Voltmeter with hold reading

Hello All,

I posted a few months ago about building a discharger for a DeWalt 18v battery. Thanks to everyone who contributed suggestions and ideas. We have our first one built and working.

A bit of background - We have a DeWalt 18v recip saw on our rescue van. We have used it during extrications where we have a great deal of water (i.e. a stream). We use our corded recip saw in less hazardous environments. We have three DeWalt batteries. Each battery rotates from 1) charger to 2) saw to 3) spare every week. It's not unusual for this recip saw to go unused for almost a year at a time. We needed some way to check the batteries. So, we decided to build a discharge unit to determine the condition of the batteries.

We would like to add one additional feature to our discharger. Some sort of voltmeter that we could hold the last reading. This would allow us to double check that we stop the discharge process at 15v.

We have a DPDT relay that we use to control a timer. This allows us to monitor how much time it takes for each battery to drop to 15v. We hope that this will allow to determine when we need to replace a battery.

Does anyone know of a voltmeter that would allow us to hold a reading via some sort of external relay or contact?

TIA, Mark Firefighter Sky Valley / Scaly Mountain Volunteer Fire & Rescue

--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world: 
those that understand binary, and
those that don\'t.

I've used an even simpler method for many years. The comparator opens a relay which removes power to an old-fashioned analog electric clock. The clock is set to 12:00 at the beginning of the test, and shows the elapsed time at the end.

Roy Lewallen

--
from your description, it doesn\'t sound like you need to hold a
voltmeter reading as much as it does to determine the amount of time
elapsed between when you started discharging the battery and when its
voltage, under discharge, fell to 15V.  An easy way to do that would
be to start a clock when the discharge process started and then to
stop it when the battery voltage fell to 15V.  A simple comparator
working against a voltage reference,  a counter with a 1 second time
base, and a seven-segment display frozen at the time accumulated when
the 15V discharge point was reached would do it.

I posted a few months ago about building a discharger for a DeWalt 18v battery. Thanks to everyone who contributed suggestions and ideas. We have our first one built and working.

A bit of background - We have a DeWalt 18v recip saw on our rescue van. We have used it during extrications where we have a great deal of water (i.e. a stream). We use our corded recip saw in less hazardous environments. We have three DeWalt batteries. Each battery rotates from 1) charger to 2) saw to 3) spare every week. It's not unusual for this recip saw to go unused for almost a year at a time. We needed some way to check the batteries. So, we decided to build a discharge unit to determine the condition of the batteries.

We would like to add one additional feature to our discharger. Some sort of voltmeter that we could hold the last reading. This would allow us to double check that we stop the discharge process at 15v.

We have a DPDT relay that we use to control a timer. This allows us to monitor how much time it takes for each battery to drop to 15v. We hope that this will allow to determine when we need to replace a battery.

Does anyone know of a voltmeter that would allow us to hold a reading via some sort of external relay or contact?

TIA, Mark Firefighter Sky Valley / Scaly Mountain Volunteer Fire & Rescue

-- Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world: those that understand binary, and those that don't.

Hi Mark, I remember your earlier posts.

Just how long do you want to hold the reading for?

Simply having a BIG electrolytic capacitor across the battery - that is disconnected by a spare set of contacts when the discharge phase ends - may do. The electrolytic will start to self discharge once it is disconnected, so it won't hold the sample for very long and will discharge (slowly) into the typically 10Mohm meter impedance once you start measuring the voltage. But you could try this out and it may be all you need.

If that isn't good enough, then you can use a simple unity gain very high input impedance operational amplifier circuit between the capacitor and meter. The capacitor can then be a lot smaller, eg less leaky. With suitable design (circuit layout as well as circuit design), that should hold a sample for the odd hundred + times longer, at a rough guess.

If that isn't good enough - yhen two other options spring to mind:

A cheap digital camera arranged to photograph the meter display at the end of the discharge phase. You can get them for under a tenner here, now. That will store the reading "indefinately".

A free-running A>D linked to a D>A converter. Simply trigger the D>A at the time the reading is needed - or do without the D>A and just use a latch and a row of LEDs.

