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Solar Charger for 18v Tool Batteries

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  • Solar Charger for 18v Tool Batteries


    I'm building a small PV system to charge 18v tool lithium ion Ryobi battery packs (both 2AHr & 4Ahr). I have two possible Scenarios that I'm considering.

    Scenario #1

    PV (87w, 12v) -> CC (PWM) -> Battery (XAhr) -> Load (12v DC "vehicle" charger)

    Scenario #2

    PV (87w, 12v) -> CC (PWM) -> Battery (XAhr) -> 150w Pure Sinewave inverter (90%) -> Load (85w 120v AC Gang Charge).

    Back of the napkin I figure the PV array can supply something like 36Ahr (assuming a 5hr day)

    Considering that the sole purpose is to charge the Li Batteries, my basic design assumption is that I'll need to charge 6 4AHr batteries on a daily basis or nominally 24Ahr (4Ahr x 6). If I assume an efficiency of say 90% for the chargers that would work out to be something like 27Ahr. Therefore:

    In scenario #1 since I don't have an inverter that's a load of 27Ahr per day (75% of PV).

    In scenario #2, based on 90% efficiency of the inverter the load goes to 30Ahr per day (83% of PV)

    As this will have long periods of non use I intend to trickle charge while stored.

    What size and type of battery should I be looking at? Do I even need a battery?











  • #2
    You can not compare different AHs that are different voltages. 6Ah at 18 volts is 108 watthours while 6Ah at 12 volts is only 72 watthours. You need to run all you calculations using watthours instead of amphours.

    You also need to calculate in at least 33% loss from the using a PWM charge controller.

    A designed neither system would be large enough.

    WWW

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    • #3
      Originally posted by ggorveatte View Post
      PV (87w, 12v) -> CC (PWM) -> Battery (XAhr) -> 150w Pure Sinewave inverter (90%) -> Load (85w 120v AC Gang Charge).
      First your plan is very flawed because you do not understand the physics. Amp Hours is a meaningless number. You need wat hours. AH is just an end result.

      Amp Hours = Watt Hours / Battery Voltage.
      Amp Hours = Amps x Hours
      Amp Hours = Amps x Hours.

      Watt Hours is what you want

      Watt Hour = Watt x Hours
      Watt Hours = Battery Voltage x Amp Hours.

      So

      18 volts x 10 AH = 180 Watt Hours.
      12 volts x 10 AH = 120 Watt Hours

      To equal 18 volts @ 10 AH requires a 12 volt @ 15 AH

      A 87 watt panel with PWM controller at best on a July day generate 200 watt hours of power. In winter half of that. On a 12 volt battery 200 watt hours gives you 200 wh / 12 volts = 16.6 Amp hours. Now you transfer that to a 18 volt LFP battery. If you could find a 90% efficient converter on a 18 volt battery is 200 wh / 18 volts / 1.11 = 10 Amp hours. In winter you are down to 5 AH or less. All that for 2-cents worth of electricity.

      No where close to the 27 AH you calculated. 18 volts x 27 AH = 486 Watt Hours
      Last edited by Sunking; 07-20-2017, 06:32 PM.
      MSEE, PE

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      • #4
        Originally posted by Sunking View Post

        First your plan is very flawed because you do not understand the physics. Amp Hours is a meaningless number. You need wat hours. AH is just an end result.

        Amp Hours = Watt Hours / Battery Voltage.
        Amp Hours = Amps x Hours
        Amp Hours = Amps x Hours.

        Watt Hours is what you want

        Watt Hour = Watt x Hours
        Watt Hours = Battery Voltage x Amp Hours.

