Charge controller vs. purge dump load:

rickeolis · January 2008

Hi guys,
I now have a halfway decent system going, but still haven't bought a large mppt controller, mainly because my solar/wind output is nowhere near the amount I am using on a daily basis. My inverter handles the 'vampire' loads in the house, like my always on computer, phone, cable box, large tv, and home theatre A\C converters. It also runs most of the lighting needs (all cf) for the place too.
While looking into what to buy for the future of my solar project, I keep seeing that larger wing based setups tend to use load dumps such as heaters or light banks to cut off overcharges in the battery systems.
So I got to thinking about how charge controllers work in that they ultimately cut full solar production down by cycling the output to the batteries, so my question is why not let the solar panels give 100% of output to the batteries, and just bleed off overproduction by means of a load dump above say 29 volts (for my 24v system.)?

-Rick-

rickeolis · January 2008

Re: Charge controller vs. purge dump load:

The edit function doesn't seem to be working and I had a couple of typos- so here's my corrected thread:

Hi guys,
I now have a halfway decent system going, but still haven't bought a large MPPT controller, mainly because my solar/wind output is nowhere near the amount I am using on a daily basis. The grid makes up the difference until I get more panels. My inverter handles the 'vampire' loads in the house, like my always on computer, phone, cable box, large tv, and home theatre A\C converters. It also runs most of the lighting needs (all cf) for the place too.
While looking into what to buy for the future of my solar project, I keep seeing that larger wind based setups tend to use load dumps such as heaters or light banks to cut off overcharges in the battery systems.
So I got to thinking about how charge controllers work in that they ultimately cut full solar production down by cycling the output to the batteries, so my question is why not let the solar panels give 100% of output to the batteries, and just bleed off overproduction by means of a load dump above say 29 volts (for my 24v system)?

-Rick-

crewzer · January 2008

Re: Charge controller vs. purge dump load:

Rick,

The dump load concept is called a "diversion load" in the solar charge controller parlance. I gather it's a rather common application. One popluar approach is to use resitive elements in a water heater tank. I sometimes use this concept in the summer to power an attic fan.

Check the instruction manuals for the OutBack MX60 charge controller and the Morningstar TriStar charge controller for additional relevant information.

HTH,
Jim / crewzer

BB. · January 2008

Re: Charge controller vs. purge dump load:

Regarding Dump Loads, from what I have read is that the dump load is typically on or off... So it can be very difficult to maintain your battery voltage at 29 volts unless you got a controller that does PWM dump load control (say to an electric heater in your hot water tank).

One reason that Grid Tied solar panels + inverter is so nice is that 100% of the output is always converted to 120/240 VAC and it is always powering your home's load, and any excess is going out your electric meter and turning it backwards.

Highly efficient (~95%), lower capital costs, no battery charging losses, no additional DC to AC conversion losses, and no battery replacement costs.

Of course, the standard Grid Tied system does not provide backup power during utility outages (generator or batteries+off-grid capable inverter required). Also, not all utilities offer (or even allow) favorable grid tied rates/plans.

-Bill

rickeolis · January 2008

Re: Charge controller vs. purge dump load:

Thanks guys-
I have already looked into my power company's ability to do grid tied, and they are about 30 years behind the curve and have already told me that they don't have many customers doing this and they'd have to "check into it". It's a local company privately owned and from what I've seen, they pay back less than wholesale as a credit only and I have no desire to go the whole nine yards with them for the trouble it's worth. I'm looking to be off-grid eventually.
I do see where finding a physical part that would do a purge based on voltage might be the hard find. I haven't seen anything as of yet, but I think I'll start looking into it.
I do see where my setup is different and why things are not as it would be for you guys, it's because I don't yet have enough panels to provide a full charge as of yet. I have my Xantrex use the grid to top off the balance, and the Xantrex is what puts my batteries through the charge cycles, such as float, bulk and all. You guys rely on the charge controller to do that job typically I guess.
Well, hopefully this year I'll buy up a good number of panels and be setup for this type of system, thanks so much again!

-Rick-

mike95490 · January 2008

Re: Charge controller vs. purge dump load:

Wind systems REQUIRE a dump load, for the times the batteries are full, and the wind is blowing, otherwise the turbine/prop overspeeds and creams the generator.

Solar PV is happy to just "throttle back" the charge current when batteries are full (afternoon when it hits float ? ) You could use a photocell or a computerized trigger to detect FLOAT, and switch on a Green Strobe, and fire up the house vac, dishwasher, clothes washer etc to use the PV output when you hit float. I've been considering a fairly well matched pump (if I expect 2500W when I hit float, fire up a 2000W pump) to pump water to an uphill tank, held in reserve for micro hydro battery charging on a rainy day.

rickeolis · January 2008

Re: Charge controller vs. purge dump load:

Hmmm, I wonder if that is an activity that can be programmed into my Xantrex 4024? I'll look tonight. Thanks-
I can simply schedule a time about once a month to run a manual Equalize charge from the Xantrex too.

