system balance

johnathanmatthew · June 2015

hey

, been reading a lot of threads here and on other forums the past couple weeks, and
it seems like you guys really know what you're talking about/ so maybe you can help clear some
confusion or doubts that I have about the system I'm setting up

I read somewhere that an Ideal balance would have the solar panels generating 5-13% of
the batteries capacity rating, with a DOD somewhere between 20-50%

something like using one kwh a day with-

(four) 100 watt panels @ around 5 amps each

(one) 30A PWM charge controller

(two) Sam's club 6v 215Ah GC2 batteries

the batteries seem to be a limiting factor here for me. In the area here, of the batteries or
brands I've heard recommended, I can only find:

Trojan SCS 12v 130Ah for $250

Sam's club 6v 215Ah for $89 each

the guy at this place Battery Post also recommended a Trojan 12v 105 Ah, though i'm not
sure about the price, so i'm gonna call tomorrow

the only batteries that i've heard people recommend on forums is the T105 and cheap
GC2 batteries, and so i'm not sure how good of a choice these other Trojans are

_____________________________________________

my goals are, at the core, to run at least one 100 watt water pump for half an hour, and a
300 watt electric rice cooker for about half an hour. I'd possibly want to run another water
pump also, with similar wattage.

right now I actually have two 100 watt panels, and I'd much rather use just these two, and a
lower capacity battery, to save $300 on the panels that I don't have. if the Sam's club GC2's
are cheaper and higher capacity though, and I need to buy more panels to keep a good
balance, I definitely want to consider it. I can always find and plan on more things
to power.

I'm looking for suggestions on how to acheive these goals, using any variation really that you
can think of. As far as batteries go, I have access to Wal mart, Sam's club, Costco, and
a few places like Autozone, but maybe you know of battery sources that i'm not thinking of.

how could I achieve these goals without buying more stuff than I need to?

lkruper · June 2015

johnathanmatthew wrote: »

hey , been reading a lot of threads here and on other forums the past couple weeks, and
it seems like you guys really know what you're talking about/ so maybe you can help clear some
confusion or doubts that I have about the system I'm setting up

I read somewhere that an Ideal balance would have the solar panels generating 5-13% of
the batteries capacity rating, with a DOD somewhere between 20-50%

something like using one kwh a day with-

(four) 100 watt panels @ ~5 amps each
(one) 30A PWM charge controller
(two) Sam's club 6v 215Ah GC2 batteries

the batteries seem to be a limiting factor here for me. In the area here, of the batteries or
brands I've heard recommended, I can only find:

Trojan SCS 12v 130Ah for $250
Sam's club 6v 215Ah for $89 each

The guy at this place Battery Post also recommended a Trojan 12v 105 Ah, though i'm not
sure about the price, so i'm gonna call tomorrow
_____________________________________________

my goals are, at the core, to run at least one 100 watt water pump for half an hour, and a
300 watt electric rice cooker for about half an hour. I'd possibly want to run another water
pump also, with similar wattage.

right now I actually have two 100 watt panels, and I'd much rather use just these two, or even
just one panel, and a lower capacity battery, to save $300 on the panels that I don't have.
if the Sam's club GC2's are cheaper and higher capacity though, and I need to buy more panels
to keep a good balance, I definitely want to consider it. I can always find and plan on more things
to power.

I'm looking for suggestions on how to acheive these goals, using any variation really that you
can think of. I'd love to do two systems of a 100 watt panel each, but I don't know if that would
make sense given what batteries are available or advisable. As far as batteries go, I have access
to Wal mart, Sam's club, Costco, and a few places like Autozone.

so you probably know something that I don't/ how could I achieve these goals without buying
more stuff than I need to?

I am not an expert, so someone jump in if I make a mistake.

Battery

300w + 200 w for 1/2 hour is 250w for one hour.
250w / 12v = 21 amp hours
5 day autonomy would require about 100 ah.
So the 105 AH Trojan 12v battery would be sufficient and simpler to hook up that 2 6V.

Charge Rate

5% is too low of a charge rate. Trojan recommends 10-13% of the 20 hr AH capacity. You don't need more than a 10 amp charge controller.

