Tow Battery Plus RV Batteries?

[Huck]

If my tow battery is rated at 40 ah and 2 trailer batteries at 70 ah each, does that mean I have a potential of 180 ah if connected?

I'm not looking for the fine points, just asking if the ah would be additive.

[Terry G]

Why would you want to use your tow battery while camping? How are you planning to start your engine when you're done? And as a general rule, you don't want to run batteries below 50% of their rating. So you actually have about 70 ah of usable battery when camping.

[Bob Miller]

Yes, and it also means that the have the potential of having a dead battery on you TV in the morning. Except in dire cases, don't count on your TV's battery to power your trailers electrical system. If you don't have a battery isolator installed on your TV you may want to to unplug when parked.

[Huck]

Just thinking there might be an occasion where you need more watts for a short time.

[Bob Miller]

The TV's battery wouldn't be of much help there unless you have a huge #000 cable running from the TV's battery to the trailer. For the most part, something less than 20 amps is about all you can count on through the TV charging line, and that's why it's called a "Charging Line".

BTW: most of us survive with a single battery, two is a real luxury.

[honda03842]

Huck,

The answer to your question is that amp-hr are additive.

If you have 2 batteries that each give you 70 amp-hours, the total available is 140 amp-hours.

Amp-hours available is a number dependent on many factors. If you discharge at a slower rate you get more amp-hrs (less amps) than if you discharge at a higher rate. (more amps) i.e taking one amp per hour gives more amp-hrs than taking 10 amps per-hr.

As to discharging, discharging below 50% for most RV batteries affects their life.

Generally if you want more amp-hrs, eventually replace your's with more powerful batteries.

In general rather than seeking more battery amp-hrs, reducing consumption is relatively easy as well as adding charging sources like solar panels or a small generator.

[SherryNPaul]

Your question was valid, and you got some good answers.
You may want to look at how to reduce power consumption in your trailer. Led lights, at least for the main lights, switiching off unused systems with "vampire" draws, etc. When we boondock, I switch off all the optional banks most of the time. Run the water pump only as needed, light mains only at night, and only if we need it. Stereo, etc., only as used. We are a bit penurious on power, even with two batteries, so we can enjoy the nature we drove so far to see and hear and smell... for longer, without tethering to power. If you're a former tent camper, or a former sailor, it's second nature. If not, think like you're tenting without power.
As Norm suggested, consider adding some solar. Solar is silent, and no umbilical cord needed if you're thrifty with the power. We love it.
Sherry

[Carol H]

Huck take a look at the 12 Volt Side of Life.

[deryk]

Norm and Ginny, A great reply!

[Huck]

What I was getting at was air conditioning. Running the ac seems possible (for an hour or 2) if I can overcome the surge wattage. I see where some ac's have a low wattage startup. From "what" I have read, a 5000 BTU ac uses about 500 watts an hour plus startup voltage.

I have a Harbor Freight solar kit I'm not using, so I was thinking of buying a separate battery to go with it. That would give me a total of 145 watts of solar power plus 3 70+ ah batteries.

At this point it is just a mind exercise as I start to understand how things work.

[deryk]

Huck, it's not worth it. If your wanting air conditioning and boon-docking your going to need a generator. Maybe a dozen batteries you can do that but after you drain all 3 batteries running the air conditioner for maybe an hour....how many days do you think its going to take to recharge the batteries with a 45 watt's of solar?

The panel I'm installing this week, (just waiting on 4 more of those z clips) puts out optimally 5.something amps of power (100 watt panel) I'm hoping enough to keep my battery topped off when running the heater at night or my maxx air hatch fan.

[Huck]

Originally Posted by deryk

Huck, it's not worth it. If your wanting air conditioning and boon-docking your going to need a generator. Maybe a dozen batteries you can do that but after you drain all 3 batteries running the air conditioner for maybe an hour....how many days do you think its going to take to recharge the batteries with a 45 watt's of solar?

The panel I'm installing this week, (just waiting on 4 more of those z clips) puts out optimally 5.something amps of power (100 watt panel) I'm hoping enough to keep my battery topped off when running the heater at night or my maxx air hatch fan.

Again, at this point I'm just playing with the idea. And I have 145 watts, not 45.

