So, my system is constantly changing its plan. As for balancing, I’m the wrong person to ask because I don’t use battery balancing; I have a slightly different strategy, but that would take too long to explain here. Basically, you have to distinguish between two things. One is the conversion losses of 10-20%; I believe the DESS currently calculates automatically with 15%. These are the pure conversion losses from alternating current (AC) to direct current (DC) and later back again. These are the losses that are released as heat energy from the MultiPlus. This is completely separate from the battery costs. The DESS automatically takes these conversion losses into account. Now, let’s move on to the second topic: battery costs. These result from the fact that a battery is also subject to a certain degree of degradation. This value is calculated by dividing the purchase price (or the cost of acquiring a replacement) by the estimated number of cycles (complete charge and discharge). Every user can enter this value (completely separate from the conversion losses) in the basic configuration. There are two options for this. Either I enter the purchase price and the expected cycles, OR I enter the cost per kWh from the outset. Personally, I’ve found a value of 2 cents per kWh to be the standard. Once this value is configured, the DESS takes it into account. Now, a personal note: in my own rough calculations, I always assume 4 cents for conversion losses and 2 cents for battery degradation. So, 6 cents total. This is my personal line of reasoning when explaining things to non-experts. Again, there are two issues! 1. Conversion losses (which are automatically factored in as a flat rate). 2. Prices for battery degradation (which each operator sets individually).
One more thing on this topic… it looks like things are changing hourly. NO… I know it’s changing hourly. I’ve been involved for 2.5 years now. I’ve been through almost the entire development cycle of DESS, from the test version to today. I’ve shared many setbacks but also successes with many testers. Even though I’m not a programmer but just an interested user… I think I know the basics of DESS. The positives as well as the negatives.
I might not be an expert but I do understand the thermal losses. We calculate ourself with 4.5 cents. In the lists I made in home assistent I can exactly see e.g what goes in from my panels, how muchin the end is feeded into the batteries, same for charging from the net, both ways.
I will put in again the lower selling prices, battery costs are set at 2 cents. And see what it will do tomorrow (after days with very low prices during the day they will be higher tomorrow).
Sorry… I didn’t mean to imply a lack of understanding with my detailed explanation. That was certainly not my intention. Observe it. It also took me some time to grasp the basic logic. Incidentally, the sell side is irrelevant to me, only the buy side (purchase side). For me, the biggest weakness is the time limitation of the prices. This means the system always limits its planning period to the timeframe for which it has available stock market prices. The system always calculates that the storage will reach the minimum SoC by the end of this time interval. But that’s the next step. First, see how the system handles the prices and changes to the forecasts.
Understood.
So, I changed the selling price. There comes a new frustration : you have to work with a %, which Ofcourse is not accurate all the way. I mean, for lower prices it’s quite good, higher it’s not. But for now fine, to see if it looks at it. I still think it doesn’t. I think it only uses these prices to calculate profit or loss? Time will tell. Now with all the kwhs coming from the solar panels it’s always interesting. So we need some very bad weather days. I am sure we will get them soon (based in the Netherlands)!
The losses, you can define them in the battery cost.
Under dynamic ESS setting you can define the battery cost.
I do see trading based on in mine case 5 cent, spread between buy and sell price.
But it would be nice if Victron could implement day ahead prediction based on weather forecast and day of the week.
Maybe use the data from https://www.wattwanneer.nl/ ?
I actually prefer the third option: I enter 0 cent for battery costs.
Why?
My battery is already purchased. Payed. It will sit there, probably working 12-15 years. With about 200 full-cycles per year, that shouldn’t bring me anywhere to cycle-count becoming an Issue. Age will be the leading degradation factor.
This leads imho to the following consequences:
If I configure realisitc battery costs, i just force DESS to utilize the battery less than pure efficiency losses would allow. But omitting these charges wont really extend my battery life significantly due to the age/cycle relevance consideration.
So, by using “0” I can force DESS to make more use of what is already payed for, and therefore actually bring the cycle count up a bit, making it effectively a bit less per kWh roundtriped when reaching EoL due to age.
Especially the last factor is kind of a recursive problem: If you configure any battery costs, utilization will be lower, bringing effective battery cycle costs up, which makes your configured costs being to low again.
