Do battery cutoffs and auto-reset breakers ...

joebedford wrote:
...fail in such a way that they reduce battery capacity?

Possibly, but unlikely.

Get a multimeter and a lonnng test lead. Check the voltage at the battery posts. Then leave the ground side connected to the battery and take the meter and the other probe and see what the voltage is at the far end (furnace blower). There should be less than 0.1V difference. More than 0.2V and you have a real problem somewhere in that path.

With my converter off, I put my meter on battery positive and the other end of the house side of the breaker. There was 5V difference. My wife turned on the furnace and the voltage differential climbed to about 7 volts fairly quickly <1 min then the breaker tripped.

I put a jumper cable on the battery and connected it to the house side of the breaker. No problem running the furnace despite battery getting down to about 12.7V by now. Moved the jumper cable to battery side of the breaker and it tripped immediately. To me that meant battery disconnect is not an issue - it's the breaker.

Got a new 50A breaker from the local auto parts store and installed it. There was still a drop of 3.5V across the new breaker but not as bad as before. (the drop across the disconnect was 1.5V of the 3.5V)

Anyway, with battery still not charged and converter off, and some lights on, the furnace has been running for the last 15 minutes. Not definitive, but it's a pretty good bet we've nailed it.

Charge battery now - longer test later.

Thanks all y'all (we're still in Texas).

joebedford wrote:
With my converter off, I put my meter on battery positive and the other end of the house side of the breaker. There was 5V difference. My wife turned on the furnace and the voltage differential climbed to about 7 volts fairly quickly <1 min then the breaker tripped.

I put a jumper cable on the battery and connected it to the house side of the breaker. No problem running the furnace despite battery getting down to about 12.7V by now. Moved the jumper cable to battery side of the breaker and it tripped immediately. To me that meant battery disconnect is not an issue - it's the breaker.

Got a new 50A breaker from the local auto parts store and installed it. There was still a drop of 3.5V across the new breaker but not as bad as before. (the drop across the disconnect was 1.5V of the 3.5V)

Anyway, with battery still not charged and converter off, and some lights on, the furnace has been running for the last 15 minutes. Not definitive, but it's a pretty good bet we've nailed it.

Charge battery now - longer test later.

Thanks all y'all (we're still in Texas).

with the breaker operating as it should, there should be less than 0.1V more like 0.01V across the breaker when it is closed. Same goes for across a closed disconnect switch.

Either the measuring method is incorrect, or something is still not operating correctly.

And if there was a 3.5V drop across the breaker with the converter off, I can almost guarantee the furnace would not operate. They won't operate much below 11V DC.

AFAIK ? You need a current to get a voltage drop. Seems like there must be a big current to get that big of a drop? Short? I still suspect the wiring from installing the new battery. If one of them is shorting (somewhat, but not completely if that is possible) could that do it?

I must be more tired than I thought: .15V and .35V DUH!

BFL13 wrote:
AFAIK ? You need a current to get a voltage drop. Seems like there must be a big current to get that big of a drop? Short? I still suspect the wiring from installing the new battery. If one of them is shorting (somewhat, but not completely if that is possible) could that do it?

well, to get a voltage drop you need current AND resistance. you can get big voltage drop with lots of current and very low resitance or very little current and lots of resistance.

for a DC circuit, V=I*R. (actually for AC as well but I, V and R are vectors, not scalars, so the math can be much more complicated)

joebedford wrote:
I must be more tired than I thought: .15V and .35V DUH!

well, those results are much more reasonable, but I would still expect to have well under that voltage drop with the load you are drawing, especially on the 0.35V measurement.

I'd check the connections to both the breaker and the switch. I suspect they may be corroded or loose, or if they used the typical crimp connector, a bad crimp that is becoming more resistive.

A single Group 27 From Autozone assuming a MARINE/deep cycle you have less than 20 Amp Hours of usable power.. Your furnace likely draws about 10 amps all by itself. Add other stuff Lights and such) and you run out of power fast

Upgrade to a pair of GC-2, that's 220 amp hours give or take a bit, over 100 usable amp hours. over FIVE TIMES teh usable power. That's your problem.

And if that Autozone battery is not a MARINE/deep cycle.. the numbers are even worse for it.

wa8yxm wrote:
A single Group 27 From Autozone assuming a MARINE/deep cycle you have less than 20 Amp Hours of usable power.. Your furnace likely draws about 10 amps all by itself. Add other stuff Lights and such) and you run out of power fast

Upgrade to a pair of GC-2, that's 220 amp hours give or take a bit, over 100 usable amp hours. over FIVE TIMES teh usable power. That's your problem.

And if that Autozone battery is not a MARINE/deep cycle.. the numbers are even worse for it.

group 27 roughly 100AH. marine, discharge to 50% with likely 50-100 cycles to 50% for cycle life, 50AH reasonable.

and how many will do 50-100 cycles in a few years? not many folks

yes furnace will draw around 8-10A. but if you don't do much dry camping, no reason IMHO to spend the bucks for a pair of GC. just get a decent marine battery or two, keep the DOD to 50% or less.

Now, if the group 27 is a starting battery, not marine, you'll kill it quick with discharges below 80%SOC, and even keeping it to 80% life won't be great. a BAD choice of batteries.

For the GC, 220AH, useable is not 100, but more like 150AH. Trojan spec's around 500 cycles to 25% SOC (75%DOD) for cycle life on the GC. close to 1000cycles to 50%SOC. and that's for the std GC. you can get double that for the renewable energy GC line.

I'd image that for all practical purposes for the majority of RV folks other GC batteries are close enough for Cycle life that cycle life isn't the life issue, just years of service.

And that cycle life to 25%SOC is reasonably accurate. Been there done that on two sets where over a 10 year and 8 year period put near 500 cycles to that DOD on them and they still were working well.

but again, which is best depends a great deal on intended use.

The group 27 I put in is a marine / deep cycle. I really don't think it's the problem. I know it won't last long, but remember it only lasted 20 minutes from full charge with only the furnace running.

I torqued down the new breaker pretty good and twisted and tugged on the terminations at the battery. I'm certainly no expert but all those mechanical things seemed fine. Zero sign of corrosion.

We usually snowbird and stay at the same park (plugged in) for a month or more at a time. We often stay at Walmarts or truck stops overnight when travelling. We have a generator and don't mind using it but basically all I need is a full night of furnace with enough juice to start the generator in the morning to recharge and make coffee.

It's kind of like RV fridge / residential fridge. We like the RV fridge but for what we do a residential is more cost effective (except for those rare occasions...)

Incidentally, I have another thread going about my RV fridge. I wonder if the 12V problems I've described in this thread have anything to do with that problem?? (but I don't want to hijack my own thread) :)