Panels

What kind of solar exposure do you have? Your total of 20 amps in the four panels is more than you need. If you discharge your battery to 21 amp hours, that is 20% DOD. It will take you 2 hours to charge @ 10 amps per hour plus 2 hours to float. You will need at least 4 hours
of solar exposure even in the winter to charge your batteries.

Inverter
=========
You will also need an inverter to take your 12v to 110v. Don't get one that is too big as they have about 80-90% inefficiency based upon the total capacity of the inverter. This is trickier. The type of load matters. Motors and compressors can have an initial surge. For 12v don't go over a 1000w inverter. In your case, that should not be a problem.

Batteries are expensive and will only last a finite number of cycles. The less expensive Sam's Club golf cart batteries will not have the same number of cycles as real deep cycle batteries designed for renewable energy. You may only get 255 cycles for those batteries, but they may last 2 years if you take care of them. So, if you don't have a need for the capacity now, your batteries will still age and need to be replaced when they have exceeded their chronological age.

I will defer to those will more experience.

johnathanmatthew · June 2015

lkruper wrote: »

I am not an expert, so someone jump in if I make a mistake.

Battery
300w + 200 w for 1/2 hour is 250w for one hour.
250w / 12v = 21 amp hours
5 day autonomy would require about 100 ah.
So the 105 AH Trojan 12v battery would be sufficient and simpler to hook up that 2 6V.

Charge Rate
5% is too low of a charge rate. Trojan recommends 10-13% of the 20 hr AH capacity. You don't need more than a 10 amp charge controller.

Panels
What kind of solar exposure do you have? Your total of 20 amps in the four panels is more than you need. If you discharge your battery to 21 amp hours, that is 20% DOD. It will take you 2 hours to charge @ 10 amps per hour plus 2 hours to float. You will need at least 4 hours
of solar exposure even in the winter to charge your batteries.

Inverter
=========
You will also need an inverter to take your 12v to 110v. Don't get one that is too big as they have about 80-90% inefficiency based upon the total capacity of the inverter. This is trickier. The type of load matters. Motors and compressors can have an initial surge. For 12v don't go over a 1000w inverter. In your case, that should not be a problem.

Batteries are expensive and will only last a finite number of cycles. The less expensive Sam's Club golf cart batteries will not have the same number of cycles as real deep cycle batteries designed for renewable energy. You may only get 255 cycles for those batteries, but they may last 2 years if you take care of them. So, if you don't have a need for the capacity now, your batteries will still age and need to be replaced when they have exceeded their chronological age.

I will defer to those will more experience.

heck yeah, thanks. I didn't know about the 80-90% inefficiency.
I'm definitely planning on getting a less than 1000 watt inverter.
I've been looking around the 400 or 500 watt range right now.

this system would be set up in South Carolina, and I see numbers
for average full sun hours here in the 4.5 - 5 range.

lkruper · June 2015

johnathanmatthew wrote: »

heck yeah, thanks. I didn't know about the 80-90% inefficiency.
I'm definitely planning on getting a less than 1000 watt inverter.
I've been looking around the 400 or 500 watt range right now.

this system would be set up in South Carolina, and I see numbers
for average full sun hours here in the 4.5 - 5 range.

Is this off-grid? Because most people need more than a rice cooker and water pumps.

johnathanmatthew · June 2015

lkruper wrote: »

Is this off-grid? Because most people need more than a rice cooker and water pumps.

Yeah, there's a house here with grid power, but this specific location will be a significant distance from any grid power source.

zoneblue · June 2015

johnathanmatthew wrote: »

something like using one kwh a day with-
(four) 100 watt panels @ ~5 amps each
(one) 30A PWM charge controller
(two) Sam's club 6v 215Ah GC2 batteries

"something like". Better be sure because youre going to a lot of trouble to build any system and you want to get it right.

12V. I almost always recommend starting with a 24V system. (The main exception is RVs that are extensively wired with 12v stuff). The reasons are simple: less wire losses, less charge controller, more expansion options.

The PWM controller worth looking at right now is the Midnite Brat. A lot of controller for not much money, and a very low tare. Beware that you need 36/72 cell panels for PWM. And shortish array cabling.

