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Voltage drop under load

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  • nfmisso
    replied
    About 3/4 of the way thru my 10 mile commute home there is a hill which I have to slow way down and coast before, then apply full throttle. Yesterday, coasting, my voltmeter read a fraction over 48V, then full throttle at low speed up the grade, the voltmeter read a fraction under 41V. This is normal.

    I have a Pswpower 13Ah, 48V lithium battery in a Hialong 2 case, a Sunwin 48V, 1000W controller, ebikling 1200W rear hub motor with 700c rim. The battery charges up to 54.5 to 54.6 volts overnight and every day at work. There is about 15" of 10ga super flexible wire between the battery and the motor controller, and about 45" of 12ga super flexible wire from the controller to the motor for the three phases.

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  • HIGHVOLTAGE
    replied
    What you're seeing is Voltage Sag

    Voltage sag is what happens when a battery is being used and actively drained of it’s capacity and will vary depending on the type of the battery, size of battery and condition of battery and how much power the motor is trying to pull from the battery.

    For example. If you have a 52v Shark Pack with a voltage of, say...51v while in a resting position and you hit the throttle to accelerate, you would see the voltage “sag” or “drop” to about 47-48v until you let go of the throttle again, where it will raise back up to slightly less than before. If you are using a high power controller such as Ludicrous this will pull more current and the sag will be greater than it would otherwise be. If you are not downshifting on hills this will also pull more current than it would otherwise and would also cause greater sag.

    So let’s say if you’re near 46v or 47v of capacity, well that means you’re pretty much dangerously close to hitting the auto-shut off mode because if voltage sags by about 3v - 4v while climbing a hill, you’ll just randomly cut-out with no explanation or warning. Once you hit about 47v, you should consider yourself low on battery and start “limping” home using minimal power with as much assist as possible to save yourself any juice for an emergency, or to keep the batteries condition at peak performance since discharging them near the cut-off also reduces the their total lifespan quicker.

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  • nfmisso
    replied
    There is nothing to fix. The indicator could be better tuned to the battery. You don't say where in the circuit you are measuring the voltage, nor where the battery indicator is located in the circuit.

    You don't mention the battery type.

    Batteries have internal resistance, the wiring also has resistance; when current increases, there will be a voltage drop proportional to the current: V= I x R.

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  • Harrypph
    started a topic Voltage drop under load

    Voltage drop under load

    I have a front wheel drive electric diy bike running of 4 12v battery in series (soon to be 8 12v battery 4-4 in series) it runs quite well getting up to about 40kmph just by itself but the voltage seems to drop from 51volts(fully charged) to 46 volts when any throttle is applied. The led battery indicator also drops from full to low as well.
    How can I fix this, should I fix this/does it affect anything will I get the same range?
    Last edited by Harrypph; 08-25-2018, 09:47 PM.
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