Thread: Battery and Alternator Issues - need some electrical experts

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.

thanks for all the helpful tips guys.

Originally Posted by rodweb

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?

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.

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.

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.