# trying to put together a system from the parts I have

Posts: 2,511 admin
i have some deep cycle, group 24 (75 amphrs), lead acid, interstate brand batteries.

i also have some panels:
max power (Pmax)- 70 watts
open circuit voltage Voc(V)- 44 volts
short circuit voltage Isc(A)- 2.99
Operating Voltage Vmp(V)- 29.80
Current at Pmax or Imp(A)- 2.35
Maximum System Voltage(V)- 600
Module voltage(V)- 24

the operating voltage is too low to really run a 24 volt system, i think. especially because it is hot where i am using these.

my questions:
is 29.8 volts too low for a 24 volt system?
is it too high to run a 12 volt system?

thanks!

## Comments

• Posts: 1,832Registered Users, Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have

How many batteries, and how many PV modules (panels)?

More later,
Jim / crewzer
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

i am needing to run about 500 watts at most, for 3 to 4 hours at a time.

i have 4 batteries. i can get as many of these panles as I need. i just am not sure if they will work.
• Posts: 6,290 admin
29.8 Vmp: too high for 12V, too low for 24V?

It is hot as hell, the solar panels have a operating voltage max of 29.8, the batteries are 12V.

will that work or is it going to be a big headache? i have heard that you need at least 17.5 volts for a 12 volt battery charging system, but can it be as high as 29.8V?

i have also heard that a 24 volt system needs at least 35 volts to charge it, so my 29.8V panels will not work. does that sound right to you?

panels:
Maximum Power(Pmax)- 70 watts. Open circuit Voc(V)- 44 volts. Short circuit Isc(A)- 2.99. Operating Voltage Vmp(V)- 29.80. Current at Pmax or Imp(A)- 2.35. Maximum System Voltage(V)- 600. Module voltage(V)- 24
• Posts: 5,099Solar Expert ✭✭✭✭
Re: 29.8 Vmp: too high for 12V, too low for 24V?

what charge controller are you going to use?
Brand of panels? Number? Parrallel / Series?
Batteries ? brand? Type?

Eric

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Posts: 6,290 admin
Re: 29.8 Vmp: too high for 12V, too low for 24V?

I am trying to find out if I can charge 12V batteries with solar panels that run at 29.8 Vmp.

I guess my real question is, is there a charge controller that will allow me to do that? Also, is there an efficiency loss if I am dropping that much voltage (from 29.8 to say 14.5 or so to charge the batteries)?

the brand, number of panels and all that are irrelevant.

the batteries are 12v deep cycle, 75 amphrs, flooded not sealed.
• Posts: 10,311Solar Expert ✭✭✭✭
Re: 29.8 Vmp: too high for 12V, too low for 24V?

the mppt type controllers like the mx60 or the xw60 can handle what you describe very well. the bluesky controllers do not handle odd voltages well for conversion as their conversions are linearly based, ie if the voltage is low for a 24v charge voltage it will translate to a low 12v charge voltage. of course you should have the battery temperature sensor for the controller you choose.
• Posts: 10,311Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have

jim,
he is running 2 seperate, but similar posted threads.
• Posts: 28,079Super Moderators, Administrators admin
Re: 29.8 Vmp: too high for 12V, too low for 24V?

Kingfisher,

You are correct that you are wasting (or losing) almost 1/2 your available power from your solar panels...

P=V*I= 29v * 10 amps (example) = 290 watts

But, if you have a "simple" PWM type charge controller, the battery is just connected to the solar panels--basically shorting them to the actual charging voltage of the battery (say 14.5 volts). This does not hurt any thing (as long as the charge controller can manage the higher voltages)--but wastes energy...

P= 14.5 volts * 10 amps = 145 watts.

If you have a MPPT type controller (say the MX 60 from Outback), it would be able to almost double the charging current to the batteries during the bulk charging phase... However, because of the extra power required by the MPPT circuits, usually the minimum size of the solar panel array should be ~400 watts. In your case, because of the large difference in PV vs battery voltages, it may even make sense on a somewhat smaller systems (200-399 watts?--just a guess)...

And, if you got a MX 60, you could put two panels in series and charge even a 24 or 36 VDC battery bank (efficiently).

By the way, a MPPT type charge controller (Maximum Power Point Tracking) is basically a switch mode power supply that can take high voltages/low current and convert them down to a lower voltage with high current (the DC equivalent of an AC transformer).