--

Sue

Just to clarify a few points.

We have the elapsed timer working well.

We discharge the battery very, very slowly. The elapsed times are running between 5.5 and 6 hours. Because of the long elapse time, we can't just have someone hang around or remember to return to the station at the correct time to check the voltage.

What we want to do is record or hold the ending voltage somehow. As this is our first attempt at a discharger, we want some way to double check that we don't drop below 15v. We are pretty green at putting this together. And let's just say we aren't 100% confident that we have everything right.

Our first thought would be to use some sort of voltmeter that had an external method of triggering a "hold". We are open to suggestions.

TIA, Mark

--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world: 
those that understand binary, and
those that don\'t.

Mark,

It makes more sense to test the discharge circuit than to monitor it, in this case. Connect it to a variable power supply set to say 17 volts, and start the discharger. Slowly reduce the voltage from the supply, while monitoring with the DMM, until the discharger turns off. Do that a few times to make sure you get the same reading, and you will know what the discharger cut off point is. You can repeat that test whenever you want.

If you don't have one, you can build a variable supply for less than 10 dollars worth of parts - a 24 volt dc wall wart from Allelectronics costs $4.50, and you need an LM317 and the circuit on page 9 of

That will give you a regulated variable supply capable of

600 mA. I don't know what the discharger load draws, but if it is taking 5.5 to 6 hours to discharge the packs, 600 mA is more than enough.

I don't remember your discharger circuit - was it the TL431 driving a transistor that operated a relay?

Ed

--
Sounds good to me!

So, why not try my idea of using a capacitor to store the value - together with a relay contact that will isolate the capacitor at the end of the dischage phase?

From the above, I guess that you want to hold the value for a few hours, so an electrolytic isn't going to manage - so use a polypropylene instead.

Try this as a quick test: Get say a 10uF polypropylene capacitor, shove it across a battery to "sample" the battery voltage - disconnect it and leave it for whatever time you think you need. Then stick it across a multimeter and take the immediate* reading and compare it to what the battery voltage was when its voltage was "sampled". You may find that the readings are near enough for what you need.

*The voltage on the capacitor will start falling quite quickly - once the multimeter is connected - so you do have to take an instant reading.

--

Sue

Ed,

A very good point about testing the discharger. I'll mention this to some of the crew tonight (we do training every Tuesday evening).

Whoa.... The circuit on page 9 went way over my head. I'm always willing to learn something. Can someone translate the circuit into simple terms?

We used a DPDT relay that looped back to keep itself energized. We have 660 ohm resistor and an adjustable potentiometer in series with relay. We used the adjustable potentiometer to set the relay to drop out at 15v. I don't understand enough electronics to use a TL431. From the research I did, a TL431 seemed like something we could use. I just didn't understand it enough. Something like a DPDT relay and resistors

- I can understand. A TL431 seemed more exact than resistors. Again, if someone can explain using a TL431 in simple terms, I'm all ears.

Just so I don't forget THANKS to everyone for the suggestions.

I hope we don't have to use our recip saw on a vehicle extrication. But, thanks to the generosity of folks here, we'll have full use of the saw and batteries.

Mark

--
Email hint - Everything after the @ is spelled backwards.

There are only 10 types of people in the world: 
those that understand binary, and
those that don\'t.

I don't know if that will work with the capacitor.However, I have a nicd battery charger/discharger for my digital camera.The digital camera uses only partially the charge of the nicd battery, so they need to be discharged first.mg however wants to find if the batteries are ok, AFAIK nicd batteries last up to 1000 charge-discharge cycles, and I think IMHO is a good idea to run the saw from time to time so that condensed humidity will evaporate.The nicd batteries according to my experience can keep their charge for months, as I have a pocket light with nicds.

-- Tzortzakakis Dimitrios major > > Just to clarify a few points.

Any particular reason why it shouldn't be worth trying?

