Batteries.

[Basement Yeti]

Looking to get a dedicated house battery bank set up soon for my camper van. I need help figuring out what power requirements I need to meet and what type of batteries to use.

I'll be running 12V LED lights and my roof vent at first but later adding a propane alarm, my stereo, water pump, and a 12V chest fridge when I can afford a solar system to supplement my alternators charging capabilities. Eventually I'd like to get a small 400w or so inverter for handling my cell phone, camera batteries, electric razor, and laptop occasionally.

Here is the list I made of power usage from appliances if it helps, I high balled most of the numbers, this is per 24hrs.

Vent fan high setting 32 Watts x 8 hours = 259 Watts
LED rope lights 5 Watts x 5 hours = 20 Watts
Propane alarm 1/2 Watt x 24 hours = 12 Watts
Water pump 36 Watts x 1 hour = 36 Watts
Stereo ?
Chest fridge 30 Watts x 6 hours = 180 Watts
Misc. charging 100 watts
= 600 Watts-ish

Also, from this I figured out 2x 12v 100Ah batteries would give me 1,200 Watts of useable power, or enough for around 2 days. Driving for about an hour a day and with 100-200 Watts of solar power would this be enough to keep me afloat?

When I get my batteries I plan on sticking them in one of these and venting it outside.
http://www.pplmotorhomes.com/parts/...m_medium=partsshopping&utm_campaign=partsfeed

 

[Herbie]

You need to get your head around the difference between watts and amp-hours. You're getting close to the right answer, but I think you're getting there by accident...

The Watt is a measurement of instantaneous work. (In this case, electrical work.) For your purposes, since this is a DC system, it is safe to assume that Watts= Volt*Amps, and you can also assume that Volts are fixed. Call it 12v or 13.2v or whatever, but until you get into the corner cases of extreme discharge on the batteries, etc, you can just roughly calculate the Amps for all of your accessories by dividing the Watt rating by your nominal voltage.

The Amp-Hour is a measurement of aggregate current delivered over time. (Literally, it's # of Amps * # of hours.) There are some limits due to pesky physics, but basically you can assume that a 100AH battery will deliver 1A for 100Hours, or 5A for 20Hours, etc. (Low current is more efficient than higher current, so look up the curve for your selected battery if you're "close").

Now, going back to your list of accessories:

Vent fan high setting 32 Watts x 8 hours = 32W/12v * 8 hours = 2.66A * 8H = 21.33 Amp-Hours

LED rope lights 5 Watts x 5 hours = 5w/12v * 5H = 0.42A * 5H = 2 AH

Propane alarm 1/2 Watt x 24 hours = 1AH

Water pump 36 Watts x 1 hour = 3AH

Chest fridge 30 Watts x 6 hours = ??
Only planning on running your fridge 6 hours a day? Letting your food get warm the rest of time time? Figure 24 hours a day for the fridge, but the good news is that the wattage rating on a fridge is usually "peak", and doesn't factor in the duty cycle. To make your numbers more accurate, figure more like 15AH for 24 hour cycle (about 0.625 amps per hour is close to what I see on my Edgestar fridge)


= 42.33 AH so far.

See where I'm going with this? In order to figure the Amp-Hour capacity you need from a battery, you need to figure out the Amp-draw for each device and multiply by the expected usage time.

If you actually had 2x100AH batteries, you'd be safe running this load for more than 48 hours. If you actually KNEW you'd be driving an hour a day, this is probably more of an "indefinite" setup, even without solar.

Of course, actually FITTING two 100AH batteries into your van is another story.

 

[dwh]

Yea, what Herbie said.

Looking good so far. 200w of solar takes up a lot of rooftop real estate, so you might have a little trouble getting that much on top of a van depending on where the vent is. On my van, the vent is more or less in the center, which really screws with my options. I wouldn't be able to get 200w on the roof unless I moved the vent toward the rear.

Running the rough numbers on say a 135w panel...

135w / 12v = 11.25a
11.25a x 4h = 45ah per day

So that would be in the ballpark, but I didn't figure in the derating of the panels from being hot or any losses in the system. So the actual harvest would likely be less.

Driving one hour won't fully charge a battery, but it'll help a lot. Combined with say a 135w panel, you should be good to go.

 

[wrcsixeight]

I am always disappointed at my alternators contribution to my battery state of charge.

It does okay if the batteries are really low, but when they are within 25% of full charge, the amps needed to keep them at 14.5 volts are so low that the 130 watt solar panel is almost as fast.

But the batteries seem to like the 25 to 35 amps they each briefly receive from the alternator when I bring them into the alternator charging circuit loop.

2 group 27 batteries or larger will serve you well, just bring a charger and extension cord so that when you can plug in and top the batteries off, you can easily do so.

The alternator is not really good at bringing the batteries from 90 to 100%, unless it is a really long drive, and the batteries will greatly enjoy being brought back to 100% weekly, otherwise progressive sulfation will reduce their capacity.

 

[Basement Yeti]

Thanks guys, any suggestions for types of batteries or where to get them?

 

[wrcsixeight]

Depends on the lifespan you expect and treatment you plan to give them. If you are not going to worry about them and just recharge them whenever, a pair of wally world marine "deep cycles" is the way to go. Always a walmart around if they still offer a warranty.

If you want a good set of batteries that can really handle abuse( deep discharges, frequent undercharges) a pair 0f 6 volt golf cart batteries wired in series for 12 volts are a good idea. Sams and Costco sell some for a good price. These are good because they are a true deep cycle design. Most 12v car jars are not true deep cycle.

