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Battery heat

  1. Jan 31, 2010 #1
    I'm having issues with overheating and need a little help.

    I have 8 6v4ah rechargeable lead-acid batteries. I wire 2 in series for 12 volts. Then i wire the sets of 2 in parallel, because I want to keep my system at 12 volts. That means I have 16 amps. Every time I add more sets of batteries, my amps increase.

    So now I'm dealing with heat issues. I'm running these batteries into a 1000 watt modified sign wave power inverter with digital monitoring. From that I'm powering a flat screen TV. After about 25 minutes, although I've still got enough power, I get an alarm due to overheating.

    I wire 4 batteries to 2 posts, neg and positive, and another 4 batteries to 2 posts, neg and positive, because sometimes I like to seperate the batteries and only use half of them at a time. These posts are rated at 10 amps, and there are 8 amps going into them. But once I cross a wire over between the posts, I've now got the total 16 amps in play. So I added a heat sink onto each post and that seems to be working.

    I'm running 14 AWG wire, about 3 feet long, because every web site calculator I've tried says that should be fine. The power inverter came with a 4 AWG cable but I don't use that, because I don't go anywhere over 250 watts with this system.

    The problem is that I've put a cigarette type adaptor between the power inverter and the batteries. I want something that I can unplug easily, and I like having the extra fuse in there. But that adaptor is overheating big time.

    I know (from reading this web site) that a car battery can put out anywhere for 45 amps to 1000 amps (depending which thread you read!), but I figure that that many amps aren't available at the cigarette jack. One question is, how many amps can you draw from your car through the dashboard? It can't be enough to overheat the adaptor, I'm sure.

    What can I do to fix my heat problems between my batteries and power inverter?

    If I run a cable directly from the batteries to the power inverter and lose the adaptor, then how do I quickly plug and unplug the thing? A wall socket can handle a lot of power. Should I try something like that?

    Thanks.
     
  2. jcsd
  3. Jan 31, 2010 #2

    Averagesupernova

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    When you have the 2 battery sets wired in parallel you don't have 16 amps, you have 8. Each battery is good for 4 amp hours. Wiring each set in series does not double the amps, only the volts. Do you know how much current you are drawing out of the batteries? A 100 watt load on the invertor assuming the invertor is 100% efficient (which it is not) will draw 8.3 amps from the batteries. You talk about posts and that there are 8 amps going into them. Do you know this for fact or do you assume it?
     
  4. Jan 31, 2010 #3

    vk6kro

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    I have 8 6v4ah rechargeable lead-acid batteries. I wire 2 in series for 12 volts. Then i wire the sets of 2 in parallel, because I want to keep my system at 12 volts. That means I have 16 amps. Every time I add more sets of batteries, my amps increase

    You seem to have this wrong.

    4 amp-hours means you could draw 4 amps for an hour or 1 amp for 4 hours. Any product of these that still gives you 4 amp hours.

    If you put two in series, you still have 4 amp-hours for each of the two batteries but only 4 amp-hours total. Putting them in parallel gives you another 4 amp-hours total.

    It doesn't tell you anything about the maximum current the batteries can supply.

    You need to measure the ACTUAL power going into the inverter when the TV is operating and you need to read the handbook of the TV to find out the ACTUAL power it uses.

    Suppose the TV uses 200 watts and the inverter is 70 % efficient.

    This would mean the inverter has to have 286 watts (at 12 volts) into it. (200 * 100 / 70 )

    At 12 volts, this is 23.8 amps.

    If you had 4 parallel pairs of 2 batteries in series, you would have 16 amp-hours of battery capacity at 12 volts.
    So, the batteries should last (16 / 23.8 ) *60 or about 40 minutes.

    If you are choosing a plug do not select one that would allow your device to be plugged into the 120 V / 240 V mains electricity supply. Even if you know not to do this, someone in the future could do it without understanding what the device is meant to do.

    The cigarette lighter outlet in cars would always have a fuse in series with it to protect the car's wiring if there was a short circuit. Some of the sockets for these plugs are poor quality and not good for anything above 5 amps or so.

    [apologies for the accidental edit -Russ]
     
    Last edited by a moderator: Feb 1, 2010
  5. Jan 31, 2010 #4

    russ_watters

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    The cigarette lighter in my car is wired to a 10A fuse in my fuse box.
    I'm not sure what "posts" are, but if anything you have the power going through (that isn't a load) is getting warm, it isn't big enough.
    And that's assuming the 4ah batteries are actually capable of 4ah. It is my understanding that typical lead acid batteries are rated full-to-empty, but aren't ever supposed to go below about 50%.

    In other words, the overheating may be due to the batteries running low and putting out a higher amperage to go with the lower voltage.

