a general design question WRT higher voltage to lower amps (off grid small workshop)

[pellicle]

Hi
I've played a little with some systems for camping (powering a 12V fridge) and have a reasonable understanding of the basics, however my problem is perhaps a little complex so I'll try to explain it as best I can.

I want to charge a battery (a collection of cells of course) and use that energy for a 240V inverter in my shed. I've done some analysis of my needs (with a power meter when using the tools in question) and my biggest single draw is the 1200W vac (although the drop saw is pretty high in transient too) most other stuff is low power. I did a "typical" days work through a power point monitor and worked out that in a heavier day I end up using 200Wh, which is of course reasonably modest when spread out. I don't work every day like that and mostly its a (electronics) soldering iron, charging some Lithium battery packs now and then and that's about it.

My question however relates to the use of the controller, the battery, the load and charging.

So, some more details. I decided to run 24V (so two batteries linked in series to make one battery) as what I feel is a sufficient step up in volts to halve the amps sucked by the inverter. I estimated that my vac would draw about 50Amps from a 24V system and after assembling it and running some tests and found that it was ball park that (some inevitable inefficiencies notwithstanding). I'm pretty comfortable that the system can tolerate that for the 5 or 10 min that I usually run the vac. I'm of course using heavy gauge wire (AWG 3) on a very short run to the inverter. There is no sign of heating under load (thermal camera).

I don't believe that I can run that from any controller that I can think of (open to input here) so instead I have the inverter wired to the battery and the solar controller output wired to the battery in parallel. I have seen this sort of arrangement used by others. My system is:

• 2 x 12V automotive batteries RC = 90
• 60cell 250W panel (claimed, measures close enough)
  • Voc = 37.51V
  • Vmpp = 30.4V
  • Impp = 8.22A
  • Isc = 8.88A
• eBay MPPT 12/24V 20Amp controller (seems to behave well)
• eBay AU$210 2000W (4000W peak) inverter (AU$ because I'm in Australia)

However this is where the "design question" problem arises. The controller has no inputs to sense the load and so it just charges the battery as it "calls it" based on seeing the Voltage. This results in the controller not pumping in hard to the battery to compensate better for loads.

I have in the past tested this controller with running a 12V Engel fridge from the controller "load" using 2 100W 22Voc panels in series and it worked like a bought one, gracefully handling the increased voltage and properly refactoring the power to give a good charging power to my (single) 12V 100Amp battery while not changing charging as the fridge cycled in and out of load. The amps measured on the panel side scaled as one would expect under the load.

This was what I call 'perfect' as it allowed my battery to be fully topped up (on a sunny day) to cope with the evening discharge only (via the "load"). However when I experimented with running the Engel of the battery directly the system was always lagging behind topping off the battery during the day (not unexpected).

I wondered how this would work when I went on to my next phase (power to my shed). Because while a 250W panel (delivering perhaps a real 200W when all factors are weighed in) should be able to fully power a 110W draw I had my strong suspicion that it would not because of the above discrepancy between using the "load" output VS only the charging.

The above behaviour showed itself in an experiment over the weekend when we had a scheduled power outage here. I decided to use the system to power my household fridge which draws about 110W when its running and over a 24H period sucks about 2kWh (not unexpected). So I dragged the batteries and inverter and a panel (as above) out of the shed and sat it at the best location in the yard (*full sunlight, sunny day) to run the fridge through a standard 10 meter extension cable from the inverter.

I started with the system nicely charged. At peak load times the fridge (meaning the inverter) was pulling 4Amps out of the battery but the measurement of Amps (clamp meter) showed that only 2A was going from the controller to the battery (probably because its algo was saying at that V apply this charge pressure. Now while the load from the fridge (lets call that 4A) was well within the 20A that the controller can provide it would pop its little fuse if that was not controlled as to what could draw.

Naturally the battery fell down to 24.5V at end of the experiment (which ran from 8am to 3pm).

So how can I avoid this issue without (say) using 2 inverters, one off the load for lighter drains and one off the battery for heavier draws?

Is there a better practice for this?

Thanks in advance

 

[HRTKD]

I could be wrong but it seems like you're running up against the charge profile settings of the solar charge controller. It sounds like it's in float and doesn't go back to bulk until the battery voltage drops a bunch.

The solution? Go with a programmable solar charge controller that you can set the voltage parameters to suit your needs. That's my guess.

 

[pellicle]

I could be wrong but it seems like you're running up against the charge profile settings of the solar charge controller. It sounds like it's in float and doesn't go back to bulk until the battery voltage drops a bunch.

Hi and thanks for your suggestion.

I was of the apprehension that its not just the charge profile but the electro-chemistry of a battery which forces a slow down (its perhaps a misnomer, but I was taught that its the increasing charge in the battery increasing the internal resistance to the charger). I have found the same with lithium (but that's a different question).