The 10uF polypropylene capacitor will, of course, start to discharge once removed from the supply - but the internal leakage resistance will typically be of the order of greater than 1,000,000,000,000 ohms (from a spec.sheet) - so it will take a while (over a year to fall 2%, if that was the only leakage). There will be surface leakage as well, but that shouldn't be too great - it isn't as if it is mounted on a pcb and will need guard rings and the rest. Its voltage isn't being constantly monitored, so won't be loaded by that circuitry. So, it should hold the voltage for quite a few hours, without difficulty.

Of course, once the meter is put across it, it will start to discharge into the meter input impedance. However, even if you take 2 seconds to read the meter, the time constant is such that the value will still be within 2% (and the meter is probably only accurate to that..). If you take 5 seconds to read the meter, the value will still be within 5%.

If anyone has a 10uF polypropylene capacitor, a 15 volt supply and a test meter, perhaps they would give this a try and report back?

--

Sue

I'll try a diagram: D1 +---------|\\ --- C2 / R2 --- \\ | | | Gnd Gnd

Ok, first D1 and D2 are diodes that are used to protect the LM317. The symbol ---|

Here's the TL431 circuit - 3 resistors, a pot, a relay a diode, and a momentary pushbutton switch:

Relay Contact2 / +---------------------o o---------------+ | | | Relay | | Contact1 | | / D1 / +----o o---+-----|

Yes, of course, but I mean it's not a *standard* industrial methode.Of course, anyone can try whatever he likes, particularly in low voltage circuits, which are a nice field of experimentation.Mains voltage is a different story.AFAIK, nicd batteries need discharging in a discharger only if they are partially used in the appliance before charging them, due to memory effect.

-- Tzortzakakis Dimitrios major > > I don't know if that will work with the capacitor.

Make that DCTX-2460 ^

I risk showing my ignorance here, but amongst the bewidering myriad functions on a digital camera's menu, isn't there one where you can set a time delay of 5.5 hours? If so, just photograph your voltmeter!

--
John Savage                   (my news address is not valid for email)

How about just measuring the battery voltage when you get around to it after the test?

Once the relay opens the battery should stop discharging.

Of course it'll slowly discharge but if it is any good at all a few hours won't make any meaningful difference in the voltage you measure.

Say that you load the battery for 5.5 hours (no comparator circuit, just a timer) and come back to measure its voltage 8 or some other number of hours later. If the voltage is still above 15 you are sure that it at least met the 15V after 5.5 hours of load criterion.

If the voltage is below 15, you know it either failed the test at 5.5 hours or was close to failure and self-discharged to an unacceptable level thereafter. Either way it is no good.

Dave

You need to let go of this mindset.

Throw the timer away. (or put it back into stock ;-) ) Turn off the discharger based on voltage, not time. _That_ will ensure that you don't drop below 15V, _and_ it will ensure that you _do_ _reach_ 15V.

Use the circuit that ehsjr suggested (or any threshold comparator- type circuit) to turn off the discharger _when the battery voltage decreases to 15 volts_. That way, you're assured that the discharge stops when the battery is at 15 volts. It doesn't matter how long it takes, and in fact, shouldn't be expected to take any particular amount of time, because it will vary depending on the condition of the batteries when they're mounted.

Now, if you think it's important, you could put an ordinary line- operated, motorized electric clock on another set of relay contacts. Set it for, say, 12:00, turn on the discharger, and go home. When you come back the next morning, the time when the clock stopped will be "how long it took for this battery to discharge to 15 V." (or that plus 12 hours ;-P ) (don't set the clock to the actual time of day, unless you want to write it down and do arithmetic in hours & minutes.)

And, of course, you can check the battery voltage at your leisure - that should hold up better than a capacitor. ;-)

Cheers! Rich

If I understand the criterion correctly, everything between 1)-6) must have passed the test.; i.e a known load for 5.5 hours did not discharge below 15.0V. All others failed. Consider result 7), at best the loaded battery dropped to 15.0V in 5.5 hours then lost an additional 0.1V with no load on it in 8 hours. Clearly this is not a battery for long term standby application.

I agree that he wants to minimize uncetrainty. The test I outlined is not very uncertain regarding ability to deliver a particular charge in a particular time and having a low self-discharge rate.