Panel/array size vs battery bank & daytime only loads?

[PNW_Steve]

Hello Everyone,

I am starting on a new endeavor. I have purchased a 40 foot bus that I am converting into an RV. I will be configuring a "hybrid" power system involving: solar, shore power, generator and coach alternator.

I have been tinkering with solar & battery sizing. I am finding that panel prices have dropped significantly since I last looked. I started getting really excited with the vision of a 1.5kw solar install. Then reality set in... (reality sucks!). While panel prices have dropped significantly since I last looked, controller and battery prices have not.

My "big loads" are woodworking tools that will (in the absence of shore power) be run from my 3kw inverter. I do not need 3kw continuous but I have found that I need a larger inverter to support the startup current on the motors.

If the weather is bad the tools don't run. I am a "fair weather woodworker".

My question:

As the majority of my power consumption occurs while the sun is shining and I have limitations on space & budget for batteries I began to wonder how practical it would be to "oversize" my solar array compared to battery capacity in order to support larger loads ONLY when the sun is shining and install just enough battery to support my typical night time loads.

Is there a downside to equipping the rig with 1.2-1.5kw on high voltage panels, a 60amp mppt controller and a 24 volt 200 amp/hour battery bank? I know that typical system sizing would call for a bigger battery bank but I do not have the space or the budget to do it. Also, that battery bank will carry my night time loads just fine. My big loads occur between 10am and 5pm and generally on sunny days. I will also have generator backup for cloudy days if needed.

To summarize: Is there any significant downside to equipping with a larger solar array & charge controller and a smaller battery bank provided that the battery bank is adequate to support my "dark time" loads?

Thanks.

S

[Sunking]

My "big loads" are woodworking tools that will (in the absence of shore power) be run from my 3kw inverter. I do not need 3kw continuous but I have found that I need a larger inverter to support the startup current on the motors.

A 3 Kw Inverter dictates minimum size requirements you cannot cheat. First thing is does it mandates a 48 volt battery which is actually a good thing because it lowers the cost on equipment, material, and wiring. Also lowers fire risk.

First thing the 3 Kw Inverter dictates the minimum battery size to handle those start up currents. At 48 volts you are looking at either 500 AH FLA battery of 250 AH AGM battery . Now that might make you think a 48 volt 250 AH battery would be less expensive than a 48 volt 500 AH battery but you would be wrong as it works out to 400% higher cost. Both a 500 AH FLA and a 250 AH AGM cost the same, but the AGM will only last half as long as the FLA.

Having said that an AGM can save you some coins initially on panel cost because it requires half the wattage to support. But there is a catch. Can you really operate on half the panel wattage? I cannot answer that because you have not stated how many watt hours you need in a day.

Any way a 48 volt 500 AH battery needs a minimum 2000 watt panel with a 40 amp MPPT controller. A 48 volt 250 AH AGM battery needs a 1000 watt panel with a 20 Amp MPPT Controller. Good luck finding a 20 amp controller made for 48 volts. Midnite Solar make the Jr which is 30 amps and would work great.

Now to your question. Not only do batteries have a maximum discharge rate, they also have a maximum charge rate. For FLA max rate is generically C/8 and some called hybrids can take C/6. Where C = Amp Hour Capacity and X = hours like 8 and 6.. So a 60 AH battery charged or discharged at C/6 = 60 AH / 6 H = 10 amps.

AGM can be charged and discharged faster than FLA. Generically C/4 so that same 60 AH battery can be charged/discharged at 15 amps. Some specialty AGMS made for UPS can be discharged at 4C, so that 60 AH battery can be discharged at 240 amps for about 6 minutes until dead. Catch is those specialty AGM batteries only have a 100 or so cycles in them.

There is another possibility using LFP batteries, but I don't think you are a candidate because you do not have the knowledge to operate them safely or without destroying them. If you are on a budget, absolutely not for you. A 48 volt 100 AH pack cost $2000 and with just one mistake they turn to boat anchors.

[Mike90250]

My opinion follows.
At LEAST a 24V system. Could be built out of 4 Golf Cart batteries. If you have space for 8 of them, then I would go for the 48V rig. Sad that neither will plug into the RV 12V system. Asking a 12V system to deliver the starting surge for the tools (even 120v house lights often dim when a 4A saw is first powered up) is nuts, It will work for a while, but eventually, something gets "tired" and gives out.
Overpaneling a battery bank calls for AGM batteries, because flooded can't accept high charge rate. But maybe you can game the system, and the batteries won't see high charge rates, as sun comes up, charging starts, and by 11am, the batteries will be close to full and not accept high rate, This problem lessens with the 48V system compared to the 24v.