        So

        18 volts x 10 AH = 180 Watt Hours.
        12 volts x 10 AH = 120 Watt Hours

        To equal 18 volts @ 10 AH requires a 12 volt @ 15 AH

        A 87 watt panel with PWM controller at best on a July day generate 200 watt hours of power. In winter half of that. On a 12 volt battery 200 watt hours gives you 200 wh / 12 volts = 16.6 Amp hours. Now you transfer that to a 18 volt LFP battery. If you could find a 90% efficient converter on a 18 volt battery is 200 wh / 18 volts / 1.11 = 10 Amp hours. In winter you are down to 5 AH or less. All that for 2-cents worth of electricity.

        No where close to the 27 AH you calculated. 18 volts x 27 AH = 486 Watt Hours
        Thanks for the physics lesson.

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        • #5
          So now I'm curious, Where did 18v x 10AH come from?

          The batteries are :
          18v x 2Ahr = 32 Wh
          18v x 4Ahr = 72 Wh
          6pcs of 18v x 4Ahr = 432Wh

          if he only wanted to charge 2 pcs 18v 4Ahr

          lets say he had a battery SLA 12V 35Ah ( 200Wh/12V=16.6Ah and we don't want to drain battery past 50%)
          if he had a PV (87w, 12v) -> CC (PWM) -> Battery 12V(35Ahr) -> Load (12v DC "vehicle" charger)
          could he charge 2 pcs 18v 4Ahr batteries on a good day in july?
          Do we need to know how many amps the 12v DC "vehicle" charger?
          Is it possible to run this PV (87w, 12v) -> CC (PWM) -> Load (12v DC "vehicle" charger)?

          It's my first post, so I expect to be slapped with a fish.

          Comment


          • #6
            Originally posted by brooklynboy79 View Post
            So now I'm curious, Where did 18v x 10AH come from?

            The batteries are :
            18v x 2Ahr = 32 Wh
            18v x 4Ahr = 72 Wh
            6pcs of 18v x 4Ahr = 432Wh

            if he only wanted to charge 2 pcs 18v 4Ahr

            lets say he had a battery SLA 12V 35Ah ( 200Wh/12V=16.6Ah and we don't want to drain battery past 50%)
            if he had a PV (87w, 12v) -> CC (PWM) -> Battery 12V(35Ahr) -> Load (12v DC "vehicle" charger)
            could he charge 2 pcs 18v 4Ahr batteries on a good day in july?
            Do we need to know how many amps the 12v DC "vehicle" charger?
            Is it possible to run this PV (87w, 12v) -> CC (PWM) -> Load (12v DC "vehicle" charger)?

            It's my first post, so I expect to be slapped with a fish.
            OK here is your Fish Slap.

            AH are meaningless. How much energy is in a 10 AH battery? No one can answer that question and I mean absolutely no one because the information is incomplete. Electric Energy is what hours, and amp hours.

            How much energy does a 12 volt 10 AH battery have can be answered without any a cell phone or calculator of 120 watt hours.

            Battery Capacity = Watt Hours = Battery Nominal Voltage x Amp Hours.

            So now you should understand Amp Hours is meaningless without a voltage being specified.

            OK back to your specific questions. When it comes to recharging power tools, the application changes a bit. For one the power tool battery will be fully discharged. Use one of the example say a 18 volt 4 AH LFP which contains 72 Wh. One of the first things to ask is how may times in a day do you want to recharge that battery. Just throwing a number out there say 4 times. That works out to 288 wh right?

            So how do we put that energy in. Most likely will use a standard 12 volt battery and an Inverter to run the charger. So in the Chain of Panels > Controller > Battery > Inverter > Charger > Tool Battery we have added two more conversion and those conversions come with losses.

            In a standard set up if we use PWM controller the panels have to generate 2 times more energy than what is being consumed, and MPPT 1.5 times more. Now we added two more conversions, so now we have more losses to deal with and overcome.

            Sure the end battery is only 288 wh, but to get it to the battery is going to take a lot more than that, more like 600 watt hours. So the design will look something like

            Panel Wattage = 200 watts
            MPPT Controller = 15 Amps
            Battery = 12 volts @ 150 AH

            MSEE, PE

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