No announcement yet.

Input vs. Output current ratings on an AC charger

  • Filter
  • Time
  • Show
Clear All
new posts

  • Input vs. Output current ratings on an AC charger

    Hi all,

    I'm trying to put together a small solar system to power an electric bike, also partly to learn more about putting together solar systems with an eye to later building a system to power a minimalist off-grid cabin. The bike has a Li-ion battery, which is charged by an AC charger.

    My initial thoughts were that the output of this charger is 42 volts at 2.0 amps, so 84 watts of power, so I've bought an inverter, a panel (a flexible panel, as I've read that these are better at generating current in lower light conditions, though I understand it may not last as long as a rigid panel), a charge controller and a battery (I understand this isn't ideal for a solar system, I bought it because it's the same one I have in my car, so I've put the new battery in the car, and the 5 year old battery is intended for the solar system), and would be good to go. I need to recharge the battery on the bike a couple of times a week, and live near the Mediterranean, so get a fair bit of sunshine (something like 2000 hours/year).

    Having ordered all this kit, I was looking at how to wire the inverter to the system, as I'd assumed I'd connect it to the load pins on the charge controller; thankfully some time spent reading this forum has educated me to the point that I realise I need to connect the inverter to the battery instead (also that I might have been better off buying a smaller inverter). What I'm currently unsure about is the difference between the input rating on my bike charger, and its output rating; the input states AC100v-240V and 1.8A, which means that the input is 414 watts, given that I'm on 230 volt AC power.

    What's confused me is that I'd previously only considered the output; 42V DC at 2.0A for 84 watts of current. Does that mean it's dumping 330 watts as (mostly) heat, given that the input is 414 watts? I'm trying to understand how much current will be drawn from the battery when the charger is plugged in, and had previously assumed that the load would be equal to the stated output of the charger, i.e. I'd be looking at 2 amp load, which I believe means that my 62 amp hour battery should be capable of powering the charger for perhaps 20 hours, given that I understand that the inverter itself might use an amp of power, and that there's some electrical resistance along the way. That would give me around 3 full charges of my bike battery, as it takes approx. 6 hours to recharge from empty. But I'm now starting to second guess myself having seen the input figures. Hence I'm looking for a sanity check, I think; have I hopelessly misunderstood how all this works, or, given my fairly minimal needs (recharging a 36 volt, 8 amp hour li-ion battery twice per week), is my 50 watt panel, 500 watt inverter, 62 amp hour car battery going to do the trick? As I understand it, my panel is producing 4.17 amps under ideal conditions, so on a sunny summer's day, with 8 hours of sunshine, I'm going to get perhaps 30 amp hours into the battery, so it's going to take approx 2 sunny days to charge my battery from empty.

    I've got a couple of other questions (e.g. my charge controller manual talks about a voltage setting for undervoltage recovery, as well as undervoltage protection; the latter I understand, but the former is somewhat mysterious, even after a search on this forum and a search engine), and why it is that my charge controller will only allow me to set an undervoltage protection of 11 volts maximum (I'd rather that figure were 12.2 volts, to maintain the life of the battery), but any thoughts on the sanity (or otherwise!) of the basic system I'm putting together are much appreciated first off.

  • #2
    So your battery is (12V x 62Ah) 744 watts total. It's 5 years old, so it's maybe 70% of original capacity - if you are lucky so 520 Watts. It's a car battery so it cannot cope with deep discharges, but even if it was a deep cycle battery you should only discharge it to 50%. That would mean you have 260 watts capacity to use. If your calculation is that the inverter has to draw 414 watts per hour then in 30 minutes you'll have drained the battery to 50% and then in one hour the battery will be totally dead and further damaged, and the bike battery will not even be fully charged. As the bike battery has a capacity of (36x8) 288 watts your car battery isn't going to be of use as your AC charger needs 3.4 hours (84 watts per hour) to charge the bike battery from empty to 288W. That's a total of roughly (3.4h x 414W) 1400 watts your battery needs to have to charge 288W of your bike battery...

    If you want to do it via solar then you need to buy a proper deep cycle battery.

    Note: If you really want to calculate how much power the AC charger draws per hour of charging the bike battery get a kill-a-watt reader. It'll tell you exactly how many watts the charger draws per hour, then you can make your decision from there.
    Last edited by DavidH; 08-20-2019, 12:43 PM.


