Changing from 12 volt to 24 volt

firefly

A friend of mine has asked me this question and I don't know the answer. He has a 12 volt system at present, which is made up of the following, two 12 volt inverters, run in series/parallel with a 240 volt transformer. Two FM80 CC, each controller has 6-170 watt panels. A Honda 6500 watt generator to help with charging. Three strings of batteries, each string has 3-4volt 1104 amp/hr Surrette batteries. Total amp/hr 3x1104= 3312amps. The daily house usage is approx. 950 amps.


What he wants to know is, if he changes his system over to 24 volt, (buying two 24 volt inverters) and dropping one string of batteries, will he still have sufficient stored amps at 24 volt to run his home? Or will he have to add another string of batteries making 12 in total. This would mean replacing the complete bank, so as not to add new batteries to his old ones. Any suggestions would be appreciated.

Steve

BB.

Re: Changing from 12 volt to 24 volt

How much of his load is day time vs night time usage? Any big starting load currents (well pump, etc.)?

To see how things would work--He could just disconnect one battery string and see what happens on his existing system.

By the way, why is he changing the existing system? Is it working for him as is, or are there "issues"?

-Bill

Cariboocoot

Re: Changing from 12 volt to 24 volt

The ol' equivalent power thing: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

Basically he's dropping 33% of his stored power by losing one string. He'd have 1104 Amp hours @ 24 Volts or up to 13 kW hours stored power. If that daily usage figure is 950 Watt hours (very low) this would be no problem. I find that number suspect, though. If it's 9500 Watt hours it would still work (at least for awhile) but be a rather dicey thing.

Now on the plus side he may not need two inverters but rather just one, depending on the way his AC is distributed. Plus, on 24 Volts one FM80 could handle all twelve 165 Watt panels (1980 Watt array).

However, that's a very small array for that much battery in any configuration. Looks to me like right now he gets a pitiful 3.8% charge rate. In the 24 Volt configuration this would be improved to between 5% and 6%.

If he's having trouble with this system the main fault may well be insufficient panels for the size of the battery bank.

Photowhit

Re: Changing from 12 volt to 24 volt

Cariboocoot wrote: »

If that daily usage figure is 950 Watt hours (very low) this would be no problem. I find that number suspect, though. If it's 9500 Watt hours it would still work (at least for awhile) but be a rather dicey thing.

...and if it's 950 amp hours at 12v nominal or around 12 Kw a day, I'm impressed that it's working for him now, must be a very sunny place or running the genny a lot.

Cariboocoot

Re: Changing from 12 volt to 24 volt

Photowhit wrote: »

...and if it's 950 amp hours at 12v nominal or around 12 Kw a day, I'm impressed that it's working for him now, must be a very sunny place or running the genny a lot.

Agreed. Especially with trying to recharge 3312 Amp hours of 12 Volt battery from 1980 Watts of array. Even if both FM80's were maxed out with panels it would be under 5% peak charge current.

Probably some performance improvement could be had by fully charging and EQ'ing the batteries and reducing the existing bank to the best six in two parallel strings with the diagonal wiring scheme.

firefly

Re: Changing from 12 volt to 24 volt

Thank all you guys for the input. The 950 amp is only an estimate and this figure is taken from the read out on a Trace Meter Battery Monitor. Their are other loads during the day which he is having difficulty in judging the draw. For example submersible water pump. sewage pump, radio phone, plus various LED light bulbs used at night. The winter time is the problem, no sun for days and yes alot of generator time, both mourning and night. Also gen time if they do laundry. He thought that going to a 24 volt system would make it easier on his equipment and by doing so he could almost double his panels for when the sun does show. Any and all the advise would be welcome.

Steve

Cariboocoot

Re: Changing from 12 volt to 24 volt

I'd suggest he doesn't spend a dime until he can get some more concrete numbers on power consumption.
A Kill-A-Watt can handle the 120 VAC stuff and 240 VAC if those loads are powered through a step-up transformer. Anything "hardwired" may need to be temporarily interrupted through plug and socket so the meter can take a reading. Any 12 VDC stuff is going to have to be measured out the hard way; Amp reading and timing.

As Photowhit said, 950 Amp hours is roughly 12 kW hours. That's quite a bit of power for off-grid, and way more than I'd recommend handling with a 12 Volt system. You need like 2,000 Amp hours of battery to run at 50% DOD. That's more than half the 3312 Amp hours he has. This would mean he could be discharging below 50% on a regular basis (depending on what's coming from the panels 'directly').

Things like water pumps may have very heavy demands for current but only run for a short time. Still this would benefit from higher system Voltage (reduced DC current).

So:
1). get firm numbers on power usage.
2). reduce usage as much as possible.
3). re-evaluate the battery bank size and system Voltage. Try for only 25% DOD.
4). adjust the PV array to meet at least the 10% peak charge rate.

westbranch

Re: Changing from 12 volt to 24 volt

Just add to Coots point#1 he should get a battery meter for his system and a DC clamp meter to be able to make sure each of those 3 banks are equally contributing or is one or more cells starting to fail.

Bat Monitors: http://www.solar-electric.com/nsearch.html?catalog=wind-sun&x=0&y=0&query=monitor

BB.

Re: Changing from 12 volt to 24 volt

And with that size of system--Should look at 48 volt too (yea, still the problem of not enough batteries--Perhaps start talking with the local battery distributors and see if there are some used batteries that they can purchase to make up the string).

