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

Picture of 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

Picture of 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

Picture of 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.


charlessenf-gm (author)2017-01-25

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

TomJ96 (author)2016-10-21

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.

JohnnyLikesApples (author)TomJ962017-01-20

So, what type of diode would be ideal?

jeffjules (author)2015-09-03

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?


DavidM15 (author)jeffjules2016-10-07

no, this will not work as you want

jeffjules (author)jeffjules2015-09-03

oh yeah, I also have a little inverter that I would be plugging the fan into.

paulclutt (author)jeffjules2015-12-14

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

paulclutt (author)jeffjules2015-12-14

Do not buy from harbor freight.i did bad choice

KeithR29 (author)2016-01-07

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

DavidM15 (author)KeithR292016-10-07

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

blast2469 (author)KeithR292016-02-18

Harbor Freight also sells an 8 channel connector so you can hook up multiple units. I have 2 complete kits hooked up to a battery array and it works well for me, just keep In mind you get another charger with each kit, now I have an extra. Here is the link.

RubenzZ (author)2016-09-20

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

Zephyr93 (author)2016-08-02

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.

KevinC303 (author)2016-06-20

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

Jbg6 (author)KevinC3032016-08-01

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

Jbg6 (author)2016-08-01

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.

JosephW63 (author)2016-04-18

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?

Jbg6 (author)JosephW632016-08-01

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?

Jerryes (author)2016-07-11

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?

allent4 (author)2016-04-28

these chargers can only handle 100 watts ..the kit itself is 90 watts total $150 on sale to $199 is a total rip off... home depot

sells a 100 watt panel for $120 a 10 amp charge controller can be had for as little as $15

DanR108 (author)2016-04-02

There IS a blocking diode in the solar panels already.

LoriP3 (author)2015-08-07

I have this same set up. It has worked really well, with no extra diode attached for 4 years. I've had it storing solar from the panels into 2 deep cell car batteries. Never a problem until recently, when i plug in to either the silver box or its companion transformer, the led read out flashes wildly thru all the numbers and then the red low wattage light goes on at 11.0 and it won't charge. Do any or all of you think my batteries are worn out? Or?

KingSlay3r (author)LoriP32016-01-31

When the battery is under 12 v it is considered a dead battery so you can try and use a charger like a car charger and see if they take a charge or even take it in to an auto parts store and have the battery tested if it is still a good battery then I would try another charge controller

dpenaherrera1 (author)2015-10-20

HI I just purchased this kit from Harbor Freight-Imagine my excitement when I saw that you had also purchased this and was using it. Forgive my ignorance - I have not even opened this up yet. I plan on setting it up in an upstairs front room with eastern and southern exposure and large windows- its sunny all day. My plan is to have it power tvs and small heaters( 10 room victorian) as an experiment this winter(gas bill runs as high as 400-500 dollars in Jan). I would enjoy this luxury of heat for a change. The thermostat never goes up past 61 degrees in this old house. Will I be able to run a heavy duty extension cord to plug in these aforementioned devices? (again forgive my ignorance I really want to take myself off the grid as much as I can)

BobD23 (author)2015-09-15

Hi. I have the 3 panel unit along with the regulator that came with it and the things seem to be working. Battery started out at 11.9 volts and is now at 12.4 and it is inside my shop with not much direct sun--for testing.

I also purchased an inverter that has battery clamps that need a place to connect.

Is it correct to just connect them to the battery where the regulator also connects? Seems like it should be OK, but...


KevinS17 (author)2015-05-16

I have this controller setup in my off-grid cabin. I have it charging two deep-cycle marine batteries in parallel and it runs 24-7. Also running 24-7 are two 5-watt LED lights. Every time I go to the cabin my batteries are fully charged. I lose almost no voltage overnight running radio, lights and charging my power tools used throughout most of the day. If the LED display is draining your battery, I suggest upping your battery capacity. If I use my batteries to run my inverter and bigger appliances (compressor, shop vac) the panels/controller will bring my batteries up from below 12V back to full charge in just a few hours on a bright day. I then switch it over to top off my spare batteries that I move around for portable power.