In the end, you also have to ask whether or not you need more DC power for your batteries... If the panels/charge controller are already working just fine--then it would be a waste to change the controller out for a more expensive MPPT controller if the power is not needed.

Does that help?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 1,832Registered Users, Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have

Kingfisher,

I merged your two related discussion threads into one thread. It's much easier on all of us to keep track of one thread rather than bounce around from one to the next.

Have all of your questions been answered?

Regards,
Jim / crewzer
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

Thanks guys,

I have a small RV camper out on a peice of property in the woods. I need to be ableto charge a couple of laptops (200watts total), runs some CF bulbs (125watts total), run a fan (50 watts), a clock (?? watts), a radioor CD player (?? watts).

I am guessing at max i have a need for 325 watts.

based on that, I think 6 of these batteries will work fine.

I am only up there for a day or two at a time, once or twice a month. I am guessing that I only need about 50 to 100 watts of charging to make it work.

In order to mminimize cost, I need to decide if I should buy 2 of these solar panels (70W, 44Voc, 29.8Vmp as described below) for 200 bucks each and a charge controller as described by Bill; or, should I buy a cheaper charge controller and more expensive solar panels that dont need such a fancy charge controller.

I think I have all the info I need to answer that question now, and thanks very much! Any further input is appreciated!
• Posts: 7,967Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have
kingfisher wrote: »
runs some CF bulbs (125watts total),

I am guessing at max i have a need for 325 watts.
based on that, I think 6 of these batteries will work fine.

I am only up there for a day or two at a time, once or twice a month. I am guessing that I only need about 50 to 100 watts of charging to make it work.

In order to mminimize cost, I need to decide if I should buy 2 of these solar panels (70W, 44Voc, 29.8Vmp as described below) for 200 bucks each and a charge controller as described by Bill; or, should I buy a cheaper charge controller and more expensive solar panels that dont need such a fancy charge controller.

125Watts of CFL's is a lot of light !! (6 bulbs ?) or is this EQUILIVENT watts,
2, 15 watt CFL = 2, 60W incandescent bulb light output ??

Using 30V panels to charge 12V batteries, with a PWM charge controller, will waste a lot of power. Look for some 18 or 20V panels if you go the 12V route.

If you went to a 24V battery system, you may not be able to fully charge on the hottest days.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Posts: 28,079Super Moderators, Administrators admin
Re: trying to put together a system from the parts I have

Kingfisher,

You need to clarify the numbers a bit... 200 watts is like saying 20 mpg--we need to know how far--or in this case, 200 watts for how long--Watt*Hours...

Batteries are frequently rated in Amp*Hours, for that it is simply W*Hr/volts=Amp*Hours...

So, a small laptop would run 30 Watts * 7 Hours = 210 Watt*Hours.

A big desktop system + printer may run 200W*1 Hour= 200 Watt*Hours... So--accurate numbers and conservation are both important here.

A Kill-A-Watt meter will be very handy here... And for other loads, choosing something like a boom-box which was designed to run from D cells and AC is more power efficient than getting a typical home stereo system. In any case, the Kill-A-Watt meter will really help pin down your details.

Another issue is that you should probably recharge your weekend load in about 1-3 days--any longer and the batteries (if wet cell) can start to sulfate and will not last you as long...

So, when sizing the solar panels, you can approach it in two ways... Sizing the panels to recharge your weekend usage (larger panels), or size them for a smaller load and use a small generator (like the Honda, Yamaha, or other quiet generator) to recharge your batteries after the weekend use, and use the solar panels to trickle charge the system (much smaller panels and charge controller)...

Lastly, you need to determine when you will want the power (summer/winter)--that will also affect the size of your array.

Solar panels are expensive, and using them (at capacity) only a few days per month may not be the best use of your money at this time... A generator+solar trickle charger may make more sense...

Your call (and your money).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

Thats a very good idea BB. I have a small generator. I think i like this solution. Thanks!
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

SO my next question is:

If I need to maintain these 6 batteries for weeks at a time without discharge, it seems that I should have a charge controller that will provide PWM and occasional equalizing charges. What is the smallest wattage panel and the smallest charge controller to deliver what I need toi maintain these batteries? I will charge them with the generator, then leave them on maintenance for weeks at a time.

6 batteries
Interstate brand
Deep-cycle, flooded, not sealed
Group 24
new
• Posts: 7,967Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have

If you leave the site with the batteries fairly well charged, you only need to maintain a float charge, you won't need to equalize while on float. Perhaps if you run a short EQ cycle when you return to the place, the solar will have kept the batteries full, and your gen will only need to run 2 hours to complete an EQ cycle, and re-mix the acid standing in the cells.

