solar with 1200 watts inverter

[Tom55555]

The main issue I have with solar on RV's is you have to park it in the sun, which also heats up the RV pretty effectively.  Can be an advantage if it's cold out but if it's summer you may end up running the genset & A/C anyway.  Or what happens to me more often than not camping in the mountains, 100' ponderosas are pretty effective at shading everything at the campsite all day.  So solar isn't a slam dunk for energy independence, it depends a lot on where you park.

Petrolium works all the time. LOL

 

[Ernie n Tara]

Just a random comment: I seems from the charts that the most use of battery life may be in the range above 50% DOD. The changes in the range of 0 to 50% reduce battery life significantly for small changes in DOD while the curve flattens out beyond 50%. In other words the loss of life is high for small changes above 50% and much lower for comparable changes below 50%.

I do understand the reduction in voltage (Watts) has an offsetting effect, but that is linear while cycles change at a different and much lower rate. In the real world it becomes a matter of pay me now (more batteries), or pay me later (less $$ and weight; higher risk). All of these factors are calculable.

Ernie

 

[Ex-Calif]

My example is with the assumption one who cycles their batteries 50 times a year.  1200 cycles is 40 months, every single day.  If you are actually doing that, and interested in maximum calendar life (as many fixed installations may do) then that's the sweet spot.  Who in the RV community cycle their batteries every single day for three and a half years straight, and to 50% never more, never less, every single day?  My guess is zero.

> the 30% line is more like 2,400 cycles or 6 1/2 years.

That's a bit over the edge of useful calendar life.  Not impossible I suppose but again we're talking daily cycles for 6+ years.  Something perhaps someone off the grid might do but not someone in an RV.  If you're not cycling daily to at least 30% then you are assuring you're throwing capacity away.

In order to make an apples to apples comparison you assume daily cycles - More apt would be "cost per discharge cycle" - The physics and data is clear the deeper you discharge the batteries the shorter life they will have.

As I said, I personally know dozens of folks who get 10 years out of trojans in an off grid liveaboard situation. i.e. They live off solar.

Not many RV folks need 400W/400ah. Most RVers are plugged in 90% of the time. I would be dumb to carry around $1600 worth of batteries for the 2 times a year on boondocks. Cheaper probably to run the generator for those times.

It is useful to know one's average energy budget so one knows what to expect when one does unplug.

 

[Mark_K5LXP]

More apt would be "cost per discharge cycle"

Cost per delivered Ah would cover it, since the delivered Ah doesn't move that much over the entire range of DOD.

deeper you discharge the batteries the shorter life they will have.

"Wear" is probably a more accurate term than "life".  You don't get less "life", it's just the rate of usage that changes.  Same goes for tires.  And tires cost a lot more than batteries.  But you never hear anyone say don't drive too fast or too far, you'll wear out your tires.

I personally know dozens of folks who get 10 years out of trojans

It would be interesting to see how they test, since it's tough to mitigate partial state of charge damage with solar.  The Trojan RE line is pretty durable, I don't have any experience with those though.  They cost more than the motive models so it's all about design tradeoffs.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

You don't get less "life", it's just the rate of usage that changes.

You actually do get less life, or more battery wear, if you prefer that term.  That's what Peukert's Law is about.  The total available amp-hours and number of cycles in a lead-acid battery does indeed vary with the rate and depth of discharge.
Tires actually have a similar phenomena, though it's not as pronounced as with batteries.  There is a small difference in total life miles (treadwear) at different speeds. It's largely the effect of heat build-up on the rubber compound

 

[Mark_K5LXP]

I would equate batteries under greater load to tires under greater load.  So it's no surprise greater loads would affect service life in either case.  You pose an interesting point though about peukert, would be interesting to compare peukert constant to lifetime Ah at different rates than just 20 hour.  Looks like I have a new spreadsheet to make.

Mark B.
Albuquerque, NM

 

[Ex-Calif]

The problem with the tire analogy is you can't/don't replenish tires.

It's like, "If I drive 60mph and the tire restores 5% of the tread worn magically overnight vs. 70mph and it restores 2% of the tread."

It's an apples and oranges comparison.

 

[AStravelers]

So we just ignore the difficulty in actually using a lead acid battery that has been discharged to 20% full (about 11.6V in a no load situation).  Yeah, you can use a battery until the lights start to dim, as long as all you want to do is use the lights.  Trying to actually live in an RV with the battery that low is another issue.

Getting down that low and you run a good chance your furnace won't run because there isn't enough voltage to run the fan fast enough for the furnace to detect the air flow and it won't turn on the gas flow. No fun at 3am, or 7am when you want to warm up the RV.

Then you are in the power range that on the cold morning and you want to start the generator to charge your battery, oops, can't start the generator.

Inverter won't power your 120V devices because as soon as they try to use the 120V the amp pull from the inverter drops the battery voltage so far the inverter cuts out because of low voltage.  Depending on what you are trying to power with your inverter the low voltage problem could start happening at 40% full or maybe higher.