I do use battery cost to compensate for inverter losses.
If there is too less difference between buying and selling you will lose money (the difference doesn’t compensate for the kWh losses)
Use 5 cents and that looks to work well.
You could also do this in the formula, but think the battery cost would be a better way.
This topic has been discussed frequently. I’d even like to add another point. In 10-15-20 years, battery technology will cost a fraction of what it does now. However, I can have a similar discussion regarding conversion losses, which are over 90% heat losses. If my equipment is located indoors, one could argue, at least in winter, that I’m using it, and therefore there are no losses. In summer, it’s a different story. If my equipment is installed outdoors, this argument is moot. But back to battery costs. The fact is, they are costs, as are, for example, a shunt, a BMS, or a balancer, and these are subject to a certain degree of depreciation. The more I use them, the higher this depreciation. So, one should certainly base a certain figure on this, but it’s equally certain that €500-1000 per kWh of capacity, which is what storage systems cost a few years ago, is too high. I experimented with this figure for quite some time. At 0, the DESS reacted too extremely; at 5 cents, it was far too low. I’m currently at 2 cents, and that seems to be leading to realistic operation with my system. It’s a compromise I’ve found that works for me. Perhaps it’s also due to the fact that Victron’s general efficiency figures are very general.
Think 2 cents is to low, you do have losses twice once in charging and one in discharging.
The conversion loss also depends on the kW charge and discharge rate, your 90% is to high.
5 cents is needed to compensate the losses, specially in the Netherlands with energy tax (also payed over the lost kWh).
That is, by itself, correct. But the DESS scheduler does already account for these losses by means of the round trip efficiency factor. Unfortunately the default value thereof seems much too optimistic at 90% whereas 80% to 83% reflects actual losses much better. Our system is set to 81% which allows setting battery cost to zero (0ct) to maximize active trading without the scheduler planning to sell at a loss. Depending on your battery costs write off philosophy you can then add back in a few cents to run the batteries less hard but I fully concur with dognose that the best strategy, after having made the financially irrational business decision to buy/build an DESS (trade) system in the first place, is to see the batteries as a sunk cost investment and therefore the least bad strategy becoming to run them as hard as possible as long as the heat losses wont actually lead to net costs.
TL;DR DESS, with all its flaws as they are, still works best when all key parameters are set reflect actual system performance values as good as possible. Not correcting the efficiency factor itself is a recepy for increased complications and less control over DESS behavior and performance.
PS, I will [EDIT: not share, thanks to Sarowe1990] my efficiency excel sheet for those interested. I reverse engineered the exact way in which the DESS scheduler calculates the delta SoC% per quarter or per hour based on the maximum grid and battery power limits. It is a plain no frills no scripting sheet, so it should not be too complicated to find the fill in cells you need to adjust for your own system (hint: different colors) and all formulas can be seen by un-hiding the intermediate columns. I won’t provide much help with it but if you really get stuck, say so in reply to this msg and I might get back to you in DM.
See also here: DESS charges only 1-2 minutes per quarter due to hourly SOC target communicated every 15 minutes - #46 by UpCycleElectric
That’s right, the DESS already accounts for efficiency losses of (according to my information) 85%. For a double transfer, that’s a bit too low in my experience. Considering that as the first argument, and secondly taking a certain amount of degradation into account, led me to the 2 cents. I used 3 cents for a while, but that made it too sluggish. Hence this compromise. In my experience, it also depends somewhat on the actual configuration and utilization of the system. Undersized cabling can also lead to heat losses. Another example would be extreme cell balancing or high charging rates. That’s why I find it tedious to have a general discussion.
There is no one size fits all solution indeed but as a starter I would advise to set efficiency to 80% and battery cost to zero for a week or so, just to experience how the buy/sell ‘cashflow’ works. And make very small adjustments increasing efficiency, 1% at a time, I settled on 81% to stay a little on the safe side. And then and only then add battery cost back in, or not. In ‘volatile’ summer weeks I may set battery cost upto 5ct to limit heat production whereas in winter time I leave it at 0ct reasoning that the heat is welcome to warm the house.