the back of the bike but I wanted a quick glance meter for the front of the bike so on long rides I dont have to keep stopping and checking the meter in the bag on the back of the bike.

I used to use a 24V one on my last rig and it worked so well, 4QD sell these little battery meters here in the UK, their website is really old fashioned but they have been around for years and are fine to deal with, the 7 Segment meter can be had for 10 UK pounds! a total bargain, they will also make them to your battery spec if you need them to.

I will attach it with a bit of sticky velcro to all my rides making it quickly removable, this works as a great cheap ebike fuel gauge, I also run my Lipo buzzers on the back too for extra safety.

So if you want a hot swappable, super cheap, reliable and durable battery meter I dont think you can really beat this?

I will post a picture of it and a little clip of it working in the week some time, they ship to the rest of the world as well.



The LEDs extinguish at approximately the following voltages (other specifications to order):-
7 LED (48v) 5 LED (24v) 3 LED (12v)
LED 1 49.7 - 49.2 Green
LED 2 47.9 - 47.4 Green
LED 3 46.1 - 45.6 25.1-24.6 12.3 - 11.8 Green
LED 4 44.3 - 43.8 23.3-22.8 Green
LED 5 42.5 - 42.0 21.5-21.0 10.3 - 9.8 Yellow
LED 6 40.7 - 40.2 19.7-19.2 Red
LED 7 38.9 - 38.4 17.9-17.4 9.0 - 8.5 Red

http://www.4qd.co.uk/prod/bcm.html

Wouldn't a $5 digital multi-meter from Harbor Freight be a more accurate, economical and versatile tool?

SamTexas said:

Wouldn't a $5 digital multi-meter from Harbor Freight be a more accurate, economical and versatile tool?

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no way, the LED concept allows you to gauge battery life "at a glance"

Exactly 8)

This is the whole point! at 30mph I need to see the level at a glance or in my peripheral vision even, unless they are big and back lit you wont see an LCD display at night either let alone notice your voltage has dropped too low, also your peripheral vision notices the colours much better than you would the digits, also did you check the size of this thing? its tiny well compared to a digital volt meter anyway the accuracy I had on the last one was good, its doesn't need to measure milivolts this thing it just gives you a visual guide to your battery health and level.

The whole point of this is not to try and replace a watts up meter or a CA even its something very cheap and very simple but very effective way of quickly and safely gauging your battery level particularly when on the move, Unless you run a shunt and a CA there isn't any cheap and simple option to display your battery level on the bars, you don't want to mount the watts up on the bars as that means extending the power leads.

the 24V unit I ran for 4-5 years was great, that was mainly on NIMH and lead but it worked so well, this unit is just what I am after and will provide, battery status when riding, then if I need to I can check the watts up meter in the battery bag, cheap, simple and very effective

And they're made from (mostly) a bunch of PNP transistors, with resistor ladder for voltage divider, so it's theoretically easy enough to fix or modify one for your particular voltage range, too.

This isn't the exact circuit for the BCMs, but it is similar:
http://www.4qdtec.com/LED/avu.html
The actual schematics for the BCMs are available to paid members of the 4QDTEC site (as are quite a number of other interesting bits and bobs, including a TON of info about the 2QD controller and PWM motor control), as long as you realize that the information from there is not to be shared. (otherwise, if abused, they could just make all the rest of it go away, preventing future access by others ).

Definitely a "cool tool" Knoxie! Thanks! Richard Torrence has written some very useful stuff on speed controls too.
otherDoc

When I had the Magnetometer Business, I had my tech build some battery testers for Metal Detectors.

He used 2 green, 1 amber and 1 red, and wired them to different capacity resistors. Drilled 4 holes in a 2" PVC pipe cap, and potted the parts.

I just saw a Club Car Golf Car, a couple days ago, that had a small Battery Indicator on it's dash. It was, maybe, ½" high X 1¼" long. They run 48V in their cars.

Yes exactly guys, I think you can tend to think you need a complicated solution with Lipo batteries when in essence you really dont (unless you are a total noob of course) then there does need to be more redundancy in the system.

