A little battery/solar experiment at the rally

[Overland]

Not much going on at the rally today so I thought I’d unplug the trailer and do a little experiment running the AC just to see how long the batteries would last.  We don’t travel much in AC weather so this is really the first time I’ve had a good chance to test the limits of our system.

 

I started at 10am with 100% charge. AC set at 72 and we’re in full sun with no clouds.  640 watts of solar and 400 amp hours of battery storage.  I’ll plug back in when the batteries hit 20%.  Any guesses as to when that will be?

 

 

[John E Davies]

What were the 10:00 AM outside and cabin air temperatures? Are your shades down and awning(s) deployed? Which air conditioner do you have? Lithium batteries? Without that info we are simply guessing. I guess 3:17 PM Eastern.

 

Are you giving a prize for the closest guess? I will take your Xtreme jack.... I’ll pay the shipping. ?

 

Please take and post some pics of the rally for those of us stuck at home.

 

John Davies

 

Spokane WA

[Overland]

I closed the rear shade partially to block some direct sun but otherwise no. But the awnings are out. It was about 78 to start and 81 now. Dometic AC and yes lithium.

 

We’re getting 500w out of the solar at noon and the AC pulls 1500w when it’s on.

 

You cant have my jack. Probably the most valuable thing I have with me that I’m willling to part with is a chocolate cookie.

[John E Davies]

You cant have my jack. Probably the most valuable thing I have with me that I’m willling to part with is a chocolate cookie.

Rats.... OK, I would take the cookie instead.

 

Do you have good enough Internet speed to upload some pics?

 

John Davies

 

Spokane WA

 

 

[Overland]

Sure, I’ll take a few and post them in the rally thread.

 

Definitely not going to make it to 3pm. Probably not even 2. I’m at 26% now and just got a low battery warning from the inverter. There must have been a brief dip in voltage that set off the alarm because after shutting down it came back on after a minute or so.

[bugeyedriver]

Nice objective test, Overland.   So, with 100% battery power to begin with, it appears you were able to achieve about 2.5 to 3.0 hours of inverter powered A/C usage given your conditions at the rally, right?  What was the lowest DC voltage level you allowed your system to see?

[Overland]

Between 3 and 3.5 hours. Apparently the limit is the low voltage limit on the inverter, which is set at 11.5 volts, per Battleborn’s instructions. The voltage was starting to dip into that range when the AC compressor was running. But then it would pop back up to 12.3 or so after the inverter switched off, and so after a minute the inverter would come back on. I let it cycle like that a couple of times but then plugged back in.

 

The battery monitor never went below 25% but then it may not be well calibrated since I’ve only had the batteries that low I think once before. It’s supposed to learn with usage so I’m curious if it would show a lower percentage if I were to try this again.

[Overland]

Personally I was expecting between 4 and 5 hours, but the AC was just working too hard. Maybe had I set it at something like 76 then it might have cycled on less and given me more time. But it pulls 1500W when running and I was getting 500W from my solar (rated 640), so to break even it would have to run less than 20 minutes per hour, whereas it was probably running more like 45 minutes per hour.

 

I can’t get the math to jive though. 400 amp hours of battery capacity should give me 4800 watt hours. And if I can use 75% of that, then I should be able to use 3600 watt hours before the inverter shuts down. The AC was pulling 1000 more watts than I was gaining from solar, but was on only ~75% of the time, so I should have only been using 750 watts per hour. 3600/750 should have given me 4.8 hours. Granted, some power was going elsewhere but not that much, relatively speaking. So either there are errors in the numbers shown on monitor or I’m getting less out of my batteries than advertised, or a combination.

[John E Davies]

 I can’t get the math to jive though. .... So either there are errors in the numbers shown on monitor or I’m getting less out of my batteries than advertised, or a combination.

Calculations on paper never equate to the real world. There are a lot of hard-to-quantify errors and losses involved in converting sun to battery to inverter to air conditioner to comfortable occupants. Many steps, many sets of losses - battery condition, monitor error, compressor start-up surges, resistance losses through wires and connections .... no way you can get what you think you should get. Sometimes close has to be good enough.

 

Thanks for doing the experiment. Your results are impressive anyway. I will take my cookie now.

 

John Davies

 

Spokane WA

[Overland]

@rideadeuce reminded me of this and I wanted to get back with why my numbers were off.  The A/C actually pulls 1731 watts, per Dometic, which would give me 3600/[(1731-500) x .75] = 3.8 hours.  Close enough.  It doesn't surprise me that the wattage number that I was seeing was off.  I've talked with Victron before and they admit that their inverter readings are just estimates.  Next time I'll pay more attention to what the battery monitor is saying vs trust the inverter, as it should be more accurate.

[nrvale0]

I was under the impression that you were not supposed to run the AC off of the batteries because they wires aren''t quite spec'ed for that sort of a load?

[Overland]

Normally, yes, but I replaced my stock batteries with LFPs, which can handle large loads.  

[HMD1056]

Overland, did you add the solar panels or did Oliver?  And your lithium batteries?  Were they diy or professionally done?  Thank you!

[Overland]

1 hour ago, HMD1056 said:

Overland, did you add the solar panels or did Oliver?  And your lithium batteries?  Were they diy or professionally done?  Thank you!

Oliver did the panels. The batteries I did myself, professionally. 😛

[BackofBeyond]

11 hours ago, Overland said:

Oliver did the panels. The batteries I did myself, professionally. 😛

Nice test, puts the solar in perspective- need more panels and perhaps a few more LFP's, and a nice quiet, high efficient AC.  For me its gonna be elevation for the cooling, and perhaps one day a small genset, if I get serious about summer in the southern sun.

[mjrendon]

Great test Overland!

I remember seeing an image of your inverter and I believe that it was a 3kw Victron and that you are using 1/0 wiring.  Any thoughts on the stock 2kw inverter that Oliver sells running only the AC with micro air easy start and 4/0 wiring?  Hoping that I can keep some of the original parts from my 2017.

Any update on the potential Li battery issues?

[Overland]

4/0 should be fine, unless you mean 4 awg which would probably be too small. I actually use pairs of 1/0 which is what Victron recommends for the Multi 3000.  That’s equivalent to a single 4/0 in cross section, but is supposed to stay cooler under load, and also just easier to work with.

Interestingly though, while talking to Battleborn earlier this week, they told me that they felt Victron’s recommendation was overkill (agreed), and that they’ve seen it actually cause problems on occasion with their batteries. The reason is that there’s a very brief outrush of current when the inverter is turned on, as the batteries charge the inverter’s capacitor; and apparently 4/0 cable offers so little resistance that the batteries can see that outrush as a short circuit and trip their protection circuit. I’ve never had that problem, but they recommended that I remove one of the 1/0 cables to prevent it from happening.