    ...When I was in high school, I took a 200w mini quadrophonic surround sound stereo system to a drive-in movie and it ran my parents' car battery almost dead after a 2 hour movie. And car batteries are typically rated for 40ah or so.
     
    Last edited: Feb 1, 2010
  6. Jan 31, 2010 #5
    6V 4AH batteries sound like sealed lead-acid type, typical for emergency light systems. 4AH is when they are new, but I'm betting they aren't new or else you simply would have bought 12V batteries, and life would have been simpler.
    Battery catalogs tell me a typical 6V 4AH is rated for 24-30 WHr, so maximum sustainable allowed current should be no greater than 4-5 Amp. If you're pulling close to that out of any single battery for any length of time, then they'll get hot as hell.

    Sounds like you are simply using batteries too small to supply sustained current for a 250W inverter. An explosive event may be imminent. You should make sure each battery pair is separately fused with something like a 3A AGC. no larger.

    BTW fault current available from a 12V car battery may exceed 1000A. That doesn't mean you should try to draw 1000A from it. It means your fusing must be capable of interrupting a 1000A fault.

    Good judgment is born of experience... experience is born from bad judgment.
     
  7. Feb 1, 2010 #6

    vk6kro

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    Battery catalogs tell me a typical 6V 4AH is rated for 24-30 WHr, so maximum sustainable allowed current should be no greater than 4-5 Amp. If you're pulling close to that out of any single battery for any length of time, then they'll get hot as hell.

    24 Watt hours is the same as 4 Amp-hour for a 6 volt battery. It doesn't tell you anything about the currents that should be drawn.
     
  8. Feb 1, 2010 #7
    Ok, thanks everyone. You provided me with a lot of food for thought. Forget about how many batteries I have, because I must be confusing everyone about that.

    This TV needs 135 watts to run. I used a Kill-A-Watt to verify that. When I use 6 batteries (12.8 volts x 12 amps = 153 watts) I can run the TV for a pretty short time, before the power inverter tells me it's being overloaded. When I use 8 batteries (12.8 volts x 16 amps = 204 watts) I can run the TV much longer, but then I get overheating problems (after about 25 minutes). I've done the math and run the test a number of times, so I'm pretty sure my numbers are right.

    I agree, I shouldn't use a 120 volt wall type receptical, because that would confuse everyone.

    I bought a higher quality cigeratte type adaptor, some kind of hard plastic, but the old one looked more like ceramic and I thought that would have work fine. I'm just guessing here, because I haven't tested it yet.

    I have another TV that only takes 125 watts and I've watched that for up to an hour off of 6 batteries, but they were at around 13.5 volts when I started out, not the usual 12.8 volts these batteries like to sit at.

    Is there any real advantage to having more volts and less amps in a system? I mean, instead of 12 volt batteries (6 volts x 2), what if I used 24 volt batteries and then only needed half as many amps? The power is still the same. Would the heat be less?

    Would switching to copper wires make a big difference in terms of heat?
     
  9. Feb 1, 2010 #8

    Averagesupernova

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    I think you need to study some of the basics on electricity. I hate to sound so tough on 'ya, but do you realize that you just suggested running an invertor that I assume is designed to run on 12 volts on 24 volts? You mention you can run the TV longer when you use more batteries before the invertor is 'overloaded'. I don't think your invertor is telling you it is being overloaded, I think it is telling you that the input voltage to it is getting too low for it to reliably work.
     
  10. Feb 1, 2010 #9
    I agree with Averagesupernova. You are drawing 12A of current from the battery approximately. Your battery will drain quickly and the voltage will drop depending on the state of charge. The http://www.calibrepower.co.uk" [Broken] is probably telling you that the input battery voltage is low.
    You also say that these batteries sit at 12.8V.
    If they are lead acids then they should be charged at 13.5V.
     
    Last edited by a moderator: May 4, 2017
  11. Feb 1, 2010 #10
    The inverter warns of overload if the appliance drawing from it is asking for more than it can supply. Yeah, it's the same as saying the input voltage is too low. Whatever. It's got nothing to do with basic electronics. It's written in the owner's manual.

    As far as batteries go, they can be charged up to 14.5 volts. You need more electricty going in to them than they actually have, or no charging will take place. But once you disconnect, the voltage will drop to 13.5 for a while. Overnight, they will settle down to 12.8. That's why they are called 12 volt batteries. My charge controller will continue to put electricity into them to keep them at 13.5 volts, after they've peaked at 14.5. But if I disconnected them, eventually, they will drop. That's physics.

    I can always buy a 24 volt power inverter. Surprised you didn't think of that.

    If you don't know anything about the difference between copper wires and aluminum wires, don't attempt to answer that one either, please. :-p
     
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