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Thread: Battery and Alternator Issues - need some electrical experts

  1. #1
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    Battery and Alternator Issues - need some electrical experts

    I upgraded from my stock 80A to a 130A alternator in December. Initially I would get a solid 14.0-14.1V with the car idling and while driving around (the carputer has a PSUmoni so I can see it live). Anyways, recently I noticed that when I first turn on the car the alternator output is 14.0V, however after driving for about 10-15 minutes it slowly makes it way down to 13.5V and stays there. Also when I turn off the car my battery voltage drops to 12V and by the next morning its around 11.7V. It's an Optima red top so I know its a good battery, but looks like it's got a chronic drain on it.

    It's possible the drain may be due to my alarm system or keeping power to the 500mA USB hub (via the p5v rigged to stay on) while the car is off. I didn't think that this would be enough to drain the battery much, but I guess it could be compounded by the fact that I only take two 10-minute trips to school and back each day (meaning maybe I'm not recharging the battery much).

    I've ran an extra ground cable from the battery to chassis, as well as an extra power cable from the alternator to the positive terminal of the battery, so I don't think it's an issue of power flow. I get no alternator whine while driving so I'm guessing my main ground is fine.

    For the time being, I've pulled the fuse from the alarm as well as disconnected the USB power so I should not have any drain while the car is off. I'm still waking up to 11.6V-11.7V on the battery. It's is only 5 months old so I'm taking it back to be recharged/replaced. Luckily the car cranks fine with the exception of last weekend when I didn't drive for 3 days and it was dead in the morning (I didn't check the battery voltage since I had to have it jumped).

    So, basically I have three questions:
    1) Why is my battery still getting drained?
    2) What is the significance of the alternator output decreasing after 10 minutes of driving.
    3) Do I need to get a separate power supply for the p5v (and what could I do for this), or possibly rig a second battery in the trunk?

    Thanks.

  2. #2
    Low Bitrate nalav's Avatar
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    Perhaps while installing the alternator whoever did it inadvertently caused a short somewhere that's causing the constant drain. It might also explain the lower voltages once it's running. Or perhaps it's a low quality alternator that just doesn't live up to it's expectations. I don't really know if either of those are it though, that just all I can come up with.

  3. #3
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    There are a few things to consider.
    1. You are drawing 1/2 an amp say 23.5 hours per day with the engine off or 11.75 Amp Hours per day.
    2. Twenty minutes running per day is not enough to charge the battery fully in the face of this constant load.
    3. Batteries with no load will settle their voltage overnight as the engine and batteyr cool down. When I have bothered to check my voltage when I switch the engine off, I see a falling voltage. 0.3 volts is probably normal.
    4. There might be a couple of power supplies to your alarm,they would not make it as easy as pulling a fuse to disable it.
    5. 13.5 Volts with the engine running sounds OK. 13.8 is the theoretical level, but 13.5 is about what mine runs at.
    6. Not sure what the charging regime of the red top, but if it accepts more charging current after it warms up, tat would pull the voltage down.
    7. The draw of the battery being charged is always there becasue you never drive long enough to fully charge the battery. This may be depressing the voltage to 13.5
    RodW
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  4. #4
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    Number 4, above sounds likely. Test this by using a multimeter. Reconnect the fuse to the alarm, then disconnect the + terminal with the car off and put the multimeter in line between the battery and the + wire. Note the amperage.

    Then, pull the fuse. The amperage should fall off. If it doesn't, the alarm system isn't powered off. Keep pulling likely fuses until you find a drop in amperage. That will help you isolate the problem.

    Also, the 500ma load on the USB hub is going to be a problem. You can't have that on all the time and expect it not to drain the battery.
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    thanks for all the helpful tips guys.
    Quote Originally Posted by rodweb View Post
    There are a few things to consider.
    1. You are drawing 1/2 an amp say 23.5 hours per day with the engine off or 11.75 Amp Hours per day.
    2. Twenty minutes running per day is not enough to charge the battery fully in the face of this constant load.
    I think this may be my biggest problem so I'm thinking about getting a wall plug-in type battery charger to juice it up once a week. Once I get my battery back today from being recharged, I'll leave everything disconnected and see what the drain is like. By the way, is leaving my USB hub unplugged sufficient to stop the power draw, or do I need to actually go and physically disconnect the power source to the p5v?

