I need more power Scotty!

[PaulT]

In a way, it is the inverse of measuring how far apart two points connected by an elastic string are. Distance depends on how hard you pull on the ends of the string and not just the resting length of the string.

By the way, thanks for the links to the pbase & Bogart articles. Each gives a lot to think on.

Paul

 

[rando]

I don't think ignoring Peukert and temperature is the correct solution, although it is the easiest solution. The other coloumb counting gauges account for Peukert and temperature (Xantrex, Victron, NASA etc). Bogarts assumption that you need to predict future loads is based on their faulty assumption that you don't actually loose capacity when under higher discharge rates. The key is you can't just apply Peukert as written - you need to invert the equation to apply it to the consumed energy as opposed to the remaining energy - e.g. if you pull out 20Ah at a 20A rate, your charge accounting records that as -24Ah, whereas if you pull out that same 20Ah at 5A it records it as -20Ah. Again, all the other manufacturers have figured this out.

As the PBase article points out - if you need really high accuracy, you need to figure out what these parameters are for your particular battery. For most of us, where +/- 10% SOC is no big deal you can just use the default values, or probably even ignore these second order effects all together (as Bogart does). My issue is more with manufacturers trying to claim their simplified approach is somehow 'better' using faulty logic - they make the same claim about their PWM charger vs MPPT chargers.

 

[rando]

Not to belabor the point too much, but another way to think about this would be to consider a real world situation - e.g. our trip on the White Rim between Christmas and New Years. Evening temperatures were around -5C, and in the evenings we were running the furnace (~3A average), lights (~2A) and charging laptops/iphones/cameras (~5A) all from our 80Ah (20hr rate) battery.

Let's say I have a BM without temperature or Peukert compensation, and conservatively want to keep my battery above 50% SOC. In this case I can run my 10A load for about 4 hours (40Ah) before my BM reads 50%SOC and I have to shut everything down and go to bed.

Now what is actually going on? Well my actual capacity at -5C is 25% lower than at +20C, so it is 60Ah. Second my 10A load is equivalent of a 12A load when you consider Peukert (this is a guess). Based on the info I was getting from my BM, I still ran for 4 hours - pulling the equivalent of 48Ah from my battery. However the effective capacity is now 60Ah, so my final SOC when I went to bed is actually 48/60 = 20%!! I am actually OK with going this low on occasion. But the real problem would be in the morning when I get up, glance at my BM and see that it is reading 50% SOC and think great, I can run the furnace for a couple of hours while I make coffee and still be above my personal limit of 30% SOC. I pull another 6Ah out in the morning but notice that my furnace blower sounds slow because my battery is now down to 10% SOC or about 10.5V. That is bad.

 

[craig333]

Just to be sure, the trimetric does offer optional temperature compensation.

 

[rando]

I got the impression from the document referenced previously that the Trimetric specifically does not offer temperature compensation - looking at their website it seems that the temperature compensation is for their charge controller. But I don't have one, so I could be wrong.

 

[Vic Harder]

If you want to calculate the Peukert exponent for you batteries you can use this calculator
https://www.victronenergy.com/upload/software/PeukertCalculator.exe

You should be able to find a chart similar to the ones for my batteries for your specific battery model:
http://www.rollsbattery.com/wp-content/uploads/batteries/S6-275AGM.pdf

My batteries calculate out to 1.13. The VictronEnergy articles reference above talk about 1.25 as being the default for their monitors as being "good enough" for most people, and 1.00 as being what the "perfect" battery would do. I guess that make mine "mo bettah" than the default/average.

 

[Vic Harder]

Update:

I exchanged my two 150w solar panels for two 100w flexible panels, because I could not figure out how to store the bigger panels while traveling. The output of the flexi panels is nowhere near as good though. In the same conditions the combined 300w panels put out about 200w. The combined 200w flexi panels only 80w.

Has anyone else had a chance to compare the efficiency of flexi vs the larger solid mount panels?

 

[rando]

I bought one the cheap 120W solar cynergy panels ($120) to play around with as a portable panel, and it seems to be producing about what you would expect - about 80 or 90W when oriented towards the sun, which is more than my flat mounted 150W panel produces at this time of year. I don't trust the reliability of these panels enough to hard mount them, but for a portable panel it is hard to beat the weight savings and ease of storage. The one thing that is a little tricky is coming up with a stand to deploy the panel. They are so floppy that a simple prop doesn't work. I ended up adding a sheet of expanded ABS to the back to make it a little more rigid and added two adjustable props to this to set the angle.

 

[Vic Harder]

Rando,

As a fellow VictronEnergy user, I'm hoping you can clarify a few things about how these units operate. Maybe show me where I missed something.

First, I have one of the MPPT 75/15 setup with the "load" output driving all the DC in the camper. I'm surprised that the gauge for the MPPT does not show the load current. Do you have yours setup in a similar way?

Second, my BMV702 battery monitor does not seem to register the current going into the battery from the PV arrays. It does register the current draw though. This is how it is wired now (small change - the load negative wire now also goes to the upper terminal on the shunt):

 

[rando]

Hi Vic

I did not set up my charge controller using the load terminals. I only have one heavy wire from the stock FWC power system to the battery box (where my MPPT 75/15 is) which provided both the power to the camper and charge current from the alternator. Because the current flow is bidirectional and the charge current could be over 15A, I don't use the load terminals. However I do see an option in the MPPT control output to display the load current, so I am not sure why this isn't working for you. I also see both the charge and discharge currents on my BMV-702. If you can't see the solar charge current the whole system won't work as it needs to add that energy back in for the charge accounting. I can't really make out the details in your photo, but the fact that two devices are not working correctly would indicate something is incorrectly wired. Can you sketch out a diagram of how your system is wired up?

