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Volts vs watts as they relate to charging

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  • Volts vs watts as they relate to charging

    I haven’t had any luck finding a differentiation in the topic of volts vs Amps as they relate to charging. I understand the formula for wattage, and perhaps I’m looking for a deeper answer than what it is, so maybe someone could extrapolate...here are some things to work from:

    nominal voltage is 12.7, float is 13.7, and (bulk) is 14.4v. Let’s go with 190ah battery, with bulk charging being 10%, or something like 274w. Put the same amperage in float voltage and you get about 260w. That’s Like negligible change in power, yet voltage is the only metric used when talking about charging a battery. Let’s say I use 20a at 13.7v (instead of 19a @ 14.4v), which yields the same ~274w.

    The power being transferred into the battery is the same. I can’t imagine 1a makes that that much of a difference in heat transfer that it has spawned a whole indistry worth of theories and technologies geared towards optimally charge a battery.

    Why is amperage left out of the dialogue almost completely when it comes to discussing charging a battery?

  • #2
    Originally posted by Dustyb View Post
    ...........

    Why is amperage left out of the dialogue almost completely when it comes to discussing charging a battery?
    They are both important but most often voltage is used as the easiest way to guess at the state of charge. Voltage can be measured when the battery is at rest and there is no current is flowing. I could measure half an Amp or five Amps going into or out of a battery and it would not tell me anything about the SOC unless I had other data.

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    • #3
      Right, but I’m not talking about measuring the state of charge. Moreso related to the rate of charge. Let’s say in a car, you start it at 12.7v, and it charges at 14.4v. It charges at 14.4v somewhat independent of of the state of charge, right? Does it charges at 14.4v at a rate of 1 amp, or 40 amps? Seems to me that would make a considerable difference.

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      • #4
        Originally posted by Dustyb View Post
        Right, but I’m not talking about measuring the state of charge. Moreso related to the rate of charge. Let’s say in a car, you start it at 12.7v, and it charges at 14.4v. It charges at 14.4v somewhat independent of of the state of charge, right? Does it charges at 14.4v at a rate of 1 amp, or 40 amps? Seems to me that would make a considerable difference.
        Rate of charge is all about the current because the voltage is determined by the state of the battery. My experience is with Lithium but I think Lead Acid behave similarly in Bulk (constant current )and Absorb (constant voltage). In your example the voltage of the battery starts out at 12.7 volts in a bulk charge even though the charger may be capable of 14.4 volts. The voltage gradually increases until the battery hits 14.4 when the voltage increase stops and the charger goes into absorb and the current begins to taper. Lithium doesn't care what the charge current is as long as it does not exceed what the battery is rated for. Apparently Lead Acid is more temperamental about current and being fully charged because funny things apparently happen with sulfation. My knowledge is with Lithium so others with Lead Acid experience may wish to weigh in on those subtleties. I am just trying to give a high level view of the relevance of Volts and Amps to see if I clarify the conversation.
        Last edited by Ampster; 02-06-2020, 10:52 PM.

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        • #5
          1) constant voltage chargers are much simpler in design than constant current chargers.

          2) with lead acid batteries, you simply supply voltage so that:
          a) voltage is high enough to complete a charge in the limited solar hours
          b) voltage is not so high you allow the battery to exceed it's safe current limits.
          There is a narrow band where things work well, when the battery is sized for the load, the solar is sized for the battery and sun hours. That's called System Design and that's why many people fail with solar off grid, they forget or muck up the design part
          Powerfab top of pole PV mount (2) | 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

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          • #6
            Originally posted by Mike90250 View Post
            1) constant voltage chargers are much simpler in design than constant current chargers.

            2) with lead acid batteries, you simply supply voltage so that:
            a) voltage is high enough to complete a charge in the limited solar hours
            b) voltage is not so high you allow the battery to exceed it's safe current limits.
            There is a narrow band where things work well, when the battery is sized for the load, the solar is sized for the battery and sun hours. That's called System Design and that's why many people fail with solar off grid, they forget or muck up the design part
            Probably the main reason off grid systems fail is because their owners either do not understand the mechanics behind solar/batteries or they have gotten something for "free" and they jump into the idea of "free" energy with both feet before they look to see how deep the water is.

            That is why I try to alert people to know what they are doing before they open their pocket books just like I did when I built my first solar/battery system and wasted my money on some of the equipment.

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