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Mezbatt

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Hello all, ok, so I've been hovering and absorbing info at all levels before I go build/ lash out thousands on a perfect setup. Here lies the problem, I've been an electrician for almost 40 years now, very capable with the building/wiring aspect BUT have tied myself up in knots with the design aspect. I shy away from the maths calculations naturally involved with design as one would. I want to put down here my concerns and hopefully someone chime in here and put me straight. I thought I had a system truly sorted in my head but oh NO...!!!

Parameters......
1. 24v battery bank running system to match the truck starter system.
2. Space for 6 solar panels up top. I will fabricate a mechanism to tilt them L to R for maximum efficiency.
3. Allowance for 3kw of 240v inverted usage.
4. Battery choice...???? I want to run 2 x LiFePO4 200ah batteries in series.
5. Main aim to be 'OffGrid' sufficient.

Q's :
1. Solar panels in series or parallel for use with an inverter..?? Main 240v usage will be the AC unit @1200watts and a washing machine intermittently used @ 2000/3000watts (diversity applied in conjunction with AC unit).
2. Battery choice, do LiFePO4 batteries like to be maintained at all times hooked up to a solar system charger or do they like to 'cycle'..??? Would I be better to use AGM batteries...???? I agonise over this choice switching constantly from one to another, maybe I'm missing a few good and obvious pointers here to help me...???
3. Absolute protection for battery life, charger choice etc..?? I DO NOT want to kill a very expensive battery bank with a novice build mistake...!!!

If anyone can do me a layout drawing of your suggested idea that would be great. I've plucked several from Google image but am hesitant to take on the construction until I've tapped into and sought advice from a 'think bank' such as here...

Thanks in advance, Mez
 

Redpacket

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Mezbatt said:
I will fabricate a mechanism to tilt them L to R for maximum efficiency.
Be sure to build for the flexing & vibration of road travel... if going ahead with this, a simple manual method might be good?

Mezbatt said:
Q's :
1. Solar panels in series or parallel for use with an inverter..?? Main 240v usage will be the AC unit @1200watts and a washing machine intermittently used @ 2000/3000watts (diversity applied in conjunction with AC unit).
The panels & the inverter aren't really related.... other than the charger you use. Look for an MPPT charger that will take the voltage & current of the panels up to Voc (STC) & Isc (STC)
"STC" is one of the panel specs. You will actually get output more like the NOCT numbers but you should spec it for the STC ones.
Typically the optimum voltage for the panels is with the panels at Vmp (NOCT) about 10V higher that the battery bank "full" voltage.
You might wire two panels in parallel to one smaller MPPT & the other two to another MPPT. This gives you a bit of redundancy & some shade handling.

Mezbatt said:
2. Battery choice, do LiFePO4 batteries like to be maintained at all times hooked up to a solar system charger or do they like to 'cycle'..??? Would I be better to use AGM batteries...???? I agonise over this choice switching constantly from one to another, maybe I'm missing a few good and obvious pointers here to help me...???
LiFePo4 cells are more expensive but are lighter & smaller than SLA/AGM batteries & will last way longer if charged properly.
If you charge the LiFePo4 cells to approx 3.40V - 3.45V/cell they are are happy to sit there all day & not "need" cycling.
They are also not damaged if left at lower SoC like SLA/AGM's are. 80% DoD is OK, <50% is better (any DoD takes some cycle life off any battery type).
Do not charge them to 3.65V/cell, also don't go below about 2.9V/cell, they won't last. Stay on the "flat part" of the voltage curve.
You could also buy individual cells instead of pre-built 12V blocks, you get better access to each cell if you do.

Mezbatt said:
3. Absolute protection for battery life, charger choice etc..?? I DO NOT want to kill a very expensive battery bank with a novice build mistake...!!!
Use a good BMS - Batrium is the best but you can probably pick a lesser one. You need an alarm output for cell high/cell low.
Use a good battery monitor like a Victron BMV-712 - tells you SoC, volts, current, etc
Use good chargers like a Victron MPPT
I've got Victron gear myself & love it.
Use proper non-polarized DC breakers like No-Arc for smaller loads & a big one for the inverter.
Use pre-charge methods.

Mezbatt said:
If anyone can do me a layout drawing of your suggested idea that would be great. I've plucked several from Google image but am hesitant to take on the construction until I've tapped into and sought advice from a 'think bank' such as here...

Thanks in advance, Mez
As long as you build it modular, you can change, add, repair, etc much more easily.
Run as much as possible (eg lights, etc, etc) directly from DC, or via DC-DC 24v > 12V converters, much more efficient.
Laptops, phones, etc can all be run this way.

