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Shopping advice: Solar battery charger and power supply

  1. Aug 24, 2015 #1
    I'm sorry if this isn't the right place for a shopping question, but I'm not sure where else to go for advice.

    I got a https://www.amazon.com/MELNOR-4-Zone-Digital-Water-Timer/dp/B0094KM4VK on sale and the unit is great, but the batteries don't hold a charge very well, especially when using rechargeables - it can register low batteries after only a week. I'd like to rig up a solar panel to the timer that will keep the batteries charged and reduce or eliminate the need for me to check on it every few days.

    The timer runs on 2 AA batteries, ~3v. Does anyone know of an existing solar charger & 3v power supply that I could use to run this timer?

    What I'm looking for is a complete integrated solution that is weather proof and fairly inexpensive. I'd like to find an existing solar power supply that charges its own batteries and regulates the voltage to an independent 3v load. Something I could just mount on the wall and run the wires to the timer and be done with it. Of course, if I do a search for anything "solar charger" or "solar power supply" related, I'm inundated with cell phone chargers (which are all ~5v).

    I don't really have the time right now for a DIY project and anything I build would not be very weather resistant. But if there's no premade solution, then I can probably sort it out. This device from adafruit would be good, except that it's "some assembly required" and its output is ~6v and I need 3v; adding a step-down to drop the voltage would just add to the cost and complexity that I would rather avoid.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Aug 24, 2015 #2


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    What kind of batteries are you using? If you just use good alkaline AA, they should last a few months. Maybe your rechargables are at end of life. Are they NiCad?

    The adafruit controller you linked to is made to charge a single 3.7V lithium cell, which won't really help you.

    You could use the adafruit to a 3.7V lithium and the follow it with a linear 3.0V LDO regulator. It's not power efficient, but it is simple.
    (but not water proof either).

    Sorry, but I don't have a better answer.
  4. Aug 24, 2015 #3


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    Last edited by a moderator: May 7, 2017
  5. Aug 24, 2015 #4
    I'm using li-ion AAs. I think part of the problem is most rechargeable batteries list at 1.2v rather than the 1.5 that the timer would get out of alkalines, so it thinks the batteries are deader than they are. It said battery "low" on Saturday so I replaced them and checked the old ones and they still register at full charge ~1.3v. Might also be some chemical differences in how li-ions discharge compared to alkalines.

    The problem with just hooking up a solar charger to some batteries and running the wires in a cheap DIY is the lack of voltage regulation and the constant charging/discharging on the batteries. NiCads would develop a memory and die. Li-ions or LiPos or SLAs don't handle overcharging very well - though I've got a spare 12v SLA and a 12v 5w panel doing nothing in a corner.

    If I expect the batteries to last, I need a charger with charge protection. If I want consistent power to the timer, I need a power supply with voltage regulation. Instead of trying to put all that together myself, it makes sense to look for a pre-made single unit. I know devices exist in 12v and https://www.amazon.com/dp/B00EM7WN1...=UTF8&colid=QWWTPR27DRE4&coliid=IJ10CT49P98B6 (i.e. for wilderness cameras) and 5v (cell phone chargers) so I'm hoping someone knows where I can find one that does 3v out of the box. I'm just not having any luck finding one.

    I did find a https://www.amazon.com/dp/B008BHAOQ...olid=QWWTPR27DRE4&coliid=I2DVA23BYXPFPH&psc=1 that I could use to step down a higher voltage to 3v, and it's small enough form factor I think it could fit inside the battery compartment of the timer, with a little modification to the battery cover. So if I'm unable to find a pre-existing 3v system, it looks like plan B might be a $15 5v solar cell phone charger and that buck converter. Bucky might put off some waste heat, but 5v-3v shouldn't be enough to worry about.

    But in any DIY case, I still have to figure out weather proofing :/
    Last edited by a moderator: May 7, 2017
  6. Aug 25, 2015 #5


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    Can you point me to some 1.5V rechargeable lithium cells?
  7. Aug 27, 2015 #6
    Last edited by a moderator: May 7, 2017
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