How do I calculate the amp hour for a device to run on battery power

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  • sam_sam_sam
    Badcaps Legend
    • Jul 2011
    • 6032
    • USA

    #1

    How do I calculate the amp hour for a device to run on battery power

    Here is what I would like to do

    I have a LED yard light that use about 360 milliamperes

    I would like the LED yard light to run a minimum of 8 hours at full brightness

    I going to use lipo 4 batteries that new / old stock which means that they do not have the maximum power output what they say they should be which probably inflated to being with

    So how do calculate how many batteries do I need with out doing a bunch hit and miss trail error

    The test I have done on these batteries so far is this

    I charged them one at time

    Put a 860 mill amp load on them until the protection board turn the load off

    Which is about hour
    Last edited by sam_sam_sam; 08-05-2018, 09:22 AM.
  • redwire
    Badcaps Legend
    • Dec 2010
    • 3902
    • Canada

    #2
    Re: How do I calculate the amp hour for a device to run on battery power

    Keeping it simple, if you assume the voltages don't change enough to matter:

    The LED yard light needs 360mA for 8hrs = 2,880mAh of energy (360*8).
    Your batteries measure 860mA for 1 hr = 860mAh capacity each (860*1).

    2880/860= 3.4 batteries worth of energy needed. Four is the fewest you could use.

    But you don't want to deep discharge the batteries every night, they have longer life if you only drain them partially. A long winter night or cloudy weather and you could run more than 8 hours of light. Cold batteries have less capacity too.

    For these reasons, you overdesign and use more batteries than really needed.

    Comment

    • sam_sam_sam
      Badcaps Legend
      • Jul 2011
      • 6032
      • USA

      #3
      Re: How do I calculate the amp hour for a device to run on battery power

      Originally posted by redwire
      Keeping it simple,
      For these reasons, you overdesign and use more batteries than really needed.
      This is why I asked the question I wanted to make sure that I over design it so the batteries will last for a long time

      Comment

      • Curious.George
        Badcaps Legend
        • Nov 2011
        • 2305
        • Unknown

        #4
        Re: How do I calculate the amp hour for a device to run on battery power

        Originally posted by sam_sam_sam
        This is why I asked the question I wanted to make sure that I over design it so the batteries will last for a long time
        I suspect that "a long time" will be "about a year".

        Keep in mind that if you are using multiple cells, you have to decide how to interconnect them to power your load.

        And, more importantly, how you will recharge them! Ideally, you would want a multitap charger so each cell is individually charged as it deems fit.

        You might be better served finding a bigger battery (battery = collection of CELLS) that more properly addresses your requirements rather than trying to piece together individual (unmatched) cells.

        Note, also, that lithium does not like water/moisture! (something you will likely encounter "in the yard")

        Comment

        • stj
          Great Sage 齊天大聖
          • Dec 2009
          • 30977
          • Albion

          #5
          Re: How do I calculate the amp hour for a device to run on battery power

          batteries are best indoors, they hate big temperature swings.

          in ww2 unexploded bombs with electric detonators where frozen to drop the battery output before disassembly.

          Comment

          • mariushm
            Badcaps Legend
            • May 2011
            • 3799

            #6
            Re: How do I calculate the amp hour for a device to run on battery power

            Originally posted by sam_sam_sam
            Here is what I would like to do

            I have a LED yard light that use about 360 milliamperes

            I would like the LED yard light to run a minimum of 8 hours at full brightness

            I going to use lipo 4 batteries that new / old stock which means that they do not have the maximum power output what they say they should be which probably inflated to being with

            So how do calculate how many batteries do I need with out doing a bunch hit and miss trail error

            The test I have done on these batteries so far is this

            I charged them one at time

            Put a 860 mill amp load on them until the protection board turn the load off

            Which is about hour

            It's a bit more tricky... your led uses 360mA but what voltage? because the light could have one led and pump 360mA through it, or the lamp could have 3-5 leds in series so still 360mA but you have 3-5 x ~ 3.2v (forward voltage of each led)


            Then moving on to batteries, you could have a single 3.7..4.2v battery or you could have a bunch of them in series resulting in higher voltage.

            This means you'll either have a buck led driver (taking the voltage from the battery down to the voltage required by the led) or you could have a boost led driver which raises the voltage higher.

            buck driver is highly efficient, with up to 96-97% efficiency
            boost driver generally is harder to optimize, will get you maybe 90-94% efficiency.

            with boost drivers, the problem is you'll have bursts of energy pulled from the battery, so the batteries will tend to discharge faster.
            In general batteries have better endurance, discharge slower etc if the current is lower.
            So for example, let's say your lamp has a forward voltage of 9v (3 leds in series with ~3v forward voltage) and 360 mA , so your light consumes 3 x 3v x 360mA = 3240mW

            If you use a single 3.7v...4.2v battery and you use a boost driver, then at some points the led driver will pull at least 3240mW / 3.7v = 875mA + around 10% due to conversion efficiencies, so let's say 950mA
            A 18650 battery with around 2500mAh rating will probably last 2-2.5h at that current.

