Solar analog charger trouble

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Hi again,

I'm a real newbie in this. I have been testing out my analog charger for a few weeks now, but the batt is not charging for some reason. I need some tips as to what might be the problem.

1. When the solar panel is charging the battery in bright sun, is the voltage supposed to go up to about 17 V ? I remember reading this somewhere..

The open circuit voltage I measured is around 12.7 V and the current 0.33 Amps and when I connect it to the circuit, the Voc is still around the same.

2. The batt voltage at rest is 12.35 V. When the battery is charging should it's voltage at terminal should be higher ?

I'm using a 12V AH SLA and 12V 5W. The charger is attached below. Thank you.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: Solar analog charger trouble

    can you give us some more details about the panels? make model etc.. especially all the data on the spec plate on the panel...

    CheersEric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    It's a very cheap, generic 5W panel. I don't have any data on it and there is no spec plate ..
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    Unfortunately what the sellers of these little panels don't bother to tell their customers, is that they aren't really meant to "charge" batteries, rather they should be considered battery maintainers. That is, they supply just about the right amount of "charge" to keep the battery from self discharging. They are of no real use in actually charging a standard deep cycle battery.
    But - - they do want to make that sale don't they :(
    A properly sized panel will indeed raise the voltage of the battery as the battery becomes charged.
    Indeed, your "5 watt" panel may also raise the voltage on your battery - - - if you have NO loads on it EVER; the natural tendency of the battery to self discharge is not too great, and - - - you live long enough to see it happen.
    Unfortunately, what you're seeing with it, is quite normal, but not useful.
    Sorry the news isn't any better, but at least you got your feet wet in Solar and I hope you will go bigger and better. By the way, generally, the cost per watt of solar panel, is much higher at places that sell these wee little battery maintainers, than it you go to the vendors of "real" solar panels.
    Don't get discouraged, we all had to start somewhere. Take some time, read and study the many solar related posts on this form. It's a real education. And the people on here are awesome any time you have questions.
    Cheers and good luck
    Wayne
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    I actually did the panel sizing and load calculations on this forum a few months ago and the result I got was that I needed a panel with 0.3A output.
    I was aware that these small panels were for maintenance charging only, but I thought it would still be able to charge the battery (although slowly) instead of not charging it at all ..


    If I want to charge the 12V batt using a power source, how do I do it ? Do I just connect it to a >12V DC supply ?

    Thanks.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: Solar analog charger trouble

    I hate to sound like the proverbial broken record (CD?) but the make and model of the SLA battery will be most helpful to get good adviceon charging voltages etc...

    cheers
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Solar analog charger trouble

    you should note that the charging circuit will pull power from the battery 24/7 even though it is small. for what it is i'd have gone with a lm317 ic regulator and used that instead. also note that voc means voltage open circuit, ie nothing connected to it except your dvm to measure the voltage. that is when the voltage should reach around 17v. with the regulator and/or batteries or even any loads connected that will load down the voltages seen. we generally don't recommend a charge rate under 3% of the ah capacity of the battery. you have indicated seeing 330ma and that would be an 11ah battery that can maximumly be used. keep in mind that batteries also tend to lose a % of charge every month and i've heard of up to 7% each month so that too is an obstacle.
    btw a 12.35v at rest voltage is low and indicates it needs charged. if the panel you have is too small to charge that battery then you can buy another panel to work in parallel with that one if in a similar voltage range.
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    westbranch wrote: »
    I hate to sound like the proverbial broken record (CD?) but the make and model of the SLA battery will be most helpful to get good adviceon charging voltages etc...

    cheers
    Eric


    Unfortunately I don't have the data for the SLA battery as well.
    What is the typical charging voltage range for a 12V battery ? It doesn't have to be a specific voltage value, does it ?

    niel wrote: »
    you should note that the charging circuit will pull power from the battery 24/7 even though it is small. for what it is i'd have gone with a lm317 ic regulator and used that instead. also note that voc means voltage open circuit, ie nothing connected to it except your dvm to measure the voltage. that is when the voltage should reach around 17v. with the regulator and/or batteries or even any loads connected that will load down the voltages seen. we generally don't recommend a charge rate under 3% of the ah capacity of the battery. you have indicated seeing 330ma and that would be an 11ah battery that can maximumly be used. keep in mind that batteries also tend to lose a % of charge every month and i've heard of up to 7% each month so that too is an obstacle.
    btw a 12.35v at rest voltage is low and indicates it needs charged. if the panel you have is too small to charge that battery then you can buy another panel to work in parallel with that one if in a similar voltage range.


