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I Meaning & limitations of Volt/Current ratings

  1. Jun 14, 2016 #1
    BACKGROUND:
    I bought an infrared monocular scope I hope to help me find Owls at night. I can output its image to an external monitor. The scope (an Armasight IR Prime 7X) can run on three CR123 batteries. These are rated at 3Volts though out of the box they read about 3.25V. (These are non-rechargeables).Three in series would provide 9.75 Volts (max). (note for future reference below: this is more than 9Volts of recommended DC power supply). The CR123 batteries I've used up so far read about 2.7V when the scope stops working. (Using a multimeter). I get about 1.5 to 2 hours usage from these batteries. If these have a 1500mAh rating, then I think my scope must be averaging between .75A and 1.5A current to last 1 to 2 hours.

    The IR scope manual says one can also use any DC 9V 500mA power supply and that is the path I am wanting to take. I notice that the primary cr123 are 1500mAh, so if the scope only draws 0.5A, those batteries should last 3 hours. Well ... in fact while the low battery (or dead battery) indicator went on at less than two hours, the scope did run for a fair while after that before stopping. Maybe I did get almost 3 hours out of them. (These estimates of current are central to my question below). Either way ... the non-rechargeable batteries cost too much to keep buying again and again. I've been told and have read that rechargeable CR123 batteries are in practice lower voltage and discharge rather quickly, (only 200 to 600 mAh in practice some say); so I'm not looking to use those.

    I took a risk and connected the USB out port of my Micro-Start PPS to the scope and used it as if it was a battery. I use this device to RECHARGE just about any of my devices (phone, Hearing aid wireless transmitter etc) because it has a huge capacity. (It's not meant to be used as a battery except when using the terminals for jump-starting a car). The scope worked using this and I ran it for over an hour with no low battery indicator. When I put it on its charger I found it was barely drawn down at all. So if it was safe to use on the scope I would just use this. However I noticed the scope was getting hot while I had it running on this device (after about 30-40 minutes). Then I hooked in a USB power meter and found the Micro-Start was running at 5V and about 1.8A. The meter said about 8Watts. The DC power supply ratings recommended by the scope manual are 9V & .5A, which I suppose would be 4.5Watts (maximum??) if the scope ran at both those voltage and current values. I'm thinking maybe I was lucky that I didn't destroy the scope running it with the high current given by the Micro-Start unit.

    QUESTIONS:
    How does one know what current the scope really draws under proper conditions? And how much current is SAFE for this device? (My power meter only works with USB type connections and it is messy and difficult - for me- to try to use my multimeter to cut into a power supply circuit to read the current). We have some kind of amperage estimate from the CR123 battery rating and performance - that suggests 500mA up to as much as perhaps 1500mA.
    What does the 9V & 500mA rating of the (suggested in the manual) DC power supply really mean? For example, does such a supply continue to supply 9V (as long as it is not over-taxed)? And is the 500mA only meant to be the MAXIMUM current that power supply would generate? I suppose this rating means that one should avoid running the scope at any current higher than 500mA; right?

    I am looking at a couple possible ways of providing power to the scope. One way is with a battery pack containing three 3.7V 18650 Li rechargeables in series, providing 11.1V. I like this solution because I have another device which uses these 18650 Li batteries. I am wondering (1) if the 11.1V would be a problem for the scope rated for a 9V 500mA supply. (2) And what current will this battery pack pump into the scope?? If this battery pack is rated for 2600mAh, all I know that tells me is that if it ran at 500mA, it would last about 2600/500 = 5.2 hours. But I don't really know what current the scope will draw from an 11V supply like this! If this set-up is safe for the scope I would be happy with it.

    The other method I am considering is:
    TalentCell Rechargeable 72W 132WH 12V/11000mAh 9V/14500mAh 5V/26400mAh DC Output Lithium Ion Battery Pack; using the 9V output port. Since 9V matches the suggested power supply this would probably work fine. While that "takes care of my problem" - I would really like to UNDERSTAND a little more about things I have been discussing; ie; how might one know what current the scope is meant to draw optimally? what happens to a device like this scope if a slightly higher voltage is applied?
    I saw that using a much lower voltage (5V from Micro-Start) resulted in a current of 1.8 Amps but I don't really know why the scope was drawing so much current when I used that!


    I know this is a looooonnnng question and I apologize for that.

    I thank you in advance for trying to help me get some clarity about these things.

    Priscilla
     
  2. jcsd
  3. Jun 14, 2016 #2

    davenn

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    Science Advisor
    Gold Member

    hi there Priscilla
    welcome to PF :smile:

    what you haven't taken into account is the voltage drop across the battery(s) under load.
    That is ... tho the battery may read 3.25V "out of the box" I would wager a bet that you were measuring this with your meter with the battery out of circuit
    in circuit, I could almost guarantee that they will drop to around 3V with the 500mA odd that is being drawn.
    Measure them again when they are in circuit and the scope is operating.

    so dropping that voltage is now going to affect your calculations of current drain etc
    and also consider that as the voltage continues to drop, with use, then the batteries will be unable to continue to supply the full current out

    have an experimental play and see what figures you come back with
    you could also break the positive line from the battery and insert your meter in there in Amps mode and see what current drain is really taking place


    cheers
    Dave
     
  4. Jun 14, 2016 #3

    Drakkith

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    Staff: Mentor

    I'd recommend this method. It is rarely a good idea to use a power source with a different voltage than what the manufacturer recommends. Just make sure whatever method you use can supply at least 500 mA of current.
     
  5. Jun 14, 2016 #4
  6. Jun 14, 2016 #5

    Tom.G

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    Science Advisor

    From the description, the IR Scope probably has a regulated, switching power supply in it to supply a higher internal operating voltage. If this is the case then it will tend to draw approximately constant power from the external power source. That would explain the higher current draw and temperature rise operating from a 5Volt source. Not knowing how much safety margin was designed into the Scope, you are best off using close to the stated 9V source.

    Here is a link for some "D" cell battery holders:
    http://www.mcmelectronics.com/product/29-6075
    http://www.mcmelectronics.com/product/28-12579
     
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