Sams club batterys are a great place to start, everyone here likes them. Dont forget to budget for some wire, midnite breakers, and a wee box to put them in.

Lets see what you could do at 24V with that gear.

4x GC2s in series. = 215Ah at 24V , target peak charge rate of 0.1C = 22A
Midnite Brat happy up to 30A, 24V.
Midnite Big Baby box, with a few 30A 150V breakers.
PV needed. Bigger panels are cheaper if you can manage the frieght and installation.
-- Ideally: 72cell 300W panels, failing that pairs of 36 cell 150W panels.
-- Vmp 36V is all good for the Brat.
-- Imp 8.5A or so, thus 3 strings needed to meet the 22A charge rate, for 900W total array, (or 600W for 2 strings will work ok at 0.08C)

Depending on climate that ought to be good for 1800Wh/d. (2Wh/d per Wp) or more specificly:
- 4 sun hours x 900W X 50% end to end efficiency = 1800Wh/d

We dont generally worry too much any more about 'days autonomy'. Using this model generally gives around 2 days up your sleeve, and for nightmare weather either use a genset, or load shed as needed . But climate does vary so check the pvwatts for your area.

lkruper · June 2015

zoneblue wrote: »

"something like". Better be sure because youre going to a lot of trouble to build any system and you want to get it right.

12V. I almost always recommend starting with a 24V system. (The main exception is RVs that are extensively wired with 12v stuff). The reasons are simple: less wire losses, less charge controller, more expansion options.

The PWM controller worth looking at right now is the Midnite Brat. A lot of controller for not much money, and a very low tare. Beware that you need 36/72 cell panels for PWM. And shortish array cabling.

Sams club batterys are a great place to start, everyone here likes them. Dont forget to budget for some wire, midnite breakers, and a wee box to put them in.

Lets see what you could do at 24V with that gear.

4x GC2s in series. = 215Ah at 24V , target peak charge rate of 0.1C = 22A
Midnite Brat happy up to 30A, 24V.
Midnite Big Baby box, with a few 30A 150V breakers.
PV needed. Bigger panels are cheaper if you can manage the frieght and installation.
-- Ideally: 72cell 300W panels, failing that pairs of 36 cell 150W panels.
-- Vmp 36V is all good for the Brat.
-- Imp 8.5A or so, thus 3 strings needed to meet the 22A charge rate, for 900W total array, (or 600W for 2 strings will work ok at 0.08C)

Depending on climate that ought to be good for 1800Wh/d. (2Wh/d per Wp) or more specificly:
- 4 sun hours x 900W X 50% end to end efficiency = 1800Wh/d

We dont generally worry too much any more about 'days autonomy'. Using this model generally gives around 2 days up your sleeve, and for nightmare weather either use a genset, or load shed as needed . But climate does vary so check the pvwatts for your area.

I am going to spec out your recommendations for myself. If the power was out in an emergency I would need about 2000 Wh/d. However I calculated that the OP only needed 21 AH per day at 12v or 252 Wh/d. Wouldn't this system be more than he needs and thus more of an expense?

zoneblue · June 2015

Well the answer to that is a question. Whats the OP's load budget actually look like? Off grid life theres usually a defining load. If youve ever lived without a fridge for any period, youll be quick to say refrigeration. 1kWh/d wont run any kind of fridge, not so as to leave anything left over. 2kWh/d might if you were careful. 3kWh/d and you are really set.

But yes if he doesnt need 1800Wh/d, he can cut the pv back ( a bit), or supplment the charging with a genset, or .... run a 12v system, knowing its temporary/limited. No need to be dogmatic with such things

lkruper · June 2015

zoneblue wrote: »

Well the answer to that is a question. Whats the OP's load budget actually look like? Off grid life theres usually a defining load. If youve ever lived without a fridge for any period, youll be quick to say refrigeration. 1kWh/d wont run any kind of fridge, not so as to leave anything left over. 2kWh/d might if you were careful. 3kWh/d and you are really set.

