Installing a Battery system

[TerryB89]

Who's on first ? I ordered a Teardrop called a New Wave out of GA. I think all it has is a power strip with the plug on the outside to plug into shore power. I just wanted to know how to hook up batteries to use if I camp where there is no shore power. I plan to buy a nice battery and an inverter and just plug it into the shore power plug and use it as long as it lasts...

[MtnDon]

Let's go back to the beginning and start over....
Start with making a list of the things you want to operate.
Two lists; 1 = 12 VDC and 2 = 120 VAC.
Maybe sub divide that into A = the "must haves" and B = "would like"
Record the watts or the amps each require.
Record the number of hours or fractions thereof each would likely be used in a 24 hour day.
Record the number of days you want to be able to run these things off the battery.

Maybe sub groups as to things that you'd only use if plugged into grid power at a campground (or using a generator) and those things you'd want to run in the boonies where there are no plug ins.

This is necessary to figure how much battery capacity you need and what size inverter you need. (This is where a good quality inverter / charger could be nice as the chargers that are an integral part of a quality inverter are better than most other battery charging systems. Also a lot more expensive.)

[TerryB89]

on the rare occasion I might camp with batteries, I would pretty much only need juice to run my C-pap over night. We would not need the tv/dvd or much lights.

[MtnDon]

OK, a life enhancing device. Important device. You should either (a) confirm the specs on the unit... the watts or amps. Then multiply by the number of hours of use to get the amp-hours or watt-hours consumed. OR (b) buy or borrow a Kill-A-Watt meter and measure the actual use. Only then the battery size and the inverter size can be calculated with reasonable accuracy. If you guess you might get lucky and hit the size bang-on, or miss and either spend too much for too much capacity or find yourself out of battery before the camping trip is over.

I don't know how CPAP machines work. If they emply a motor then a pure sine wave inverter may be best. Mod sine wave inverters might work. If they do they are not as efficient electrically and may cause any motor to make more noise. Might be a factor to consider.

[TerryB89]

Thank you !

[troubleScottie]

Stolen from a CPAP community forum:

ResMed S9 AutoSet with heated humidifier. No heated hose.

Auto 18-20, EPR=3, humidifier=4. Fairly humid night. Room temp around 75F.

Kill a Watt power meter measuring AC power into the power brick.

4:32 hours.
0.15 Kilowatt hours
Average power 34 watts.

Looking at the instantaneous power readings as I inhale and exhale and the heater runs, power varies all over the place. Between 15 and 90 watts read on the meter.

OR: 34 watts / hour of usage

Converting to 12V means 2.8 Amp draw per hour ( Amp * V = Watts or Amp = Watts/Volt = 34/12 ) at 100% efficiency






Inverter efficiency: Taken from solar-facts.com:

By efficiency, we are really saying, what percentage of the power that goes into the inverter comes out as usable AC current (nothing is ever 100% efficient, there will always be some losses in the system). This efficiency figure will vary according to how much power is being used at the time, with the efficiency generally being greater when more power is used.
Efficiency may vary from something just over 50% when a trickle of power is being used, to something over 90% when the output is approaching the inverters rated output. An inverter will use some power from your batteries even when you are not drawing any AC power from it. This results in the low efficiencies at low power levels.


A 3Kw inverter may typically draw around 20 watts from your batteries when no AC current is being used. It would then follow that if you are using 20 watts of AC power, the inverter will be drawing 40 watts from the batteries and the efficiency will only be 50%.
A small 200W inverter may on the other hand only draw 25 watts from the battery to give an AC outpur of 20 watts, resulting in an efficiency of 80%.

Larger inverters will generally have a facility that could be named a "Sleep Mode" to increase overall efficiency. This involves a sensor within the inverter sensing if AC power is required. If not, it will effectively switch the inverter off, continuing to sense if power is required. This can usually be adjusted to ensure that simply switching a small light on is sufficient to "turn the inverter on".
This does of course mean that appliances cannot be left in "stand-by" mode, and it may be found that some appliances with timers (eg washing machine) reach a point in their cycle where they do not draw enough power to keep the inverter "switched on", unless something else, eg a light, is on at the same time.

Another important factor involves the wave form and inductive loads (ie an appliance where an electrical coil is involved, which will include anything with a motor). Any waveform that is not a true sine wave (ie is a square, or modified square wave) will be less efficient when powering inductive loads - the appliance may use 20% more power than it would if using a pure sine wave. Together with reducing efficiency, this extra power usage may damage, or shorten the life of the appliance, due to overheating.

So at 50% efficiency == 5.6 Amp / hour.

A 100 A Hour battery should only be run down to 50% of its rating so 50 A hours

So you loss 50% to the inverter and 50% to the capacity of the battery.

So about 9 hours before a full recharge is needed. And remember, no other power usages on the battery.
With no drop in the inverter you are looking 18 hour usage.

[MtnDon]

The best small inverter there is; virtually no idle losses (55mA) and consumes only 450 mA while powering loads.

Pure sine wave power. 300 watts, so lights, TV can be run too.

User manual

One place to buy

and another

[tony.latham]

The second teardrop I built was for a CPAP user. Her's is a Resmed. It's a 24v device with an inline inverter in the cord so that you can plug it into the wall. Resmed makes a 12v cord for it. That's the obvious way to go for a 12V system.

I don't think she uses the humidifier portion while camping. The amp draw depends a lot on the settings, but she does just fine a a group 24 marine battery.

Tony