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Tim F.
September 20th, 2008, 19:18 PDT
I have a PV system. IN general do I need a voltage dump (power dump?)?

BB.
September 20th, 2008, 19:33 PDT
Technically a "dump load" is used for generating devices that cannot be safely turned off and you need a place for the energy to go so that you don't overcharge the battery.

The most common device (that I can think of) that needs a dump load is a wind turbine. If, you "stop charging" by opening a switch, the turbine will over-speed and self destruct. Here is a youtube video (http://www.youtube.com/watch?v=tcrO4USqRfo) of a LARGE wind turbine whose blades failed when its brakes failed during a wind storm.

So, your charge controller turns cycles on a "dump load" so that there continues to be a load on the wind turbine and the speed stays within specs. (wind turbines should also have one or two additional methods to prevent over-speed for safety).

The other discussion on this board about "dump loads" is to use them as "opportunity loads"... For an example, your Solar PV panels (which do not need a dump load) have fully charged your battery bank... Now you can turn on an electric heater (for hot water) or turn on a well pump to fill the cistern with the "extra" power available.

In general, using a dump controller to charge a battery is probably not a good as a regular 3+ stage charge controller (most dump loads are either on or off--no "float" setting).

Of course, this is all based on a guess that this is an off-grid system, using solar PV (Photo Voltaic) panels and some sort of modern PWM or MPPT type solar charge controller...

If you have a solar panel just connected to a battery bank--then you will need some sort of charge limiter to prevent your batteries from being damaged/over charged/boiling away your electrolyte.

If you have a "Grid Tied" system (typical solar power system for a home with utility power)--then there is no place to install a dump controller+load.

-Bill

SolarJohn
September 20th, 2008, 21:12 PDT
You've no doubt spent quite a bit on your PV system and it's a shame to waste a portion of the energy it produces. I'm going to use excess energy to maintain the charge on a spare battery bank. My main battery bank powers loads on a daily basis and is often depleted by morning, while my spare battery bank will be available to power loads in the event of a grid power failure.

mike95490
September 20th, 2008, 23:09 PDT
I have a PV system. IN general do I need a voltage dump (power dump?)?

NO, but you will need a Charge Controller. PV's do not need a "dump load" Wind and Hydro need dump loads to keep turbine speeds down, when batteries are fully charged.

Tim F.
September 21st, 2008, 6:22 PDT
Thanks for the responses. I was thinking that was the case as far as wind/hydro.

That brings up the next question. I do have a PWM CC. Can I put in a dump load device or use my CC that it will "turn on" after the batteries are full? It seems like a waste to lose that "excess" energy? As SolarJohn mentioned he can charge a different battery bank. How is that "wired" in? Thanks.

thoughtforward
September 21st, 2008, 7:56 PDT
You've no doubt spent quite a bit on your PV system and it's a shame to waste a portion of the energy it produces. I'm going to use excess energy to maintain the charge on a spare battery bank. My main battery bank powers loads on a daily basis and is often depleted by morning, while my spare battery bank will be available to power loads in the event of a grid power failure.


I would think tha tif you are depleating your batterie bank qvernight, then it is not big enough. Why are you using a seperate bank? By having the two banks sepatate you are shortenning the operational life of both banks.

n3qik
September 21st, 2008, 8:38 PDT
That brings up the next question. I do have a PWM CC. Can I put in a dump load device or use my CC that it will "turn on" after the batteries are full? It seems like a waste to lose that "excess" energy? As SolarJohn mentioned he can charge a different battery bank. How is that "wired" in? Thanks.

Most CC's do not have this function. SolarJohn and I have an additional controller that when the battery reaches a voltage level the controller turns on other loads. SolarJohn is using the Morningstar relay drive controller and I am using a industrial PLC.

SolarJohn
September 22nd, 2008, 7:08 PDT
Quote: I would think tha tif you are depleating your batterie bank qvernight, then it is not big enough. Why are you using a seperate bank? By having the two banks sepatate you are shortenning the operational life of both banks.

I could certainly use a bigger battery bank, but I'm not shortenning the life of both banks.

