bearsnob wrote: Further, I was told that you could increase the amps by replacing that wire with a thicker gauge.
The thicker wire reduces voltage drop which increases amps (up to the charging sources maximum). But with an alternator, you don't just have a voltage drop problem, you also have heat. Alternators will reduce power output when they get hot. If you could effectively cool the alternator PLUS reduce voltage drop in your wiring, you would get the max output from the alt. But how do you get effective cooling in the hot engine bay of a car?
bearsnob wrote: Further, I was told that you could increase the amps by replacing that wire with a thicker gauge.
The thicker wire reduces voltage drop which increases amps (up to the charging sources maximum). But with an alternator, you don't just have a voltage drop problem, you also have heat. Alternators will reduce power output when they get hot. If you could effectively cool the alternator PLUS reduce voltage drop in your wiring, you would get the max output from the alt. But how do you get effective cooling in the hot engine bay of a car?
Maybe I wasn't clear or maybe I'm just not understanding. I used the analogy of what I was told about my truck not to try to get the max out of the alternator but, instead, to try to logically figure out what might be possible with this car in terms of charging a second battery without stressing the alternator. Again, my problem is that I have to go mostly on logic because electricity and auto electric are weak areas for me. I went and checked and my memory seems to be right in that two headlights draw about 10 amps from the alternator. Obviously, the car runs just fine with the headlights on constantly. I know that headlights decrease gas mileage, but I also know, because I have a meter in my car that gives instantaneous mpg, that this decrease is hardly noticeable. So if I wire things so that the second battery is being charged at 10 amps while driving, and my headlights are off, logic tells me that I should be fine. Moreover, because I am not dealing with a lot of battery, 10 amps should be a pretty good charge. One formula I found said to divide the battery's amp hours by the charge rate and then add 10 percent to determine how long it will take to fully charge a "dead battery." So I think that means that if I go with the 35 amp hour battery, and the 10 amp charge rate, it would be fully charged in less than 4 hours of driving.
Am I on the right track or am I still off? I started this thread with my thinking based on logic and I was way off. So I wouldn't be surprised if I was still way off. The only thing I have going for me right now is that I understand a lot more based on what you all have taught me.
Bill, Carol & Striker The Cat
2005 Everest 323k, Anderson gooseneck adapter
2007 Dodge Crew Cab, 5.9L, auto, drw, 4wd, 3.73 axle, B&W gooseneck hitch
Two standard headlights are, depending on the lamps, 6 amps on "Bright" or 10 amps on "Dim" yes, Dim is brighter than bright.
On some of the newer 4-lamps sytems the draw is the same high or low
and on others it goes UP on high. (12 amps)
Yes, use of a heavier guage wire should increase current flow from the alternator to the battery however ... be careful of overloading the alternator. it is kind of expensive.
Nothin adds excitment like something that is none of your business
Kenwood TS-2000 housed in a 2005 Damon Intruder 377
Am I on the right track or am I still off? I started this thread with my thinking based on logic and I was way off. So I wouldn't be surprised if I was still way off. The only thing I have going for me right now is that I understand a lot more based on what you all have taught me.
Still a bit off but you could substitute your battery for headlight use. That would be fine. It would not take 4 hours to recharge that battery at a 10 amp charge rate because the battery resists current the more it's charged. Typically it takes twice as long (or longer) to charge a battery from 90-100% than it does from 50-90%. That's why most boondockers don't go for 100% while camping. It could take a couple of days or more to get that last 10% in there. And it seems most batteries need an overcharge (like 16V) to get to a true 100%. And then there are other issues like progressive capacity loss (PCL). Basically the more 50-90% charges you do, the less overall capacity your battery will have. People with enough solar don't get PCL (one reason why it's recommended).
In short, boondockers usually end up with multiple batteries and/or solar or live with the shortcomings. If you want to use just one battery, then you'll have to work within its parameters or plan on replacing it often.
* This post was
edited 03/17/12 06:15pm by mena661 *
mena661 wrote: It would not take 4 hours to recharge that battery at a 10 amp charge rate because the battery resists current the more it's charged. Typically it takes twice as long (or longer) to charge a battery from 90-100% than it does from 50-90%. That's why most boondockers don't go for 100% while camping. It could take a couple of days or more to get that last 10% in there.
That makes a lot of sense. I do a lot of boondocking and I had already figured this out without knowing what was happening. If I was using a generator to recharge, I found that I'd be better off just shutting things down before full charge because it took forever to get that last bit into the battery. But I had not made the connection between that situation and this one until you mentioned it. So now I also have to take into account the fact that I'll usually be starting with a battery that is at 90 percent charge and can only go down to 50 percent before having to recharge. Realistically, I think this means that I only have available 40 percent or less of the battery's amp hours in the situation I am describing. I never realized how complicated all of this is. Thank you.
