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Thread: Portable Car Charger as Power Supply?

  1. #1
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    Portable Car Charger as Power Supply?

    Not being familiar with Portable Car Chargers, can I use one for this application (see attachment)?

    I need both a compact 12VDC, 6-8Ah and a 5VDC, 2Ah power source for the attached circuit diagram. I saw this DBPOWER portable car charger.

    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

    It's small, has both voltage outputs from one package, and I can easily disconnect it for recharge (no special charger needed, just a standard 5V USB wall wart.)

    Pros - low cost, single small package geometry fits available space, convenient connect/disconnect and recharge, plenty of capacity.

    Cons - 5V charger port requires minimum maintained 100mA to stay "ON". 12V port is constant on and capable of 300A output if shorted.

    To resolve "cons"...

    A 50R resistor across the 5V output keeps the power on for the processor and relay modules. That only drains 100mA, so no problem for the planned three hour max. run time.

    The 12V will be fused and switched, with max. draw of near 15A@12V. Total "ON" time at 15A about 0.3h per session....about 4.5Ah. So the 8Ah capacity is good.

    Thanks for any advice.
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  2. #2
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    This can probably be made to work, but there are some more cons. There is a high probability that the pack actually punches out ~14V to start cars and that voltage is probably poorly regulated. For heaters that probably does not actually matter. While pulsing the heaters the 5V output probably gets noise on it. If this is purely a timers based system that probably does not matter as long as the rise past 5V is minimal, but if you are using the analog functions to read a temperature or similar this may need more design work to get good reads (possibly simple by not reading while heater/s are on).

    The starter pack would have been designed for short duration pulsed loads and may need cooling to support operation for hours. Given the relatively low on times you are planing it will probably be fine but would suggest monitoring the temperature the first time you run it, especially if the eventual plan has it enclosed. You may also want to look at your code and work to make it as unlikely as possible that things will hang up with a some/all heaters on, and if failure mode for stuck heaters include 'house burns down' consider some form of slave CPU or similar hardware watchdog. https://en.wikipedia.org/wiki/Watchdog_timer

    The system efficiency of the 12V boost circuit is probably low with a high standby current which will eat into your run time calculations by 10-20%.

    The pack is almost certainly not 8000mAh, simply because everybody in the supply chain for batteries seems to round capacity up by a bit, and often measure the starting capacity under generous circumstances. Being a branded product with an actual warranty this is probably close enough you can use it for maths, but if buying from more nebulous online sellers assume 40-50% of claimed capacity and if possible actually test it.

    You are using a product contrary to the design intent so surprises are to be expected, but is probably still easier than making your own.

    If you want to reduce that 50R resistor loss you can probably keep the output running by pulsing the resistor only every every couple of seconds with a transistor but that is probably not adding much to something like this.

  3. #3
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    why not use a DC-DC converter that takes 8-40V input and gives a regulated 3.3/5V output?
    Do you need a "battery" type of system in the vehicle? If so, along with this, you could goto any local alarm supply store and pickup a TM1-PLUS

    http://www.keryx.ca/Labco-TM1PLUS.pdf

    it's programmable from 1 second to over 4 hours.

    I've used both units in automotive environments fine. In the past, since my old car didn't have auto shutoff lights, I used a TM15 (1-15sec timer) to control the headlight wiring in the steering column. This way if i left the switch on when i turned the car off, after the timeout the relay would disconnect the wire at the switch. Whenever I turned the ignition on, it would restore the switch immediately since i had the ignition triggering the timer trigger.

  4. #4
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    Quote Originally Posted by GremlinWrangler View Post
    ... high probability that the pack actually punches out ~14V ....
    ...While pulsing the heaters the 5V output probably gets noise on it. If this is purely a timers based system that probably does not matter.
    ..... may need cooling to support operation for hours. Given the relatively low on times you are planing it will probably be fine but would suggest monitoring the temperature ....
    ...You may also want to look at your code and work to make it as unlikely as possible that things will hang up with a some/all heaters on, and if failure mode for stuck heaters include 'house burns down'
    ...The pack is almost certainly not 8000mAh,.
    ...You are using a product contrary to the design intent so surprises are to be expected, but is probably still easier than making your own.
    Thank you for the extensive reply.
    - Pack VDC=12.3V LiPO based, rated for 300A draw on cranking car for 30 sec up to 5 times. Measured no discernible V drop at 12A draw for 4sec (near max for this application), so overheating should be very unlikely. 5.12V output drops to 5.03 at 12A max load on 12V output. Surprised me.
    - Code....I'm a total rookie, so code is simply a step by individual step thru a series of on/off commands. Hopefully robust.
    - Failure mode - The main power will be switched on/off with a physical R/C switch. R/C controls are totally separate circuits from this system. Thinking of adding thermistors on each of 9 heaters to provide input back to controller to shut down a power switch separate from the relays. Good heads up on the disaster scenario.
    - Calculated power during total run time is less than 5Ah. Pack is brand name, LiPO based, so V available after 5Ah draw should be fine. TBD.
    - Easy, compact and both 12v and 5V stable output from same package made this very desirable.
    Will be looking at the safety aspect....thanks again.

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