Battery disconnect

[Byron Kinnaman]

Originally Posted by John in Santa Cruz

care to clarify where I was wrong? I've been working with electronics, including battery charging systems for 40+ years.

the 4th stage on a 4-stage charger is an 'equalization' phase, to be used ONLY on wet cells.

no WAY the charger stops there. That statement.

Want to compare credentials?

[David Tilston]

Originally Posted by Byron Kinnaman

no WAY the charger stops there. That statement.

Want to compare credentials?

Oh the things that I typed... and then erased. I got a good chuckle.

[John in Santa Cruz]

under what sort of sane conditions would a charger stop charging at 12.2 volts ?

I did make one error, 12.2V is considered 50% discharged for a battery at rest at room temperature... I don't like to let my batteries ever get below 12.1V so I use a compressed scale in my head.

[ntsqd]

I long ago concluded that converter/chargers make good anchors and not much else. I concluded this when I found the unit in our camper boiling the T-125's electrolyte. Anytime a company sets out to do many things and offer the product on a budget you can be assured it doesn't do any of it very well. If you're lucky it will do one of those things well.

I'd look at solar with a charge controller if that is an option.

The C-Tek Dual 250S looks to be a good, low current multi-stage charging option if a large solar panel is out. It will accept power from a solar panel and also from a simple power supply or the TR's alternator

[workhorsed]

Originally Posted by D-Robbie

Hi, I am D-Robbie. I have a new 17 ft Casita Freedom Deluxe. As I have had problems with my battery running down while not in use I want to install a disconnect switch. Not much room in battery cubicle. Any good ideas?

Princess Auto, Harbor Freight, Northern Tool etc all carry the same simple battery disconnect, brass colored with a green knob. Simply disconnect neg batt cable, install disconnect to batt, reconnect batt cable to disconnect. Job done!! Cost? Less than ten bucks!!

[Steve L.]

It has always been my impression that the battery disconnect should be in the positive line, just like fuses and switches.

This is not about avoiding issues during removal of the battery but rather isolating downstream of the battery. Interrupting the negative line means you still have a hot (i.e. charged) connection at every 12v device which would provide a chance to ground the line to some other alternative ground near the devices while working on them or installing them.

Said another way, interrupting the negative is for folks worrying about shorting while removing the battery. Interrupting the positive is for people worried about phantom drains on the battery or shorting while working on the electrics downstream of the battery.

[gordon2]

Originally Posted by Steve L.

.... Interrupting the negative line means you still have a hot (i.e. charged) connection at every 12v device which would provide a chance to ground the line to some other alternative ground near the devices while working on them or installing them.
...

If the battery negative is not connected - not to the wiring, the frame or anything, then exactly what alternative ground are you referring to?

I have read a lot on this debate, and opinions are mixed (for this type of application). I have done it both ways. With a low voltage DC circuit I have concluded it makes no difference if in fact the switch is the first thing in the circuit, such as at the battery post, mounted like the OP has.

[ntsqd]

Didn't I see mention of a breaker at the battery? I use Series 285 Breakers for disconnects on a lot of 12 VDC vehicles. Dual purpose, fewer parts to fail.



I also have a keyed type Hella battery disconnect* on one vehicle for making it harder to steal. That switch is in the ground cable. An open circuit is an open circuit, it doesn't matter where the circuit is opened. It CAN matter where it is closed though. I say better to close it downstream of the loads than to risk a voltage spike in the system.


*In these keyed battery switches there are a lot of chinesium copies of these sold out there. Buy a brand name like Hella if you want parts that will work and last.

[pete_britt]

Originally Posted by Casita Greg

Unless you've upgraded to a 4-stage "smart charger", your old 3-stage converter charger will gladly fry your battery in short order.

I kept my original 3 stage Paralax converter plugged in continuously for a couple of years. I checked the battery with a hydrometer every couple of months. The converter did a fine job. I never had to add water, the battery stayed fully charged, never fried.


I think the 3 stage converter may have been maligned too often.

[k corbin]

Originally Posted by gordon2

If you only want to know where to mount the switch I can’t help much. I do like the idea of the switch in the camper instead of outside where it is subject to corrosion, weather and vandals. I also suggest that it be easy to reach (but not where children will switch the battery off without you knowing it). Yesterday I moved my switch from under the bench hatch to inside the “Porta-potty” storage area because it was often a pain for me to move things around, lift the cushions and access the hatch. Now I can open one door and use the switch.

More notes that you probably already know about:

Check your converter manual to ensure the system will function properly without a battery attached to it. Some converter designs require the battery to be attached.

Positive or negative side? There is much debate and it usually makes little difference in a small RV. But if things are wired straight to the battery on the positive side, and they use a common or frame ground elsewhere (not at the battery), and if you want these things off when the switch is off, then the ground side makes sense. I wont debate this anymore since like I said, in a small camper it usually is OK either way.

But if you do use the negative / ground side and use a shunt for monitoring the battery, then it should be on the load side of the shunt so that the battery monitor can continue to monitor the battery when the switch and loads are off. The battery monitor does still use a few milliamps, and of course the battery will self-discharge, so you still need to charge it occasional and/or use a battery tender type of maintenance charger.

Also consider solar if you have it, or might get it in the future. I would (and did) wire it so that, when the switch is off, it still allows the battery to be charged. In other words the switch disconnects the loads but not the solar charger. The charger is connected at the load side of the shunt, but before the switch, so that the TriMetric can record the solar charging current.

