My Prius solar hack installation

I wanted to use the solar cell coming from my Prius 2013 not only for the venting fan as it coming from factory, but to charge a battery. After reading several post and forums, I realized that connecting to the main traction battery will be expensive, complex and not so effective. Traction battery uses high DC voltages and the small charge coming from the solar cell will be almost imperceptible. Instead, I considered the secondary battery as my target, it  supplies current to all the accessories, including the electrical computer that controls if the Prius turns on or not.
     This battery is smaller than the typical car battery, and there are reports that it does not have a long life, probably less than the 2 expected years. When left parked for more than 2 weeks, the charge could be depleted. This car has a lot of electronics, so by maintain the battery charged on acceptable levels, it can last more.


The first step is to locate the point where I can intercept the current coming from the cell. Reading some documents posted for emergency response teams, it is mentioned that the solar cables are running from the left back corner of the roof (driver side). So I opened the hatch door, took off the left plastic panels and moved the ceiling cover a bit down so that I can see there a filter and then a connector coming from the cell.

(Be careful, this has to be performed with battery disconnected. As you can see on below picture on the left you can see the side air bag and any error can cause a catastrophic accident)

(A fellow from prius chat made it easy this step by finding the line running on the back left door panel)
With a quick connect splicer, I run a wire from there to the right side of the compartment where the secondary battery is. I noticed that the negative cable from the cell was grounded , so I decided not to waste cable on it.

The negative cable from the battery is connected to ground with a bolt nearby so there I hooked my negative cable. For the positive from the battery I used also another screw that was nearby the fuse and the connection to the battery. I placed a 7.5 amp fuse, so that I can protect the battery from overcurrent. I read max current accepted by the battery to avoid damage is 4.5 Amperes. Then from the +V coming from the cell I placed a switch so that I can disable that voltage. After that I was ready to install a solar controller.

I looked on ebay and found a cheaper controller for less than $10 USD, it was the CMP12

I was happy with it, until I had the chance to verify voltages and current flowing on it when it was a sunny day... and for my surprise this device was not working at all... seller also claimed it was a MPPT that of course it is not.

First, I checked the current and it was flowing about 2.5 amps on direction of the controller, and also on direction of the battery... later I sow -2 amps, meaning current was flowing on the cell direction!!!!!
What happened was that the automatic vent was turned on, and there was a cloud shading the solar cell, so the vent was sucking current from the battery controller and battery because solar cell was under shade...

I was a bit disappointing by the lack of protection of this controller, so turned the solar switch off and ordered a 2 dollar NTE573 60V 5 Amp Schottky Barrier Diode Rectifier #:NTE573
which included shipping on Ebay. A month later, it arrived, I soldered on the cable coming from the solar cell, and waited again for a sunny day to test the system again. A week later, I was able to test it.. now there was no backflow current... great... then I measured the voltage.. and o my god!

Device protection for over-voltage supposed to be 14.2V, and when verified the voltage at the battery, it was more than 15V... exactly the same voltage at the solar cell input... So this controller also did not protected for overcurrent!!!! It has a led indicating if it is charging or not, and it turned always off when reaching the 14.2v threshold , but higher voltage and current was still flowing to the battery...

Dissapointed, I made a claim on ebay so that I can receive my money back... some times those chinese guys refund all the money paid, some times they not... so I was not worry because it was a cheaper device... finally I got my money back..

Not learning my lesson, I decided to buy the next controller in price, that was about 13 dollars. It was the controller CMTP02

No matter it was a couple of dollars more expensive than the previous dummy unit, it looked of worst quality and worst design. Just recently I opened both units, and the CMP12 at least have a formal heatsink because the CMTP02 uses only the back tap as heat dissipation.

Short story with this controller, I tested just the over-voltage protection (black flow current already protected by my schotty diode), and confirmed that is as dummy as the CMP12... They don't protect at all, and seems their leds are the only thing that works on them...
On this unit, I am still negotiating with the EBAY seller the refund, and asking to stop selling fake controllers.

Finally, I decided to look for something of a better quality (and higher cost), and find theMorningstar SunGuard SG-4, 4.5 Amp 12V Charge Controller

This unit does not have control of the loads added to the system like the previous ones, it does not have any leds at all, but it does just what I needed: To control the charging of my battery with the solar cell. The unit is sealed, so it can be installed outdoors without any concerns.

I installed the SunGuard SG-4 on my car, tested for overvoltage protection... and it worked flawless. When I turn the car on, the car starts charging the battery increasing the voltage to 14.something volts, and checked the voltage of the cell it was on 20v... meaning no connection to the battery... checked the current and also there was no ampere flowing ... so it was working!!!!! It costed $30 usd... but at least it does what it supposed have to do.

I will continue my testings... since i want to make sure that when charging the battery there is still enough juice to let the vent to work and refresh the interior of my car on the hot summers of Texas.
(Link to my testing)