Load shifting and rebulk question

vtmaps

My off grid system was designed for our domestic needs and is working quite well. When the sun comes up we're typically at 80% SOC. My system was not designed to run my workshop... I use a generator to run table saw, chop saw, etc.

So far, I have only been shifting loads that are equal to or less than my fm60 is producing (laundry, for example). I would like to begin shifting workshop loads that exceed what the fm60 is producing.

Please assume its 10 AM on a good solar day in March (like today). I just reached absorb and I will be in float mode at noon. Trimetric shows that the fm60 is charging 25 amps. If I run the table saw, trimetric will show a 30 amp deficit charge. The saw usually runs less than 5 minutes at a time.

Question 1) Does it matter (to the welfare of the battery) what time of day I run the saw? Should I wait until I am in absorb? or wait until float?

Next, suppose I would like to run larger loads longer... (sander or saw with dust collection vacuum). I will not permit the sustained draw on the batteries to exceed 50 amps. The energy use of the tools will be less than the solar energy that is wasted in absorb and float, but the tools will certainly drop the battery voltage and SOC.

Question 2) Once again, should I wait until absorb or float?

Question 3) If I am in float, there is a possibility that I will might want to turn on the rebulk function. The rebulk on fm60 is currently turned off (by default). Should I turn it on? What voltage should it be set at?

Question 4) Should I abandon the idea of load shifting and just continue to run the generator for larger, longer duration loads? I would probably run the dust collection on the generator and the saw on the inverter.

thanks in advance, vtMaps

BB.

Re: Load shifting and rebulk question

Technically, a battery is more efficient in the 20% to 80% state of charge range... Cycling a battery (particularly recharging) in the upper ~80-100% range is much less efficient.

I would guess that if you get >90% state of charge back in the battery several times week... Try to get >75% state of charge at the end of every day... And avoid going (to often) below 50% state of charge--You are doing fine.

The loads on the battery are being reduced by the Solar Charge Controller--Which is always a good thing.

-Bill

vtmaps

Re: Load shifting and rebulk question

Bill, the premise of the questions is that I achieve float every day, and if I use my tools today I will still achieve float today... in fact, if I wait until noon to use the tools I may float twice today. The questions are about what time of day (and whether or not) to use the tools.
--vtMaps

westbranch

Re: Load shifting and rebulk question

VT, the way I read BB's note is that you should use the tools while in Bulk, when your battery is in the 20-80% SOC state

BB.

Re: Load shifting and rebulk question

As long as you don't break the rules, use your tools when you need to use them.

My two cents.

-Bill

But during bulk would seem to be more efficient.

westbranch

Re: Load shifting and rebulk question

Coincidentaly SolaRevolutioin just posted an interesting PDF- "Coupling Efficiencies" here http://forum.solar-electric.com/showthread.php?569-photovoltaic-schematic-design&p=111641#post111641

It gives another perspective to when to use the 'free' power.

I guess it all depends on how much you want to 'exercise ' your batteries, Big Bank/big array / low DoD- exercise in the morning, small bank / balanced PV / Hi DoD or looking to keep the excess power efficiency-of-use high, wait for float

make sense?

vtmaps

Re: Load shifting and rebulk question

BB. wrote: »

But during bulk would seem to be more efficient.

I certainly do understand that bulk charging is more efficient, but efficiency is not my concern... I can't use or store most of the power that I will produce today.

My chief concern is the battery's health and longevity. I am discharging the battery when I use the tools. If I use the tools before sun up I will drive my SOC lower than its 80% starting point. If I wait until I'm at 85% SOC I might drive SOC back down to 80%. If I wait until absorb, I will spend more hours today at a high absorb voltage than if I use the tools during bulk. If I wait until float to use the tools, the voltage may go low enough (under load) to initiate another bulk phase, even though the SOC has only dropped 5 percent. Presumably the second bulk would be fairly rapid and I would achieve float today.

BB. wrote: »

use your tools when you need to use them

The whole point of load shifting is that I will use the tools at the time that works out best for the battery's health and longevity.

--vtMaps

BB.

Re: Load shifting and rebulk question

From the math, it appears a 2x battery bank will last 2x as long and cost 2x to replace.

Sort of a wash--but needs 2x solar array and balance of system too.

Cheap vs a really nice system with higher costs. No right answer.


-Bill

vtmaps

Re: Load shifting and rebulk question

westbranch wrote: »

Coincidentaly SolaRevolutioin just posted an interesting PDF- "Coupling Efficiencies" here ...

westbranch, maybe I'm missing something there... all three scenarios in that pdf do not have the battery discharging. It is more about efficiency while load shifting, but the loads are all less than the charging power.
--vtMaps

westbranch

Re: Load shifting and rebulk question

vtmaps wrote: »

the voltage may go low enough (under load) to initiate another bulk phase, even though the SOC has only dropped 5 percent. Presumably the second bulk would be fairly rapid and I would achieve float today.

--vtMaps

I am thinking that if you go low enough to get to a second bulk you will also get a second Absorb as the FM has a minimum time for Absorb, doesn't it?

westbranch

Re: Load shifting and rebulk question

Re the PDFs , yes efficiency was my point. If you are short on power (PV) you get more bang by waiting till float and use the excess directly,if that is a factor for you. result is a fully charged battery and you get some work done without impacting the battery SoC tooo much...

SCharles

Re: Load shifting and rebulk question

I've been dealing with a similar situation for a lot of years. My table saw is now the only thing I do not ever run, or even try to run, from my PV system. It is just too much. So, I have a Honda 3500 w. generator I use for the saw. If it's a cloudy day, and I need the air compressor, I also use the generator for that item, but not on good sun days. Now, besides the table saw, I do not have any real power-hungry tools [no welder, etc.]. I have a healthy collection of the usual: Sawzall, drills, chainsaw chain grinder, sanders, etc. Everything runs fine on PV power. If you have other big-electricity items this system may not work for you [don't know how big that dust collector is.]

I tried to get completely off the generator and have done pretty well...used to have to run it for laundry and vacuum cleaner op's, plus the air compressor and etc. I've since upgraded my array and the only thing left is the table saw...no way I will even get that 220 v. thing running off my array.