dump load settings

[defed]

currently, i have a 4' test turbine, 700w solar on a xantrex mppt and 440ah @ 24v battery bank.

the xantrex has an aux function that does various things, but for now, i think i want it to divert to a dump load.  the only problem is, i'm not sure how to set it up.  i don't recall the EXACT figures w/o looking them up, but the controller charges in bulk and absorb (approx) between 28 and 29v, and floats at 26.5.  the controller regulates the panels so they won't overcharge the batts, but the turbine could.  at 4', i'm not  too worried about it, but want to have it just in case.

also, since there is not much draw on the batts at this time, i am wondering if i could force a load on at xx voltage just to keep the output up.  for example, today it was clear and sunny, and i only made .20kw in 8 hrs because the batts were in float all day...basically i was getting about 7w out of each panel all day when i had 160w available.  if i was to set the dump to come on at 28.5v (for example), would this force the solar to 'work' more?  the reason i would like to have more output is for monitoring the system.  i won't have it hooked up to real loads until the spring, so all winter it will basically be used to heat the small compartment housing the controller and other electronic devices.  i would like to know what kind of power WOULD be available each day had i needed it, not just how much i made that day to keep the batteries on float.

hope this makes sense.

thanks.
Dave

[ghurd]

I am not very familiar with the aux output properties of a Xantrex.
If it does not do what you want...

ghurd controller?
That is what it does.

The problem will be the MPPT is constantly changing voltages.
ie:
"(for example)"
The dump load comes on at 28.5V.
The MPPT is in float at 26.5V,
and the battery never reaches 28.5V,
the dump load does not come on,
and the panels still only make 7W.

I put a lot of though into it, discussed it with others, and the best solution I have for your type of case is to trigger the dump load at 26.4V.
It is not a great solution, just the only one we can figure out.
There can be issues with the MPPT trying to reach bulk absorb for a set amount of time, if it tries to auto equalize it can't, etc.
Basically, it will make it into a single stage controller, but the PVs will be making every watt they can.

If it has temp comp, then it gets past impossible to do in a simple way.

If anyone has a simple way to do it, I would like to hear it too!
G-

[defed]

If it has temp comp, then it gets past impossible to do in a simple way.

that's what i figured.  yes, it has temp comp.  i think every morning when the controller powers up, it goes thru the charge stages.  even if the battery didn't use any power, i believe it will start in bulk.  full batts will quickly move thru the stages.  i was thinking that if i had the dump come on at 28v, it wouldn't advance charge stages since it was using power....but i guess that would have to draw alot of power off the batts to keep it from advancing?

i think the diversion mode works much like your controller, just not sure how to use either in a mppt setting.  worse case i will set it for 28.x volts and only use it for when the turbine starts cranking.

i had both producing today at the same time.  solar controller was in float at 40w (26.5v @ 1.5a) and when the turbine would hit cut in, the charge controller would idle the panels back (almost no output at 3w).  when the turbine slowed and wasn't making power, the controller would ramp back up to 40w.

[Bruce S]

If anyone has a simple way to do it, I would like to hear it too!
G-

G-
Nothing more simple than that, in my numbers either.
Even if the NiCds I have were setup this way this would have them at 1.3 which is 90% full which is perfect so more complicated step down algorithms aren't needed.
Cheers;
Bruce S

[ghurd]

Bruce's line of thinking is the same as mine for this one.
You do not have a real load.
As long as the batteries stay above 25.0V, and below 26.4V via dump load, and the MPPT gets all it can from the PVs, then all is good?
The batteries will stay between full charged and float.
They may not be 99.9% full depending on the temp, but they won't freeze.

Put the dump load resistors in the house.   
G-

[Bruce S]

Put the dump load resistors in the house.   
G-

G- It's been said great minds think alike 
I don't have a great mind I'm just mental 

OR go get a few of those cheap-O HF 12V ceramic heaters , or use the dump load resistors and a nice slow fan to add a little warmth to house and keep resistors cool at same time.
Cheers;
Bruce S

[defed]

thanks for the ideas.  unfortunately, at this time, everything is in a remote (100' from the house) enclosure.  if i had a good way to get the heaters in the house, i'd probably have all my other control electronics in here too!  i built a 2 compartment box, fully insulated.  one holds the batteries and the other the charge controller, battery distribution blocks, turbine rectifier, turnigy meter, thermostat/relay and my heat source (a pair of 35w mower headlights).  as of now, all of that power will just be used to keep the controller side of the box warm, which shouldn't take much.

on thinking further, i think i really only need to dump the turbine power if i was to get too much during the windy winter.  i only need to monitor 'full output' on a handful of days...like a dark overcast rainy day, a full sun day, partially cloudy day, etc....i just want to know what production would be under these conditions...so next year when i have the system on its permanent load, i would know that on the overcast day...i will only have 500wh of production that day.  i have access to a cheapo 24v inverter, so i think i could even use this on those specific days to provide a decent load to the system and get the output near its capability.

i'll do some messing around w/ the dump function and see what happens.  i believe in dump mode, the xantrex has:  voltage on, a programmable time delay (be at xx voltage for xx secs before activating), the voltage off, and another time delay setting (might be same as the on setting, don't recall offhand).

[defed]

as discussed in another thread, i have a xantrex solar controller w/ aux function to activate a dump load.  i have it hooked up as the manual describes...to a solid state relay which opens the 24v from the battery bank to the load.  it seems to be working as expected (and like i expect the ghurd controller to work)...it hits activation voltage, load comes on, voltage drops, load goes off.  gusty winds, it cycles on and off as the wind changes (on/off delay timer set to 0 secs).

now, to the topic of this thread....how would i want the voltages set?  the solar controller bulk charges at 28.8v, and floats at 27v.  if i set the aux output to come on at 28.9v and off at 28.8v....that would prevent it from coming on during normal solar charging....but do i want 28.8v going into the batts when they are supposed to be floating?  or is the amperage more important?

example...the turbine makes 4 amps @ 29v, let's say dump load draws 3 amps...so i essentially have 1 amp @ 29v going into the battery when dump is on?

similar example, turbine making 4 amps @ 29v, dump load draws 10 amps, would that then be drawing 4 amps from the turbine and 6 amps from the batteries?

still a bit confused on the intricacies of the dump load size and settings.