Skip to content

Do Solar Panels Drain Battery At Night? [A Guide]

    We all know how solar panels are supposed to work — the sun shines, the panels lap up all that light, convert it into electricity (much like when we metabolize the nutrients in our dinner), then pass that power on to a battery for safe keeping.

    We can then use that battery as and when we need some free, green solar power.

    Do Solar Panels Drain Battery At Night? [A Guide]

    However, on occasion, you may notice that after going to sleep with a full battery, you wake up to a, well… not so full battery.

    But why does this happen? Do solar panels go full vampire when the sun goes down, or is there another reason that charge dwindles when your battery is idle overnight?

    That’s exactly what we’ll be discussing today, so stick with us, and all will be revealed!

    Will Solar Panels Drain Your Battery After Sundown?

    You’d be forgiven for thinking that current has a single direction, a course set by the components involved in its transportation, but this isn’t actually the case.

    Electricity is a rather odd soul, or perhaps mercurial and lazy would be more apt descriptors.

    It doesn’t only travel in one direction, but rather from high potential to low potential. In other words, current flows in whichever direction is easiest.

    If there’s a path of least resistance, even if it’s where the current originally came from, that’s the path current takes — It just wants an easy life.

    It actually behaves a lot like water in this way. Water will always follow the easiest path downhill to a low point, which is why you don’t see streams flowing uphill.

    Granted, it’s an imperfect analogy, as gravity isn’t a contributing factor in current flow, but you see what I mean.

    As such, in theory, yes, your solar panels could absolutely turn into vampiric battery drainers when the moon rises; here’s how:

    • Your solar panels deal with the lion’s share of the power during the day = current flows to your battery.
    • Your panels have no power at night and your battery has lots = current flows from your battery to your panels.

    So, technically, it’s not that your panels are actively siphoning off your solar juice, but that they’re not doing anything to stop the current racing back from whence it came and essentially leaking out.

    This is known as “back-feeding”.

    Carnivorous Solar Panels

    It’s not just your battery that solar panels drain when their own power dwindles, but their fellow panels as well.

    Let’s say, for example, that you have multiple discrete panel arrays all connected up in a single system; should shade fall on one array before another as the sun goes down, the powerless panel will leach power from those still sunbathing.

    Even though this is panel vs. panel pilfering, it’s just another example of back-feeding, of the current being lazy and following the path of least resistance.

    However, just because back-feeding is a thing, doesn’t necessarily it’s your panels to blame.

    Back-Feeding Protection

    Unless you’re dealing with an incredibly rudimentary solar setup, you’re most likely fully protected against back-feeding, as there are two common components that all but eliminate the process:

    • Bypass Diodes/Blocking Diodes
    • Charge Controllers

    Let’s take a look at these nifty gizmos in closer detail.

    Bypass Diodes & Blocking Diodes

    If you’re familiar with wiring, then you likely already know what a diode is, but for the uninitiated, a diode is essentially a small component that prevents current from passing.

    You can think of them kind of like a valve in a water network. You’ll find these electrical valves throughout a more fleshed-out solar panel system.

    As for the difference between bypass and blocking diodes… there really isn’t one. Their function is always the same, but the lingo changes depending on the particulars of their job.

    For instance, if one has been wired into your system to prevent solar panels leaching off one another, then it’s known as a bypass diode, but if one has been wired between your panel and your battery, it’s a blocking diode.

    Charge Controllers

    The primary role of a charge controller is to regulate voltage to keep the battery safe from peaks, troughs and overcharge, but many, such as the Renogy Rover, are also kitted out to ensure power goes in one direction and stays there, even at night.

    Will I Need Diodes And A Charge Controller To Prevent Drainage At Night?

    Will I Need Diodes And A Charge Controller To Prevent Drainage At Night?

    As long as you have either blocking diodes or a quality charge controller, you won’t have to worry about back-feeding at night — you don’t need both.

    Having said that, if you only have diodes, I’d highly recommend investing in an MPPT charge controller too, as it will keep your battery in top shape, extend its service life, and optimize your solar yield.

    A lot of panels will arrive with integrated diodes, but if you’re looking for more, I’d recommend checking this Aukenien kit out.

    I Have Back-Feed Protection But My Battery Is Still Hemorrhaging Charge Overnight… Help

    If you’re seeing significant charge leaching despite having the proper components locked and loaded in your system, then there’s most likely a problem with one of the constituent parts of the circuit.

    Solar Charge Controller Chaos

    There are a couple of things that can prevent a solar charge controller from doing its proper duties.

    Cheap Or Old Controllers

    Poor quality, cheap charge controllers will often end up on the fritz sooner than you’d like, and it’s not unheard of for the blocking technology to go caput.

    The same is true if you bought a quality controller but you’ve had it for a great many years.

    Vampiric Charge Controller

    On rare occasions, it might be that your solar panels aren’t the culprits of the back-feeding after all, and that it’s actually your charge controller that’s pulling the current back.

    If this is the case, you may be able to get it repaired, but you’ll probably need a full replacement.

    Blocking Diodes Dilemma

    There are no perfect electronics; most will fail after a long enough period, and diodes are no exception.

    With no charge controller in place to pick up the slack, busted diodes are going to let that current flow willy-nilly.

    The good news is that diode replacement is well within the realm of the DIY fix, but unless you have some expertise with electronics and wiring, I really can’t recommend going it alone.

    If you have any doubts about the process whatsoever, call in a professional technician to sort the issue out safely.

    Battery Botheration

    Have you checked your charge controller and/or diodes and found everything’s working just fine? Then the problem is almost certainly your battery.

    Low quality composition or installation can often cause alarmingly rapid self-discharge.

    Just in case you’re in need of a replacement, I can’t speak highly enough of this Ampere Time LiFePO4 battery, but if that’s a little too rich for your blood or too large, this more affordable alternative from Renogy will be just the ticket.

    For something in between those two price points, I’d shoot for the Lossigy LiFePO4 12V 200AH.

    Final Thoughts

    A solar battery draining overnight is a total pain, and if you have neither diodes nor a charge controller in place, it’s almost certainly due to your panels causing back-feed.

    Thankfully, though, it’s an easy fix. Simply invest in a quality charge controller, or failing that, a blocking diode, and your power will be safe and sound when the sun goes down.