In essence, all you are making is something called an expanded scale voltmeter - there are heaps of different designs out there, just have a look. Dot/Bar drivers come readily to mind, with a high voltage LM317K or something for voltage regulation for the IC.

Here is a car battery "kit" for 9 bucks:
http://www.futurlec.com/CarBatteryMonitor.shtml
I imagine it could be modified to fit a variety of voltage ranges

hi guys
At $5 cheap enough I thought.
http://www.ebike-bmsbattery.com/

Regards John.

Got them both in the post today, great service and speedy delivery now just got to pot them in a couple of little boxes and we are done. 8)

jmack said:

hi guys
At $5 cheap enough I thought.
http://www.ebike-bmsbattery.com/

Regards John.

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Thats a great find. Thanks for posting

I think this is a more direct link for those that didn't click the BMS tab.

http://www.ebike-bmsbattery.com/index.php?url=products.php%3Fdid%3D6&did=6

Post up component values and a pic of the rear of the board

jmack said:

hi guys
At $5 cheap enough I thought.
http://www.ebike-bmsbattery.com/

Regards John.

Click to expand...

Is that a single cell monitor ??

Knoxie, if you haven't potted em yet, can you see what those devices are? are they TL431/LM431's? Cheers

Hillhater said:

jmack said:

hi guys
At $5 cheap enough I thought.
http://www.ebike-bmsbattery.com/

Regards John.

Click to expand...

Is that a single cell monitor ??

Click to expand...

I believe it is a whole-pack voltage indicator. Not meant to save your Lipos, just to let you know how much more your pack will take you

I thought it could probably use the popular LM3914 chip.. it can control up to 10 leds




The meter look like the old VU from the 80' tape recorder

Doc

jonathanm said:

Knoxie, if you haven't potted em yet, can you see what those devices are? are they TL431/LM431's? Cheers

Click to expand...

Ill get my magnifying glass out :lol: yes no problem will have a look later

They are PNP transistors, for the most part. Take a look at my post above for a link to a similar device by 4QD.

amberwolf said:

They are PNP transistors, for the most part. Take a look at my post above for a link to a similar device by 4QD.

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YEP BC556 transistors they are dirt cheap as well

Thanks. A few options to make these little guys then. I see there's cheapish PCBs on ebay for those LM3914 chips, too...

The LM3914 is a nice circuit but its pretty linear in that the LEDs will simply extinguish when the voltage threshold drops below the split in the internal divider circuit where as the circuit in the 4QD unit means that the top led in the chain will gradually fade and then switch off, it gives a nice smooth feel to the level in the battery if you like!

I managed to get it all in to a nice little box last night, I had to shorten those legs on the leds though, will post some pictures later.

Today I will receive a parcel from G-Power with a ebike kit and an Fuel Guage;

At product description :

">Apply to PCB/PCM of 1~13 cells li-ion battery pack.
>Apply to PCB PCM of 1~16 cells LiFePO4 battery pack.
>No external power is need.
>Can connect directly to PCB/PCM via special connector on
board or connect directly to output of battery pack.
>5 LED display battery pack's voltage.The voltage is listed below
is base on battery during discharge.(the actual voltage might vary for LED 2~4 during charging.)
>Power consumption: 80uA
>Dimension(LxWxH):51mmx26mmx4mm
>Net Weight: 6.5g

0% LED Flashing for singel cell voltage < 3V (must recharge battery)
1 LED = 20% for singel cell voltage: 3.0~3.3V (battery need recharging)
2 LED = 40% for singel cell voltage: 3.3~3.5V
3 LED = 60% for singel cell voltage: 3.5~3.8V
4 LED = 80% for singel cell voltage: 3.8~4.1V
5 LED = 100 for singel cell voltage: 4.1~4.2V (battery is full)"

I will see if it's real

Had to tweak the meter a little to make it match perfectly my battery, slipped a 160 ohm base resistor in there now its perfect, made a little label for the front of it, I am going to print it off and cover it with some clear plastic to seal it from the elements, here is my battery label made with the fantastic Gimp image editor which is the next best thing to photo shop and its free~!!