-Rick-

BB. · January 2008

Re: Charge controller vs. purge dump load:

Rick,

If your utility would just give you 1 month net metering (and wholesale price for any positive balance at the end of the month)--that is still not too bad of deal. You may not want to over build your array (so that you stay a bit negative every month).

The worst would be if you had to install a second meter and they would only pay you for generated power (not net metering).

Tell them they can contact their local newspaper and get some "free" green advertising too.

There is always the first guy that has too push the envelope...

-Bill

rickeolis · January 2008

Re: Charge controller vs. purge dump load:

Hi Bill, I'd love to find the time and money to take on a new challenge like that, but my hobby is road-course car racing and it leaves little else for me in that respect...
Thanks!

Everyone: This is a device that I found to do just what I was talking about. I don't know if I can get my Xantrex in tune with it, but it's worth a try.

http://www.hydrogenappliances.com/batteryregulator.html

I figure if I can tell my Xantrex to stop charging up to the point where this device begins it's dump: somewhere around 28 volts, then let this device dump out until down to about 25 volts, then get the Xantrex to begin it's job again, maybe this will work.

-Rick-

mike95490 · January 2008

Re: Charge controller vs. purge dump load:

I don't know what kind of a "magic box" that thing is, first guess is a zener diode and a small relay, potted in epoxy, so you don't see that you are buying a $10 gadget.
I also think at it's 15.1v setpoint, you will boil a lot of battery juice, and maybe fry VRLA batteries.

I'd like to see some "user reviews" of folks having used one for a season or 3.

It still does nothing useful with the dump load, electric heater, water pump....

rickeolis · January 2008

Re: Charge controller vs. purge dump load:

I'm with you Mike; I was going to see what other types of similar devices I could find. This one does look quite cheap. But I figure it's basically doing the same job as an old car voltage regulator, simply bleeding off the higher voltage in order to protect the batteries.

Personally I like that approach over a charge controller for two reasons: One, that it can keep the batteries topped off more often, and secondly because I have a hybrid solar and wind system, and this would keep me from buying controller types for both and figuring out how to keep them in synch with each other.

-Rick-

rrbv · March 2008

Re: Charge controller vs. purge dump load:

And if your dump load burns out or is disconnected you may ruin your battery bank. I've seen it happen.

nigtomdaw · March 2008

Re: Charge controller vs. purge dump load:

Hi Im getting interested in this harnessing excess solar energy thing, now that Ive got 2.4kw solar up and running Im seeing and hearing my C40s buzzing as they regulate my fully charge 2100 ah battery bank every day the sun shines. Im not for one minute going to go down the diversion load route as I cant afford to toast my batteries. Id thought about putting a buzzer on my solar shed tofire off when the batteries get to float but dont know how to do it, If I could ? then it would allow us to add some AC loads to the inverter .ie washing ect to cream the excess energy, the bank voltage would fall and the C40s would start pumping the solar back into them. The problem is only going to get worse damm:roll: when I fit a further 1620 watts of panels up later this year. (for planned swimming pool pump). As it stands now I have to visit the solar shed see Ive hit float and get the missus to plug the clothes iron in switch on the washer and cook my lunch, she hates sunny days now:grr. Wants to emigrate back to grey UK:p

Given a bit more time Ill be able to guess float reasonably well but a buzzer sounds better, wont bother with a strobe in sunlight .

nigtomdaw · March 2008

Re: Charge controller vs. purge dump load:

To our electronic experts out there the 2 x C40 charge controllers each have a flashing LED if the system reaches float ie fully charged one of the flashing LEDs with light solid and remain solid for the rest of the solar day. Could this be utilised ?

n3qik · March 2008

Re: Charge controller vs. purge dump load:

nigtomdaw wrote: »

To our electronic experts out there the 2 x C40 charge controllers each have a flashing LED if the system reaches float ie fully charged one of the flashing LEDs with light solid and remain solid for the rest of the solar day. Could this be utilised ?

Funny that you mention that. I just did that very same thing this Sunday, Hooked a photo-detector to the LED and ran it to my PLC input. Just have to add the input to the new program.

The same ideal using a photo-detector,op-amps,relay,etc. can be done also.

rickeolis · March 2008

Re: Charge controller vs. purge dump load:

What about this: Does anyone here have a Xantrex 4024 or similar, and does it have the ability to turn a switch on once a certain voltage has been reached? (Say 28.5v)
When I get home tonight I'm going to re-read the setup manual and see if that can be done from the inverter itself...

-Rick-

Ralph Day · March 2008

Re: Charge controller vs. purge dump load:

Rick,
My 4048 has 3 relays (that's besides the gen control relay), all programable to voltage values you can set up. I use none of them, will sell cheap. I wonder how many people use them once they get the aux feature of the MX 60 they have as a charge controller.:p

Ralph

Charge controller vs. purge dump load:

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