    3. Batteries with no load will settle their voltage overnight as the engine and batteyr cool down. When I have bothered to check my voltage when I switch the engine off, I see a falling voltage. 0.3 volts is probably normal.
    I'm getting more like 1.5 V drop :-(

    4. There might be a couple of power supplies to your alarm,they would not make it as easy as pulling a fuse to disable it.
    I installed the alarm myself and it's a remote start type so it is tapped into the ignition and ACC cables, but the power to it only comes in from one main line.

    5. 13.5 Volts with the engine running sounds OK. 13.8 is the theoretical level, but 13.5 is about what mine runs at.
    6. Not sure what the charging regime of the red top, but if it accepts more charging current after it warms up, tat would pull the voltage down.
    7. The draw of the battery being charged is always there becasue you never drive long enough to fully charge the battery. This may be depressing the voltage to 13.5
    The weird thing about the alternator was that when I first installed it, I would maintain 14.1V, and now it drops to 13.5 only after driving for about 15 minutes. I figured that if it was the battery sucking out the extra voltage that would happen sooner rather than later (but I guess temperature may play a factor). I'll see what the voltage reads when I get it back freshly juiced up today.

    Also regarding the post about checking the voltage going into the alarm unit with the battery disconnected: if I take off the positive post from the battery altogether, how would anything be receiving power? Maybe I am misunderstanding.

    Anyways thanks for all the tips. Any advice on installing a second battery in the trunk, or maybe a separate small power supply for the p5v?

  6. #6
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    you clould also have the alternator load tested to see if it is still working properly.

    I read somewhere once that a typical car has a parisitic load of about 0.5-1 amp from all the engine computers that the battery is keeping up.

    personally, my current setup is running some of the same voltages. I have a 80 amp facotry alt. but it load tests to 20 amps. it starts up at 14.0-13.9, and as everyhting warms up, it drops down to 13.5-13.1. I have a optima yellow top, and a sound system that doesn't stay quiet, and typically drive in 15-20 min intervals, so all of these could contribute also.

    all voltage measurments were from my radar detector, so unsure how accurate they are.

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    so i got the battery back after they charged it but the voltage was the same meaning it didn't take the charge. i guess once it gets drained enough its permanently damaged and won't charge up? anyways, they replaced it for me. the alternator voltage now stayed 13.8-14.0 while driving. now as far as preventing this from happening again, i've disconnected the alarm and usb power (just the output from the p5v, not the input going to it), and i'm going to monitor the voltage. i may get one of those plug in trickle chargers and juice it up once a week. do you guys have any recommendations on a good type or brand? keep in mind its a gel battery.

    also is there ANY way to hook the p5v up to a rechargeable power source that will let it stay on independently for at least a day or two? that source could be charged as i drive the car and then provide power to the p5v while the car is off.

  8. #8
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    Probably the C-Tek microprocessor controlled charger if you can get it in your country.

    You could consider a dual battery with a smart battery isolator. I am a bit lucky as my 4WD has two 80 amp hour batteries in it and one of them is dedicated to the accessories I run and isolated from the cranking battery. This means so I can flatten it without worring about not being able to start my car. If it has been run down a bit, I notice the voltage is lower when driving because of the current draw it robs from the alternator while charging.
    RodW
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    Hmm.. I was thinking something more along the lines of a simple device to hold charge for a while (like a capacitor). Tell me if what i've show below makes sense, i only have very basic knowledge of electronics:

    Current Setup, P5v operates independently of CNX-p2140 and whether car is on:
    Batt +12V --> P5V --> USB Hub

    The way its supposed to be, with the p5v only on when the 2140 is on:
    Batt +12V --> CNXP2140 --> P5V --> USB Hub

    Idea 1, powering the p5v only when ignition is on but having something recharge in between to provide a day or two of power. The diode prevents reverse current:
    ACC 12V --> Diode --> Capacitor/Some sort of 12V rechargeable device --> P5V --> USB Hub

    Idea 2, essentially the same, but let the 2140 decide when the p5v should be powered (but it would be harder to stick a diode and a rechargeable device in between the 2140 and p5v):
    Batt +12V --> 2140 --> Diode --> Capacitor/Some sort of rechargeable device --> P5V --> USB Hub

    Please let me know if any of these ideas make sense or are possible? Would a standard capacitor serve this function? Keep in mind the draw of the p5v is pretty low as its only meant to provide a max of 500mA at 5V (2.5 watts). I'm not sure what the current draw is on it, but my 2140 is 90% efficient so I am guessing the p5V would not to too far off that (maybe 3-4w total). Thanks.

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