I note that among the well respected chargers that Victron does not put "load" points on its higher voltage and higher output charge controllers. Perhaps there's something in that I need to understand?

 

[HRTKD]

I don't know what you mean by "load points". My MPPT chargers are programmable to whatever I want, within reason.

Yes, the battery will accept fewer amps as it gets full. But if you're down at 24 volts, the MPPT should have kicked in with higher amps/volts.

 

[pellicle]

I don't know what you mean by "load points"

on most controllers I've seen there are three pairs of places (points) to attach wires:

1. panel
2. battery
3. load

 

[HRTKD]

on most controllers I've seen there are three pairs of places (points) to attach wires:

1. panel
2. battery
3. load

Correct, there is no Load Output on a good number of the Victron MPPT units. The data sheet linked below is for a line of Victron MPPT that do have the Load Output feature.

https://www.victronenergy.com/upload/documents/Datasheet-SmartSolar-charge-controller-MPPT-75-10,-75-15,-100-15,-100-20_48V-EN.pdf

 

[pellicle]

Correct, there is no Load Output on a good number of the Victron MPPT units.

which does not answer the question I asked:

"I note that among the well respected chargers that Victron does not put "load" points on its higher voltage and higher output charge controllers. Perhaps there's something in that I need to understand?"​

within the context of charging the batteries

 

[Intheswamp]

I think most of the higher-end inverters are simply designed toward AC production and that any DC connections that are desired are made via more substantial DC busbars fed directly off of the battery.

 

[HRTKD]

which does not answer the question I asked:

"I note that among the well respected chargers that Victron does not put "load" points on its higher voltage and higher output charge controllers. Perhaps there's something in that I need to understand?"​

within the context of charging the batteries

Probably because the Load Output isn't intended as a port for charging a LiFePO4 battery. It's intended for powering minor loads.

 

[pellicle]

I think most of the higher-end inverters are simply designed toward AC production and that any DC connections that are desired are made via more substantial DC busbars fed directly off of the battery.

agreed ... but my question is about maximising the solar input into my system given that it has the capacity.

 

[pellicle]

Probably because the Load Output isn't intended as a port for charging a LiFePO4 battery. It's intended for powering minor loads.

where did this come from and where did I mention

1. charging a LiFePO4
2. charging anything off the Load connection?

are you just teasing / trolling me?

 

[HRTKD]

where did this come from and where did I mention

1. charging a LiFePO4
2. charging anything off the Load connection?

are you just teasing / trolling me?

You asked about the Load Output within the context of charging batteries. It sounded like you were looking to charge the batteries from the Load Output instead of the Battery Output.

 

[pellicle]

You asked about the Load Output within the context of charging batteries. It sounded like you were looking to charge the batteries from the Load Output instead of the Battery Output.

I refer you again to my original post for context where I wrote:

However this is where the "design question" problem arises. The controller has no inputs to sense the load and so it just charges the battery as it "calls it" based on seeing the Voltage. This results in the controller not pumping in hard to the battery to compensate better for loads.​

​

I have in the past tested this controller with running a 12V Engel fridge from the controller "load" using 2 100W 22Voc panels in series and it worked like a bought one, gracefully handling the increased voltage and properly refactoring the power to give a good charging power to my (single) 12V 100Amp battery while not changing charging as the fridge cycled in and out of load. The amps measured on the panel side scaled as one would expect under the load.​

​

This was what I call 'perfect' as it allowed my battery to be fully topped up (on a sunny day) to cope with the evening discharge only (via the "load"). However when I experimented with running the Engel of the battery directly the system was always lagging behind topping off the battery during the day (not unexpected).​

anyway ... thanks

 

[WYtreasure]

Welcome to the show pellicle FYI: HRTKD is as far from troll as it gets.

I too have been a bit confused throughout the thread.
What is "vac"?

"I note that among the well respected chargers that Victron does not put "load" points on its higher voltage and higher output charge controllers. Perhaps there's something in that I need to understand?"​

Is the "load" point a number or an actual terminal to attach a wire?

What does your Manual say the load terminal is meant to be used for?

Like I said I got confused so if these questions have already been answered, I missed it.

 

[pellicle]

What is "vac"?

Sorry, an abbreviation for vacuum cleaner, in this case it's a 1200W one.

What does your Manual say the load terminal is meant to be used for?

Operating a load, please see the picture I included above in answer to the question.

Here https://diysolarforum.com/threads/a...mps-off-grid-small-workshop.35441/post-445103

I feel like my post is not being read, nor my answers.

Perhaps this forum is not for me...

 

[pellicle]

Welcome to the show pellicle

sorry, I missed adding this before because I was out and using the phone

Thank you for the welcome

 

[Intheswamp]

within the context of charging the batteries

"within the context of charging the batteries" I missed that part.