Look for electronically controlled appliances, & lower wattage eg washing machine, microwave oven, etc will draw less current.

Post some links to diagrams you like & we can comment on them! :)
 

Mezbatt

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Redpacket said:
"You might wire two panels in parallel to one smaller MPPT & the other two to another MPPT. This gives you a bit of redundancy & some shade handling"
Can you expand on redundancy and shade handling please..????


"Do not charge them to 3.65V/cell, also don't go below about 2.9V/cell, they won't last. Stay on the "flat part" of the voltage curve.
You could also buy individual cells instead of pre-built 12V blocks, you get better access to each cell if you do"

I thought about building a bank up but would it be as robust and fit for purpose in a truck (Race truck Volvo FL6)..???

This is where i get lost a little, do I buy individual MPPT controllers and an inverter or get a combined unit as per the Victron..?? My truck will do quite a bit of 'offgrid' if not mostly should the system prove good. I have to get this right. I have an onboard generator which may also be figured in SO:
1. Solar. 2. Battery bank. 3. 240v hook up. 4. 240v generator supply. ....?????

Mezbatt said:
3. Absolute protection for battery life, charger choice etc..?? I DO NOT want to kill a very expensive battery bank with a novice build mistake...!!!
Use a good BMS - Batrium is the best but you can probably pick a lesser one. You need an alarm output for cell high/cell low.
Use a good battery monitor like a Victron BMV-712 - tells you SoC, volts, current, etc
Use good chargers like a Victron MPPT
I've got Victron gear myself & love it.
Use proper non-polarized DC breakers like No-Arc for smaller loads & a big one for the inverter.
Use pre-charge methods.



I've fitted several shunt type monitors like the BMV-712 to systems when i worked on superyachts many moons ago so that will get figured in and a good prompt..... Also everything fitted was indeed Victron...!!!

Have a look at these batts that i have attached. Two of these in series and a good set up which I'm trying to config........


image_wsvgey.jpg

image_wzhxcj.jpg

image_zskhdt.jpg

image_vdljej.jpg
 

Redpacket

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That Volvo FL6 looks like quite a large vehicle so should be able to do plenty of things.
Yes I'd suggest getting separate individual modules eg MPPT's, BMV-712, inverter.
You would be able to integrate a generator with switch over, etc.

Re shade, etc, if you cable all the panels in one lot to one controller, you may not get as much power collected.
So you put say 3 panels into 1x MPPT unit & the other 3 into another MPPT.
This way if some panels are shaded, you should get better results.
Also for redundancy, if eg a panel got damaged or one MPPT failed, you would still get some power input.

Cells like these would be good:
https://www.ebay.co.uk/itm/8-x-LiFe...-8kW-Battery-Solar-Bike-Car-CALB/193680672776

Maybe find a diagram you liked & link it here.
 

not2bme

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Messages
497
Parameters......
1. 24v battery bank running system to match the truck starter system.
There's no real benefit for matching since you would never power off the truck's SLA batteries. It's possible to charge the batteries off the truck, but you'll have to make sure you have a beefed up alternator and a separate charge controller to maintain the current.

2. Space for 6 solar panels up top. I will fabricate a mechanism to tilt them L to R for maximum efficiency.
Good idea. Go for mono panels as well for maximum efficiency.

3. Allowance for 3kw of 240v inverted usage.
Assuming you're not in the US otherwise you'd choose 120v? My take is to go with Victron stuff.Good efficiency and decent units. However if you are on a budget there's other stuff around.

4. Battery choice...???? I want to run 2 x LiFePO4 200ah batteries in series.
Not sure your budget but we have quite a variety of builds here from all sorts of different battery sources. Not sure what 200ah LiFePO4 batteries are since that's just current. You might want to specify the voltage as well so we can calculate the kwh.

5. Main aim to be 'OffGrid' sufficient.

Q's :
1. Solar panels in series or parallel for use with an inverter..?? Main 240v usage will be the AC unit @1200watts and a washing machine intermittently used @ 2000/3000watts (diversity applied in conjunction with AC unit).
Solar panels are rated based on the MPPT controller and the battery voltage. Typically you want to size the panels close to the MPPT zone for best efficiency. For example if you use 2 panels in series to get let's say 90V (Vmp) but you have 24V batteries, it's probably better to wire panels in parallel to get 45V (Vmp).
I would choose an AC unit that has an inverter (basically a variable speed compressor) as it will be less stress on your inverter. That way there's no hard starts or surge. Plus a lot more efficient so save more power. I've seen folks on tiny homes put actual home type mini-splits on their camper.

