Semi-mobile garage

Re: Semi-mobile garage

Welcome to the forum,

We can look at designing the system two ways... First is to define your loads, peak watts and average watts*hours per day of operation.

The other is to take a limit (like batteries or solar panels) and design the optimum system around it.

For the most, a small system will not usually be expected to supply a lot of power (maybe 200-500 watts for upwards of 10 hours of battery, or maybe 800-1,00 Watt*Hours per day based on ~400 watts of panels).

Batteries are heavy and need to be replaced every 3-8 years or so in that type of application. So keeping the number/size of batteries on the "small side" is usually a good idea (cost/weight). However, if you have periods of bad weather, a larger bank you can recharge (genset or truck) is sometimes worth it.

Also, do you have a season/area that you will be using your system (winter in Seattle or summer in New Mexico).

There are "choices" that can be made to give you options (like using more expensive AGM batteries to run a good sized electric impact gun with high surge current).

A good place to start is a) conservation and b) measure your loads with a Kill-a-Watt meter (for AC loads) or similar.

It is pretty easy for us to throw a system or two at you based on what you have given us so far, but I would like to know a bit more about your loads so that we can start down a path to a system that will better meet your needs/expectations.

-Bill

Re: Semi-mobile garage

OK, had to come back later--Last time my cookies got messed up and would not let me post.

For now, lets design a 4x 6 volt @ 220 AH "golf cart" battery system. Cheap, pretty reliable, and if you mess something up or want to change out for another battery bank, you will not feel too bad.

Using various rules of thumbs to get you a quick basic design...

First, how much "usable" power would a 4x golf cart battery bank give you... Note, two 12 volt strings or one 24 volt string will give you the same amount of power (power=voltage*amperage). But, for higher power (roughly, over 1,200 watts, you should step up to 24 volt bank to keep the wire sizes workable--I.e., not much larger than 100 amps of current suggested).

So, for a flooded cell battery bank, a couple "magic numbers". First, the maximum continuous current should be ~C/8 (run battery dry in 8 hours with X current). The other is the maximum surge current (power) that a battery bank can output before its output voltage will collapse is ~C/2.5 -- So:

• 4x 6 volt * 220 AH * 0.85 inverter eff * 1/8 rate of discharge = 561 watts max continuous AC power recommended
• 4x 6 volt * 220 AH * 0.85 inverter eff * 1/2.5 rate of discharge = 1,795 watts max surge AC power recommended

So, for this battery bank, the maximum inverter that should be supported is in the ~500-900 watt range (assuming the inverter can surge 2x rated power).

Note, a "220 AH" battery rating is (typically) done at 20 hour discharge rate. If you discharge a battery faster, it is less efficient and will have less "apparent" capacity at C/8 (for example) discharge rate.

For solar panels on a typical battery bank, we would look at ~5% to 13% rate of charge for good battery chemistry and relatively quick recharge of a battery bank. Note that in solar, +/- 10% is usually "good enough"--I am carrying 3-4 digits so you can reproduce my math (and avoiding roundoff error in longer calculations):

• 4 Batteries * 6 volt * 220 AH * 1/0.77 panel+charger derating * 0.05 rate of charge = 343 watts minimum array
• 4 Batteries * 6 volt * 220 AH * 1/0.77 panel+charger derating * 0.10 rate of charge = 685 watts nominal array
• 4 Batteries * 6 volt * 220 AH * 1/0.77 panel+charger derating * 0.13 rate of charge = 891 watts maximum "cost effective" array

And what would a XXX solar array produce. Using PV Watts, for Salt Lake city with a "flat to roof array", we get:

1      2.00     
2      2.96     
3      4.05     
4      5.29     
5      6.84     
6      7.32     
7      7.47     
8      6.72     
9      5.24
10      3.78     
11      2.35     
12      1.64     
Year      4.65      

Assume you get a minimum (March through September--although, if you tilted the array you could get "more sun") of 4.05 hours of "noontime" sun per day. In terms of usable AC power, a minimum of:

• 4.05 hours of sun * 0.52 system derating * 343 Watt Array = 722 Watt*Hours per day
• 4.05 hours of sun * 0.52 system derating * 685 Watt Array = 1,443 Watt*Hours per day
• 4.05 hours of sun * 0.52 system derating * 891 Watt Array = 1,876 Watt*Hours per day

So, does 1,443 Watt*Hours per day for March (average) of 120 VAC power make sense to you?