So there are 2 possibilities,
Solar Float only ( ?? amps required )
Solar Float, with an EQ cycle mid week ( ?? amps required, more panels. )

An EQ cycle is often defined as holding a high voltage (Bulk setting, plus 1 extra volt every 12V) for 2 hours. The battery will determine how much current (amps) will flow under these conditions. Afterwards, the cells may need water added, from all the bubbling.

Float current may only need 2 or 3 amps, but mornings at sun-rise, it may take a tad more to bring cells back up to float, after they were dark/idle all night. You haven't said what your bank capacity is, float current should be some percent of that (2% ?)

Are these going to be in a 24V config (3 strings of 2) or 12V config (6 in parallel) ?
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Posts: 28,079Super Moderators, Administrators admin
Re: trying to put together a system from the parts I have

Jim or somebody else can probably give you a better answer--But, you probably don't need to have a "fancy controller" for this. I would even suggest that equalization with a "trickle charge" sized solar array is not really possible.

Ideally, you need a controller that is temperature compensated (if the controller is in the same "bay" as the batteries, it probably does not need a temperature probe). And it needs to be set for "float" voltage of around 13.02 to 13.20 volts (using an accurate voltmeter... Since it will only charge with the sun up (~5 hours per day), setting towards the 13.20 vdc range is probably warranted (you can set it a bit higher--just monitor your water levels--you don't want to see them drop hardly at all while on float).

The amount of current you will require... I have seen numbers of self discharge from around 1-2% per month, to 2% per week to 1% per day (for old batteries)...

So, the worst case average current would seem to size the panels for (assuming 12 vdc):

solar PV=6x75AH * 0.01 discharge/day /(0.85 (PTC PV rating)) * 1/5 hours per day of sun * 17.8 VDC (typical 12 vdc solar panel voltage rating)=19 watt panel...

So, a 20+ watt panel would seem to be the minimum size--you might want to go to 40-60 watt to allow for winter (less sun) and a bit of charging if you don't get them fully tanked up with the generator.

If you want to be able to perform equalization with the solar panels, you should be about 3%, minimum, of the AMP*Hour rating of your battery bank (to get good bubbling in the electrolyte for mixing) and be able to get to ~15.0-15.5 volts (depending on temperature).

=6*75AH * 0.03 /0.85 * 17.8 = 283 watts minimum for equalization by solar PV...

Get a good AC battery charger (like the IOTA brand) to make the best use of your generator time (lots of current early in the cycle, able to perform equalization with the generator).

I am sure Jim / Crewzer will have lots to add...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

BB,

Yeah, that was about what i had arrived at. The Sunguard Charge Controller by Morningstar costs only \$28, and is:
Rated Solar Input 4.5 Amps
Max. Input (5 min.) 5.5 Amps
System Voltage 12 Volts
Max. Solar Voltage 30 Volts
Regulation Voltage 14.1 Volts
Accuracy 60 mV
Self-consumption 6 mA
Temp. Compensation -28 mV/°C
Reverse Current Leakage <10 mA
Operating Temperature – 40 to +85°C

And since I probably wont always be able to or want to stand around for long enough to charge the batteries completely with the generator, Wi was figuring on getting a panel with 50 to 75 watts output and hoping that would finish the job.

I like this solution a lot, but, the next problem is: Does that Sunguard CC protect my batteries from overcharging? Do I even need to worry about overcharging with only 75 watts coming in?
• Posts: 28,079Super Moderators, Administrators admin
Re: trying to put together a system from the parts I have

Jim would probably be the person to answer this problem...

But, if you leave the voltage at 14.1 volts it will probably eventually boil the batteries dry if you did not check the electrolyte levels...

If it was going to sit for a few months without you coming by (every few weeks), I would adjust the voltage down to 13.2 volts (assuming it is adjustable). The 14.1 volt setting is probably a bit high for long term trickle charging. But if you can keep the water levels up--it might be OK...

I had a 1 amp trickle charger (12 watt--made for long term storage) and it boiled two storage batteries (probably 80-100 AH rated) dry over many months (don't know exactly how long--was long before I got on this board and really paid attention to this stuff)...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 1,832Registered Users, Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have

Kingfisher,

My recollection is that your location is heavily wooded. Woods (shadows) and PV don't mix particularly well, as the shadows/shade can dramatically reduce chrage current from the array. At a minimum, your PV array location should be shadow-free from 9 AM to 3 PM (minimum).

