MPPT Controller Sizing

Alaska Man · March 2014

I have a battery bank of 8-220ah 6 V batteries wired in series and parallel for 12V. A Zentrex Freedom Series Inverter(1,500W)/Charger (150Amp) with a 300Amp disconnect to the house. The house is 120V AC. This past winter I used my generator to recharge my bank, but I now have (3) 230W 12V panels I would like to hook-up this spring.

We are pretty conservative on our loads. All Led lights and T.V. with a 12 volt dc pump for water and an on-demand propane water heater. Our nomal loads run between 40W-80W for the house. There are times that we do draw up to 280w-350w (Stereo Cranked).

So two questions.

1. Would a 60A MPPT be about right?
2. When producing power from the solar array to charge the batteries would the excess power pass through the inverter and power the house like when I'm running my Generator?

westbranch · March 2014

Re: MPPT Controller Sizing

a bit more info will help evaluate your proposal.
First, loads, what is it you are running? Please tell us in watt * hours, as watts only tells us the instantaneous draw, time is an important factor here...

Your choice of charger will be dependant on the amount of power you will need to produce from your batteries.

Without knowing the exact specs of those panels, I estimate that those 3 panels will generate ! 48 Amps in 12 v configuration, at about 17V for recharging. so they would supply about a 5% charge rate for the entire bank (4 x 12v @ 880Ahr), so they are at the low end for charging another 2 or 3 would be good. Depending on what you want to do.

hth

Alaska Man · March 2014

Re: MPPT Controller Sizing

westbranch wrote: »

a bit more info will help evaluate your proposal.
First, loads, what is it you are running? Please tell us in watt * hours, as watts only tells us the instantaneous draw, time is an important factor here...

Your choice of charger will be dependant on the amount of power you will need to produce from your batteries.

Without knowing the exact specs of those panels, I estimate that those 3 panels will generate ! 48 Amps in 12 v configuration, at about 17V for recharging. so they would supply about a 5% charge rate for the entire bank (4 x 12v @ 880Ahr), so they are at the low end for charging another 2 or 3 would be good. Depending on what you want to do.

hth

Thanks for the reply

I have plans to add three more panels at some point. We run Led lights and an Led T.V. The lights draw 4w x 12 hours = 48Wh each x 4 lights =192 Wh/day The T.V. draws 40W X 12 Hours = 480Wh/day This is the normal load.

** I should note that the sun shines pretty much 24/7 up here in the summer and only 3 hours during the deep of winter. I will still use my generator to float once a week if I have to. We are totaly off the grid up here.

westbranch · March 2014

Re: MPPT Controller Sizing

You might want to consider getting those extra 3 panels now and cut your gen run time as much as possible. I would wager that you could use as much as 12 panels in the winter based on only getting 3 hrs of sun, as I bet a lot of it is just squeaking over the horizon...
How far will the panels be from the battery bank? How far to the house? There are some design features to consider...

Alaska Man · March 2014

Re: MPPT Controller Sizing

westbranch wrote: »

You might want to consider getting those extra 3 panels now and cut your gen run time as much as possible. I would wager that you could use as much as 12 panels in the winter based on only getting 3 hrs of sun, as I bet a lot of it is just squeaking over the horizon...
How far will the panels be from the battery bank? How far to the house? There are some design features to consider...

The panels will have a max 20' run to the Charge Controller. The Charge Controller will be mounted next to the battery bank. 12 panels would be a dream. I just don't have that kind of money. It's taken me 5 years of scimping and saving just to get where I'm at with my system now.

Like I said, I'll add 3 more panels and another controller in time.

westbranch · March 2014

Re: MPPT Controller Sizing

Look around you should be able to find panels at $1 or less / watt, quality ones too...

back to the question of an MPPT controller, the max wattage, Vmp and amperage will help you choose the CC for your setup. Depending how you see the end product. A MidNite Solar KID might be more than adequate http://www.solar-electric.com/midnite-solar-kid-controller-accessories.html

Alaska Man · March 2014

Re: MPPT Controller Sizing

westbranch wrote: »

Look around you should be able to find panels at $1 or less / watt, quality ones too...

back to the question of an MPPT controller, the max wattage, Vmp and amperage will help you choose the CC for your setup. Depending how you see the end product. A MidNite Solar KID might be more than adequate http://www.solar-electric.com/midnite-solar-kid-controller-accessories.html

Right on!! Thanks

vtmaps · March 2014

Re: MPPT Controller Sizing

Alaska Man wrote: »

I have a battery bank of 8-220ah 6 V batteries wired in series and parallel for 12V.

Welcome to the forum,
just have a comment on your system...

I suggest you invest in a DC current clamp meter.

You have 4 parallel batteries. That is not considered best design practice. Parallel batteries can and do work, but they are unstable and ultimately lead to a shortened life for the bank. The reason is that the life of the bank is determined by the weakest, shortest-lived cell in the bank. You have 24 battery cells in your bank and one of them will go bad before the others. When one cell inevitably fails, at first it will be masked by the other parallel battery strings. By the time you notice something wrong, it has damaged the rest of the bank. It happens quickly.

A DC current clamp meter will allow you to quickly check (every week) to make sure the charging current is equal among your four battery strings. When one string ultimately fails, you will see it in the current readings. You can disconnect the bad string and get by on 3 strings while you think about new batteries. If another cell goes bad in another string, you can take the two good batteries (from the two bad strings) and make a string.

If you pursue the strategy of regularly removing bad batteries from your bank, you will get significantly longer life from the bank... a bad cell is quickly bad for the entire bank.

Short discussion here about parallel batteries.

--vtMaps

MPPT Controller Sizing

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