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  • Solar driven E-bike motor.

    I have made a small motorised field-trolley with a small 24V E-bike geared motor. The battery I use at the moment is a Ni-ion 6S1P 2Ah with Battery Management System (BMS).
    For normal use the motor uses ~50W (with bursts of 1-2 minutes I can use up to 140W). After using for 1 hour (mainly parked) the power consumption has been ~ 7-10Wh.

    I would like to charge the battery with solar panels to avoid taking the batteries home for recharging - and for fun. Solar panel charging will ONLY happen when the trolley is in use. When stored the solar panels are disconnect or the trolley is standing in the shed.

    I play around with 2x10 W solar panels from eco-worthy that together is giving me ~ 10-14 W at this time of the year.
    (Panels are made for trickle charging with a diode)

    [B]My question is:[/B]
    Can I connect the solar panel directly to the battery (and motor controller) with out a solar controller? (to make it as cheap and simple as possible? )
    I know it is possible when solar panels are used for trickle charging and the capacity of the batteries are much bigger than what is delivered from the solar panels.


    [B]My worries are:[/B]
    1) Can the motor damage the solar panels when there is no solar charge controller in between ?

    2) The solar panels will only be connected when the trolley is in use and the power consumption will then be similar to what I can get from the solarpanels.
    At ~24.8V the BMS seems to cut off the connection as my cheap wattmeter goes from ~12W -> 0W. It all seems to work fine but I have a feeling I am missing some important stuff as it all seems too easy and simple?

    3) I am considering to use a 7S1P battery (li-ion) to keep the minimum voltage higher. Then I ideally need 29 V for charging. With out having a clue I feel that the solar panel will work better with a 6S1P li-ion battery. Any opinions on this?

    4) For solar panel charging it seems that LiFePO4 batteries is the normal/better choice. I know the safety of LFP batteries are higher than for Ni-ion and they do recharge more times, but is there any other reason to choose LFP batteries for this application? (I do like the higher energy density of the Li-Ion cells as small size matters for me. Unfortunately we do not have sunshine everyday and I probably end up with a 5-6Ah battery pack to be able to run the trolley for 3 days with out any sun)

    5) Things I haven't though off.

    regards Carsten

  • #2
    At a minimum,...
    a) You must stop charging a Li-Ion battery when it reaches a specific Voltage.
    b) A small & inexpensive Over-Voltage circuit could be used to auto-disconnect the Solar Cell

    No, you should NOT continue to Float Charge a Li-Ion battery with a Solar Cell, after the battery has reached 100% charge.

    I know that you stated that the Solar Panel will only be connect to the battery when the bike is "in use",
    but we both know what will happen in the real world.
    You should never design a Li-Ion charging circuit that requires the human to remember to manually shut-off the charger.

    A 20 watt MPPT battery charging circuit would be very small, inexpensive and good insurance.
    Last edited by NEOH; 08-20-2019, 08:30 AM.

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    • #3
      It sounds like your existing BMS cuts off charging voltage at 24.8 volts. For a 6s pack that would be 4.13 per cell which implies NMC chemistry. If you go with a 7s LFP the high voltage cutoff should only be 25.5 volts because that chemistry has a lower charging voltage of 3.65 per cell.

      i dont know the parameters of your solar panel so I can't tell you if it is capable of charging at that voltage.

      To answer your question about connecting the trickle charging solar panel to your battery motor controller I would only be able to guess that it might work if the voltage of the solar panel is enough to charge the battery. Normally as [USER="4075"]NEOH[/USER] has said it is bad on Lion batteries to float charge them for any length of time. In your case you say you are only connecting them when the device is running. My only suggestion when doing that is that you monitor voltage to make sure it does not go over the correct voltage for your batteries. It sounds like your existing BMS does that so there would be some automatic control in the event you forget to turn off the switch.
      Last edited by Ampster; 08-20-2019, 10:08 AM.
      9 kW solar. Driving EVs since 2012

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      • #4
        Motor <— Speed Controller <— Panel (w/ diodes) —> Charge Controller —> BMS —> Battery
        (Battery —> Speed Controller —> Motor)

        (shouldn’t have to worry about switching, etc. motor uses panel before battery. battery charges (balanced) when the charge controller allows it. diodes protect panel when there’s no draw—otherwise cover the panel when not in use. make sure all components can handle 24v and max draw from the motor (peak amps)—usually regulated by the speed controller)

        edit: wiring, etc. (MOSFETs a.k.a. throttle, should handle any voltage fluxiations from the panel—make sure the speed controller can handle the maximum current/amps; might be a weird ride though—maybe soften that with supercaps? mppt?)
        Last edited by HadrianKross; 10-08-2019, 09:33 AM.

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