Let's say you have 2 100 watt solar panels. And let's say that during the summer you collect 1600 watts of power per sunny day. Excluding startup wattage, that's enough power to run a 5000 btu ac for about 3 hours. The estimates I have seen are that a 5000 btu ac uses 350-550 watts/hr.

If you run the ac for an hour, say around 8 pm, wouldn't that cool down the trailer for several hours? I find I can sleep fine if the temperature is max 80 and I have a good fan, so what I am looking for is a way to run the ac for an hour without having to carry a generator and gas,

[deryk]

it might temporarily remove humidity but trailers are far from being adequately insulated. If it was 90 some degree's that hour will remove the humidity from the air but it will get stifling in there again fairly soon. Also don't figure in max amps of power from the solar panels unless its tracking the suns path all day on perfect clear cloudless days.

[Bob Miller]

I sympathize and empathize with having to live in the wonderful heat and humidity of VA, something I too did for more than 10 years before escaping back to the Golden State. But that said, usually by 8 PM it was cooling down after peaking 5-6 hours earlier.

In working with and on mini-motorhomes and FGRV's for over ten years, I listened to this same problem, that of wanting to run an a/c off of battery power, many times. In that time I have seen several attempts and zero successes. Here's what I have learned, albeit at others expense:

1. Stuff that works great on paper don't always translate to real life:
Solar panels seldom puts out the stated power rating except when perfectly positioned on a perfect day. For real life you may have to reduce that figure by 50% or more.

2. Rapid draining of batteries, with upwards of a 40 amp continual load, can quickly lead to a reduction in charge capacity, warped plates from overheating and reduced battery life.

3. The "Locked rotor current" (aka Restart Current) in inexpensive a/c's can be 3 times run current. This is why it is almost always stated to use a dedicated 15 amp circuit to run what looks like a 4 amp running load. To you this means that you will also need an inverter that can handle 1500 watt momentary inductive loads as well as a huge surge draw on your batteries.

Comments I have heard after these attempts have included:

"Killed off the batteries in less than 1/2 the time I figured"
"Burned out 2 inverters so I quit"
"A/C unit and Inverter kept tripping off"

As I best recall, those that had to have a/c, resorted to generators or only stayed where power was available.

Good Luck and let us know how it works out....

[Carol H]

Originally Posted by deryk

Also don't figure in max amps of power from the solar panels unless its tracking the suns path all day on perfect clear cloudless days.

Deryk's point is a REAL important one.....

Huck the best advise I can give you is to learn to love your Maxi or Fantastic Fan.

[David Tilston]

To run AC, I would get at least 1000W of solar. At current prices, that would be expensive. A tracking array might also be a good, if expensive, idea. Size matters as well. A figure I have read is that you can expect good solar cells, under ideal conditions to make about 10 W / square foot. For 1000 W, that would be a 10' x 10' tracking array. Where do you pack that?

The prices are coming down for solar, and they are getting more efficient. So your idea is not wrong, just early. Give it a few more years.

[beemerphile1]

Simple answer to the original question;

Batteries wired in parallel you add the ah rating.
Batteries wired in series the ah rating stays the same and you add the voltage.

ie; two 6v batteries in series give you 12v but the ah stays the same
or two 12v batteries wired in parallel stay at 12v but the ah is added.

[Denece]

Don't forget you can only use half of the amp hours listed, or risk ruining the battery. Look into marine A/C units, Mermaid come to mind. Mucho dinero, but built for 12V.


Sent from my iPad using Fiberglass RV

[Timber Wolf]

Originally Posted by Bob Miller

As I best recall, those that had to have a/c, resorted to generators or only stayed where power was available.

Sad to say, as I would love for all this to work with solar but this is a situation where a good generator is called for. It will be expensive for a nice quiet one but no more than all the batteries, wire, controller, panels, invertor, etc., etc. to try and make solar work. And you will have a nice gen-set for other uses like storm outages and such.

[Borrego Dave]

Yup Tim a generator is in your future. I have a 9000 BTU AC on my SD and my Honda 2000 runs it great. Only running half throttle max during the AC cycling on and off. One plus is that it will do that for 8 hrs 15 minutes on a full fuel load.