2. Battery choice, do LiFePO4 batteries like to be maintained at all times hooked up to a solar system charger or do they like to 'cycle'..??? Would I be better to use AGM batteries...???? I agonise over this choice switching constantly from one to another, maybe I'm missing a few good and obvious pointers here to help me...???
There's not much maintenance to the batteries if properly matched (we deal with a lot of used cells here in this forum so mismatched cells are a common theme here). Lithium cells are not like SLA/AGM batteries. They do not suffer any memory effects like nicads either. However the lifepsan suffers if cycled 100% daily. That's why a tesla car only charges up to 80%. At 100% depth of discharge every day they will run between 500-1000 cycles. If you reduce that to 70-80% you might increase your cycles up to 2000. A deep cycle SLA has probably 500-1000 cycles as long as you only do a 50% depth of discharge. So typically you need 2-3 times more 'capacity rating' for SLA than you need for Lithium. That means 4 times the size with 1/2 the capacity and 1/2 the life. So no SLA! Just don't do it! We won't like you and you wont' be welcome here!

3. Absolute protection for battery life, charger choice etc..?? I DO NOT want to kill a very expensive battery bank with a novice build mistake...!!!
That's what a Battery Management System (BMS) is for. But really if done right there's nothing to worry. The inverter you use would cut off before your batteries would wear out, and if not then your BMS would be your failsafe and have a secondary trip to cut off the voltage. If you however bypass this BMS by let's say powering up some 24v lights directly connected to the batteries, then all bets are off.
 

Mezbatt

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Thanks guys,

Starting to gain some momentum now, all posts greatly appreciated and read several times to absorb....

Reading reviews on equipment is proving a plus. Found a guy called Will Prouse on Youtube who's enthusiasm for LiFePo4 cells is very infectious indeed and the use of BMS. I'm now looking at cells from Ali Express as below, great reviews and delivery times etc....

It won't be for several weeks or even a couple of months until i take the plunge and purchase, I have a healthy budget but do not want to squander funds on unnecessary equipment either.

This is a great forum and some good advice coming my way, thank you..!!


image_xtcash.jpg
 

Redpacket

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Will Prowse's videos are great & give good information. +1 for them :)

200ahrs at assumed 24V system volts is actually a measure of storage capacity (a quite reasonable amount) & should run a van like yours well.
Your main power usage over time is likely to be a fridge so dig hard to get the most efficient & best insulated one there. Avoid small "bar" fridges like the plague!
 

ajw22

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Solar side 37.5v for 24V system sounds a bit too low. Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V. And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?
 

Mezbatt

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ajw22 said:
Solar side 37.5v for 24V system sounds a bit too low. Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V. And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?

I have some new Dimplex 230w panels at my disposal, so, how to work out the current value for the two MPPT controllers then...

Is it 920(watts) / 75v = 12.26amps ...???
 

OffGridInTheCity

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ajw22 said:
Solar side 37.5v for 24V system sounds a bit too low. Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V. And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?
Agree with @ajw22 here. I'd series them as you suggest "....should I link them in series/pairs to give 75v a...." and get a controller that can accept up to 100v.
 

Redpacket

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Mezbatt said:
ajw22 said:
Solar side 37.5v for 24V system sounds a bit too low. Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V. And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?

I have some new Dimplex 230w panels at my disposal, so, how to work out the current value for the two MPPT controllers then...

Is it 920(watts) / 75v = 12.26amps ...???

+1 to posts above, two in series is likely to to be better here.
Your diagram looks OK. You need a suitable DC breaker at the input to each MPPT & another at each output to the battery.

You need the full spec sheet for your panels. It should show STC & NOCT values for Vmp & Imp & also Voc & Isc.
The MPPT controllers input voltage should be rated for Voc STC with a bit of margin, eg 10-15V to cover for highest voltage on cold days ie the max voltage input.
The current input & output should also be rated to cope with the Isc of the panels in and the total max power out current.
In actual use, in the middle of a clear sunny day, the panels should produce near the NOC Vmp & Imp values, less current if not facing the sun eg installed flat vs tilted, etc
 

ajw22

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Mezbatt said:
I have some new Dimplex 230w panels at my disposal, so, how to work out the current value for the two MPPT controllers then...

Is it 920(watts) / 75v = 12.26amps ...???

I'm guessing it's the DimplexRenewablesdxpvm230p6-30 , spec sheet:
https://db.photovoltaikforum.com/PvForum/api/download/1/54962/1/pdf/54962_DXPV_M230_P6_30_EN_09_10

It's a common misunderstanding of the different voltage/current specification with PV panels:
"Open circuit voltage" (Voc ; 36.65V in this case) is the maximum voltage the panel can output if _nothing_ is connected to it. This value is just used for safety calculations, and does not occurunder normal running operation.
"Maximum power operating voltage" (Vmp ; 29.0V in this case) is the voltage the panel is most efficient at. This is approximately the voltage under normal running operation.