Then as I wrote much earlier, there is the problem of trying to get that battery from 20% to 80%-95% full so you can get through your next days power usage requirements.  That doesn't happen with just an hour or two of generator run time.

This is the practical side of trying to use a battery at below 40% or 50% charge. 

 

[solarman]

I would equate batteries under greater load to tires under greater load.  So it's no surprise greater loads would affect service life in either case.  You pose an interesting point though about peukert, would be interesting to compare peukert constant to lifetime Ah at different rates than just 20 hour.  Looks like I have a new spreadsheet to make.

Mark B.
Albuquerque, NM

you will find the Peukert exponent increases with battery age, things just get worse with sulfation effects and then you finally end up with a 100% performing boat anchor.  LOL

 

[Mark_K5LXP]

The previous two posts are very true.  It's a reason to maintain a given SOC based on your operating requirement but not for battery longevity (a position I have stated too).  They're also a telling example of proper system design/battery selection.  Batteries that are overloaded to the point of not operating equipment as desired isn't a battery fault, it's a design fault.  Yep, once batteries get long in the tooth it gets harder and harder to get them through an absorb phase and fade quickly under moderate loads.  But by the time they reach this point capacity is well on it's way out and will fail a capacity test.

Mark B.
Albuquerque, NM

 

[AStravelers]

The previous two posts are very true.  It's a reason to maintain a given SOC based on your operating requirement but not for battery longevity (a position I have stated too).  They're also a telling example of proper system design/battery selection.  Batteries that are overloaded to the point of not operating equipment as desired isn't a battery fault, it's a design fault.  Yep, once batteries get long in the tooth it gets harder and harder to get them through an absorb phase and fade quickly under moderate loads.  But by the time they reach this point capacity is well on it's way out and will fail a capacity test.

Mark B.
Albuquerque, NM

Mark, I'm confused.  In most of your posts in this thread you have been advocating to ignore the "rule of thumb" of only taking the battery down to 50% full and recommending using the battery down to 10% to 20% full, or even 0% full. 

Now you seem to be stating that using the battery down to 20% full or even 0% full is not good.

 

[Ex-Calif]

Changing tack a little bit...

The title of the thread is using a 1200W inverter with solar. If one fully utilizes a 1200W inverter that's in the neighborhood of 100 amps. With 200-250 amps of battery for example the discharge rate is going to be extreme to say the least. And discharge rates are a huge factor in heat and sulfidation.

100 amps is in the neighborhood of fixed home solar installations.

 

[Gary RV_Wizard]

Can't get 100A from a 200-230AH lead-acid battery bank without dropping the voltage below the inverter LBCO threshold. Maybe with lithium batteries, though.  A 170 watt panel isn't going to produce enough amps (maybe 10-12?) to make a difference even in peak sun - the inverter is drawing from the batteries. So [in my opinion] this system is never going to actually deliver 1200W. Might manage 700 or so, though.

 

[Mark_K5LXP]

Now you seem to be stating that using the battery down to 20% full or even 0% full is not good.

What is happening is folks are exchanging extreme or unusual operating conditions for depth of discharge.  They're not the same.  You can only squeeze so much out of a turnip and if the system is being operated outside of ratings then "results will vary".  80% DOD is 80% at that level of draw.  At the 20 hour rate you'll get a given number of Ah, at the 5 hour rate you'll get some number less.  Per this 20hr vs 5 hour example, if one where to run a battery down to say, 70% DOD at the 20 hour rate, then suddenly apply a 5 hour or greater rate load the battery will immediately give up - 70%DOD at the 20 hour rate is well beyond 80% DOD at the 5 hour rate.  This isn't a battery or DOD problem, it's working exactly as expected.  There are ways to accommodate this, whether it's with more or different batteries, or yes, even limiting DOD so that you stay within operating range with the expected loads.  So my position on DOD and practical cycle life stands, whether one can effectively realize that in their operational profile depends on how the system is configured and user expectation.  If one chooses to use DOD to accommodate a given system or usage then fine, but that's not part of the DOD vs cycle life discussion/knife fight.

Mark B.
Albuquerque, NM

 

[Ex-Calif]

We are getting nowhere fast on this so I will bow out with this.

The battery makers post charts defining the life of batteries based on discharge depth cycles. Don't listen to me, listen to them.

1 - If one wants lower life cycle cost of their batteries they will avoid deep depth of charge cycles
2 - If one wants efficient boondocking one will match their solar generation to their energy storage
3 - If one plans to hook up an inverter the inverter draw should be reasonably matched to the energy storage capacity (i.e. 1200W with 2 batteries won't work)

If one wants to discharge the batteries until the lights go out all the time, personal choice and based on need I would also do so.

An example of this would be occasional boondocking where one might run the batteries to 50% before firing up the generator for a few hours to restore the charge. If I were going to boondock "a lot" - I would size my solar to become more energy independent.

I am out...

 

[TonyL]

At the end of the day, batteries are a fuel source just like propane or gas. The more you use it whether deep discharge or 50%DOD, the sooner you need to replenish / replace them
TonyL