[PNW_Steve]

Thanks for the replies.

My question:

As the majority of my power consumption occurs while the sun is shining and I have limitations on space & budget for batteries I began to wonder how practical it would be to "oversize" my solar array compared to battery capacity in order to support larger loads ONLY when the sun is shining and install just enough battery to support my typical night time loads.

My "big loads" are woodworking tools that will (in the absence of shore power) be run from my 3kw inverter. I do not need 3kw continuous but I have found that I need a larger inverter to support the startup current on the motors.

Is there a downside to equipping the rig with 1.2-1.5kw on high voltage panels, a 60amp mppt controller and a 24 volt 200 amp/hour battery bank? I know that typical system sizing would call for a bigger battery bank but I do not have the space or the budget to do it. Also, that battery bank will carry my night time loads just fine. My big loads occur between 10am and 5pm and generally on sunny days. I will also have generator backup for cloudy days if needed.

To summarize: Is there any significant downside to equipping with a larger solar array & charge controller and a smaller battery bank provided that the battery bank is adequate to support my "dark time" loads?

[PNW_Steve]

I am planning on 4 Trojan T-105 batteries in series. That is ALL I have space for. I DON'T need to run big loads from battery. I DON'T need any more than 1000 watts continuous from the inverter. My most commonly used tool that will run from that inverter consumes about 600 watts running. The 3kw inverter is due to the high LRC of the motor.

Did I give too much information in my first post? Or was my question not clear? Let me know I will try and phrase my question in an easier to understand manner.

Thanks again.

[Mike90250]

Overpaneling a battery bank calls for AGM batteries, because flooded can't accept high charge rate. But maybe you can game the system, and the batteries won't see high charge rates, as sun comes up, charging starts, and by 11am, the batteries will be close to full and not accept high rate, This problem lessens with the 48V system compared to the 24v.

[PNW_Steve]

I am not looking for high charge rate. I am looking to adequately charge 4 x T-105 batteries in series as well as have the ability to support an additional DC load from the panels during the day as well as charging the batteries.

Perhaps I need to start over and rephrase the question. It appears that I am failing to communicate effectively what I am trying to accomplish.

[Sunking]

I am not looking for high charge rate. I am looking to adequately charge 4 x T-105 batteries in series .

You have no choice or say about it. You are trying to say 2 + 2 = 3. You cannot change the math, its the law of physics. Batteries will take all the current the panels can provide. Panel wattage and battery capacity have to be matched. It is part of the design process. If the panel wattage is to low, the battery charge current will be to low and not capable of fully charging the battery allowing the batteries to stratify sulphate. FLA batteries need a minimum C/12 charge current, On the high side a T-105 can handle up to a C/6 charge current. If you go higher your batteries are going to spend way to much time at GASSING VOLTAGE which causes them to overheat, warp the plates and corrosion.

So regardless what voltage you use a T-105 requires a minimum charge current of 225 AH / 12 H = 18.75 Amps, and a maximum of 225 AH / 6 H = 37.5 Amps. Panel wattage and battery voltage determines the current. Charge Current = Panel Wattage / Battery Voltage. So with 4 T-105's regardless of voltage (6, 12, or 24) the minimum panel wattage is 475 watts and maximum is 950. watts. Consequently the largest load 4 - T-105's can handle if you include noon panel wattage is 1800 watts if you have a 900 watt panel and that is only at Solar Noon. Less and less at all other times. Panels do not produce their rated power, and max power is just for a few minutes around noon. Can you do everything in 30 minutes to an hour? If not you need more batteries and panel power.

No reason to rephrase your question, we understand. The issue is you do not understand the physics. You can try your way and initially you will get away with it. But the fiddler is going to come collect come around October or November.

[PNW_Steve]

Apparently I need to further my education.

I was under the impression that the charge controller would regulate the charging of the batteries not the panels. I am embarrassed that I missed the mark on such a basic concept. I wonder why they call it a "charge controller"?

From what you are saying I must undersize my panels to avoid boiling my batteries.

What I was wanting to do is install panel capacity in excess of what is required to charge my batteries. and use the excess capacity, when available, to run "opportunistic" loads.

Thank you for setting me straight.

[Mike90250]

plain simple english. You can do it. There is a slight risk of boiling batteries if they are low, a foggy morning, and then full sun comes out at 11am

it is not the best situation, but doable. 4 batteries wired for 24VDC is the best with what you say you have. 3Kw 12V inverters are fire bombs waiting to suddenly go off. Your usage of 4-5Kw starting surge puts about 220A through the system @ 24V, or 450A @ 12V. Boom someday.

