Lt.Dan
Solar Wizard
Id suggest you go back and read this thread all the way through, it will explain a lot. Its definitely not a "camping car".
Cinergi's 28 kWh / 4 kW Solar / 10 kW inverter RV build
- Thread starter cinergi
- Start date
Is a 5th Wheeler not being a camping car relevant to the questions i was asking ? i do not think so, appart if i'm missing something.
There is near no differences between them appart from the 5th wheeler not able to move itself.
In our country, those are camping cars : Something you can live in that move on the road.
Hedges
I See Electromagnetic Fields!
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- Mar 28, 2020
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Is a 5th Wheeler not being a camping car relevant to the questions i was asking ? i do not think so, appart if i'm missing something.
There is near no differences between them appart from the 5th wheeler not able to move itself.
In our country, those are camping cars : Something you can live in that move on the road.
It's like this. My "base" load (no A/C, no heat) is roughly 6kWh/day ... satellite TV, internet, computers, yadda yadda.. plus the idle load of 2 5kva inverters.
Now add in a heated water hose (300 watts) and heat pump for heat (bingo!) and a fan or two to keep things circulating and you get to 40 really easily. This thing isn't particularly well-insulated.
If you got Chinese all in one (like i do) they roughly take 50W => 100W for 2 and 2400Wh a day already (24 Hours)...
Heat pump got a COP that decrease low temperatures... that plus the lack of insulation and those low temperatures are taking a heavy toll on your batteries.
Maybe you could use "PowerCalc" to track your power usage and optimise it (if you're looking to do so), it's like a virtual power sensor... it got a database of Appliances-Power and guess what has been powered on/off, then cumulate consumption.
Sometimes low power devices that work too much hours..... waste a lot of energy.
Lt.Dan
Solar Wizard
I'm not complaining about my system at all -- just stating the facts for readers here. I'm happy with my system.
I understand that you are happy (not completly cause batteries are low) but ... wouldn't be better to have a correctly insulated living space instead of wasting energy heating it in winter and cooling it in summer ? Cause in the end, the lack of insulation generate needs for more batteries, more powerfull heating and cooling devices, more space for those devices... more planet heating... all is connected.
But i suppose this kind of habitat is already kind of insulated ... isn't it ?
Most people in our country, when they transform their van, insulate them with Wood fiber, cork, foam, fiberglass (not amazing)
My cousin got one insulated with wood fiber (the best option imo) and it's pretty effective. Of coruse, since the living space is small, their own body temperature tend to heat the space... i also understand that you live all year long in yout home so it can't be compared in that regard.
Is it that your particular 5 wheeler is not insulated or .. they simply do not build insulated 5 wheelers or maybe .. even insulated ones got pretty bad insulation ?
luckielab
New Member
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- Nov 26, 2020
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- 91
Hooo thanks, interesting, R = 11 on walls, floor and roof 25, converted to Europe R values that's R1.8 for walls and R4 for floor and roof.
That's equivalent to 5cm of polyurethane in walls and 11cm Floor/Roof, not a lot for something that is supposed to go to -23°C ....
The inside is very beautiful .... ! Should be very pleasant to live in this home.
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Yeah it's insulated and I could probably tweak a few spots here and there. I'm not normally in these kinds of temperatures and needing this much energy. I'm usually 100% self-sufficient (using < 20kWh/day). Again, just spewing numbers here for the benefit of others.
I don't think two separate batteries with solve your drift issue. The primary issue causing the drift after an extended period without a top balance or float, is due to differences in self-discharge between the cells. This is due to manufacturing differences but also due to temperature differences between various cells in the pack.
Our 38 ft fifth wheel with mini split typically needs about 17 -25KWhr/day when the temperatures are in the teens F. We keep the back third just above freezing, and we have added an inch of insulation everywhere, and improved the slide seals.