            But if you go with 4 batteries in series you have a buck driver with higher efficiency and and the current is much lower, as now you have 3240mW / 14.8v = ~ 220mA and even with let's say 5-10% losses, you're looking at around 235mA of current.
            A 18650 battery with 2500-2800mA rating will have no problem deliver 0.1C (~250mA) from fully charge to basically depleted, so it should last for at least 10 hours.

            Comment

            • sam_sam_sam
              Badcaps Legend
              • Jul 2011
              • 6032
              • USA

              #7
              Re: How do I calculate the amp hour for a device to run on battery power

              UpDate on these batteries that are new / old stock

              Using a BSM board to charge them I have run into a problem

              The cut off voltage is at about 3.7 volts

              Well some of the batteries can reach this voltage without getting warm while some of them are getting some what warm and do not reach the cut off voltage with the current set at 200 milliamperes

              The way I am going to charge is as follows

              Set the cut off voltage at 14.8 volts
              Set the cut off current at 200 milliamperes

              Just in case there is a problem with any of the batteries under charge

              Here how I going to determine which batteries are good as follows

              Set the voltage at 3.7 volts an the current to 100 milliamperes and watch the temperature the battery gets so I can match the batteries better

              If anyone has a better idea of how to do this please feel free to post your ideas I am open to any and all ideas

              I have a RC fast charger but it is not telling me that the battery is having a problem with reaching the cut off voltage with out the battery get warm this to me is a real problem
              Last edited by sam_sam_sam; 08-12-2018, 07:17 AM.

              Comment

              • stj
                Great Sage 齊天大聖
                • Dec 2009
                • 30977
                • Albion

                #8
                Re: How do I calculate the amp hour for a device to run on battery power

                you need to analyse the capacity of each cell with something like a liitokala Li500 and then only use cells with matching or very similar capacity.

                Comment

                • sam_sam_sam
                  Badcaps Legend
                  • Jul 2011
                  • 6032
                  • USA

                  #9
                  Re: How do I calculate the amp hour for a device to run on battery power

                  Originally posted by mariushm
                  It's a bit more tricky...
                  Then moving on to batteries, you could have a single 3.7..4.2v battery or you could have a bunch of them in series resulting in higher voltage.

                  This means you'll either have a buck led driver (taking the voltage from the battery down to the voltage required by the led) or you could have a boost led driver which raises the voltage higher.

                  buck driver is highly efficient, with up to 96-97% efficiency
                  boost driver generally is harder to optimize, will get you maybe 90-94% efficiency.

                  with boost drivers, the problem is you'll have bursts of energy pulled from the battery, so the batteries will tend to discharge faster.
                  In general batteries have better endurance, discharge slower etc if the current is lower.

                  The problem is finding a BSM board that is 42 volts
                  The closest that I can find is 48 volts and still would have to use a PWM switching regulator set to 42 volts

                  Comment

                  • stj
                    Great Sage 齊天大聖
                    • Dec 2009
                    • 30977
                    • Albion

                    #10
                    Re: How do I calculate the amp hour for a device to run on battery power

                    no, the initial problem is you need to sort and group/match your cells.
                    if not, you will be constantly having problems.

                    Comment

                    • sam_sam_sam
                      Badcaps Legend
                      • Jul 2011
                      • 6032
                      • USA

                      #11
                      Re: How do I calculate the amp hour for a device to run on battery power

                      Originally posted by stj
                      you need to analyse the capacity of each cell with something like a liitokala Li500 and then only use cells with matching or very similar capacity.
                      Thanks for the information

                      This is what I ordered

                      4 Slot 110-240V Liitokala Lii-PD4/Lii-PL4 LCD Smart Rechargeable Battery Charger

                      https://www.ebay.com/itm/4-Slot-110-...53.m2749.l2649

                      Comment

                      • stj
                        Great Sage 齊天大聖
                        • Dec 2009
                        • 30977
                        • Albion

                        #12
                        Re: How do I calculate the amp hour for a device to run on battery power

                        possibly useless,
                        AFAIK only the 500 Engineer model does the NOR charge-discharge test and then tells you the true capacity.
                        i think that one is just a charger.

                        https://lygte-info.dk/review/Review%...-500%20UK.html
                        Last edited by stj; 08-12-2018, 10:33 AM.

                        Comment

                        • sam_sam_sam
                          Badcaps Legend
                          • Jul 2011
                          • 6032
                          • USA

                          #13
                          Re: How do I calculate the amp hour for a device to run on battery power

                          It might be but the batteries ( LiFePO4 26650 3.2 v ) that I have that charger does not support it



                          But I am going to buy the one you posted because I also have a lot of batteries that it does support
                          Last edited by sam_sam_sam; 08-12-2018, 02:59 PM.

                          Comment

                          • stj
                            Great Sage 齊天大聖
                            • Dec 2009
                            • 30977
                            • Albion

                            #14
                            Re: How do I calculate the amp hour for a device to run on battery power

                            well do some research, there are other chargers that can do a capacity test that are newer - and probably support lifepo4

                            if you really want a good one, look at skyrc stuff like this:
                            https://lygte-info.dk/review/Review%...3000%20UK.html

                            Comment

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