Make the Harbor Freight 45W Solar Panel Charge Controller Useful With a Simple Mod.

Introduction: Make the Harbor Freight 45W Solar Panel Charge Controller Useful With a Simple Mod.

Like many of us who are interested in solar power, I bought a Harbor Freight 45 Watt Solar Panel kit for about $150 which came with three solar panels, and a charge controller.  It was exciting times when I set it up several months ago to see the front panel lit up.  I thought that I was getting enough solar juice to charge my SLA batteries.  However, the excitement was short lived.  I was foolish enough thinking that the light was all powered by solar for the very first night, and forgot the fact that the battery was fully charged already before I connected it to the solar panel.  Just like many found out, the charge controller doesn't appear to be working as we expected.  For example, my SLA battery originally had 12.5 Volt.  During the day, the front panel shows about 13-14 volts so I leave the charge controller on, but when I come back from work, I found that the battery wasn't charged at all.  In fact, the voltage would go down about 0.1 volt to become 12.4 volt.  What's going on?

As usual I searched for answers on the internet but most people simply complain that this particular charge controller is not efficient, or it can only be used as a power distribution device.  To get a better charger, one would need to invest another $100.  Basically, the solar panel kit is only good for demonstrating the concept (I have to say that from outside, the charge controller looks nice and solid).  

Recently, I got some spare time and started making diagnosis on this charge controller and I found that while it indeed charges during the day, it discharges as the sun goes down.  I suspect that when the voltage from the solar panel is below the voltage of the battery, it may be discharging the battery.  I don't know this for sure since I haven't open the device to see the design.  This device does discharge at night at about 30mA if you leave it on (the LED display uses power unless you turn the LED display off which is recommended).

Step 1: Add a Blocking Diode to Stop Discharging the Battery

One experiment I did was to simply add a blocking diode on the back of the charge controller.  I initially placed the diode on the battery side but based on recommendations from "evilmunkey", the diode should now be put on the solar panel side.  The diode I am using is 1N5822 which happens to be in my toolbox so I guess many other diodes would work as well.  1N5822 can handle 40v 3A so I use two of them in parallel to make sure it can handle the current from the solar panel.  Also make sure that the cathode lead of the 1N5822 should be connected to the charge controller side while the anode should go to the solar panel side.  Previously I experimented with putting the diode on the battery side but I was concerned that it might overcharge the battery so now the diode is on the solar panel side.  After the modification, my SLA battery is charged better. 

Step 2: Caveats With the Blocking Diode

Since I don't know the charging circuit inside the box, the effect of this added blocking diode on the solar panel and battery charging is not clear.  Theoretically the charge controller should already have blocking diodes on the solar panel side, otherwise it will give all back the solar power at night.  The ultimate solution would be to look at the components inside the box and see what it's doing.  There are many different kinds of controllers and they all work differently.  I assume that this is a PWM type but I don't really know.  For example, does it have a microcontroller to control the voltage?  What about current limiting?   Given all the unknowns, I wouldn't risk charging any expensive batteries.  In my case, I salvaged some old 12V SLA batteries by refiling with Epsom water.  I also leave the battery outdoor in case of outgassing.  So far it works fine and the battery is actually revived to a certain extent.  This is very much an experiment so be careful not to overcharge.  Finally, make sure to add an inline fuse near the battery terminal (such as 5A).  It's better to be on the safe side.  If anyone has any better suggestions improving this unit, please let me know.  I may open the box in the future and write another instructable later.  For now, I need to move on to the next project with desulfator.



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    39 Discussions

    I have the same kits. A 45watt and a 100wat kit. I have the block that u put 8 panels on also but im only getting 15.4 volts outta all this and i have to unhook the box at night because i dont have the diodes on mine yet. Should it be putting out more volts with 145watts? And how much does the volts go up in summer when u have higher amount of uv rays?

    I would suggest that, if you setup the system correctly and find it does discharge (the battery) when the Sun goes down, the unit is defective. Since they offer a 30-day (or better) replacement I would have suggested you simply return the kit.

    If you let too much time pass before discovering the problem, go in and buy another kit, swap out parts to determine if it is the controller or another defective part (in the old kit) and then swap out the defective part and return the latest purchase and get your money back.

    Good news is these kits are currently advertised for $119.00 w/coupon 11123222


    1 year ago

    You can't use two diodes is parallel.

    One diode will always turn on before the other - which can result in burning one diode then the second parallel diode when the load is greater than what a single diode can carry.

    You actually need a single diode that can carry the entire load.

    Also - keep in mind - that an average diode will have a .7 (point 7) volt drop. Thus 24.0 volts through a diode will actually give you a working voltage of 23.3 volts.

    1 reply

    Hello all,

    I have a question that hopefully somebody here can answer. I am looking to get this solar kit from harbor freight. I have a jayco travel trailer that gets very damp in the winter. I bought a 100w fan that should take care of this. I will be getting two batteries for it. Is there something else I am missing? The kit, two batteries, fan. Would this set up be adequate to run the fan 24/7 for (and absolutely nothing else) 6 months?


    4 replies

    Amazon has a 100w pannel for about 15dollars more much much better i purchased one and it works very good your choice why I got a 45 watt system when you can get a hundred lot system for ten fifteen or twenty dollars more

    Do not buy from harbor freight.i did bad choice

    Can I add more panels to this charger controller or does the controller only accept x amount of input and if so what is the x amount

    2 replies

    the charge controller can only handle 100 watts. If you are already using the 3 panels that come with the system, you have 45 watts, so all you could add is 55 more watts

    I have a 20×35 square foot home in baja how many panels do I need or what setup I have fridge window air con 12btu and 4 rooms light

    This is kind of old, but just in case others are finding it, I'll offer some notes.

    1) Do not put blocking diodes on the battery - the whole point is that this system stores energy into the battery as well as taking it out again, so that flow needs to be two way.

    2) Blocking diodes on the solar panel MIGHT make sense, to avoid draining the battery into the panel at night. However, some solar panels include an internal diode, and some controllers include an internal diode, so adding another one might or might not be needed for that purpose (I don't have the unit and cannot check).
    3) The blocking diode reduces the voltage from the solar panel, and thus the charging power available, so adding one if not needed is actually a bad idea. If you do use a diode, make sure it has enough current rating (and that it's placed so it's OK to get hot). A Schottky diode has less voltage drop than a standard diode. One diode with a sufficient rating is somewhat preferable to two under-rated diodes in parallel (because the two may not divide the current evenly). Since OP says he got better results with a blocking diode on the solar side, then for at least his unit it may have been needed.

    Which way is the stripe on the diode supposed to face? Closer to the box or away from the box?

    1 reply

    Stripe is the cathode or negative. Op states to place the stripe towards the charge controller.


    2 years ago

    I have the same problem...i suspect the inverter portion of the box is always on and may be constantly draining about 10 watts offof the battery. I'm thinking of switching to a $15 charge controller to eliminate this wasteful power loss.

    I have one in my garage that u have yet to setup, my question is if ran to 2 deep cycle marine batteries would it provide enough power to run a above ground pool's pump in an area with plenty of sun?

    1 reply

    Short answer, no. These put out 45 watts, 4 amps peak... Adding a second battery doesn't increase your charging. How many amps does your pump pull?

    I have 4 complete systems using one regulator and solar panels 12 hooked into a round box with one wire coming out to the regulator and it wont light up one light without a battery ? What have i done wrong guys?