    I'm actually building a solar tracker for a project for college. Unfortunately due to budget constraints, I cannot afford another panel. So, I guess I'll just have to charge the batteries using a DC supply instead, but right now I'm still unsure about how to do that.
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
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    Re: Solar analog charger trouble

    Google is your friend , using the search "sla charging voltages"

    first hit using Google is http://www.powerstream.com/SLA.htm

    14.4V at 77F is considered the norm for lead acid batterys for bulk/absorb and 13.5V for float charge.
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    niel wrote: »
    voc means voltage open circuit, ie nothing connected to it except your dvm to measure the voltage. that is when the voltage should reach around 17v.

    Hmm, when I measure Voc I only get around 12-13V ?
    Google is your friend , using the search "sla charging voltages"

    first hit using Google is http://www.powerstream.com/SLA.htm

    14.4V at 77F is considered the norm for lead acid batterys for bulk/absorb and 13.5V for float charge.


    Thanks. I'll try Google as well.


    ps. How come those small panels on backpacks, etc can charge their batteries, but this 5W one can't ? Is it my battery size/voltage ? Could I charge a 6V battery using this 5W panel ?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Re: Solar analog charger trouble

    Yes you can charge lower voltage batteries using the panel (assuming all is good with the panel). You can also put two panels in series to up the voltage...

    A solar panel voltage, once it has a minimum amount of light, will be at its Voc/Vmp (depending on load). However, the output current, at the Vmp voltage will, more or less, be proportional to the amount of sunlight hitting the panel (up to full noon time sunlight--then you will get ~rated current). Panels normally are not damaged by open circuit or dead shorts... Of course, if you connect it wrong to a battery--that can fry your panel (and/or protective diodes).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Solar analog charger trouble

    that must be the problem as it isn't reaching a good charging voltage so it must be made for a smaller output voltage. yes, you could get another one like that one and put it in series with this one and a small diode of about 1amp capacity would be good too. a good place might be inbetween them where the series connection would be for the 2 modules. if you put it in backwards no current will flow. if you go to do this and are unclear about this then ask us again.
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    BB. wrote: »
    Yes you can charge lower voltage batteries using the panel (assuming all is good with the panel). You can also put two panels in series to up the voltage...

    A solar panel voltage, once it has a minimum amount of light, will be at its Voc/Vmp (depending on load). However, the output current, at the Vmp voltage will, more or less, be proportional to the amount of sunlight hitting the panel (up to full noon time sunlight--then you will get ~rated current). Panels normally are not damaged by open circuit or dead shorts... Of course, if you connect it wrong to a battery--that can fry your panel (and/or protective diodes).

    -Bill

    So you're saying if I use a 6V battery instead of my current 12V battery, it would charge ?

    I see datasheets for '12V 5W' panels indicate that open circuit voltage is around 20V and peak voltage Vmpp 17V. My panel's Voc is only 12-13V. Is it not 5W but 3W instead ? I'm very confused ...
    I thought it could output around 17V and thats why I bought a 12V battery.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Re: Solar analog charger trouble

    Possibly... You need to know the Vmp (Maximum Power Voltage level) for your panel, plus the voltage drop across your circuit.

    For example:

    Voc=12.5 volts (12-13 from your measurements). Assume standard voltage ratio drop Voc/Vmp... KC120 specs: 21.5 / 16.9

    12.5 volts (Voc) * 16.9v / 21.5v = 9.83 volts estimated Vmp

    Assume that you want about 7.5 volts to charge your battery...

    9.83-7.5=2.33 volt drop allowed across your voltage regulator (for 77F/25C)

    If your regulator drops more than 2.33 volts for proper operation, then you will get less than maximum power from the solar panel.

    Regarding 5watt vs 3watt and such... The panel rating is typically Vmp*Imp=Watt rating... So, a 5 watt panel with Vmp=16.9volts is going to have less current output than a 5 watt panel rated at Vmp=9.83 volts.

    W/V=I=5w/16.9v=0.3 amps rated panel

    5w/9.83v=0.5 amps rated panel

    Does this help?

    -Bill

    PS: you can get an estimate of the panel's current by measuring Isc (Short Circuit Current)... Just put your meter on Amps (start high scale and work down) and you can see how much current you will obtain.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Solar analog charger trouble

    Actually, since you charge current (in this case) is so low, you likely don't even need a charge controller circuit at all. Just a diode somewhere, to keep the battery from discharging into the panel at night.

    Sorry you got sucked into a situation where a poorly labeled panel has proved incapable of charging your 12V battery. You'll need a panel rated higher than 16V, to charge your 12V battery.

    5W @ 16V = 312 mA charge current. This is hardly more than a trickle charge, depending on the size of your battery, and the amount of power it's expected to deliver.