If you do not want to worry about offgassing( within reason) or watering the batteries occasionally, a pair of AGM Die hard platinums from sears is the way to go. But Universal battery make some cheaper AGMs.\

Google 'battery distributors San Diego' for the freshest batteries. You do not one that has been siting on a shelf. My Crowns were less than 2 weeks old from a joint in Escondido

 

[Captm]

I would replace your vent fan. I have 2-12v fans in my pop-up that along with 6 led bulbs use .9 amps per hour. This would be a big decrease in your amp hr. needs.

 

[desertrover]

Herbie's explaination of power usage is spot on.. However, I think that even driving for one hour per day, you will run your batteries flat in no time.

I run a very similar setup: the big Edgestar fridge, water pump, vent fan, interior lighting, and computer charging. My dedicated house batteries are a pair of walmart "maxx" group 29's. The pair of batteries was under $200 tax included and they have an 18 month free replacement policy with few questions asked. These batteries will run my entire house setup for over three days before hitting 50% state of charge by hydrometer this time of year, and a solid two days during the summer in the desert.

My house batteries are wired to the truck with a marine battery switch, and I normally start on the chassis batteries, then switch to charge the house batteries after a bit of driving. Driving three hours a day will not maintain my batteries.

Lead acid batteries have a varying coefficient of charging efficiency dependent on their state of charge. Without getting into too much math, charging from a low state of charge up to around 75-80% is very efficient. Most of the energy fed to the batteries goes to actually charging them, given appropriate voltage and c/rate. However, above that level, charge efficiency tapers off greatly. Once your state of charge is in the mid 90's, a battery may well be less than 50% efficient. An alternator, producing a nearly constant voltage, will also charge at a lower current the higher the battery voltage. Combine these two elements, and while a little bit of driving may take very deeply cycled batteries up to 75%, that much driving again will almost surely fail to take them to a full charge.

If your charging system is failing to attain a 100% state of charge, the underutilized plate surface will be subject to sulphation, greatly reducing battery life and further reducing charge efficiency.

This has been the most problematic part of my house electrical system. The vehicle's engine is great at bulk charging the house batteries, but performs poorly at the absorption and float phases of the charge. Right now, I have to use mains/generator power and a charger every few days to keep my batteries in top shape. Remember, lead acid batteries have no "memory" and like to be taken to a full charge as frequently as possible (just check your water levels).

Please excuse my repeating what others have said above, but I wanted to offer some anecdotal information about a very similar system. The bottom line is that you need to either run your engine many hours per day, or have another source of charging to bring your batteries up to 100%. I am planning on installing a 200 watt panel, which will give me enough margin to charge my batteries from 75 to 100% even without the bright days we always have out here. 200 watts would be be close but probably sufficient to run my entire house system indefinitely in the desert in the winter, and should perform well in the summer given 1 hour or more of driving per day.

 

[wrcsixeight]

It should also be said that without adequate thickness cabling leading to the distant house battery bank, bulk charging from 40 to 80 % will be severely limited by thin cabling. When I added 2 awg cabling directly from alternator (+) to my battery switch, the amps I saw when the batteries were low increased tremendously. It went from a quickly tapering 62 amp maximum at high rpm to a 107 amp at medium rpm that tapered much slower until the voltage hit 14.5.

But it had no effect when the batteries were above 80%.

10 awg wire leading to a distant battery bank will be inadequate for bulk charging low house batteries by a factor of ~4, IMO

 

[DKB2GO]

It should also be said that without adequate thickness cabling leading to the distant house battery bank, bulk charging from 40 to 80 % will be severely limited by thin cabling. When I added 2 awg cabling directly from alternator (+) to my battery switch, the amps I saw when the batteries were low increased tremendously. It went from a quickly tapering 62 amp maximum at high rpm to a 107 amp at medium rpm that tapered much slower until the voltage hit 14.5.

But it had no effect when the batteries were above 80%.

10 awg wire leading to a distant battery bank will be inadequate for bulk charging low house batteries by a factor of ~4, IMO

In doing research for the buildout of my own setup, this often-overlooked nugget is an easy way to maximize the system's output. Remember, the guy selling solar panels is concerned with selling you more panels.

You may want to take a look at this thread. I found it full of valuable information not offered elsewhere.

David

 

[desertrover]

As far as wiring a 200 watt or smaller solar panel, 10 gauge wire would be sufficient for the panel. your charge current to a 12v battery would be less than 15 amps in direct overhead sunlight. With a run of less than 8 feet, you would only have about 2% loss in the wire.

For an alternator hookup, you may be able to put near 80 amps towards your house batteries in some circumstances. Figuring for an 8' run, you will loose over 10% at 80 amps, generating in the ballpark of 100 watts of heat in the wire. In practice, this would not happen because the voltage available at the battery would be significantly lower, causing a lower charging current. Going to 2 gauge wire would put you back around 2% loss for the same run.

You might save some long hand by playing around with one of the several online calculators, like this one.

How are you planning on switching which batteries will receive charge from the vehicle? Will you wire both camper batteries in parallel and use some type of marine switch? I'd be a bit wary of putting two lead acid batteries in parallel permanently if they aren't identical in make, model, date, and history. I would recommend wiring your camper batteries to the chassis with a minimum of 4ga, with some way of switching or isolating your starting battery. This can be as simple as a kill key style disconnect somewhere inline with the wire from your chassis to house batteries if you are sure to mind it. Just keep the runs as short as possible.. Saves wire, money, and electricity!