Cann you tell us more about your site?

Also, which model are your Group 24 size Interstate marine batteries: the SRM-24 or the HD24-DP? (They have different specs.)

More on PV array size and a controller later,
Jim / crewzer
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

Site:

frighteningly unshaded (but i have a shady spot for the batteries)

Battery model: SRM 24
• Posts: 6,290 admin
Re: trying to put together a system from the parts I have

Lat: 40.7355 (40:44:7.73)
Lon: 122.9663 (120:57:55.9)

actually, though the properety lacks much shade, but there is probably ~20% shade at noon at the spot where I will put the trailer (and the batteries go under the trailer). I can put the panels about 30 feet away and get virtually no shade, but that may be of no benefit.
• Posts: 1,832Registered Users, Solar Expert ✭✭✭✭
Re: trying to put together a system from the parts I have

Kingfisher,

I use semi-custom 12 V LED lamps to provide night guide illumination for my driveway, front walkway, and rear deck. The lamps draw ~0.5 A total for anywhere from ~ 8 hours/night (summer) to ~16 hours/night (winter). So, the light system's
worst case energy requirement is ~8 Ah/day.

I have a single Interstate SRM-27 12 V flooded-cell marine/RV battery for energy storage. The battery is located next to an outside north-east wall of my house. It’s recharged from a BP/Solarex 40 W PV module (16.8 Vmp x 2.38 A Imp) via a Morningstar SunLight 12 V x 10 A two-stage controller. Maintenance has been to add water two- or three times a year. But, after 6-1/2 years of service on my RV and then for the night lights, it’s starting to fade, and will soon be replaced.

Average December insolation on a south-facing module tilted up at latitude in my location is 3.1 hrs/day. My south-facing 40 W PV module is tilted up at 45 degrees. This provides a bit better winter performance. So, my small system should be able to generate an average of 3.1 hrs/day x 2.38 A = 7.38 Ah/day. This is ~50% more that my average worst day energy requirement.

The module’s mid-day output current of 2.38 A is ~3% of the battery’s rated capacity of ~85 Ah. That’s about the minimum charge current I recommend for a lightly discharged battery that is cycled daily (including charging during the day and self-discharge at night) and has no significant daytime loads.

Your SRM-24 batteries are rated at ~80 Ah each. Four wired in parallel would be rated for 320 Ah nominal. 3% of 320 Ah is 9.6 A, which would require a “12 V” module rated at ~17.4 V x 9.6 A = ~170 W STC.

For your standby application, I would think that you should be able to get by with mid-day recharge current in the 2% to 2-1/2% range. This would require a “12 V” module in the 123 W to 140 W (STC) range from BP, Kyocera, Sharp, Mitsubishi. See: http://store.solar-electric.com/hiposopa.html

The 12 V x 10 A Morningstar SunSaver is probably the minimum controller to use. It’s a two-stage controller (bulk and absorb) with a 14.4 V setting. That’s probably OK considering the low afternoon charge current available and the fact that your batteries aren’t floated at night. This controller includes built-in temperature compensation, so it’ll automatically adjust the absorb stage charge voltage for ambient conditions. If the controller and batteries are located in same environment, the charge voltage should be close enough. The SunLight controller’s additional night-time load control feature could come in handy at a later date and so might be worth the extra cost.

When connecting the charge to the battery bank, makes sure you connect the controller (+) lead to the (+) post on battery #1, and the controller (-) lead to the (-) post on battery #4. Do not connect both controller leads to the same battery, as this will cause the batteries to charge and discharge unequallly. Also, don’t forget a fuse (~10 A to 15 A) in the (+) lead between the controller and the battery bank, as well as a similar size fuse between (+) lead between the PV module and the controller.

Finally, your best bet for a high-quality battery charger for charging the batteries from the generator is the Xantrex TrueCharge 20+. It ain’t cheap, but, with an isolation transformer, battery-type selector, four-stage charging (bulk, absorb, float, EQ), and manual- or automatic temperature compensation, it charge your battery bank correctly on land or on a boat. This charger can get pretty warm while in operation, so it must be mounted vertically to promote convection cooling and avoid over-temp shutdown (first-hand experience!).

HTH,
Jim / crewzer
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