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What does Battery Voltage Mean?

Discussion in 'General Technical Questions and Answers' started by KR1CKM, Dec 2, 2010.

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  1. KR1CKM

    KR1CKM Premium Subscriber QRZ Page

    Ok, hear me out, before you respond with "Its the voltage of the battery!" :)

    I recently bought a Yaesu VX-170 from the Swapmeet here (and I love the radio, btw). It comes with a "7.2v" battery - its written right on the little label inside the battery. Seems pretty easy, so far, right?

    Right.

    So, when you turn the radio on, it generously provides you with the current voltage level of the battery. Right after a charge, I have seen it read as high as 8.4V. WHA?? :confused: How that happen?

    After blabbering on for way to long yesterday, the little "low batt" indicator started blinking. Eventually, the level got so low that the radio was kind enough to cut off my transmission, rather then deep-discharging the battery, I assume. So, I turned the radio of, turned it back on to see the reading and it said "7.2V".

    Now, wait just a minute!! How is 7.2v "low" for a 7.2v battery?

    What am I missing? Is that rating on the battery meant to describe its lowest possible charge level? Or am I just being too literal?

    Thanks, folks.
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    >>For one thing, it may not be a terribly accurate gauge. Measuring the battery with a known accurate DC voltmeter will tell the story. But also, recently charged batteries commonly run about 10% over their standard "float" voltage -- even the one in your car, which is a 12V battery. If you've just taken it off a charger it will measure over 13V. Draw some current from it, or let it sit a while, and it will be back at 12V again.

    Again, the indication may not be all that accurate; but even if it is, 7.2V in "standby" (not transmitting) doesn't mean the battery has the capacity to deliver transmit current, which is a much larger load. When it reads 7.2V as in your example, try transmitting and see what it reads.
     
  3. W1VT

    W1VT Ham Member QRZ Page

    Going from 8.4V down to 7.2V is perfectly normal for six NiMH cells. It could be that the labels were printed for NiCd cells, but the manufacturer decided to use them rather than print up new labels.

    Zack
     
  4. VE6JKV

    VE6JKV Ham Member QRZ Page

    Battery voltage is actually a nominal voltage. NiCD or NiMH cells are rated at 1.2 volts, but their fully charged voltage can easily range to 1.45 volts per cell. Your pack has 6 cells, which gives a nominal voltage of 7.2 volts, but at 1.45 volts, that would add up to 8.7 volts. Now, this is an unloaded voltage, so once the radio puts a little load on the batteries, 8.4 would usually indicate a fully charged pack. You're probably never going to see 8.7 volts even without a load, but that really doesn't matter. You now know that once the voltage gets down to 7.2 volts, the battery needs charging. The accuracy of the voltmeter isn't really at issue, but even if it was inaccurate, as long as it's consistant it will tell you when the batteries need charging.

    Hope this clears things up a little

    Jim
     
  5. AG3Y

    AG3Y Ham Member QRZ Page

    Various re-chargable batteries will tend to "bounce back", which is to say that their open circuit voltage will tend to rise up toward a higher value even when they are fully discharged. However, there is no current capability, so the moment you try to draw any power from the battery, the voltage will drop, again.

    I have some electric razors that illustrate this very well. If you run them until the motor stops turning, you can turn the switch off, wait a few minutes, turn the switch back on, and the motor will turn for a moment or two. But it very quickly stops again. This can be done a couple of times before the motor finally refuses to turn.

    No doubt, if you measured the voltage it would be higher, until you turned the switch back on again.

    YMMV but that is the general idea.

    Jim AG3Y
     
  6. KD0CAC

    KD0CAC Ham Member QRZ Page

    A true measure of any battery is a load test , having an amp & volt meter readings as the battery is loaded , within its normal use range .
    Back to the car battery , a fully charge 12v battery at rest [ sitting lets say for 8 hrs. after charging ] should be 12.75 volts , then as long as many circumstances are kept to normal use , temp , load being applied [ engine & battery matched ] a automotive battery should never be used more than 20% of its charge to start a car , best case - warm weather maybe 10% .
    These batteries for your HT are designed for extended use , so not having the data sheet on that battery only guessing to its rated draw down higher percentage of discharge is part of the design .
    Back to flooded lead acid [ because its the ones I know more ] deep-cycle batteries [ similar in use to yours ] are intended to be use to 80% discharge .
    Its all about watching volts & amps , a discharge battery may show close to good volts at rest , but if near its design for charge , the amps are gone .
     
  7. K8MHZ

    K8MHZ Ham Member QRZ Page

    Hopefully the answer sought

    Your radio will operate on voltages from 6 to 16.

    The Low Batt indicator will come on when the voltage drops to around 6.

    The battery packs are (6) 1.2 volt AA cell NiMH for a total of 7.2 volts at rest.

    1.2 volt MiMH cells will show 1.4 - 1.6 volts each after a full charge and take some time to settle down to 1.2 volts each.

