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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 10:46am 18 Oct 2009
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Hi All,
1) What is the draw backs to dumping excess power through the inverter by using a bar heater, (ac 750 W ) in the colder months of the year.
I have put together a little circuit that will monitor battery voltage, and when the voltage at the battery reaches a settable level, it will turn the heater on.
The current to the heater is controlled by the circuit.
As the battery voltage rises the current to the heater is also increased (or decreased) effectively reducing the voltage at the battery. (ie:- watts in verses watts out )
In most cases the heater would barely be on, and would emit only a small amount of heat.
In the cold months any amount of heat in the house is better than nothing at all, and it would normally be wasted in a dump load elsewhere.
The circuit is not intended to replace the existing diversion system but operate in between battery full and diversion cutin.
With the test circuit , I have the heater off at 13.8 volts, and heater cut in at 13.9 volts.
The battery voltage is maintained at 14 volts +/- 0.1 by the load on the inverter.
This beings me to the next question.
2) What voltage settings for above would be best targeted for operation in a 12v, 24v, 48, system.
3) What sort of havoc would this system cause with battery equalization, and how is a diversion normally managed with equalization.
As the circuit controls ac 120/240 volt, I wont post the circuit yet till all the bug are out of it, so some darn fool dont hurt themselves.
All thoughts welcome.
Pete,
Sometimes it just works
davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499
Posted: 06:00pm 18 Oct 2009
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As a thought for more investigation . . . my understanding is that batteries only need equalisation periodically. Like once every 4 months or ?
I don't see anything wrong with your idea. Perhaps divert to the inverter all the time and once a week or ? allow an equalisation to take place.
What do you understand as the best equalisation period?
Cheers
davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499
Posted: 06:09pm 18 Oct 2009
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P.S.
How are going to control the current to the heater? Is it necessary? As long as the heater can dissipate more energy than you are generating, just have it cycle as you propose.
You will probably find that the cut-in and cut-out limits are too close. Battery terminal voltage drops as you suck current out of them and even monitoring the voltage right at the battery terminals I have about 0.5 volt hysteresis in my system.
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 07:29am 19 Oct 2009
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Hi Davef
The current to the heater is controlled by little bursts of power to the heater in a pwm fashion.
The idea is to control the heater current so the battery stays always at full charge, and only use the excess power that would be normally dumped elsewhere.
This way if a light or other appliance is switched on and the battery voltage drops below full capacity (ie 13.8v ) then the heater will switch off.
If the battery voltage increases above the 13.8v then the heater will cut back in.
When the heater cuts in, it is at low current and is ramped up or down to maintain a constant voltage at the battery.
Cycling the heater is one option but this would draw the batteries down then allow them to recharge.
I didn’t like this option as it can leave the batteries below full charge if the wind drops and you stop making power.
Better to have full batteries for when needed for other purposes.
The cut in voltage is set by a trimpot and the hysteresis can also be adjusted.
I was after some figures to better setup the span of both cutin and hysteresis for different voltage systems.
I have no knowledge of best equalisation periods.
I was hoping someone else would comment on this.
Pete,
Sometimes it just works
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 08:15am 19 Oct 2009
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The only way to know what is best is to know how much time the battery spends above, or below the nom system float voltage. If the battery spends some portiuon of the time below the float voltage, then time will be required to hold the battery voltage above this to prevent gradual battery sulphation.
Battery maintenance controllers perform the task with minimal fuss. If you can build a unit that performs boost, equalize and float functions, with settable timing and hysterysis values, I commend you. I made many battery maintenance ccts that performed a similar function to a plasmatronics unit, but I have used the micro's for other tasks now and have gone back to the plasmatronics for battery maintenance.
I use the load switch output to control power to an external inverter. My dump load is my house power ccts, fridge, lights etc. My windmills do not provide more than the base load, so no export. That is what solar is for.
Things would be a lot simpler if the politicians got around to fixing the grid feeding legistation, with no stipulation on how the RE was made.
Gordon.
become more energy aware
davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499
Posted: 09:06am 19 Oct 2009
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Technically, using a PWM to finely control the surplus power is the best. But, I've found that with a FET or relay switching my dump on and off in periods of around 30-60 seconds means I am not really cutting into the stored energy significantly.
Good luck
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 09:49am 19 Oct 2009
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Gordon,
I am not intending on building a ” unit that performs boost, equalize and float functions, with settable timing and hysteresis values” just a axillary dump control system that would allow it to be used to power a heat source in the home rather than a diversion load elsewhere.
In doing so I do not wish to interfere with the boost, equalize and float functions and was asking how, when, and to what voltages these functions occur at.
As there is normally a diversion load in place, my intension is it to have the circuit cutin a fraction under the set point that normal diversion would take place.
In a way the circuit will do a simular job as to what you do without the need for a second inverter.
As you use as much as you generate, than this system would be of no use to yourself, but your input is always valued.
Davef,
Where do you dump your power and what do you use?
At what voltage do you start to divert power?
Pete
Sometimes it just works
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 11:48am 19 Oct 2009
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Hi Downwind,
Do not underestimate the requirement of maintaining batteries, but these settings would work.
for a 24V flooded Pb-acid system, set the voltage to approx 29.5V.
for a 24V Gel type battery opt for approx 28.6V.
Gordon.
BTW I prefer to use electricity for high value applications. I have a slow combustion fire for heating, only when needed. I resort to recommending heater type diversion loading only when there is no other option on an RE setup.
become more energy aware
herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258
Posted: 08:35pm 19 Oct 2009
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Hi
I have used this dumping via the inverter for over 15 years.
Normally via a PLC that senses voltage and if above set point dumps to my water heater for about 10 sec if this does not lower volts it will continue longer(not that this happens). I also have developed a program with the Zeilo PLC that changes the bulk charge rate (approx 29Volts to a floating rate 26 volts.
However recently I decided to evaluate the Tristar PWM controller that has got really cheap. I connect this up as normal to a DC dump (12metres 10g fencing wire in concrete)
but have a 24volt timer relay triggered by the dump output that turns the water heater on for 10 sec , this basicly over rides the DC dump but if there is a failure it reverts to DC PWM.
I belive that the switched dump has advantages over PWM in that current flow in connections is larger pulses and does not cause corroded connections that seem to be prevalent on PWM systems. Batteries I have old forklift batteries that are still working after 15 years so this does not harm batteries.
Herb
Ps I have set up 7 systems with this control with no come backsEdited by herbnz 2009-10-21
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 03:52am 20 Oct 2009
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Herbnz,
Sounds like a good reliable system you have designed there.
I like the idea of more than one source to dump power to, and had never thought of a flintstone heater (wire in concrete)
You commented on corroded connections, which connections do you find are corroding?
As I am looking to use pwm to control ac power the corrosion of terminals should not occur .
Like your system the power being dumped will be diverted to a useful source and if that source is switched off or has failed then the power will be dumped through the primary dump load ( in your case the flintstone heater) as per normal.
A 2400w ac dump to your water heater is about 100 amp drain for 10 sec on the batteries, a fair bite.
Pete.
Sometimes it just works
herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258
Posted: 06:27pm 22 Oct 2009
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Hi Pete
Was refering to battery connections, I believe if connections get a little less than perfect a small current increases causes corrosion and failure, a large pulse welds and is self healing. PMW in most cases is only very small currents as it controls voltage.
Usually only fit 1500watt element in w/h to go easy on inverter really but have one client who is using 2000W
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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