Again, I think their upper-end inverters are mostly designed for AC production by focusing on battery charging. My understanding that the load connection on a SCC is for *DC* loads.

agreed ... but my question is about maximising the solar input into my system given that it has the capacity.

Your original question: "So how can I avoid this issue without (say) using 2 inverters, one off the load for lighter drains and one off the battery for heavier draws?"

I think @HRKD was on the correct trail...needing a programmable controller. It seems your controller isn't programmed correctly for the efficiency you seek. The first experiment was a DC refrigerator, the second experiment an 110 AC refrigerator. It seems that your SCC is built to allow unfettered nominal DC (12.6-12.8v) load output up to the controllers rated limit and if there is a sag in power going through the charger due to the DC current load the controller appears to increase/regulate the current to the battery. The sensor for the "boost" residing in the "DC load" circuit. Running AC loads from the battery It appears the regulation in the battery output/charge circuit of your controller isn't regulating in the range that is optimum. This is all just thinking out loud and no idea if this is possible or if I'm even in the realm of reality. If what I'm saying is possible and this is what is happening, I wonder if something like a photo-cell that turns a 100W light bulb on when there is bright light (again, I have no idea if this "reverse photo-cell" exists) so that the controller senses the "load" current and that triggers it to increase the charge rate to the battery?

A programmable scc, as HRKD spoke of, would probably handle the sensing/regulating for you, though. Just chunking ideas at you and those ideas may make absolutely no sense...I'm not an electrical engineer, I've never been accused of having a full load of bricks, nor have I slept at a Holiday Inn Express in the last few years.

 

[pellicle]

My understanding that the load connection on a SCC is for *DC* loads.

Good morning.

yes, that's what it is there for .. but let me come back to that.

Your original question: "So how can I avoid this issue without (say) using 2 inverters, one off the load for lighter drains and one off the battery for heavier draws?"

As you may guess I'm aiming to have this thing power most things in AC (which is 240V in Australia) simply because most electric things in my ownership are ultimately powered by AC no matter how little power they draw (even my cordless 18V tools are ultimately charged by AC).

This is where I'm a little bit confused with how commercially available things seem to work: meaning 12V as a goal seems to be it unless one is talking about grid tied solar to a suitable grid tied inverter. I'd like to have my system AC at the outlets but powered by solar. My present situation is pretty similar to what others have done

• DC battery to store (in my case a pair of 12V batteries {which are 6 cells in series} in a series making a 24V battery)
• a solar panel
• an inverter

Now getting to the point of "lighter drains" my controller is up for 20Amps wihch @ 24V is a meagre 400W (not flying so tight to blowing a fuse) This is actually enough to power my fridge which I spoke of which if I'd wired it that way would not have resulted in my battery falling to a lower level.

I know that sulfation of cells and if not dealt with earlier than expected death of the battery (meaning higher costs and undesirable waste).

I think @HRKD was on the correct trail...needing a programmable controller. It seems your controller isn't programmed correctly for the efficiency you seek.

I'm skeptical but willing to consider it. Why am I skeptical? Well the problems I see are these:

1. due to the limitations of charging a Lead Acid Battery (lets leave lithium chemistries out of this for the moment) as you approach (say) 2.3V per cell (under charge, not a reflection of the actual state of charge which is usually lower) the amount of current a battery will accept will be lower (the internal resistance goes up I believe) which will in my understanding still form the final limit to how much can be put in see figure 1 below (poached from Battery university and augmented). Perhaps a programmable Solar Charge Controller (I assumed that's what SCC meant) may address this issue but I would be skeptical about the actual efficacy of that without actual measurements to show how many amps are going in.
2. I alluded to "sensing the load" in my original post (as did you in your above reply) where I wrote "The controller has no inputs to sense the load and so it just charges the battery as it "calls it" based on seeing the Voltage" I wrote this because I have exactly seen controllers (typically quite expensive) which do have such capacity and indeed scale amps to balance that and provide in as much as the load is pulling (subject to the limits of the system) to allow the battery to be as full as possible going into the evening (when the needs for power remain present).

Fig1​

Just chucking ideas at you and those ideas may make absolutely no sense...I

and that's great because chucking (pardon me fixing your sp in a quote) ideas around is where things go forward.

Anyway, where I'm satisfied with this system and its limitations (because at the moment I do not anticipate drawing so much as to leave the system in a low state (encouraging sulfation) for more than a day (I don't work down there every day) I have friends who are looking at what I'm doing (and the ideas I'm chucking around with them) who have actual needs for off grid systems (including on-site power for a van and another an RV). Right now I'm steering towards advising them (based on this experiment) to strongly consider a 48V system (which would provide sufficient power from the "load" circuit of the SCC to power a fridge all day without interrupting optimal charging even with a lower cost system (not everyone is wealthy).

Thanks for your inputs.