    What is likely happening is that when you transmit your pack is dropping to the min level (6 volts) and recovering when you un-key.

    Since you have almost no head room with a 7.2 volt battery, one cell starting to fail will render the entire pack bad and cause the readings you are seeing.

    NiMH batteries will work quite a while as they fail. That being said, once a pack is suspect and an new one is purchased, the old one can still be used for shorter periods of time and as a back up pack. I use mine until they puke right out. I have several for my FT 50, along with 2 AA cell cases. Some last years, others not so long.
     
  8. WA9CWX

    WA9CWX Ham Member QRZ Page

    Think of voltage ratings as campaign promises.
     
  9. KE5FRF

    KE5FRF Ham Member QRZ Page

    It sort of helps to think of a battery like a balloon. When you blow into the balloon and inflate it, the balloon stretches out to allow for more capacity to accomodate the pressure. But once the balloon is inflated to the point that the rubber becomes tight, the pressure inside does not increase by a great amount, even as you continue to inflate.

    Voltage can be seen as much like the balloon pressure. Once you charge the battery up to its rated voltage, you start consuming its capacity. No, it doesn't physically stretch to accomodate more, but the principal is similar. And then, there is a capacity limit too that can be exceeded and damage the battery. (overcharging)

    So, when your battery is low and still putting out 7.2 volts, you can think of it as the balloon that has been deflated to JUST THE POINT that the rubber walls are tight but not stretched. Release just a little bit of the air, and the pressure will drop off very quickly as the balloon deflates to ambient pressure.

    "Not enough oomph anymore".
     
  10. KM3F

    KM3F Ham Member QRZ Page

    There is another way to look at the effects of battery charge.
    Think if you had a battery that had a constant voltage output but then you added a series resistor between it and the radio.
    With no resistance the voltgae would be that of the battery.
    As you add resistance the radio would see a lower voltage.
    What this become equivelent to is battery impedence gets higher as the battery discharges, lowering it's voltage "available" at it's terminals (while under the load) from the radio.
    As the impedence increases, the radio reaches a point the circuits won't operate.
    When you charge the battery, you reverse this higher impedence back to a lower value by reversing the chemical composition within the battery.
    Chargeing allows the chemical change to reverse but quickly falls back to the design nominal value from internal leakage, temperature, time and load use.
    Many batteries left discharged and open circuited are sensitive to temperature changes and leakage so you may see this with a voltmeter that loads the battery very lightly.
    As well, overvoltage from charging can also be seen and will bleed down just from looking at it with a voltmeter.
    It's not a mistery but one of knowing how various types batteries react in various states of charge and loading.
    From the above, you may see that 'cells' in series are never better than the worst cell in the chain.
    The worst cell becomes the the highest circuit resistance and becomes the limiting cell to the final output voltage and current flow ability of the whole battery assembly.
    Hope this helps you without getting to deep.
     
  11. KD0CAC

    KD0CAC Ham Member QRZ Page

    Km3f , that's why when ever one of my multi-battery units [ flashlite whatever ] dies , I take out the batteries check them all with a meter , I almost always find one bad cell , just replace one battery .
    Really cuts down on buying batteries .
    Back in the 70s-80s I use to work with an old Oky [ someone from Oklahoma ] he was a radar tech in the Navy a long time ago .
    And he was into Government & utility auctions , when we found a bad battery [ old ones with tar between the cells ] then late the plastic cases also , we would find the bad cell and replace it with one of the same size and put it back together , the tar batteries we heated the tar and poured it back into the gaps between cells , the newer plastic case batteries we us a hot glue gun to reseal the case .
    With 20+ acres of equipment , he save a lot of money doing these kinds of fixes .
     
  12. W2ILP

    W2ILP Ham Member QRZ Page

    Batteries must be tested ubnder full load conditions.

    A battery that has been discharged theoretically looks like a battery with a series resistor inside of its case. With no load on the battery it will read the nominal voltage that it would be rated for...but with a load on the battery its internal resistance causes an IR drop and the voltage being fed to the load is decreased.

    If you want to test a battery you can not simply do it unloaded with only a voltmeter. You have to load it with a resistance that would be equivalent to the load that your radio presents when you push to talk. The greatest current load exists when you transmit. For an SSB radio, when you transmit and modulate. You can not use the radio itself to make the test. You will have to find out how much current is drawn from the battery when you transmit with a good charged battery; then you can calculate the required resistance that would be required to simulate the load....and get a power resistor of that many ohms to draw that current and run future battery tests.

    Weeding out bad cells is not always a good idea. If a cell is bad it is best to replace gthe entire battery, as others may also be going bad. All cells in the battery should be in similar condition, otherwise you will get unequal charge rates, which will lead to more cells going bad in the near future.

    A charged battery will measure nominal voltage with full load.

    w2ilp (Internal Load Potential?)
     
    Last edited: Dec 4, 2010
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