Promising initial Off-grid AC test

[BradKW]

For the first time today, I ran my AC unit off battery only for an extended period of 3 hours, where compressor ran 100% of time (no cycling) and from battery only (disconnected solar input).

I have 4 Trojan L16 batteries (6 volt) wired in series for a 24v bank. The most common measurement of a deep cycle battery's capacity is Amp Hours (Ah), and we use the 20 hour discharge column when talking about Ah. For example, a 200 Ah battery gets that rating from a tested 20 hour discharge cycle...the number changes if the discharge cycle is less or more time, but that's not really important for discussion purposes.

But Amp hours are really not an accurate measure of capacity for anything other than a 12v battery or battery bank...it's more a descriptive artifact that just happens to work for the most commonly used voltage, 12v. A more accurate measure is Watt Hours... that same 12v 200 Ah battery is really 2400 Watt Hours (Wh). This becomes more useful when talking about non-12v systems, such as my 24v battery bank. Having grown used to talking about Ah, I'm not 100% confident with my math having to convert to Watt hours.

A single 6v L16 battery is rated at 370 Ah. Four together is 24v and still 370 Ah. That should equate to a battery bank of 8,880 Watt hours (370 x 24). Divide by 2 to find what is usable without dropping below 50% SOC (state of charge) and we get 4,440 Watt hours usable. So if my AC draws 430 watts (which it does on high), then in theory I should get nearly 10.5 hours of operation before hitting 50% SOC. That doesn't take into account inverter inefficiencies or Peukert effect. 

So this is what I got from my 3 hour test today:

Starting Specific Gravity (SG) after 5 days rest, no charging: 1.280 ...which is 100% full/charged.  Voltage reads 25.6v, also indicating 100% charged.

Ran air conditioner with doors open so it had no chance to cycle off, and wattage varied between 390 and 430 according to KillaWatt meter. dunno why the variation, but after first hour it stayed pegged closer to 430 Watts.  After 3 hours, I shut the system down and took readings after 15 min:

SG: 1.260 and 25.2 volts.  Both readings would indicate an SOC of 90% remaining capacity. So -10% for 3 hours operation. So 9 hours operation = -30% ? Dunno yet.

Also had a couple cells read at 1.255 best I can tell...not sure why variation. 

Overall though, I think that the results are promising and feel encouraged.

 

[sushidog]

Let's do the math. I assume that the 430 watts was on the A/C side of the inverter. Assuming you have an inverter with a typical efficiency of 85% 17.91 amps (at 24 volts) / .85 = 21 amps draw on your 24v 370ah bank. Let's not consider Peukert (to simplify the math because the discharge rate is close to the 20 hr battery rating) 370/21 = 17.6 hrs run time x .5 = 8.8 hrs to 50% battery discharge. The question is how do you plan on replacing the battery's charge? Solar, alternator, generator, land line or a combination thereof?

Chip

 

[BradKW]

The question is how do you plan on replacing the battery's charge? Solar, alternator, generator, land line or a combination thereof?

Chip

For the moment, the only charging source is 1000 watts of solar which performs well. I've never yet seen it actually put out 1k watts, but during peak hours it has no trouble hitting 800+.

At 4pm I the solar still pulls 535 watts, so it can run AC and still add to battery charge. I've been delaying getting a dedicated 24v charger for a few reasons...further testing will determine immediacy of needs. Fair to say, in Key West I'll be pulling significant solar well into evening hours during summer I'd expect...

 

[jimindenver]

Sounds good Brad. For the better part of the day you should be able to run it straight off the solar.

 

[BradKW]

SG still reading 1.260 this morning, which is good because I wasn't sure how battery activity effects the reading. I have found that voltage is very inaccurate even up to 72+ hours after charging. Don't know if simply depleting battery without charging also messes up voltage accuracy.

Also, the SG variation between cells seems to have evened out overnight...

 

[Queen]

Your post could be written in Swahili for all I understood of it, but I think your solar set up is letting you run your AC, so yay!!

 

[mariasman]

Thanks for sharing the results. What kind of inverter are you using?

I have zero doubts about the viability of providing AC with an off grid solar system. It just has to be properly configured and properly operated.

 

[mariasman]

Double post.

 

[SternWake]

Must feel good to have set so much of this up having known so little about it all, how long ago?



Voltage should always be taken with a large grain of salt.  If you were to monitor voltage closely during discharge over many cycles then you could get a better idea of how it relates to state of charge, but most people think voltage is like a fuel tank gauge, and the only way that analogy works is if the fuel tank is suspended on rubber bands in a windstorm.

Also the first few deep cycles usually exhibit different behavior.  It is said the batteries actually increase capacity for the first few cycles, assuming full recharges of course.

