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Battery Bank

Discussion in 'Power Electronics' started by Ryan Heuss, Dec 4, 2014.

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  1. Ryan Heuss

    Ryan Heuss

    2
    0
    Dec 4, 2014
    Hello, I just purchased a pickup truck and am planning on spending a bit of time camping out of the cab. I would like to set up a primitive battery bank. I would just like enough power to run an LED light, charge my phone and other small electronics. I am on a pretty tight budget and hope to spend no more than $150. I have a small solar panel I could mount on the roof but it is probably not worth the effort for the power. Ideally I would like to wire the battery bank into the alternator to charge while driving.

    I am seeking some advice as I am new to larger electronic projects. I can wire basic circuits but my knowledge of batteries is limited.

    Many thanks
     
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Step 1. Requirements.
    Determine if you want AC 110V, or just 12V...
    Determine at most, how many devices you want to connect.
    Determine how long you want the batteries to last when all devices are plugged in. (Or when a desired subset of devices is connected)
    Guess how large/small the bank should be.

    Step 2. Work on determining if requirements fit expectations/budget.

    Step 3. Build!

    You can do it no problem! Of course, you need to concern yourself with two things:
    -Isolating your battery bank from the main vehicle battery so you don't accidentally drain it.
    -Managing bank power level... if you kill your batteries... when you start the car there will be a very high current draw to begin charging the bank. There are two solutions for this. Monitor the charge of the bank, and cut off power when it drops too low. Or build a controller to limit the amount of charge current to the battery bank.
     
  3. Ryan Heuss

    Ryan Heuss

    2
    0
    Dec 4, 2014
    I would Ideally like 110V. Could I just hook an inverter up to the batteries to achieve this? If i wanted more than one battery would I need a transformer to lower the voltage before entering the inverter. I would like to be able to get at least four hours of light use out of the bank without recharging. If however I was able to figure out how to charge the batteries from the alternator, an hour of charge would be fine. How would I "build a controller to limit the amount of charge current to the battery bank".
    Thank you so much for the help.
     
  4. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    You skipped over the decision to buy or build. I am using one of these right now and can recommend them, but only if you need all of their features. If not, you can get a similar Watt hour rating for less money with only a 5V USB output and a single charging voltage. Then again, if you don't need lightweight portability, sealed lead acid batteries are still the most cost efficient.

    http://www.powerstream.com/PST-MP3500.htm
     
  5. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    -Yes. You would choose an inverter with a power rating based on the devices you wish to power... You can over-engineer here and choose a 250W inverter even if you will be below 100W. This give you the ability to draw more than 100W if needed, at the expense of cost. And of course... when you draw more than 100W the time goes down. (ie. batteries run out quicker)
    -No, a transformer only works with AC anyway... and a battery or battery bank will put out DC. There are 'Buck' converters that will lower the voltage for you, but that introduces more complexity, as you will now need an even higher voltage to charge your bank. The ideal solution here is to make your bank out of batteries connected in Parallel. This way the battery bank will charge just like a normal car battery, and will provide 11-14V to any device you connect directly to it. (ie. the inverter)
    -I am unable to provide too much detail here... as I am unsure how many devices you want to power at the same time... Here is some food for thought though to determine what kind of battery or the quantity you may need.
    Say you want to power a laptop and a cell phone for 4 hours. The laptop will draw 90W (depends on model) and the cell phone will draw at most 10W. So you would need a total of ( 90W+10W ) x 4Hours = 400WattHours. The part that may not click in immediately is determining how that translates to automotive batteries (or any other for that matter)... Lets look at a 12V 50Ah battery. If you take the batteries voltage, and multiply it by it's capacity, you end up getting WattHours. So a smallish 50Ah car battery will pack around 600WattHours. This already sounds like a good option for 4 hours with the devices we picked in this example. In reality. I guessed when I picked a 50Ah battery we could have easily required more... so what if you require more WattHours? Simply add more batteries in Parallel, and the AmpHours or WattHours will simply add together, and the output voltage of the pack will stay at 12V which won't require anything funky to step back down.