The charge controller regulates the VOLTAGE to the batteries, not the amps. If you limit the amps with a deep menu setting, you limit the amps the controller can deliver to the saw and you end up running off the batteries and not solar.

[Sunking]

I was under the impression that the charge controller would regulate the charging of the batteries not the panels. I am embarrassed that I missed the mark on such a basic concept. I wonder why they call it a "charge controller"?

You are only half right. Charge Controllers control the voltage, not the current. Panel Power determines the current.

With MPPT Output Current = Panel Wattage / Battery Voltage. That is what you are missing. So if you have say a 2000 watt panel and a 24 volt battery you will need a 2000 watt / 24 volts = 83 amps. That means you need to buy at a minimum an 80 Amp controller which would be foolish, because at 48 volt battery you would only need a 40 Amp controller which is a lot less expensive. If you tried to do this at say 12 volts with 2000 watts is a fools game. Do the math, 2000 watts / 12 volts = 160 amps. You would need two very expensive 80 amp charge controllers and cables as big as your wrist. The controller you just need to make sure meets the minimum charge current output. Now you can over power a charge controller, and the charge controler would just the current of to save itself, but again that is somewhat foolish. If you have 1000 watts of panel on a 12 volt battery with a 60 amp controller you only get 800 watts of power from a 1000 watt panel.

So what you are missing is Charge Current is determined and controlled by PANEL WATTAGE. All the controller does is control the voltage. Here is what you can quickly learn and why we know instantly If you have say a 400 AH battery, you want a C/10 charge current and C/10 on a 400 AH battery is 40 amps. Real damn easy to find the required panel wattage, all you need to know is the Battery Voltage. Power = Amps x Volts.

12 volts x 40 amps = 500 watts
24 volts x 40 amps = 1000 watts
48 volts x 40 amps = 2000 watts.

So darn easy a 5th grader can do it.

[Sunking]

Now that I beat you up enough, I will help you up off the turf and give you a chance to take another shot at it.

You have goe about the wrong way. If you saw or gizmo has a 3000 watt starting power doe snot mean you need a 3000 watt Inverter. Not even remotely close. If you buy a quality 24 volt TSW Inverter it needs to be rated 1500/3000 watt which means 1500 watt continuous, 3000 watt peak for x seconds. Does that make you smile? It should.

At 1500 watts is doable with 24 volts and a 24 volt 250 AH battery. This you will like even more. Look at a US Battery model US 145-XCT. Those are 6-Volt 250 AH battery and will cost a little less than a Trojan T-105 @ 225 AH capacity. US Battery is Trojans competition and has slightly lower Internal Resistance and can easily handle a C/4 discharge current. To do that you have to give up some cycle life so instead of a T-105 lasting 3 to 4 years you get 2 to 3 years.

A 250 AH battery at 24 volts, all you need is 600 watts of panels and a minimum 25 amp high quality MPPT Charge Controller. In fact I would recommend a MS TS-MPPT-45 as that will allow you to grow to 1200 watts @ 24 volt battery. If you want to shave a few more penny's then the TS-MPPT-30 will work and allow you to grow to 800 watts @ 24 volt battery.

Here is your Inverter. The 24 volt model.

I just saved you a lot of money, should work, and in your budget.

[Wy_White_Wolf]

I am not looking for high charge rate. I am looking to adequately charge 4 x T-105 batteries in series as well as have the ability to support an additional DC load from the panels during the day as well as charging the batteries.

Perhaps I need to start over and rephrase the question. It appears that I am failing to communicate effectively what I am trying to accomplish.

I think you're failing to understand that there will be times your bank has to absorb the full output of the array. Such as when you shut off a tool. The battery will not be fully charged then as it had to supply at least the startup surge. If the battery is not large enough to handle that surge it will go

KABOOM!!!!

Go back and read SunKings original answer for what you need.

WWW

[Mike90250]

So, last count, 3 VERY knowledgeable people have said it can work (for a while), and all 3 have said that someday it will go Boom ! (or kaboom)

Eventually, events will stack up, and the batteries will be hit with a large surge of power from the PV panels. When the are weak (old, tired) this surge will be fatal to the bank, Till then, all will be fine. (unless the motor starting surge fries the inverter)

[bcroe]

Isn't there an MPPT controller out there, which can be set to limit the output current (like my grid tie)? Bruce Roe