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Charging a 12v battery permanently wired in a circuit

  1. Jun 15, 2015 #1
    First off, I am a ME student, and I only know enough about electricity to get myself into trouble. I am looking to build a flashlight using a 100w LED chip, a step up constant-current transformer from 12V (or 24V) to 34V and either one or two 12V batteries (higher voltage to improve efficiency). My goal is to make this project portable and easy to recharge without taking the batteries out to charge.

    It would be ideal to plug the flashlight into the wall after the batteries have worn down and let the batteries charge while not having to monitor the voltage. I'm sure there are many AC to DC rectifiers that sense voltage and shut off at the appropriate voltage. But would there be a safety risk or LED damage if, say, I were to loan this to a friend and they decide to turn on the flashlight while plugged into the wall? Would the AC to DC converter just simply be connected directly to the battery terminals? Are there other safety considerations that I have not thought of?
  2. jcsd
  3. Jun 16, 2015 #2


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    Hi there
    welcome to PF :smile:

    what type of battery(s) did you plan to use ?
    this will determine the charging system and requirements

  4. Jun 16, 2015 #3

    jim hardy

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    There used to be a hobbyist forum
    i think CandlepowerForums.com
    where guys customized flashlights for fun. They used power tool batteries, home made voltage converters and aircraft landing lights.

    If it's still there you will get lots of ideas.
  5. Jun 16, 2015 #4
    Thanks Dave. I would likely use two 12v lead acid batteries for cheapness, and the lighter the better so probably ~3Ah. This brings up another question, will they provide the continuous 3-3.5A needed to power the LED? If I remember correctly, any battery can provide any current required of it. How much the voltage drops and the battery heats up (battery life) is the question. How far am I off base there?

    Thank you, I will check that out.
  6. Jun 16, 2015 #5


    Staff: Mentor

    The 12 volt battery in a car fits your description. It is connected in the circuit always, and it is charged by the alternator.

    You must add time to any question about how much the battery can deliver. 3.5 A for how long?
  7. Jun 16, 2015 #6
    Charging any battery can be tricky. Lead acids have their own quirks. If you follow the information in the link, you shouldn't have any trouble with your LED burning out. Even at the topping charge the LED should be fine. It will dim during the constant current phase though.

    Lead acids are about 80% efficient, meaning you need to replace the current you draw plus about 20% extra. They take a long time to top off which can be a problem in constant use applications. But they are cheap and great for intermittent use. It's easy to figure how many amp hours you need. Take your current draw, multiply it by the amount of time your circuit needs to run, then double that (it's a bad idea to run lead acids all the way down).

    So a 3 amp hour battery will get you 30 minutes of 3 amp usage.

    Building the charger is a bit tricky. The battery charges in three phases. Initially you need a constant current source. That needs to switch to two constant voltage sources, first a high voltage, then a slightly lower maintenance level. I would have thought there was a chip out there somewhere that did this. It's a common enough need. Does anyone know of one?
  8. Jun 16, 2015 #7
    Well if I got one hour out of the flashlight I would be happy. Mostly intended for use at the lake, camping, etc. So that's why I said around total 3Ah at 24V should be sufficient.

    Obviously I could find a 24V DC charger that would work (I saw some mobility scooter chargers), and hide everything but the wall plug and cord inside of the flashlight. I would feel 100% comfortable charging an unconnected and isolated battery (no load) with a charger like this. But something about the battery being connected to the flashlight circuit while charging is making me second guess things.

    Is it really safe to leave the flashlight plugged into the wall (possibly for a long time) without any additional safety circuitry?
  9. Jun 16, 2015 #8
    With a proper charger it is safe.

    Without one the battery could explode from overcharging. An overcharged lead acid battery generates hydrogen gas. In a sealed battery this can lead to a blowout. Fire is a minor issue as well. (Hydrogen gas tends to float away rather than burn -- usually.)
  10. Jun 16, 2015 #9

    jim hardy

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    We don't know what is your level of expertise.

    Battery University has tutorials to help one grasp the basics.

    There exist specialized integrated circuits for making "smart" battery chargers.

    Here are datasheets for two of them.
    Reading them will give you insight to the approach for safe charging.

    Both sense battery's state of charge by its voltage and both apply controlled current.
    So, provided the charger is capable of more current than the light bulb draws, leaving the light on would just lengthen charging time.

    Be careful - lithium batteries catch fire if charged too hard.
  11. Jun 16, 2015 #10


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    I was going to link to them earlier but didn't get around to it. Looks like they have added some good stuff. Here's their information page if the OP wants to take a peek.

  12. Jun 17, 2015 #11


    Staff: Mentor

    Three stage chargers for lead acid batteries are the exceptional "premium" way to do it. Tens of millions of vehicles charge with a simple alternstor, no three stage. The simplest charger is called a trickle (or float) charger. It puts in a current so small (<2A) that it can continue indefinitely without damage to the battery.

    Thus, there are four levels of cost and complexity available for lead acid batteries. The benefit of higher cost is more rapid charging.

    1. Trickle charger. Even an old wall-wart laptop charger 13-14v will do.
    2. Single stage manual charger. Not suitable for unattended operation.
    3. Single stage automatic charger
    4. Three stage "smart" charger .
    Last is the choice of lead acid battery. "Starting" batteries are optimized for very small percent discharge, but high number of cycles. "Deep discharge" batteries deliver more energy to the load before needing recharge, but they are designed for fewer lifetime cycles.

    If your only use is powering LED lights, then the least expensive used battery and the least expensive trickle charger are probably optimum. I think you can do both battery and charger for less than $10.

    For $15, you can buy a Luci Light, very bright, lasts 12 hours, waterproof, built in solar recharger.


    I suggest that (solar with nickel cadmium batteries) is a more sensible flashlight project than lead acid batteries recharging from an AC source. Buy a solar garden light at the dollar store and take it apart to see how it works and what components it has.

    Good luck
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