Sate of Charge LED Battery Meter

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Sate of Charge LED Battery Meter

Postby Dee Bee » Wed May 17, 2006 12:20 pm

Here is a do it yourself plan

http://www.homepower.com/files/bat-o-meter.pdf

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Postby Q » Wed May 17, 2006 1:14 pm

I built one of those years ago and it works just fine. Comparing the price of electronic parts and the price of a $3.99 Harbor Freight digital VOM, I think the VOM would be cheaper and more useful.

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Postby EZ » Wed May 17, 2006 3:32 pm

Ya, do-it-yourself is cool, but the Harbor Freight digital is only $2.99 on June 2,3 and 4. I think that's in-store only (Sidewalk Sale). I am mouting one of those on a wall connected to my main DC line-in (fused) along with the little colored card that I printed out from The 12 Volt Side of Life that gives some indication of the battery condition by the voltage output.

Hey, can still build it anyway, put it in a nice little case and it would be custom.

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Postby apratt » Wed May 17, 2006 3:56 pm

I looked on the harbor freigh site, can't find it. None for 2.99 or 3.99. Exactly what is it called.
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Postby bledsoe3 » Thu May 18, 2006 2:12 am

I paid more than $2.99, but it looks cool.
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When hooked up it has red LED's to show battery voltage.
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Postby GeorgeTelford » Thu May 18, 2006 5:14 am

Hi

Just a quick point, unless the battery as been left to rest for 12 hours with no charging or drain then a voltage meter is not going to tell you state of charge.

Also to guage state of charge from a meter you need to perform the following.

First fully charge the battery (with a proper 3 or 4 stage charger, not automatic or any other kind)

Then leave the battery unattached to anything for 12 hours, then take an accurate voltage reading.

for example let us say that after the above it reads 12.8 Volts that is your 100% fully charged point, if you then count every tenth of a volt lower as 10% less charged you will not go far wrong.

ie 12.3 Volts would be around 50% discharged (when measured after a 12 hour rest)

The DIY meter at the start is no good at all for battery monitoring, d9 to d11 are the only three leds in the zone and still do not match what you need to know.
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Postby Kaos116 » Fri May 19, 2006 12:13 pm

It's interesting that this thread appeared when it did. A friend of mine is into amateur radio and asked me if I knew of a circuit that would give him a quick view of the state of charge of his emergency battery system that he uses for 'Skywarn' activity. I did understand the circuit design in the link above, but do not know what 'useful' readings would be. I feel I could modify the circuit to get it to read a 1v range. So, the questions are...

1. What would be a useful range ie. 12.8-11.8? 13-12?

2. For a ball park reading as he went about his business of transmitting, would a volt reading that didn't have a 12 hour stabilization period be useful? For example, he's been operating for 3 hours and gets a break in the activity, removes the load to the battery, takes a reading. Would this reading be of any use?

His concern is, as I understood it, was to know at what point a battery is 'empty', to swap it out and not deplete it to a damaging point. This circuit would be great for him as it could give him a color code indication and not have to remember 12.3 is x% remaining

Thanks,

Todd
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Postby Sonetpro » Fri May 19, 2006 1:49 pm

I just bought a automotive voltage guage. $20. Not a precice as as George descibes but close enough. I don't need to know the exact voltage to the % of a volt, I just need to know when to charge it. But If I really needed to know down to that exact state (don't know why I would), I would just borrow a Midtronics test set from work and test it.
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Trimetric Meter

Postby Q » Fri May 19, 2006 1:53 pm

I've had one of these installed on my home solar power system for many years. Works great but it's a bit expensive for a teardrop.

http://www.bogartengineering.com/trimetric.htm
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Postby GeorgeTelford » Fri May 19, 2006 4:50 pm

You couls have the most expensive Voltage tester in the world and it still will not give you the state of charge on a recently charged or discharged battery.

The only way to do that with a voltage meter is outlined in my last post.

If you are out and about in a tear waiting 12 hours to check state of charge is a non starter.

That said you can with experience gain a little insight to the batteries condition.

Lets say that under a proper full charge test as outlined in last post that 12.8 is fully charged then 12.3 is 50% (roughly speaking)



If under load the meter is showing 12.2 ish switch all loads of for 10 mins and then measure it should recover enough to give you a clue say it goes up to 12.4 in that time then you have some usage left yet.