 

[Vic Harder]

If you can't see the solar charge current the whole system won't work as it needs to add that energy back in for the charge accounting. I can't really make out the details in your photo, but the fact that two devices are not working correctly would indicate something is incorrectly wired. Can you sketch out a diagram of how your system is wired up?

I CAN see the solar charge current on the MPPT controllers. NOT on the BMV - it just shows discharge. JPG of my VISIO drawing:

To clarify, the roof and portable +ve "Batt" wires from the MPPT both go to the battery +ve terminal. The -ve wires from both "Batt" outputs on the MPPT controllers go to the BMV's shunt. The only thing connected to the other end of the shunt is the battery post -ve cable.

Roof and portable PV +ve and -ve wires are isolated from the other parts of the camper wiring, and only terminate at their respective MPPT controllers.

The +ve "Load" output on the roof MPPT controller feeds a 40A breaker and then the DC fuse panel. The -ve "Load" used to go to chassis ground as per the diagram, but based on input from VictronEnergy, I hooked it up to the shunt as well. That didn't make any difference.

 

[rando]

So you do see the load current on your BMV, and you see the charge current on your MPPT controllers, but you don't see the inverse - the charge current on your BMV and the load current on your MPPT controller?

Your diagram looks OK to me, but I don't see where the grounds from your loads go - do they go back to the MPPT controller or straight to the shunt? I also don't see where the +ve or -ve line from the truck/alternator comes in. I don't know which side of the load circuit the MPPT controller measures current on (-ve or +ve) or if it measures net or total load current. If you are drawing more power from the loads than you are producing from the solar panels, you will still see charge current on the MPPT control, but you won't see charge current on the BMV as the charge current is flowing from the panels straight to your load.

What do you see if in the two boundary cases - loads on and no solar (should see same load current on MPPT and BMV), and no loads battery discharged and solar panels in the sun (should see about the same charge current on both).

 

[Vic Harder]

Rando,

The load ground goes to the shunt. It used to go to battery -ve. No difference.

Truck +ve and -ve come in via an Anderson Powerpole, the -ve goes right to the camper battery -ve, the +ve goes through a 100A breaker, then a Blue Sea ML ACR.

Next time the sun is out I will deploy the portables again and check. Camper is still hiding on my dolly in the garage.

 

[rando]

OK, so I see two potential issues now:

1. All the current needs to flow through the shunt for the BMV-702 to work properly. This includes the negative from the alternator, otherwise the BMV can't keep track of how many amp-hours the alternator is putting back into the battery. It is not shown in your photo or wiring diagram, but the negative from any of the loads not connected to the MPPT controller (see point 2) also needs to be wired to the shunt, not the battery. To think about it simply - the only thing wired to the negative terminal of your batteries is the shunt - everything is wired to the opposite terminal of the shunt.

2. The MPPT 75/15 load terminals appear to be wired incorrectly. The negative load terminal needs to go to the DC fuse panel where your loads (lights, fridge etc) are connected, not to the battery negative or your shunt - see the wiring diagram on the last page of your manual. The return current from your devices flows back to the MPPT controller, into the -ve load terminal, through the controller and out to the battery on the -ve battery terminal.

Fix these two issues and I am guessing your issues will be solved.

 

[Vic Harder]

Good info Rando, thanks. Where did you find the info re: the mppt measuring the load on the negative side? I'm not sure which wiring diagram you found?

 

[rando]

I don't know that the MPPT Controller measures on the negative side for sure - but electrically it is easier to measure current on the negative side, and the fact that the controller has a negative load terminal would suggest this to be true. It would also explain the issue you are having. The wiring diagram is on the last page of your MPPT manual, past all the other languages.

 

[Vic Harder]

I didn't think that picture was worth much.... but, I see now that in principle what they are saying is "no shared grounds"

This is going to take a bit of work to redo.... Thank goodness for these forums, its knowledgeable and gracious members and heated garages!

 

[Vic Harder]

OK, so... I am feeling a bit better, and also more confused. I re-did those two connections, and now the MPPT control DOES read the load current! Yeah! And the BMV and the MPPT agree on the amount of current. However, the confusing bit is that the BMV reads that it is charging instead of discharging. I am pretty darn sure I got the shunt wired correctly, as it was reading current draw correctly before.

And I thought I was pretty good with sparky things.

Body and mind are tired.... that's my excuse and I'm sticking to it. Time for an adult bevy and have another go tomorrow.

 

[rando]

Looking at the photo your shunt, it is installed backwards - the UTP connector (the phone jack) side of the shunt should be towards the battery. Flip it around and you should be good to go.

 

[Vic Harder]

well, color me embarrassed.

Just checked, and yes, in one of my revisits to this wiring I did reverse those wires.

Also realized that the load -ve wire needs to also attach to the chassis (camper frame) to pick up the return current from the stuff that was in place when I got the shell. Factory wiring grounds to the camper frame for the interior ceiling lights, fan, porch and flood lights.