Some rules of thumb that might help understand the issues:
Lead acid batteries have a _usable_ energy density by weight of about 12 watt hours per pound (50% DoD cost efficiency point as per smartgauge)
Battery ratings are for a draw of about a watt per pound. Higher draws reduce the available energy capacity. The charging system needs to have the capability to supply about 4 watts of charging for each pound of battery. Solar systems need at least 1 watt of solar panel per pound of battery.
A typical household uses about 30 kWh/day of electrical energy.
The residual loads in an RV (control boards, alarms, etc) runs at about 10 watts so that's 240 watt hours per day. The RV furnace runs at about 100 watts.
An inverter is about 80% to 90% efficient in converting 12v DC to 120v AC.
You typically want sufficient battery reserve to handle 3 days.
Watts are the proper unit for power and allow for comparisons no matter the voltage involved. For energy, use watt hours as it is easy to calculate and can be compared to your household energy bill.
A full and complete battery charge takes 8 - 12 hours (re batteryuniversity.com) - as noted, above, batteries are not simple electrical energy storage devices (like capacitors). They need proper care and due consideration for the chemical processes they use to store energy. They are generally only about 80% efficient in terms of electrical energy in compared to what they can give out.
bearsnob wrote: I never realized how complicated all of this is. Thank you.
You're welcome! I think some complication is necessary up front in order to make things simpler in the long run (Bill Cosby said it takes great thinking and work to keep from working.). BTW, 50% is not a hard limit BUT battery life shortens significantly below that point (actually below 80%). IMO, in your usage, I would spend more money on a battery that can be taken below 50% continuously and still provide a lot of cycles. The only problem with that is those batts are probably too big for your space PLUS you might need other measures to bring it back to 100% after your camping trips.
I want to thank everyone who contributed to this thread and update you on what happened. Turned out that the initial calculations everyone made were amazingly spot on. The fridge ran three days on 1 kwh of household current. That led me to use the auxiliary battery in my RV as the source battery. It is a large 12 volt deep cycle (125 ah). Under approximately the same conditions, the fridge ran three days off the battery and I was very pleased to see that the fridge turned itself off at almost exactly a 50 percent discharge. I then measured the kwh's that it took to recharge the battery and it was just a little more than one kwh. Amazing. I then started experimenting with using this battery as the second battery in my car. This is where I learned a lot but also wasted a lot of time. I was hampered quite a bit by the fact that my multimeter has a 10 amp current limit DC. I found a better one with a 20 amp limit, but that only got me a certain distance because the darn thing broke before I could get things figured out. I eventually bought one of these: EWNX:IT&_trksid=p3984.m1439.l2649#ht_2492wt_1166">these
That turned out to be a great purchase. I quickly figured out that the battery would only draw 15 amps at most off of the alternator while charging when it was 50 percent depleted and then, within a matter of minutes, that rate would drop to around 10 amps. So there doesn't appear to be a problem with straining the alternator. Just to be sure, I replaced all the lights except the headlights with LEDs. That will get me at least 5 amps if I ever need to charge the second battery while driving at night. I also bought a voltage sensitive relay (VSR) that wires in easily and prevents the starting battery from losing charge while I am running things off the second battery. Then I did something that I was kind of impressed that I was able to figure out how to do. Granted, I was able to find a youtube video that pretty much showed me what to do, but I still would not have thought I could do this without explicit instructions tailored to my exact situation. What I did was installed a 50 amp digital ammeter in the circuit and mounted it in the dash. I did this because I will sometimes want to run some other things off that circuit and I want to know exactly how many amps are being used when I am running things simultaneously. It works great and I think the whole setup was less than 10 bucks purchased off ebay from China. Part of the reason why this one was hard to figure out for me was that it came with no instruction beyond a wiring diagram. So I had to find out what a shunt was, how it worked, why it was needed, etc, and then plug all of that into what I was doing. The whole setup seems to be working great. Hopefully there are no disasters on the horizon. Thanks again everyone. I love this forum!
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BTW, 50% is not a hard limit BUT battery life shortens significantly below that point (actually below 80%). IMO, in your usage, I would spend more money on a battery that can be taken below 50% continuously and still provide a lot of cycles. The only problem with that is those batts are probably too big for your space PLUS you might need other measures to bring it back to 100% after your camping trips.