    You could easily use this same panel to charge a 6V battery. You charge current would still be the same, and the battery will load the panel down to 6V. Again, if you have modest loads, you likely still don't need a charge controller, as 312mA is a trickle charge. Only if you leave this charging for days, and no loads, would you likely need a charge controller or regulator.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    Do you know how much voltage will be dropped across the lm317 regulator ?
    So if I get a Vmp of around 7.5V and that voltage is connected to the 6V batt, will I be able to actually usefully 'charge' the battery ? Instead of just compensating for the self discharge, like Wayne said earlier.
    I just want to make sure before I go out and buy another battery.

    The Isc I measured is around 0.33 Amp.
    mike90045 wrote: »
    Actually, since you charge current (in this case) is so low, you likely don't even need a charge controller circuit at all. Just a diode somewhere, to keep the battery from discharging into the panel at night.

    Sorry you got sucked into a situation where a poorly labeled panel has proved incapable of charging your 12V battery. You'll need a panel rated higher than 16V, to charge your 12V battery.

    5W @ 16V = 312 mA charge current. This is hardly more than a trickle charge, depending on the size of your battery, and the amount of power it's expected to deliver.

    You could easily use this same panel to charge a 6V battery. You charge current would still be the same, and the battery will load the panel down to 6V. Again, if you have modest loads, you likely still don't need a charge controller, as 312mA is a trickle charge. Only if you leave this charging for days, and no loads, would you likely need a charge controller or regulator.

    Yea, its a bit frustrating also because I spent a lot of time thinking about that charge controller and now I probably wont be able to use it.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Solar analog charger trouble
    rngd wrote: »
    Do you know how much voltage will be dropped across the lm317 regulator ?

    about 2 volts at low current, as current goes up, the drop increases.
    There are some Low Dropout regulators (LDO), but I wouldn't bother.

    What's the capacity of your battery ? 30Amp Hours ?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    The battery is just 7 Amp hours. I'm trying to build a small solar tracker.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Solar analog charger trouble
    rngd wrote: »
    The battery is just 7 Amp hours. I'm trying to build a small solar tracker.

    And what is the motor load going to be, will the motor aim the 5W PV panel ?

    If the motor runs much, to track the sun, that may be your "charge controller" It's won't scale up, but would be proof of concept.

    Do you have a circuit that does the tracking? Does it use green LEDs as photo sensors ? (green LEDs are slightly photovoltaic)
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    mike90045 wrote: »
    And what is the motor load going to be, will the motor aim the 5W PV panel ?

    If the motor runs much, to track the sun, that may be your "charge controller" It's won't scale up, but would be proof of concept.

    Do you have a circuit that does the tracking? Does it use green LEDs as photo sensors ? (green LEDs are slightly photovoltaic)


    Yup I will be using two servos to aim the panel. I am going to use photodiodes probably and use a PIC to control the movement.

    I'm not sure how to use LEDs as sensors. I tried to shine a torchlight onto a green LED and measure the open circuit voltage output but I only got around 0.03 volts..


    edit : So if I get a Vmp of around 7.5V from the panel and that is connected to the 6V batt, I will be able to actually usefully 'charge' the 6V battery up instead of just compensating for the self discharge (like Wayne said earlier) ?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Re: Solar analog charger trouble

    The 6 volt battery itself needs around 7.5 volts at its terminal to really fully charge (and around 6.8 volts to "float" charge and not boil dry for long term storage).

    The output voltage (Vmp) of the solar panel needs to be, at least:

    Vmp=Vbatt-max + Vdrop-charger + Vdrop-wiring + Vdrop-blocking-diode + etc...

    Example:

    Vmp=7.5v + 2.0 volt + 0.1v + 0.2 v (if Shottky diode required)=9.8 volts

    So, for this example your solar panel needs a Vmp of 9.8 volts (or more) or approximately 12.5 volts open circuit (assuming ratio of Vmp/Voc is similar to other solar panels).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    niel wrote: »
    we generally don't recommend a charge rate under 3% of the ah capacity of the battery.

    What is the reasoning behind this? Thanks!
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    Thanks Bill.


    Also, which diode is good/commonly used as a blocking diode for solar panels ?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Re: Solar analog charger trouble

    Regarding the 3% minimum charge rate... There are several issues that I see.

    For flooded cell batteries, higher charge rates will provide enough hydrogen/oxygen bubbles to properly stir the acid and prevent stratification (especially a problem on tall batteries).

    Self discharge rate of batteries, as they get older, increases. Add discharge from use, and self discharge of an older bank, it becomes pretty difficult to get the batteries fully charged and have proper equalization.

    Lastly, charging batteries slowly (especially those flooded cell batteries that have been discharged below 75% state of charge), allows the sulfates deposits (that naturally occurs with battery discharge) to fully harden and this will eventually lead to battery failure (sulfates begins to harden after, roughly 24 hours).

    Regarding the solar panel blocking diode:

    Any Schottky diode that has a minimum Vreverse rating that well exceeds your battery voltage and the maximum forward current of your solar panel will work fine.