    • #3
      The Charger states ...
      AC 100 v - 240 v ~ 1.8 amps MAX

      It does not state 1.8 amps continuously at 240 volts.
      It states 1.8 amps MAXIMUM.

      Nothing is 100% efficient, therefore it is guaranteed that the INPUT WATTS will be more than the 84 watts ( = 42 vdc x 2 amps ) output.
      Exactly, how many watts input is required for 84 watts output is unknown.
      If it is a switching type power supply then use 80% efficiency, so 105 watts input will yield 84 watts output
      1.05 amps = 105 watts / 100 volts

      I do not believe that power supply is wasting 330 watts as HEAT = extremely hot ... no, not happening,

      Last edited by NEOH; 08-20-2019, 10:18 PM.


      • #4
        Originally posted by NEOH View Post
        If it is a switching type power supply then use 80% efficiency, so 105 watts input will yield 84 watts output
        Yeah, that makes much more sense.
        50lar53b, you could use a kill-a-watt meter to get the exact number, but if you go by 105 watt input per hour then, (x 3.4h) = 357W to charge 288W of bike battery. That drains your car battery until it only has roughly 30% left - probably less due to inverter inefficiency. I would get a 12V 100Ah deep cycle battery which using 357W (plus inverter inefficiency) will drain the battery until it has probably 65-70% remaining - your battery will last a very long time.
        Last edited by DavidH; 08-20-2019, 12:56 PM.


        • #5
          Thanks for your reply. I don't have a kill-a-watt meter, but I've just discovered a video on youtube where a guy in the US is recharging the bike I have with one of these meters, and the charger is actually drawing 55-60 watts. So it seems the stated input and output on the charger is perhaps more the maximum capacity that it's able to convert from AC to DC, rather than what the bike is actually using.

          The video also shows that it took 250 watts to charge the bike battery (as you note, its capacity is 288 watts, which in practise seems to be closer to 250 watts, as he did that charge from a completely drained battery). So if I assume that my 5 year old car battery has a capacity of 500 watts, with a full charge, it should charge the bike battery, and be at 50% charge. I understand that it's not a deep cycle battery; is discharging a car battery to 50% going to damage it?

          If I assume I'm going to get 5 hours of sunshine per day on average, given my 2000 hours per year of sunshine per year, that should give me 20 amp hours per day from my 50 watt panel, or enough to recharge the battery in 1.5 days. I'm hoping for to recharge it twice a week, i.e. every 3.5 days, so this is looking like a reasonable possibility to me, assuming I don't have weeks of overcast weather, of course.


          • #6
            Yes, deep discharging a car battery is going to damage it.

            Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).

            So if a new car battery can fail with a minimum of 30 deep cycles and your car battery is already 5 years old... It's impossible to know how long yours will take to fail if you are doing two charge/discharge cycles per week. Each cycle will also reduce the capacity of your battery by a certain amount, meaning that each time you have to discharge it deeper past 50% which in turn damages it more... you get the picture.
            Last edited by DavidH; 08-20-2019, 01:32 PM.


            • #7
              David, thanks for your input. I read that discharging an SLI battery by more than 25% is a good way to reduce its life significantly. If I believe that video I linked to earlier, the bike battery will take me 25km if I don't pedal at all, and about 50km if I use pedal assist mode. I'm planning on a 25km ride, twice a week, so if I'm not lazy and actually do some pedalling, the bike battery should be about half empty by the time I arrive, which in turn means I'll be using around 25% of the capacity of the car battery. So hopefully I'll get at least a few dozen charges this way, and I can buy a proper deep discharge battery when it dies. When I've put the system together (I'm waiting on a final Amazon delivery, having not considered that I'd need wire to connect the various components!), I'll update this thread with some real data.


              • #8
                That sounds like a plan. If you were planning to throw out the old car battery anyway then it won't matter when it fails. The system will also give you a good idea of how much power you are likely to generate with the solar panel. Remember it's a 50W panel only under lab conditions - with the sun directly overhead around midday and a temperature of 25C you'll be getting close to 50W. Anytime the sun isn't directly overhead the output will be less.


                • #9
                  A car battery is designed for around 10% depth of discharge and then be recharged immediately. It won't last long recharging it over a couple of days to reach full charge.

                  An old battery requires more power to get a charge, as the internal resistance had increased. I doubt your 50W panel will be enough.

                  As an experiment it'll be educative I guess.