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  • Author Author: jw0752
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Back Up Battery failure inspires new test tools.

jw0752

The computer in my lab is very important to me. This of course is very subjective as its contents and utilities are understood and utilized only be me and so someday, when I can no longer use it, it will go directly from my lab to the computer recycle facility. In the meantime I do everything in my power to keep it clean, backed up, and with a protected UPS power supply. The original battery of the 650 Watt UPS is long since failed and when it did I bought a car battery and rewired things so that I can monitor the voltage of the battery and the discharge rate when it is being used.

 

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Here is the battery in its case with the volt/ammeter mounted in the cover. Every couple months I pull the mains electrical plug and let the system run for an hour or so. Two nights ago I performed this routine test but within 10 minutes the UPS was beeping frantically for power and before I could get to the mains plug it died.

 

I took the battery case and opened it to find that while I was checking everything else I had not been monitoring the water level of the battery and 3 years of constant float charging had finally dropped the fluid level to about an inch of the bottom. This is not a good situation and there was not much else to do except to replace the battery. This time I got a battery with ADM maintenance free technology and an 8 year warranty. We will see how that goes.

 

I am not one to give up too easily and my thoughts went to the possibility of resurrecting the old battery. Just to show how bad the water situation had gotten the battery took almost 3 liters of distilled water before it was full again. I put the battery on a small charger and left it to charge.

 

The next problem that caught my attention was how I was going to test and measure the health of the car battery. When I have this problem with small batteries I hook them up to the Process Duration Timer

 

https://www.element14.com/community/videos/22548/l/first-test-of-process-duration-timer?sr=search&searchId=f826766b-bfaf-4e39-bdc2-afd26ffc3843&searchIndex=4

 

Short video of the Process Duration Timer (PDT) unit in operation. You can also search on this site for the original 4 blog series if you want to know more about the Process Duration Timer build.

 

The size of this battery however seemed likely to strain my present test abilities. One thought that seemed to make sense was that I would use a spare UPS Backup unit with a 75 Watt incandescent bulb and use that for the load.  The next problem would be how to interface the PDT to the system so that I could time the draining of the battery until the shut off voltage (which I didn't know) of the UPS was reached.

 

This was the inspiration that I needed to build an Interface for the PDT. As it sits the Process Duration Timer (PDT) was designed to monitor and time a voltage level and respond by stopping the clock when the monitored voltage level drops below a given threshold.

 

At its core the Interface is very simple with an LM393 comparitor, a MOSFET output and the required resistors and hardware needed to allow a sensor  to monitor a given process by watching other physical changes in the system and reporting back to the PDT with a High or Low voltage output which in turn is used to stop the timing of the process.

 

Here is a schematic for the Interface:

 

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It was my plan to build the Interface and let its ability to monitor a light level watch the 75 Watt bulb on the output of the UPS that was being powered by the damaged battery. As long as the bulb was lit the interface would send 15 volts to the PDT and it would happily keep on timing the process of battery discharge. As soon as the battery level dropped below critical for the UPS operation however the bulb would go out and the interface would send a Low voltage signal to the PDT and the timer would stop and wait for me to return and read its output.

 

Before I get to the actual battery test however here are some picture of the Interface:

 

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I am using a DPDT toggle switch on the unit to give me the flexibility to transition from High output to Low output on the Interface in either direction, Light to Dark, or Dark to Light. The LED tells the state of the output and the potentiometer allows the operator to adjust the sensitivity to fit local light conditions.

 

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Here is a picture of the experiment set up. The battery has been charged for several hours and is showing all the signs that it is bad. The UPS is hooked to the battery and has a 75 Watt bulb plugged into one of its outlets. In a preparatory experiment I have measured the current demand on the battery to power the 75 Watt bulb. This turned out to be about 8.5 Amps. For some reason I don't understand right now the UPS's power demand is not stable but fluctuates +/- 1 amp while it is working. If anyone knows why please clue me in. The photo resistor sensor of the PDT Interface is pointed towards the 75 watt bulb. The Interface is set so that the output will transition from High output to Low output when the sensor no longer sees the light.

 

The PDT has been set to watch the Interface which should put out about 14.86 volts and continue timing the process until it sees less than 5.81 Volts.

 

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The timer indicates that we are 1 minute 23 seconds into the test at this point.

 

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We really did not have long to wait as 12 minutes and 30 seconds after the process started the battery gave up. If we do the math for 8.5 Amps over 12 minutes and 30 seconds we get about 1.7 Ah of power. For a battery that should have upwards of 45 Amp Hours this is a clear indication that the low fluid level has permanently damaged this battery beyond redemption.

 

Since I was still in the mood to upgrade the Interface I also made a Thermistor Sensor for it. The thermistor was chosen to work with the Interface in the range of 40 C to 130 C as I guessed that this might be a range that would be usable for me. Now I will be able to monitor and time processes that involve things that gradually heat up or cool down.

 

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The sensitivity control was labeled with approximate transition temperatures for future use of the thermistor sensor.

 

Overall I am pleased with the added flexibility of the PDT that the Interface has provided. It is the silver lining of the cloud that was the failure of the backup battery.

 

John

 

 

  • in reply to jw0752

    13.6v is appropriate for Gell Cells.