I've programmed the system to stop using power from the main battery bank when the SOC drops to about 80%. I could have used a lighter load, but then I would be under-using my system. I want to use as much free energy from the sun as I can, without stressing my battery bank.

My spare battery bank consists of older batteries, and they're not the same type as my newer battery bank. Connecting them together would not be a good idea. However, by maintaining my spare bank at full charge, I always have stored energy to use in the event of a grid power failure.

I use a Morningstar Relay Driver, controlling a relay, to divert energy from the charge controller to the spare battery bank. The first priority is to recharge the main battery bank, but when that bank is nearly fully charged, excess energy from the PV array is used to charge the spare battery bank. The relays are controlled by a Morningstar Relay Driver, which allows me to establish voltage setpoints, and therefore charging priorities.

To summarize: Charging my main battery bank is top priority, and I use as much energy from that bank as I can (without stressing the battery bank too much), in order to get the most from my small system. The spare battery bank is maintained at full charge, but seldom used to power loads. It is my emergency power supply.

icarus
September 22nd, 2008, 9:34 PDT
John,

Just out of curiosity, what criteria do you use to determine 80% soc? Voltage? if so how does that compensate for load? Total amp/hour draw from a meter like a Trimetric? Specific gravity? I don't know anyway to measure sg and translate it to do any switching.

Tony

SolarJohn
September 22nd, 2008, 11:00 PDT
John,

Just out of curiosity, what criteria do you use to determine 80% soc? Voltage? if so how does that compensate for load? Total amp/hour draw from a meter like a Trimetric? Specific gravity? I don't know anyway to measure sg and translate it to do any switching.

Tony

Battery voltage, under a significant load, according to information from the battery manufacturer. I depend on a compressor kicking in, and the resultant voltage drop as my trigger to remove the load. I also compensate for momentary motor-starting voltage sags by programming in a high-to-low transition delay of 10 seconds. This system is not terribly accurate of course, but I've monitored it and am satisfied that it's fairly close. Once the load is removed, no-load battery voltage gradually increases to 12.5 volts or more. This seems to indicate that I've not gone lower than 80% SOC. I could probably use a little more energy from them each night without stressing them too much.

I wrote more about it here: http://solarjohn.blogspot.com/2007/03/measuring-battery-state-of-charge.html and in more recent blog posts.

John

BB.
September 22nd, 2008, 11:05 PDT
I have not ever used one of these battery monitors (grid tied system on my home)--but I like the Specs. for the new Xantrex battery monitors (http://store.solar-electric.com/xabamoac.html) which have a programmable output--that can be set to turn on/off at specific State of Charge levels. Use the output for an alarm/load control (highly recommended for helping to prevent killing of batteries on automatic/spouse/employee/somebody who does not pay for batteries/etc. operated installations).

The high-end Xantrex BM also appears to have battery temperature compensation--highly useful for those installations with wide temperature swings.

-Bill

icarus
September 22nd, 2008, 11:40 PDT
Battery voltage, under a significant load, according to information from the battery manufacturer. I depend on a compressor kicking in, and the resultant voltage drop as my trigger to remove the load. I also compensate for momentary motor-starting voltage sags by programming in a high-to-low transition delay of 10 seconds. This system is not terribly accurate of course, but I've monitored it and am satisfied that it's fairly close. Once the load is removed, no-load battery voltage gradually increases to 12.5 volts or more. This seems to indicate that I've not gone lower than 80% SOC. I could probably use a little more energy from them each night without stressing them too much.

I wrote more about it here: http://solarjohn.blogspot.com/2007/03/measuring-battery-state-of-charge.html and in more recent blog posts.

John

John,
You have nicely quantified my own intuition. While perhaps not 100% accurate in all cases, it certainly gives a pretty good general idea of condition. As Bill suggests, the Xantrex meter can trigger a lvd, and I think the new Bogart series will do so as well. In my case, my nightly draws are so small, I seldom see voltages, even under load below ~12.5. As I said, I use the Bogart Tri-metric %meter, coupled with the amp/hours from full to guide me in much the same way.

Tony