Personally, I would suggest you stay with a 12 volt battery bank (at least for now--compatible with your 12 volt tools). Also, I would suggest your first inverter should be a MorningStar 300 watt (600 watt for 10 minutes) 12 volt TSW (true sine wave) inverter. If you need more power for drills, etc... I would suggest you get a cheap MSW type inverter (again, much more than 1,200 watts is overkill for this size system).

And, since this is going to be a highly variable power type situation, I would suggest you get a Battery Monitor. It will display in XX% how much power you have left in your battery bank. Normally, I would suggest not discharging below 50% before recharging (or reducing power use), but since this is a portable setup and you only use it (mostly) for weekends, you could drain down towards 20% state of charge when needed, such as a cloudy day (less battery life, but they will probably "age out" rather than "wear out" from heavier discharge--if you quickly recharge the bank).

If you wish to read about the "optimum" charging setup for such a battery bank, this thread is really nice:

Question about battery charger selection with EU2000 generator

Charging from the truck--Sort of difficult to do a 12 volt DC run from the alternator to the trailer battery bank. OK for "low current" charging (maybe 10 amps maximum with reasonable sized wire). But, to charge faster, you would need a inverter on the truck side and a 120 VAC to 12 VDC battery charger on the trailer side. Sending "battery voltage" between an alternator and trailer usually just ends up with too much voltage drop for fast charging (and deep cycle batteries really want higher charge voltage than Starter/Ignition batteries--14.5-14.8 volts vs 13.8-14.2 typical for car/truck batteries). Some 12 volt car electronics (and 12 volt adapters for computers and such) will stop working when the deep cycle batteries are charged at 14.5 to 15.5 volts (equalization)--so you don't want your vehicle alternator to crank up the voltage that high either.

Questions?

-Bill

Re: Semi-mobile garage

You have a life too? :roll:

Yes, the basics you have defined should work fine. Note--Ensure that you can get those Evergreen panels as they are no longer made and there are few high wattage panels (>100 watts) that have Vmp in the 17.5-18.5 volt range.

If you end up getting different panels with higher Vmp (>20 volts or so), you may have to go with a more expensive MPPT type charge controller.

Also, for basic 120 VAC power--such as charging battery packs, radios, hand tools, etc... A TSW inverter is just much easier on may of the smaller appliances. MSW are just (more or less) square waves and can cause some wall mount transformers, small electronics, induction motors to overheat. Also, some audio/visual equipment will have a 120 Hz buzz and interference on video with MSW supplies.

All About Inverters
Choosing an inverter for water pumping

For reliable power, I would still suggest a smaller TSW inverter to supply your "critical" loads and get a larger MSW to supply power to your corded tools--as needed. The reason I really suggest the MorningStar 12 volt TSW 300 watt inverter (PDF spec sheet) is because it can be configured for automatic on/off. If there is less than ~6 watts of AC power required, it will "go to search mode/sleep" and only draw ~55milliamps (0.055 amps) when not powering any loads. Most AC inverters will draw ~6-20 watts when powered on and no loads--As you can guess, more than a few people have killed their battery banks when they forget to turn off their inverters when the loads have been removed.

-Bill

Re: Semi-mobile garage

One panel--Would be better for storage than really using the single panel to generate significant power for daily/weekend use. Back to the rough guide:

• 4 Batteries * 6 volt * 220 AH * 1/0.77 panel+charger derating * 0.05 rate of charge = 343 watts minimum array
• 4 Batteries * 6 volt * 220 AH * 1/0.77 panel+charger derating * 0.10 rate of charge = 685 watts nominal array
• 4 Batteries * 6 volt * 220 AH * 1/0.77 panel+charger derating * 0.13 rate of charge = 891 watts maximum "cost effective" array

And then there is the amount of power that would be generated from your array (assuming you don't use a genset/shore power):

• 4.05 hours of sun * 0.52 system derating * 343 Watt Array = 722 Watt*Hours per day
• 4.05 hours of sun * 0.52 system derating * 685 Watt Array = 1,443 Watt*Hours per day
• 4.05 hours of sun * 0.52 system derating * 891 Watt Array = 1,876 Watt*Hours per day

It really depends on what your expect from your battery bank / solar panels...

If the system is really intended to be solar powered for much of the time--then you are really looking at a minimum of two panels.

If the system will use the solar panel mostly for when the trailer is parked in storage--Even a smaller panel will work too (down to 100 watts to float that battery bank).

-Bill