Are there any other things that should still be powered when the switch is off? Obviously, a break-away switch for example should not be wired upstream of the switch (and should be wired straight to the battery).

Kudos for your decision to add the switch. It not only helps to manage your power use, and extend the life of the battery, it is also a good safety device.

This is what I ended up with, using the ground side to disconnect the trailer's 12 loads (the same polarity one should disconnect first when disconnecting the battery at the battery posts). All trailer loads run off the buss bars.

Thank you for this advice. I will be installing all new electrical soon and this advice will make the protect easier to plan out. I do have an electric fridge that should be wired direct from the battery as well as a solar charger and I do want a battery cutoff switch. The current alternator in my tow is not large enough to charge the house battery, someday an upgrade in the future to plan for so I needed to know about that as well.

[ntsqd]

Voltage drop in a charge wire from the TR's alternator is a whole new topic. I'll digress & guess that 90% are using far too small of a wire for less than 3% Voltage Drop at the camper's battery.

[David Tilston]

I ran a #8 from the vehicle battery, (through a #8 fuse holder with a 40A fuse) to the 7 blade connector. Trouble is I had to cut some of the strands to get it to fit into the terminal on the connector. I guess that will be the highest resistance point. I should expect heat. I considered using the center pin for some of the conductor, but decided against it. I still might.

What I should have done, that I didn't, was to also run a #8 negative wire as well.

[ntsqd]

12 VDC 30A over 30 feet (total circuit length) at 3% Voltage Drop needs 6 ga. per:


Circuit Wizard - Blue Sea Systems


Not going to, nor should anyone try to get that into a std 7 pin connector. Needs one of the various flavors of Anderson connectors.


Other excellent reference materials here: https://www.bluesea.com/support/reference

[gordon2]

Originally Posted by ntsqd

12 VDC 30A over 30 feet (total circuit length) at 3% Voltage Drop needs 6 ga. ...

But who needs 30 amps? Maybe someone... but I get 2 - 2.5 amps at the battery from my tug with 10 gauge charge line, and it helps. 5 amps would be plenty for me. And at most, a small camper with a 3 way fridge running on 12 volts might need 10 - 12 amps to maintain the camper house battery at 100%. Agree?

[David Tilston]

Originally Posted by gordon2

But who needs 30 amps? Maybe someone... but I get 2 - 2.5 amps at the battery from my tug with 10 gauge charge line, and it helps. 5 amps would be plenty for me. And at most, a small camper with a 3 way fridge running on 12 volts might need 10 - 12 amps to maintain the camper house battery at 100%. Agree?

Agree.

Also, I have never seen a trailer with an anderson connection. But I don't get out much. I also expect that the trailer plug should be able to handle 15A or 30A? if the center pin is used. but it could possibly burn up if I get to 40A. The 30A guess ignores the fact that the same current will have to come back on the negative blade, and there is only one of those. I use a 40A fuse, not because I expect to supply 40A to the trailer, but just to protect the conductor.

[ntsqd]

Nope, I don't agree. I'm sure that the 30A charge rate isn't long term, but I'm also sure that with a 10 ga. just getting a bulk charge on a reasonably sufficient Ah battery(ies) will take a long time. Probably longer than a day's drive if they are approaching 50% SoC in the morning. That 30A guess ignores nothing of the kind.

And I have seen 40A on the TriMetric in our camper. Granted, that was after the converter/charger had basically destroyed the Trojan T-125's that were in it.

More important is what is the alternator voltage at the battery(ies)? You need the Alt's voltage at the trailer battery(ies) to be enough higher than the battery to actually charge it/them. This is what really drives the wire size, not the ultimate current carrying capacity of the wire. If the alt's output voltage is meager and not adjustable then you'll need to adjust the allowable Voltage Drop percentage to compensate.

I own a trailer with Anderson connectors, and I didn't install them. It came to me with them already in place.

[Uplander]

All travel trailer owners should read “the 12 volt side of life” on the internet.
A real education on batteries, especially for extended use in campers.

[ntsqd]

Followed shortly by Handy Bob's solar blog.

[Byron Kinnaman]

Originally Posted by Steve L.

It has always been my impression that the battery disconnect should be in the positive line, just like fuses and switches.

This is not about avoiding issues during removal of the battery but rather isolating downstream of the battery. Interrupting the negative line means you still have a hot (i.e. charged) connection at every 12v device which would provide a chance to ground the line to some other alternative ground near the devices while working on them or installing them.

Said another way, interrupting the negative is for folks worrying about shorting while removing the battery. Interrupting the positive is for people worried about phantom drains on the battery or shorting while working on the electrics downstream of the battery.

Does it really matter which side? Kerchoff's laws indicate any interruption of current loop will stop the current flowing.

[Steve L.]

Originally Posted by Byron Kinnaman

Does it really matter which side? Kerchoff's laws indicate any interruption of current loop will stop the current flowing.

I agree but my thinking is that disconnecting at the positive means you can't short the 12v devices to some other alternate ground in the vicinity of the device. Say, you wanted to swap out a 12v light. You could switch/interrupt the positive at the battery and both sides at the light would be dead. But if I switch off at the negative then one of the wires at the lamp would still be hot and if I were sloppy with a tool I could potentially short it on something nearby.