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My DIY 5v power source project from garden lights - is it safe?

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  • My DIY 5v power source project from garden lights - is it safe?

    I've been playing around with solar panels from broken garden lights to learn a few things and to give me something to do. Since I'm still in the learning process and based on other threads in this forum, I'm questioning the safety of my little project here so I thought I'd ask here if I'm doing something that is not safe. I don't know if this is the right place to ask this but I'll ask anyways.

    Every few months one or more of my dollar store garden lights stop working or break. They break from kid's Halloween candy bags, school bags, stop working for unknown reasons, whatever. I had a growing supply of mixed sized solar panels and batteries from these broken lights. I wanted to do something with them. Not necessarily something useful with them but that would be an added bonus. After doing lots of research on solar panels, battery charging, things not to do, etc., I came up with an idea to create a solution with the solar panels that I could use to power or charge 5v USB devices but with the ability to add more solar panels to the solution as more garden lights break. This is what I've built, not very useful but I have been using it for a small few things.

    For each solar panel I've created a circuit like this:

    +V:
    [Solar Panel] - [Blocking Diode] - [Battery] - [Blocking Diode] - [+V Shared line]
    -V:
    [Solar Panel] - [Battery] - [-V Shared line]

    Several of these solar panels are all connected to the same shared line. The shared line is copper cabling that currently runs from the window about 15 feet to a table but may run longer in the future. From the shared line there is a DC 2 DC converter to boost the voltage to 5v but may have a few more of these cheap converters in the future as more solar panels and batteries are added.

    Here's the details behind this plan:

    The solar panels are different sizes, different voltages, different everything. The design I set for this project is that the voltage output from these solar panels do not exceed 4v in direct sunlight.

    The batteries are different. They are all 1.2v Ni-CAD or Ni-MH cells only. No lithium or anything else other than 1.2v batteries. They are different sizes in their capacities, some are AA, some are AAA and some are 2/3 length cells but they're all 1.2v rechargeable batteries. In most cases, they're the same batteries that were with the solar panel as a garden light. In some cases though, they're rechargeable batteries I found in the battery recycling box we have at my office that still work fine.

    The theory (in my mind):

    Each solar panel charges a single AA or AAA battery. The blocking diode between the solar panel and the battery is to prevent draining the battery at night. Each battery is then connected through another blocking diode to the shared line. This is for a couple of reasons. First, it prevents parallel charging of the batteries from the other solar panel / battery groups in this solution. I read that it was bad to parallel charge batteries because of the risks if one battery in the set died and shorted out so I added this to my design. The second reason for the second diode is to keep the shared line at the highest voltage possible. The batteries at the highest voltages should be discharging together but if a smaller sized battery drops in voltage because it's almost dead, the diode should block it from the shared line preventing the shared line voltage to drop and should then be charging from it's solar panel (if it's sunny).

    Overall, this create a bunch of isolated solar panel charge circuits to charge the batteries and then a set of batteries discharging in parallel.

    Testing observations so far:

    I had about 12 - 15 strings of 'solar panel to single battery to shared line' set up and running for a couple of months. It had no problem charging small devices like my wife's Fitbit device or powering small USB power lights like Christmas decorations. It didn't do so well trying to charge larger things like an Android tablet. The tablet reported that it was charging but in reality, it was only slowing down the tablet battery's drain during usage.

    The batteries didn't get warm or anything to show they were having any sort of problems. The solar panels were quite warm though during the daytime. If a AA battery was removed during the day, the voltage increased on the shared line since the solar panel was now sending power to the shared line through 2 diodes instead of a battery but once a load was put on the shared line, voltage dropped back down to the typical 1.2v from the batteries.

    I had not found any signs of problems or anything that would cause concern for safety but I'm asking here in case I'm not aware of something. I did disassemble this in the spring time / early summer time (I live in Canada if that matters) because the solar panels were held to the window with suction cups that were drying out and kept falling down, etc. I'm planning on setting this up again so I have free power for Halloween & Christmas lights and decorations.

    I expect that the life of the AA and AAA batteries will be short with the constant charging and draining cycles. More if two batteries are discharging at different rates and blocking diodes open and close as their voltages drop at different rates.

    I do have some ideas for future upgrades and learning possibilities with this but I want to be sure this is safe so far and that I don't come home to a smoking pile of debris some day.

    I couldn't find anything when searching for expanding solar power supply designs like this so I don't know if that's because there's risks or if it's because it's not worth the effort. Are there any safety concerns here that I might be missing? Are there any limits I should be aware of as I add more solar panel / battery strings it?

    Thanks.

  • #2
    Is there enough energy to require a fuse ?

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    • #3
      I don't think so but I don't know for sure. Even with 12 solar panel & battery strings in parallel, I'd guess that only 3 or 4 will actually be putting out power at anytime but I'm not sure of an easy way to figure out the exact numbers.

      I'm thinking that if for example, 3 batteries were 2400mah, 3 batteries were 2000mah and 5 were 600mah, with all being fully charged, they would all be active. Within a few minutes the 600mah batteries would drop in voltage as they discharge faster than the others and would quickly be blocked by the diodes. They would recharge at different rates so they would return at different rates. The same theory would go for the 2000mah batteries next. Once all of the batteries are drained, the 600mah batteries that are probably going to recharge the fastest being the smallest batteries would be active first the next day. Again, this is the theory in my head but could be wrong.

      Now, if eventually I did have enough power having a few amps (probably talking 100 garden solar panels), where would a fuse go? Since it's a collection of strings sending power to a common 1.2v shared line and may have a few DC converters pulling in parallel from the same shared line; I don't know where a fuse would go to make it useful. I could put a fuse on each DC converter feed but that really only protects a small extension of the whole solution right? If the shared line shorted out, those fuses wouldn't do anything and the only placement for a fuse that would work would be on each string of [solar panel - diode - battery - diode] line. But which each line only putting out at most 4v but low current (for the short period of time when there is no battery so the power from the panel goes direct to the shared line) I can't see the value in that.

      Other than something shorting out in the wiring, would there be a risk or need of having a fuse?

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