Specific gravity also has a little bit of lag, so I've read.  I've honestly never checked it during discharging and have no words of wisdom on its behavior during discharge.

On charging many people told me that Spcific graivty would rise slightly after resting, but I never observed that on the battery whose cells i dipped often.

The more voltage readings and Hydrometer readings you take the more you will come to know how much pressure your batteries can maintain when deflating.  Knowing the deflation rate( amps) is also pretty key to this learning of 'trends and tendencies' of batteries under load and when being charged.  A battery temperature reading can also lower  curious eyebrow when former observed 'trends and tendencies' are outside one's expectations.

The sheer thermal mass of your L-16s will steady this, but a high amp charging source used on a hot morning and a 100% full charge attained by afternoon, could have voltage readings seem abnormally high overnight.

 

[BradKW]

Thanks for sharing the results. What kind of inverter are you using?

Go Power! GP-SW3000-24 3000-Watt Pure Sine Wave Inverter


[img=325x250]https://images-na.ssl-images-amazon.com/images/I/81N9nfWzQYL._SL1500_.jpg[/img]

 

[jimindenver]

Ran air conditioner with doors open so it had no chance to cycle off, and wattage varied between 390 and 430 according to KillaWatt meter. dunno why the variation, but after first hour it stayed pegged closer to 430 Watts. After 3 hours, I shut the system down and took readings after 15 min:

As the head pressure rises, so does the watts needed to run it.

 

[RVTravel]

It'll work with the appropriately massive solar/battery bank.

 

[BradKW]

It'll work with the appropriately massive solar/battery bank.

The question is more how big does it have to be...because there's an amazing lack of real-world example or information about off grid AC.

The system I have is a result of doing calculations for the minimum necessary to power AC for 8 hours off grid, and recover the charge from solar in one day. The size of my bank + panels is easily doable for any Class C, A, TTs, 5th wheels, and some B's ...and of course any DIY truck conversion. So...not massive, just big  

 

[jimindenver]

I have been able to run the 5000 BTU A/C off of the 750w flat mounted system without losing battery voltage during peak hours in the summer. I have been using the 435w tilted panel to allow me to get it going earlier and keep it running later as well as create enough power through out the day combined with the roof to compensate for having to run it after dark for a hour or so. It's currently 100 F outside and 84 F inside the trailer. I don't use a lot of power otherwise, just charge the computer, hotspot, phone and the LED lighting. The important thing is keeping the trailer cool for Max so I haven't been running the hot water heater or hotplate.

Your system should easily be able to run the A/C like my roof system, even better actually but having the truck panel tilt and track through out the day does more than make up for the differences in roof system wattage.

 

[sushidog]

That's a sweet pure sine inverter for the price. I am planing on building a 48v system, (because I plan on using a 48v DC mini-split air conditioner.) This is the one I will use http://www.geinnovations.net/HSAC_Productline.html The HSAC-12H/C is a super efficient heat pump that only consumes 560 watts to produce 12,000 btus of air conditioning, with no inverter loss. I'll still need an inverter for my other a/c needs, but there are not many 48V inverters to choose from. I really like the Outback VFX3648M 3,600 watt pure sine inverter charger (but its about $1,800) so I will probably settle for an AIMs 2,000 watt model because of price, (only $628) though it only comes with a 20 amp charger vs the Outback's 45 amp charger.

Chip

 

[amwbox]

Awesome results...but what model of AC unit are you using? 430 watts to me seems miraculously low. My little 5000 BTU window unit draws almost twice that after its been running a while...

 

[mariasman]

Awesome results...but what model of AC unit are you using? 430 watts to me seems miraculously low. My little 5000 BTU window unit draws almost twice that after its been running a while...

450-500 watts electrical is expected for a 5000 BTU/hr window a/c unit. I suggest double checking your unit.

 

[jimindenver]

The newer Frigidaire energy star 5000 BTU is rated for 410w.

 

[BradKW]

It is definitely on the low side. I purchased the Frigidaire 5,000 BTU based upon Jim's experience with it...

 

[blars]

I'll still need an inverter for my other a/c needs, but there are not many 48V inverters to choose from. I really like the Outback VFX3648M 3,600 watt pure sine inverter charger (but its about $1,800) so I will probably settle for an AIMs 2,000 watt model because of price, (only $628) though it only comes with a 20 amp charger vs the Outback's 45 amp charger.

One advantage of ordering factory direct like I did is customizability, 48v input to the inverter was an option, not even extra cost like 72v or 110v IIRC. I paid less than $200 for a 2kw pure sine inverter on aliexpress. I would prefer better output voltage regulation, but I am happy with it so far.