It's a decent, capable setup, but without adequate battery capacity you will be frustrated. Adding the second battery made all the difference for me.

AmandaM36 (author)KevinS172015-08-25

Do you have a picture of your set up, I am wanting to set up one for Just incase situation

gaieb (author)2015-06-11

I would consider putting a 12v coil
relay on the battery side with the coil tied to the solar side, that way if the
voltage on the solar side is too low it will disconnect the battery.It doesn't appear that the controller has a boost circuit, I didn't recognize one anyway from the schem.

shootr (author)2014-10-19

I've been researching this topic since needing to use a small solar panel to keep a 12v automotive style lead acid battery topped off. I want to wire the panel (18v max / 170mA max) dirctly to the battery.

Can this diode be put directly into the positive lead to keep the battery from being discharged at night?

Tom Hargrave (author)2014-09-07

The schematic for this charge controller shows it already has an internal blocking diode. The blocking diode is D1 and D2 wired in parallel in the attached schematic.

The real issue is the display continues to draw power when the switch is on, even when the solar panels are not producing power. The simple solution is to buy a large enough reserve battery to carry overnight with the display on if the display is important to you, or just to open the case and unplug the display.

astral_mage (author)2013-12-24

ok here wat i use. i bought thier seperate charge controller. the 30 buck unit. an that inplace of the big box unit. it works way better.

ThriftStore Hacker (author)2013-11-26

i have this same kit and am having the same problem.

would it work if you put the blocking diode on the solar panel side? so power could come out of the solar panels to the box but not back the other direction?

It might. However, I believe that it does some kind of blocking on the solar panel side when no voltage is produced by the solar panel. I actually measured the voltage on the solar panel side in the evening when the unit is turned on. As expected the voltage is zero which suggests that it doesn't discharge the battery into the solar panel at night. However, what I noticed after I install the blocking diode on the battery side is that the unit will still be on as the sun is going down. The voltage can be as low as 4V and the front panel LED is still on. I suspect that if I didn't have the blocking diode, it would be draining the battery at this time when the solar panel is producing voltage below 12V. However, I don't know for sure because I haven't seen what's inside the box. I might just open the box one day to find out if I become really curious about it. In any case, the blocking diode seems to work on the battery side.

hmmm. well i am picked up some diodes and i am going to install one today. i will let u know how it goes.

also u said the display on yours eats power. did yours come with the on/off just for the display (black button right of the display)?

Glad you pointed out. That switch is rather inconspicuous and it looks like a small LED by itself. Indeed that is the switch for the LED display. However, the LED display consumes about 30mA which is small compared to the 45 Watt capacity. Therefore, I can't think of why it would be the main cause for discharging the battery, but we never know. Look at it another way, one can buy a small outdoor solar LED walk light for $5 and it would still be able to keep the LED light up almost all night, and why would one need a 45 Watt solar panel for that.

i tried out the diode connected to the solar panel input. works great. i noticed i lose a lot less power overnight. i have my 45w kit hooked up to two 72ah AGM batteries (rescued from an old wheelchair) it runs a 12 watt LED light overnight that lights my side yard. it also runs multicolor decorative lights in my garden (instructable coming) but not all the time.
with as much storage i have hooked to it i can run my fridge and a few lights when the power goes out.

I agree that putting the diode on the solar panel side is a safer bet. This way it will less likely to overcharge the battery. However, I had to change my diode to two 1N5822 (in parallel) so that it can handle the high voltage (about 23 V). Previously the 1N5820 can only handle upto 20v safely. I use two diodes in parallel because each can handle 3A, while the solar panel can produce more than 3A in theory. I just made the change today and will see how well it works. The voltage drop across the diode is about ~0.2v, which is more inline with the spec. I don't know whether this has any impact on the solar panel. I don't see how but without seeing the circuit, it's hard to say. If someone has the circuit diagram and willing to share, it will be very helpful, or I will open the box when I get some time.

At this point, I think putting diode(s)on the solar panel side, and turn off the LED display are two simple things that one can do to save some solar energy. I plan to revise my instructable to acknowledge your contributions. Thanks a lot, evilmunkey.

your very welcome. if you come up with any more cool mods let me know.

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