Battery cut off switch

[garbonz]

Originally Posted by ko777

Because of my TV tongue weight 350# limit, I may have to store a spare battery at the rear storage. Just connect it temporary when needed. The extra weight will help decrease my TW which I need badly. I currently have over 50# of TW limit when I travel.

What happens if you connect the spare to the weak battery in parallel, will it charge the weak one?

sort of but not a good idea as it will pull the system voltage way down.

What setup could you recommend for temporary connection of the spare battery to the TT? I assume a cut off switch is necessary for this set up.

you would disconnect the main one and hook a harness to the spare.

[ko777]

Originally Posted by garbonz

you would disconnect the main one and hook a harness to the spare.

That is simulated in my diagram from previous post, it makes it easy to connect and disconnect the spare battery.

[ko777]

Originally Posted by ko777

Observation: Solar panel

Yesterday it was raining and no Sun, the TT battery was draining naturally...

Today the Sun is bright and the Solar Panel is pumping out 14.2V charging power but the TT battery is reading 12.2V.

I'll check tonight if the battery gets fully charged by the Sun.

I now know when the battery is fully charged by the solar panel, the volt meter reading spikes to 13.5V. It started from 12.2V to fully charged but took almost the whole day.

I purposely drained the battery by running the fridge using battery for 2.5 hours, it dropped to 12V. Turn on the amber light for 3 hours, it dropped to 11.8V.

At one point I had the furnace on for 10 minutes at the same time the fridge was on, it dropped to 11.2V but after I turned off both devices the battery recovered back to 12V.

Tomorrow will be another sunny day, let's see if the 40W solar panel will charge the battery to 100% from 11.8V. This time the cut off switch will be enabled, so the battery is not being drained by the TT internal devices while it is being charged.

[ko777]

The results are in, the battery got charged to 12.4V on a full day of sun light:

Using this chart, the battery from 35% and was charged to 75% by solar.

Luckily I have a gas generator which I'll bring in my trips but I like the idea of using solar panel to recharge the battery for free.

My challenge on my next dry camping trip is conserve energy, use the solar panel and avoid using the gas generator.

[pgandw]

Originally Posted by garbonz

Cmon, you lose valuable capacity and service life if you keep them separated. and if you buy them and use them together, they will equalize capacity very quickly. ALL of the good reference books recommend a single large house battery bank. I have had sailboats for 40 years in all kinds of situations, and been crew on long time full timers and none of them every kept separate banks, except for having a starting battery dedicated to starting the engine. If you want an emergency spare get a small separate 12 volt and keep it unconnected from anything unless you need it.

IMO, of course.

Yes, I am normally operating on the "spare" battery concept, except that the spare is the same as the regular. Whether it's a good idea or not depends upon loads relative to battery capacity. If the load is small enough that the single size 24 is adequate 90% of the time of better, then the spare concept works out well. Yes, one battery will likely out-live the other - not necessarily a bad outcome to me.

Using 2 size 27s wired through a selector switch on my outboard-powered (9 amp alternator) 25ft sailboat also worked well. Of course, a single battery could go a week or two (and did) when the only draws were cabin lights, and during an occasional night sailing, compass and running lights (6 amps draw there). Anchor light used separate 6 volt lantern battery. Quite successfully cruised the northern Bahamas for 4 weeks with only 3 nights with dock power and 3 nights sailing all night.

As has been pointed out, you cannot know a battery's charge level from a voltage measurement unless the battery has been idle for a while. In my scenario, I can measure the state of the backup, but I cannot accurately measure the battery in use.

But what would be a lot more helpful is to know the current draw of the furnace when running, the anticipated duty cycle, and the normal (furnace not running) parasitic load of the camper. Then I can make a good assessment of whether or not I need to combine the batteries for a weekend boondock. Hope to get some of these measurements on an outing in 2 weeks - our first trip was lost to a leaking propane fitting where the lines connect to the trailer plumbing.

If I truly normally need 2 size 24 batteries paralleled to handle the load for a 3 day weekend, then I should be looking at wiring upgrades to more efficiently charge the larger battery bank. Or, switch to a catalytic heater to drive the load way down.

Fred W (keeping things simple so we use the camper more)

[ko777]

Does the solar panel drain the battery when cloudy and at night?
The rate my 40W solar panel is going, it will never fully charge the battery.

My guess the 12.4V I got yesterday was just the surface charge. Today was mostly cloudy and the charging results is disappointing, it started at 12.1V am and now it still 12.1V pm.

The only good news is the cut off switch prevented the battery to drain.

Is it safe to store a spare battery in the back storage of the TT? During travel, can the floor handle the focus weight of the battery?

[thehamguy1]

There should be some diodes in the solar panels or the charge controller to prevent current back flow.

[D-mo]

For these types of solar panels, in order for them to run/charge at full potential you have to have all the cells collecting sunshine, the output will drop even if you have shade from a tree on part of the panel.
Im using an 80 watt panel ... and during cloudy days and at night i leave the entire system hooked up. There is a blocking diode in the panel itself . Some charge controllers are pretty piddly ... like the one for a 40 watt solar ... so basically its just a charge controller that cuts off and prevents overcharging of the battery.
I also have a 40 watt that i use to charge up a Xantex power box .. similar to the one in the pic provided ... but this power box is used exclusively for a CPAc machine ... when camping off line.
The nice thing about the power box is that you can plug the panel directly into the unit without a controller .. which helps deliver more raw power and speeds up the charging of the unit.

d-mo

[ko777]

What wire size for the connector do you recommend for the spare battery?