Supremely accurate? no, but an indication and better than most battery abusers do.
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Postby Sonetpro » Fri May 19, 2006 5:51 pm

George,
We have huge DC plants at work -48V 5k to 10k Amps. Im talking 24 2.2 volt battery's that weigh several hundred pounds apiece. These are always on float. When the technicians test them they use a Midtronic's test set and a biddle meter. They are never taken off charge from the rectifier to be tested. They are tested once a month. They are tested for Voltage and conductance per cell. Like I said they are never rested before being tested. Any battery will self discharge after 12 hours.
So I am missing your point. I don't think most people on this board are interested in the theory of battery's They just would like to know at what point you need to recharge them. I think that's pretty straight forward. If it gets to 12 volts recharge it. I certainly am not going to be without power for 12 hours while camping just because I want to know the exact voltage at which I should charge.
Theory is grand, but real life is somewhat different.
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Postby Q » Fri May 19, 2006 8:22 pm

George,

The link in my last post is to a cumulative amperhour meter. It's pretty darned acurate. It monitors the amperhours drawn from the battery and the amperhours of charge. It also figures in the age of the batteries, charge efficiency, etc..

Q

GeorgeTelford wrote:You couls have the most expensive Voltage tester in the world and it still will not give you the state of charge on a recently charged or discharged battery.

The only way to do that with a voltage meter is outlined in my last post.

If you are out and about in a tear waiting 12 hours to check state of charge is a non starter.

That said you can with experience gain a little insight to the batteries condition.

Lets say that under a proper full charge test as outlined in last post that 12.8 is fully charged then 12.3 is 50% (roughly speaking)



If under load the meter is showing 12.2 ish switch all loads of for 10 mins and then measure it should recover enough to give you a clue say it goes up to 12.4 in that time then you have some usage left yet.

Supremely accurate? no, but an indication and better than most battery abusers do.
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Postby asianflava » Fri May 19, 2006 10:30 pm

Sonetpro wrote:Theory is grand, but real life is somewhat different.


There is a big difference between book knowledge and practical experience. We have engineers (usually fresh outs) who tell us how it is supposed to work. When they finally leave their desk and make it into the factory they find out that the machines don't run exactly as spec'd out. Boy are they suprised.
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Postby GeorgeTelford » Sat May 20, 2006 5:59 am

Hi Asian

I Have the practical experience as well as the knowledge, my motorhome has a 10 X 110 Ah battery bank weighing one third of a ton, I could run full mains for over a week in the boonies without ever using a genny. Including running a fullsize mains freezer.

Fully qualified electricians seek my advice on setting up motorhome mains and 12v electrics and the interactions between Invertor mains and gennies.

Q

You meter is good and is one of the best available device's for measuring SOC, I was discussing the others misconceptions that battery SOC can be measured easily with a voltage meter. BTW I have a Sterling 4 channel unitdoes the same Job

Sonetpro

If your engineers were measuring voltage alone when its on charge they cannot gain any useful info on state of charge, all they will be measuring is the output of the charger. BUT with inteligence and known cell conductance they can get an aproximation of SOC, but this is totally impractical for people in tears. because they have no way of measuring individual cells, the rate of resistance only moves a tiny amount.

The idea of waiting 12 hours is that surface charge will skew your readings, surface charge dissapates after 12 hours, similarly due to peukerts law a batteries voltage will be lower than its actual/real steady state if the battery as been discharged (used) recently, BUT if the discharge was not heavy (like in a tear just for lights say) then a close approximation can be had by leaving for 10 mins.

All

To see this effect in action measure your truck battery voltage before any use, then start and immediately switch it off now check the voltage, it will be well down typically below 12 Volts, then check again after 10 Mins, it will be pretty much settled again.

now try start leave running for 1/2 hour then switch off keep checking voltage, in 10 mins it will be lower nut not at its settled state check again the following morning, back to square one.
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Postby Sonetpro » Sat May 20, 2006 7:07 am

George You are right I am wrong.
Since I only have 25+ years experience in industrial DC Plant design and hands on testing and maintenance. And measuring voltage while on rectifier without resting is only the industry standard. I will say nothing further. :MLAS

I will have to go to my senior engineer's and explain to them this is why our wet cells only last 25 years, and our AGM's only last 15-20 years. :D
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