Same pattern with "Short circuit current" (Isc ; 8.59A) forsafety calc, and "Maximum power operating current" (Imp ; 7.99A) in actual use.


So with a 2x series arrangement,the charger needs to support at _least_ 73.3V (36.65V * 2), but doadd some safety buffer here.
And with 2x in parallel, the potential max current if something should short circuit is 17.18A (8.59A * 2), so the cables have to be sized for that, and the breaker sized to trip before the cables startto overheat.

Actual operation values for this 2p2spanel arrangement will be (Vmp * 2) * (Imp * 2) = (29V*2) * (7.99A*2) =58V * 15.98A = 927W
Of course only under ideal conditions.
 

Mezbatt

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Thank you again gents, this is the stuff i need......

So layout of panels reached, one MPPT controller or two..??? Just run this by me again please the need for two..??

I'll get a picture of the panel spec labeland post up.

Now my issue of "do i get a combined charger/inverter" or do i buy separate units.? Any pro's & cons appreciated.....

I have been seduced by Victron (Goddess of power control) so looking to use as much of it as i can afford but open to other product references as well.

I will draft up a diagram using all the info gathered so far ready for approval and marking...!!!!

Thanks...


image_aedhbc.jpg
here's my truck that's getting a full revamp and solar install.
 

ajw22

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ajw22 said:
potential max current if something should short circuit is 17.18A (8.59A * 2), so the cables have to be sized for that, and the breaker sized to trip before the cables startto overheat.

Just wanted to clarify that the breaker is not really for theprotection of the cables, since the panels can't magically produce more current to melt the wires. But it is usually a regulation requirement, and a very useful safety switch for doing maintenance work.
Normal AC breakers may fail catastrophically with DC currents, so make sure to getDC rated breakers. And with many DC breakers, current must only flow in one direction, so make sure to check the wiring specifications.
 

Mezbatt

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ajw22 said:
ajw22 said:
potential max current if something should short circuit is 17.18A (8.59A * 2), so the cables have to be sized for that, and the breaker sized to trip before the cables startto overheat.

Just wanted to clarify that the breaker is not really for theprotection of the cables, since the panels can't magically produce more current to melt the wires. But it is usually a regulation requirement, and a very useful safety switch for doing maintenance work.
Normal AC breakers may fail catastrophically with DC currents, so make sure to getDC rated breakers. And with many DC breakers, current must only flow in one direction, so make sure to check the wiring specifications.

Bloomin typical, about 7 years ago I slung out a huge box of breakers, S,D&Tpole AC & DC rated from when I worked as a panel wireman. Now I got to buy them...!! I'm presuming and myself unprompted would use DP breakers for the solar panels.
 

Redpacket

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ajw22 said:
Normal AC breakers may fail catastrophically with DC currents, so make sure to getDC rated breakers. And with many DC breakers, current must only flow in one direction, so make sure to check the wiring specifications.
+1 look for the non-polarized ones (better).
Needed for safety & really useful for isolating gear.
The Voc from 2 panels gives a good bite if you can't isolate it!

Basically you need:
Panels > DC breaker > MPPT(s) > DC breaker > battery
battery > big DC breaker > inverter > AC breaker(s) + RCD(s) > AC loads
+ battery > DC breaker(s) > other DC loads
 

Mezbatt

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Quick question here..... I'm planning a setup whereby the automation will allow only one bank (2S2P x2banks in total) of panels to tilt at a time to pick up a more efficient angle to the sun. They will tilt in opposite directions (one way or the other) via a clever method we have come up with but this will give rise to different charging currents feeding the batteries almost all of the time. IE: A lead bank and a trailing bank with reference to charging potential. Any thoughts or input on this idea please..????
 

Redpacket

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Mezbatt said:
Quick question here..... I'm planning a setup whereby the automation will allow only one bank (2S2P x2banks in total) of panels to tilt at a time to pick up a more efficient angle to the sun. They will tilt in opposite directions (one way or the other) via a clever method we have come up with but this will give rise to different charging currents feeding the batteries almost all of the time. IE: A lead bank and a trailing bank with reference to charging potential. Any thoughts or input on this idea please..????

Should work fine, probably should use two separate MPPT chargers for best results.
Maybe you could run this length-ways along the van with the hinge side down the centre line?
Lift drivers side row or passengers side row up from outside edge.
 
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