Our 38 ft fifth wheel with mini split typically needs about 17 -25KWhr/day when the temperatures are in the teens F. We keep the back third just above freezing, and we have added an inch of insulation everywhere, and improved the slide seals.
John Frum
Tell me your problems
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Is this due to differences in internal resistance between the cells?
Yes i understand, it's very informative, thanks for sharing, i'll sleep a little smarter tonight...?
The active balancer will prevent drifting of the weaker cells (depend on the drift though).
On my 16s Eve i never go over 15mV delta between cells. Of course it's also because i charge up to 90% only, cells show their differences at hich SOC, the higher...the worse it is.
But you are right, if those cells are really badly matched...then the BMS can't do much.
If the pack is operated continuously in the flat area of voltage curve, no balancer, active or otherwise, can counteract differences in self discharge. This is why regular top ups to the knee area are often required to avoid significant SOC drift with small parallel cell count packs. EVs, which often go extended periods without hitting the knee, get around this issue by using lots of well matched cells in parallel. This allows for the cell block to average out the self discharge variance between individual cells
Internal resistance differences are often correlated with self discharge variance. However, they are separate parameters. Each cell has its own self discharge rate, which can vary significantly, even between well matched cells. For example, my well matched (A, but not EV grade) pack was top balanced within 10mv at 3.55Vpc prior to a recent trip. After 2 weeks of daily cycling, without getting a full charge, and the pack was about 0.5% from high to low cell, with regards to SOC. This is partly due to cell variance, but some of it is due to temperature. The cells on the ends of the pack are always 3-6 degrees colder than the ones in the middle. This pack is insulated, and mounted under a vehicle. There is a well placed heating pad, but perfectly equal heat transfer is not possible without cell level heating pads. Which isn't really necessary anyways.
Self discharge rate is highly dependent on temperature, and it builds up over time.
This is part of the reason that packs which are in standby, (not long term storage), should be floated at a voltage which prevents most self discharge, but avoids keeping the cells at >97% SOC.
When you have multiple cells in parallel for each block, it is possible to have additional energy losses parasitically between parallel cells, if there is major difference in IR, or contact resistance. Where one cell supplies most of the current, then during low current periods, the higher SOC cell passes a bit of energy to the lower cell. However, LFPs voltage curves are so flat, that it takes a very long time, or a very large IR difference, combined with significant cyclic current, to causing this to happen. So its rarely encountered, and probably isn't an issue here.
The reason I think splitting it into two batteries would be a lot better from a balancing perspective is because the cells in parallel are fighting each other with respect to who gets/gives the electrons (due to mismatched IRs). It would be very easy to draw extra energy from one cell vs. the adjacent (parallel'ed) cell and then when you go to charge it, that cell pair will be out of whack. Would be a fun test to do. But I ain't gonna do it lol
Mmm i certainly missed something in your sentence... cause my balancer counteract perfectly the difference in self discharge if the voltage delta between highest and lowest go over my parameter (0.01V in my case) then BMS get those voltage in line.
Or maybe it's a word definition problem ..?
I never top balanced cells in my last pack and they are perfectly balanced.
So yes BMS counteract diferent self discharge on different cells till they are at the same voltage (definition of balancing ?). Being at same voltage of course do not mean they got the same SOC or capacity cause they are all born different.
I do not know what "top ups to the knee" mean .. :D
I posted a thread, i've let my cells 2 years without touching them at 19°C, they lost under 0.01V in this period .... 3.29V to 3.28V. So .. self discharge is pretty low.
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Lt.Dan
Solar Wizard
.01v in that voltage range can be upwards of 10-20%. There's no real gauge of SOC in that voltage range.
Agreed. In fact due to cell to cell variance, in the middle range two cells of the same soc can have 1-2 mv of difference as well. Which is why attempting to balance in this range can actually create a worse imbalance.
This imbalance can be unseen until the pack is taken to high or low SOC.
Also self discharge is highly dependent on SOC and temperature, a low temp, low SOC cell discharges much slower than a high soc high temp one.
