John (and Mr. Tweety, if you're reading),
We've only been out once this year, and we had electric hookup, so I haven't had a chance to really run the batteries yet. September it should be nice enough out for us to go to the non-hookup locations. Nick's Lake might very well be our first time out on battery this year.
I know you mentioned you have a clamp-on ammeter. I do as well. If we're all going to be up there, I can bring that along and maybe some other test equipment and we'll do some measuring. I'll have the little genset along as well.
Now, the charge rates - I'm far from an electrical engineer (I dropped out after a few semesters, but that's a different story). In other words, I know enough to be dangerous
That said, here's a few of my thoughts, for what they're worth..
First, a little google action found this post (
https://www.goodsamers.com/members/f...g/22696582.cfm) on another forum, where a gentleman contacted WFCO in regards to current limiting on their converters. He supplied the following answer from WFCO:
"For any of our converters there is no way to control current entering into the battery. If the battery resistance is low it will draw up to the limit of the converter minus the electrical load of the RV.
Be Well,
Daniel Derrow
Technical Support
Phone: 877 294 8997 ext. 106
d.derrow@wfcoelectronics.com
www.wfcoelectronics.com"
Interesting, since my 8945 states a maximum of 30A fuses on the charge connection. Those haven't blown yet, but I think that's what started you guys on this discussion before
Of course, other brands may differ. But in this world of cheaper-to-build-is-better, maybe not?
The WFCO answer mentions Battery resistance, which turned on a lightbulb in my head. Battery chemistry is really complex, and I won't claim to know much, but I do remember reading that internal resistance of a lead-acid will change with State Of Charge. Back to google, where I found this (
http://www.batteryuniversity.com/partone-22.htm):
"The resistance of lead acid goes up with discharge. This change is caused by the decrease of the specific gravity, a depletion of the electrolyte as it becomes more watery. The resistance increase is almost linear with the decrease of the specific gravity. A rest of a few hours will partially restore the battery as the sulphate ions can replenish themselves. The resistance change between full charge and discharge is about 40%. Cold temperature increases the internal resistance on all batteries and adds about 50% between +30°C and -18°C to lead acid batteries."
There's a pretty little graph included on the page showing the curve of internal resistance vs. SOC.
OK, so the SOC of the battery would play some role in the observed charge current.
Next, I was thinking about what made me do the wire upgrade - The effect of voltage drop across the charge cables. I like examples, but to keep it simple (and me sane), a few caveats: We'll only calculate the positive cable run, since the negative could get very complex being that the chassis, several connections and at least two seperate wire runs are invovled. We'll also assume there is no added resistance on the positive charge cable from connections, connectors, corrosion, fuses and/or circuit breakers.
A Wire chart tells us the following:
8 AWG has a resistance of .6405 ohms per 1000'
4 AWG has a resistance of .2533 ohms per 1000'
0 AWG has a resistance of .096 ohms per 1000'
Assuming a charge rate of 30A and a positive charge cable run of 20', voltage drop is (V=RI):
8 AWG: .3843 Volts
4 AWG: .15198 Volts
0 AWG: .0576 Volts
Voltage drop will decrease as charge current decreases. Run the numbers again, same length wire, this time with a 10A charge current:
8 AWG: .1281 Volts
4 AWG: .05066 Volts
0 AWG: .0192 Volts
Conversely, if charge current increases, so will voltage drop. Say you have a nice beefy converter and can charge at 50A:
8 AWG: .6405 Volts
4 AWG: .2533 Volts
0 AWG: .096 Volts
I think 20' is a pretty fair cable length on most of our coaches. 30A seems like a reasonable current. So the stock cable would chew up .3843 volts. That's probably enough to prevent some chargers from entering the high-rate mode. I've read a few posts on other forums where guys with WFCO's were complaining they never saw boost mode; they all say "problem solved" when they beef up the charge cable.
One last thought - connections. When working on the 2363, I've learned that Sunline (and from what I hear, a lot of manufacturers) have a love of wire nuts. Wire nuts are great on sticks and bricks wiring that doesn't move, but IMO, using them in something that bounces down the highway at 65 mph is probably not the best idea. My unit had several wire-nutted connections on the charge line, all a possible place for a less than ideal connection and more resistance. Also, when I crawled under the coach, I found the negative battery cable lug pretty corroded where it attached to the frame. Again, more resistance.
PS: John, I got the Breaker from this dude:
http://cgi.ebay.com/Bussman-DC-Circu...3286.m20.l1116, came fast, price was reasonable. They also make a surface mount version, like this 50A model:
http://cgi.ebay.com/Bussman-DC-Circu...4.c0.m14.l1262. I *think* the surface mount one comes in 30A as well, but he don't have them.
PPS: An inexpensive way to get an high-current ammeter into the coach:
http://cgi.ebay.com/Blue-LCD-Digital...3A1%7C294%3A50. Bunch of different styles and shunt ratings on ebay, all very reasonable around $20 for the meter+shunt package.
PPPS: Norm, I dont recall off the top of my head. I will try to take a peek under there today and see if they are marked.
- Frank
Linear Mode