    -This is where things get a little interesting... there are multiple methods to achieve this. Cheap dirty and not so reliable would be to hook up a simple relay to disconnect the powerpack from the vehicle when the engine is not running. This is not ideal, because if the power pack runs low enough, the initial surge of current required to begin charging it could kill the relay, damage the wiring, or any other device related to the alternator or automotive charging system. To protect against this threat, you need to make sure that the voltage on the battery pack does not fall, which could severely limit your expected time out of the pack. This is NOT an ideal solution by any means... but if you're stuck in the bush and need to rig something up that will work for a night, this may just cut it if your careful.
    The ideal solution (to me at least) is to find or build a charging circuit. The operation is fairly basic. If the battery pack is low... say 7V because you really killed it by forgetting to turn off the 12V fan you had hooked up in the cab ;) The charging circuit will not simply connect the 14V volts from the alternator as this would be that 'less than ideal' solution above. It would instead supply a smaller controlled voltage to make sure that the battery pack is being charged at exactly 1Amp. The exact mechanism may be difficult to understand right away... but we will cover that later. As the battery pack's voltage increases, the charge circuit will increase it's voltage as well to try to keep the current flowing into the battery at 1Amp.

    From this, we can determine how long it will take to charge the battery pack. Remember that these numbers are just examples and will need to be adjusted based on your desired use...
    The battery we used up above is a 50Ah battery... so from dead, it will take 50Hours to fully charge... now we can decrease that number... lets say our battery charge controller is setup to provide 10Amps, this will cut the charge time down to 5Hours... This does scale pretty evenly to your use of the battery though... This example is for a 'dead' battery, but perhaps you only use the laptop for an hour to write down some notes. The laptop would draw less than 90W because it's not working very hard to write text, so we can say at MOST the laptop would take 90Wh from the pack. Here's the conversion again... 90Wh from a 12V battery is 7.5Ah .. So our 10A charger could top up your pack in 45-minutes.
    If you were simply going to take the pack home and charge it, you can avoid making or buying a charge controller and simply using an off-the-shelf automotive battery charger.

    Word of warning though! Even if you take special care to wire everything nice and heavy duty and get a bigger alternator, a battery can only charge so fast. Attempting to charge a battery too fast can cause it to 'vent' which could be very dangerous. This is why the first example I had given is very poor!

    You're very welcome.

    **Disclaimer ;)
    The times and values used are for example only and do not account for losses in wiring, charging systems, inverters, or any other sources. Nothing is 100%, so you will never be able to actually pull out the calculated power from the battery. You can easily accomplish your goal, but don't expect to achieve it if the devices and batteries chosen are equal to or just good enough for the numbers calculated.
    We can go over some more details in looses to determine some more realistic values later.
     
  6. Scotophor

    Scotophor

    48
    6
    Oct 8, 2014
    Camping and RV supply stores carry battery isolators, so that when (for example) a tow vehicle is connected to a camping trailer, the running engine and alternator can charge the trailer battery, but when the engine is stopped while the rig remains hitched, the trailer loads can NOT kill the tow vehicle battery. Battery isolators are used the same way in motorhomes, to protect the starting battery yet allow charging of the "house" battery.
     
  7. Bluejets

    Bluejets

    4,208
    893
    Oct 5, 2014
    Also in emergency vehicles such as ambulances.
    I'm fairly certain there are kits out there just for this purpose.
     
  8. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    *facepalm... I forgot about those isolators.
    Do you know how they would behave if you external pack was dead or very low??
     
  9. Scotophor

    Scotophor

    48
    6
    Oct 8, 2014
    I imagine pretty much the same as if the external battery was simply hard-wired into the system... the isolator must be sized to handle the alternator's full output current.
     
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