My revised plan, the spare battery will be my power pack. I plan to use this for emergency only when the TT battery run out in a cold night, the power pack has enough power for several hours taking care of the furnace fan, I think.

[D-mo]

you may want to rethink that again ... i have the same battery box as you do.. and it only powers my CPAC machine. In the morning the battery box is sitting at 60% charge, and requires recharging .. or attachment to the solar panel. no heat is used in my CPAC when camping .. so while these boxes are a great idea .. i wouldnt use it as a backup.
Just trying to save you some issues in the future..

d-mo

[ko777]

I'll give this a test next weekend camping. The power pack has a rating of 33AH, this should be enough for 1 night, I hope.

Can I use any metal part of the TT for ground? The power pack has a clamp which I use for ground connection.

[ko777]

The 40W solar panel charged the TT battery to 12.2V or at 60% charged level, this is several days of charging.

If I parallel another 40W solar panel will it fully charge the battery?

[Herk7769]

Originally Posted by ko777

The 40W solar panel charged the TT battery to 12.2V or at 60% charged level, this is several days of charging.

If I parallel another 40W solar panel will it fully charge the battery?

This info comes from here:

BatteryStuff Articles | Everything You Need to Know About Solar Chargers

In short:

As a rule of thumb a rating of 15 watts delivers about 3,600 coulombs (1 AH) per hour of direct sunlight; Panels loose 20-25 percent of rated power during the course of a charging day due to sun position) so a 40 watt panel will deliver an average of 30 watts or 2 AH per hour of daylight (say 10) or 20 AH.

TWO panels (assuming they receive the equivalent sun) will generate twice that or 40 AH per day.

An average OEM Deep Discharge battery is about 70 AH total capacity so with a 40 watt panel and good average sun, will take 70/20 or 3.5 days to recharge a used up battery. (less obviously if charging one that is only 50%).

So 80 watts will take a bit over a day and a half.

HOWEVER,

1) You can NOT use the battery(s) while it is charging for these numbers to work.
2) LARGER batteries (banks) take LONGER to recharge since there are more amps to replace.
3) The solar charge controller will manage the charging rate to avoid boiling the batteries as they fill resulting in longer recharging times than above.
4) (With PWM controllers) If the panel's voltage drops below battery voltage the charging will stop. Shade will rapidly decrease generated voltage and if the battery has a higher voltage that the incoming charge voltage it just won't accept it. This happens "a lot" as the battery nears "full". (More expensive MPPT controllers will trade charging current for voltage as the sun sets)
5) Always size your panel to generate your anticipated usage plus extra to charge that battery.

Hope that helps...

[ko777]

Yes, thanks for the information. I will wait for a solar panel sale and get another to accomplish what I plan to do. After my next trip, I'm leaving my trailer there for a week or more then come back to camp their again...repeat. I am hoping the solar panel will fully charge the battery before I get back. For now I'll use my gas generator to top up.

Can I mix different Watt sizes solar panel parallel charging?

[camaraderie]

Originally Posted by ko777

The 40W solar panel charged the TT battery to 12.2V or at 60% charged level, this is several days of charging.

If I parallel another 40W solar panel will it fully charge the battery?

80 watts of solar panels will provide approximately 20 amp hours of juice per day.
Lets assume you have an 80 amp hour battery discharged to 50% (which MOST charts will tell you is 12.2V for a wet cell deep cycle battery)...which you should never go below. That means you need to put back in 40 amp hours or 2 days worth of sun to recharge IF you have no draw on the battery during that time. If you do have a draw on the battery...you need to add the necesscry extra time into the mix.

It does not matter WHAT size panels you use...just stick to the same chemistry & output max voltage to keep things easy.

[Herk7769]

Originally Posted by camaraderie

It does not matter WHAT size panels you use...just stick to the same chemistry & output max voltage to keep things easy.

Ditto. If the max voltage is different I am not sure what the controller will do if you pigtail them together.

[ko777]

Started working on the cut off switch today, here is my final schematic

When the key is taken out, the line is cut off.
The spare battery connection is temporary, to use this spare, take the key off for the main battery.
When the spare is not use, take the key off for the spare battery, this will make sure the wires to connect the spare battery is not hot.

Take both keys off when TT is parked or when charging the main battery with a solar panel; the spare battery could be on while charging the main battery.

I finished installing the first cut off switch but had to stop because the first test failed. I use a multimeter to find the fault and found the switch to be defective. There goes an hour wasted. I learned my lesson...always check the parts for defects before using it.

The wiring connections to the battery is completed.

A battery tender with a fuse is installed for connecting the solar panel directly.

Cut off switch part

The test failed, no current going through when key is on.

I have to get a replacement part and should be able to complete this project tomorrow.

[ko777]

Cut-off switch mod is complete. I tested both main battery and spare battery connections, both passed.

Both keys off, this is how the TT will be set when parked and not in used. The blue and black cables are for the spare battery connections

For emergency use only, use the power pack battery to power the TT, the booster cables worked like a charm

I'll test the power pack this coming weekend, the last night of my trip. If this last for the night that will be awesome. I'll post the outcome.

Happy Spring Camping!

[JimR65]

Here is a photo of the way I setup the cutoff switch. I use the neg. side for the wiring. For what it's worth.