But yes if he doesnt need 1800Wh/d, he can cut the pv back ( a bit), or supplment the charging with a genset, or .... run a 12v system, knowing its temporary/limited. No need to be dogmatic with such things

I thought his needs were unconventional, which is why I asked him why he only needed 1/2 hour of a 300w rice maker and 1/2 hour of a 100w water pump (or possibly another similar pump for 1/2 hour).

It almost sounds like a question on some kind of exam

lkruper · June 2015

zoneblue wrote: »

"something like". Better be sure because youre going to a lot of trouble to build any system and you want to get it right.

12V. I almost always recommend starting with a 24V system. (The main exception is RVs that are extensively wired with 12v stuff). The reasons are simple: less wire losses, less charge controller, more expansion options.

The PWM controller worth looking at right now is the Midnite Brat. A lot of controller for not much money, and a very low tare. Beware that you need 36/72 cell panels for PWM. And shortish array cabling.

Sams club batterys are a great place to start, everyone here likes them. Dont forget to budget for some wire, midnite breakers, and a wee box to put them in.

Lets see what you could do at 24V with that gear.

4x GC2s in series. = 215Ah at 24V , target peak charge rate of 0.1C = 22A
Midnite Brat happy up to 30A, 24V.
Midnite Big Baby box, with a few 30A 150V breakers.
PV needed. Bigger panels are cheaper if you can manage the frieght and installation.
-- Ideally: 72cell 300W panels, failing that pairs of 36 cell 150W panels.
-- Vmp 36V is all good for the Brat.
-- Imp 8.5A or so, thus 3 strings needed to meet the 22A charge rate, for 900W total array, (or 600W for 2 strings will work ok at 0.08C)

Depending on climate that ought to be good for 1800Wh/d. (2Wh/d per Wp) or more specificly:
- 4 sun hours x 900W X 50% end to end efficiency = 1800Wh/d

We dont generally worry too much any more about 'days autonomy'. Using this model generally gives around 2 days up your sleeve, and for nightmare weather either use a genset, or load shed as needed . But climate does vary so check the pvwatts for your area.

I read in the specs that the Brat has a built in battery temperature sensor. Does this adjust the voltage when charging based on temperature? Also, how does one adjust the amps for charging? And finally, at what battery voltage does the load controller shut down?

Also, don't you lose some capacity without MPPT on these 300W panels?

zoneblue · June 2015

As bill says, power use is a "highly personal set of choices". Rereading the OP:

"my goals are, at the core, to run at least one 100 watt water pump for half an hour, and a
300 watt electric rice cooker for about half an hour. I'd possibly want to run another water
pump also, with similar wattage"

The thing about mains power is there is almost no overhead cost (or marginal cost for that matter) to providing a small load like this. To build a system that guarantees those loads 365d/y, you end up spending and building a bigger system than you need. You can get around that with a genset, or other backup solution, but then thats more overhead cost.

If thats really the extent of the load budget mid to long term, then, well youve got, per day:

2 x 100W x 0.5hrs
1 x 300W x 0.5hrs
20% contingency
total 300Wh/d

Comments:
- thats a fair bit less than 1kWh/d
- if those are shurflo 2088/ 4008 style pumps then they draw closer to 50W, and are available in both 24v and12v
- no lights? device chargers?

Ok a load of 300Wh/d. Assuming, daily cycles to 25% DOD, Wh end to end efficency of 60%, then the indicated battery capacity is:
= 300Wh / 0.6 / 0.25
= 2000Wh
= 167Ah at 12V
= 83Ah at 24V

So thes OP's pair of GC2s is fine, especially as theres no inverter.

lkruper wrote: »

I read in the specs that the Brat has a built in battery temperature sensor. Does this adjust the voltage when charging based on temperature?

Yes.

Also, how does one adjust the amps for charging?

Theres dipswitchs to set a number of charging options, its not infinately adjustable but should most bases (except lithium).

And finally, at what battery voltage does the load controller shut down?

Try the manual :
https://www.google.co.nz/search?q=midnite+MNBRAT+manual

Also, don't you lose some capacity without MPPT on these 300W panels?

Panels are now cheaper than charge controllers. It wasnt alwasy thus.

system balance

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