    The blocking diode is typically part of the solar charger's function and many solar panels designed for use with battery chargers have their own blocking diode(s) already installed in the panel.

    You can probably check if you need a blocking diode (for the panel or the charge regulator) by hooking the battery up to a "dark solar panel" and just put a good current meter in the circuit to measure current flow. If the discharge current is too high for your application, you will need the blocking diode.

    Remember if you add the blocking diode to your battery charger, you will need to account for the diode's voltage drop so that the battery voltage is still correct.

    Lastly, regarding your home made charger, the spare op-amp should have some sort of feed back loop (output to input) to prevent it from oscillating because of unconnected inputs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Solar analog charger trouble

    in addition to what bill said so well is that batteries are typically designed around 20hr and sometimes 10hr periods. 20hrs is a long time in terms of solar charging as many in the summer will get 5 hours of full sun (based on intensity) and that equates to 3% for 5hrs equals 15% of the charge in the battery per day. assuming you aren't drawing from the batteries during this charging period it will take nearly 4 days to fully charge the battery assuming it to need no more than 50% of its charge replaced. so based on this, during bad weather days, and winter of course, this lengthens the time needed to charge the battery even considerably more. for some that low % may work for them, but for most it will not and the battery industry usually goes by the 20hr timeperiod for discharge and charge (5% rate). this is fine for a power source that is consistant like utility power, but with pv you must replace what is used daily or you will slowly (or quickly) be running the battery dead. i believe higher charge percentages are better and most batteries can take it up to 13% of capacity with many more being able to take far in excess of that, but not without the ok from the manufacturer as overcharges can be detrimental to some and could minimumly increase maintenance. that makes the general charge percent range to be 5-13% and this is without any loads on the battery that would detract from that percentage. we do understand the high costs involved with solar and so we do realistically say it can be done in some cases with smaller percentages than 5% and below 3% is approaching the area of trickle charging, float charging, or maintenance charges as these are sometimes words of the same thing being described. of those 3, trickle charging is probably the only one i would interpret as having the possibility of a slight gain to the battery charge though it will be very small.
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    BB. wrote: »

    Lastly, regarding your home made charger, the spare op-amp should have some sort of feed back loop (output to input) to prevent it from oscillating because of unconnected inputs.

    -Bill


    Thanks for the tip. How do I do that ? Can I just ground the 2 inputs and one output ?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Re: Solar analog charger trouble

    Boy... It has been so long since I worked with op-amps... Depends on the op-amp and what you may want to do with it the latter. But the simplest is to probably to take one input and tie it high and the other to ground through a pair of resistors.

    You don't want to let the two output be tied to the same voltage--remember the Op Amp just multiplies the difference between the two voltages and any leakage current/offset/noise in the circuit will just be amplified and possibly cause oscillations. Using high value resistors (typically 10k or larger) is to prevent any large current flows if there is a power or circuit failure (local bypass caps can store energy and cause reverse biasing--again depending on op amp, circuit, etc...).

    The other issue is you need to look at the range of the input voltages... Check the spec. sheets to see if the inputs can go to +/- rails, or if you need a resistor divider of some sort to keep them within spec.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble

    I see. Alright, thanks everyone for all the replies, you guys have been really helpful.
  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
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    Re: Solar analog charger trouble

    rngd
    I don't know if you have seen this web site yet but you can get scematics and lots of ideas on building led based solar trackers here: http://www.redrok.com/electron.htm#led3x
    Good luck,
    Larry
  • System2
    System2 Posts: 6,290 admin
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    Re: Solar analog charger trouble
    rplarry wrote: »
    rngd
    I don't know if you have seen this web site yet but you can get scematics and lots of ideas on building led based solar trackers here: http://www.redrok.com/electron.htm#led3x
    Good luck,
    Larry

    Hi larry,

    Yup I've been to that website many times but I still dont get how the leds work. Are you familiar with them .. ?

    I understand the concept of using blockers to cast a shadow on the led, but how do those with 4 leds without the blockers sense the position of the sun ? Even the ones with the blockers, the blockers are so small that I dont see how a shadow is cast on the led ..

    Also, I tried using those regular green leds but it couldnt output any voltage in bright light. Does it need to be the ones with the clear casings ?

    Thank you.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Solar analog charger trouble

    stated overly simply the minute power generation the leds can produce in bright sunlight is amplified and allows differences to be evened out so to speak by the activation of the motors. i would have concern if that evening out doesn't occur what may stop the excess power from being used? minimumly power could be wasted and maximumly damage to the batteries could occur by depleating them.
    do note that diodes, and that includes leds, are pn junctions and those junctions can generate small amounts of electricity in strong light. this was how the solar cell was actually born because they expanded the junction to allow for more power to be produced in a wider area. leds do have a narrow field making sensitivity angles to light upon it reciprocal to the angles of light it can produce.