    I suspect that is why it 'cooked' your battery.

     

     

    If anyone else is planning to use Lead Acid batteries, there are some for mainly European cars that have an external vent via a 1/4inch tube.

    I used one once in a vehicle when the battery got relocated and I didn't want acid fumes in the wrong place.

     

    There are some sealed versions of the case John used, but they aren't as cheap or readily available.

    Strangely most of these are sold for boot/trunk mounting of batteries ....without the option to seal. image

     

    Mark

  • in reply to mcb1

    Hi Mark,

     

    This particular UPS floats the battery at 13.6 volts. I am sure you are correct it would be much better if it were dropped to 13.2 volts. This might be the primary cause of premature battery failure in UPS units, float voltage a tad too high. Of course the original battery was a completely sealed UPS specific battery and I was the one that put a regular car battery on it as a replacement. Your explanation of the reason for the fluctuation made sense too.

     

    John

  • I'm curious to know the float voltage you had for the battery.

     

    When we were maintaining remote phone systems way back when, they used a normal car battery and float charger.

    The initial setting from the electrical people meant they always needed water, and finally we settled on 13.2v float to keep them charged but not requiring constant watering.

     

    As as said, I suspect your battery is now scrap, and given that you want it to work when required, not something you need to keep.

     

    Why does it fluctuate.

    Most of these small UPS are designed to shut down the computer, either by a connection to it, or that beeping noise.

    I suspect that 75w is at the lower end of it's design range and the unit is struggling to regulate because the load is too small.

     

    You also have a battery that has a high internal impedance, and this may be impacting on the inverter as well.

    But essentially these things are simply and inverter and a AC generator and not a precison voltage source.

     

    For the 650w models we used, they were a SPS and the incoming mains was fed via an auto transformer and then provided 'inverter' mains when the voltage was outside the limits.

     

    Mark

  • in reply to jw0752

    I always enjoy your builds John.

    You are developing into a very competent maker.

     

    DAB

  • I should probably start a new blog for this but you are all probably getting sick of my Process Duration Timer so I will add it to this stream. After I finished with the interface I saw another simple improvement that I could make to the PDT unit. Up until now the PDT only worked on situations where the voltage from the Device Under Test (DUT) was falling towards the Target Voltage. By adding a switch that the Arduino could read as High or Low I was able to add another Mode whereby the PDT can also process DUT voltages that are rising towards a Target Voltage. The software of the Arduino also had to have a simple upgrade to accommodate the reversed process. Here are some pictures of the upgrade:

     

    image

    A little hard to see but the blue SPST switch in the center of the picture will pull the 5 volts supplied by a 10K resistor on Pin 11 of the Arduino down to 0 volts when it is in the closed position. This directs the computer to process the input voltage data to the PDT as Rising or as Falling.

     

    image

    A view of the end control panel of the PDT

     

    After I finished with the upgrade I performed a test of the unit. I set up an experiment with a 10,000 uF capacitor which the PDT monitored. I set the bench power supply to 10 volts and the Target voltage on the PDT to 6.32 volts which is one Time Constant. I selected 2 random 10K resistors from the bin. I then charged the capacitor through each resistor while the PDT monitored and timed the process as it went to the Target Voltage. Resistor "A" took 142 seconds and Resistor "B" took 145 seconds. The Ratio of Resistor "A" to Resistor "B" was 142 / 145. I next took an ohmmeter reading of Resistor "A" which turned out to be 9,869 Ohms. Using the ratio predicted by the PDT I calculated the value of Resistor "B" to be 10,077 Ohms. Finally I measured Resistor "B" and found that it was actually 10,038 Ohms. The reading of the PDT was therefore off by 0.4% which is acceptable for any experiment that I would likely do.

     

    I have begun to gather the materials and my thoughts as I hope to build another PDT and put it into one of the more professional looking cases. This is the usual way that I develop an idea into a tool. If you remember, my first iteration of the PDT was a device with a electro-mechanical clock and a comparator circuit that had to have an external target voltage supplied to it from a bench supply. Little by little as the short comings were recognized and the ideas came improvements were made. In case you want to look at the first PDT here is the blog from April 2015.

     

    https://www.element14.com/community/people/jw0752/blog/2015/08/16/battle-of-the-batteries--e-vs-d-vs-bargain

     

    The current model is still a hodgepodge of commercial clock, power supply, and microprocessor modules interfaced together to do a job but it is much more stable and accurate than the original build.

     

    Eventually I will try to work towards the following goals:

     

    A timer that will cover more than one 24 hour period before cycling to 00:00:00

    Accuracy to 0.1 or even 0.01 second.

    Auto start that better coordinates initiation of the process with zero seconds. Currently there is a slight delay in the start sequence (about a second) as the processor resets the clock.

    Slightly higher input voltage.

     

    It is all a process and it takes time. There was about a year between the latest upgrades to the unit and the previous one in which I installed a switch, a relay, and a software upgrade where I could choose to have the unit disconnect the external load or not when the Target voltage was achieved. If I finally get my mind in the right place to proceed with the building of a second unit in a more commercial case this might provide the opportunity to make some other as yet un-thought-of upgrades.

     

    John