-Bill

firefly

Re: Changing from 12 volt to 24 volt

Thanks guys, I have made a correction on the following, panels should read 6-170 watt on each controller. He does have a battery monitor, a Trace battery meter and I have a good AC/DC Clamp on. It has been suggested to him, that if he goes 24 volt, that one controller could handle all present panels, or maybe only 11 panels @ 170 watt. If he were to add 8-230 watt Solar world panels (VOC36.9,Vmpp29.6,Isc8.46,Impp7.76) on the other controller, would their be a conflict between the controllers? Also trying to find 3-4 volt batteries of the same age, is going to take some time. In the mean time can you suggest any way he can run this place on just the 6-4 volt, 1104amp @ 24 volt. He is trying to get exact usage numbers now, will get back with those when I have them. As many of you know, the winter time is a problem, with no sun for days at a time. Thanks again for your help.

Steve

Cariboocoot

Re: Changing from 12 volt to 24 volt

Okay, so there's another 60 Watts of panel. That won't make much difference to the final figures.

There would be no problem putting all twelve 170 Watt panels on one controller for a 24 Volt system. About 65 Amps peak output. There also would be no problem loading the second controller with eight 230 Watt panels. About 60 Amps peak output from that. The two controllers will work together, even if they are not connected via a HUB.

Before making the big switch up I'd check those batteries' specific gravity readings and see if they should be kept at all. Perhaps it would be more prudent to change them for a new set of matched cells.

Whether or not he can run the place on 1104 Amp hours @ 24 Volts in the interim depends on how much power is used and how often the generator is run. That's about 10 kW hours AC stored power at 50% DOD, which is really quite a lot for off-grid usage. Meanwhile the panels would only harvest less than half that; around 4 kW hours on a good day. If he can keep consumption below that it works from solar alone. If he can't or the weather is particularly uncooperative then the generator gets used.

firefly

Re: Changing from 12 volt to 24 volt

Thanks all, Carl has decided to go with the following. Changing from 12 volt to 24 volt and for the time reducing consumption (still no exact figures yet) and going with the 6-4 volt batteries 1104 amp/hr total.. The first FM80 will have 12-170 watt Suntech (Voc 43.8, Vmp 35.2, Isc 5.14, Imp 4.83) panels and the second FM80 will have 8-230 watt Solarworld ( Voc 36.9, Vmp 29.6, Isc 8.42, Imp 7.76) panels, a total wattage of 3880 watts. When he gets the other three batteries and increases to 12-4 volt, do you think his PV array will be sufficient?

Steve

BB.

Re: Changing from 12 volt to 24 volt

What does he have for backup charging (AC/DC genset, etc.?). No matter the (reasonable) size of the solar array, during this time of year the genset/alternate power source is going to be needed (I presume).

-Bill

firefly

Re: Changing from 12 volt to 24 volt

Hi Bill, the backup charging is a Honda 6500i generator. He has been using this for 2 hours in the mourning and the same at night, for almost 6 weeks. System will be changed over next week.

Steve

BB.

Re: Changing from 12 volt to 24 volt

Steve,

What is he using for the charger itself? I.e., a 100 amp @ XX volt--And what types of DC current (and AC if known) is he seeing during charging?

Basically, I am getting at is 1) he using fuel efficiently (running >50% of rated generator power vs 25% or less) and 2) is the DC charger behaving as expected/desired (running near max rated current at relatively high Absorb Voltage setting when batteries are less than ~80-90% State of Charge).

Also, is the generator being operated "efficiently" vs needed power... I.e., lots of DC Charging current in morning. Possibly less DC Charging current + other AC loads (washer/drier/lighting/cooking/etc.) to keep the generator utilized.

I like to suggest getting an old utility kWH meter and installing it on the generator output (and/or amp/Watt meter) and measuring fuel usage... Get utilization numbers (like % load and kWH generated per xx liters of fuel, etc.) to see if simple changes (such as generator scheduling, new/different AC Battery Charger, etc.) can help reduce fuel costs during this time of year (and ensure the battery bank is being well taken care of for longer life).

Fuel has got to be a killer--Probably something like 1/2 - 1 gallon per hour for that class of genset.

-Bill

firefly

Re: Changing from 12 volt to 24 volt

Hi Guys

I am keeping this thread alive, so questions and answers are easy to find. Due to factors beyond my control, this is the first opportunity to reply.

Charging of batteries is being done via 2 VFX2812 inverters, rated at 125 amp each. There are no DC loads on the system. We have found the batteries are very sulphated, EQ correction is being implemented. There is a Generator bypass on the system, so he can run the house separately and have the solar do the EQ. The system has not been changed from 12v to 24v yet, felt it was more important to get batteries back in shape, if we can.

A Kill O Watt meter was purchased, here is the latest figure, 4.5 Kwh so far of daily usage. Still have to get info on Fridge, Bell Radio Phone, Computer, Small battery chargers for Vacuum and drill.

They do try to be as efficient as possible, running heavy loads when the generator is in use. Some of his items are now on timers, ie. Freezer, Blue light for water system and TV/Satellite. He also have dropped one string of batteries, so he is running on 6-4 volt 2204 amp total.

At present he has three FM80 Charge Controllers, two in use one as spare. Each FM80 has 6 170 watt Suntech Panels for a total of 12, 2040 watts.

Now Outback says, 6 of these panels are to much for their controller, max 850 watts or 5 panels. Don't understand since their manual says 1250 watt Max for FM80.

After ice out, we plan on adding 8 Solarworld 235 watt Poly Voc 37, Vmp 30, Isc 8.35, Imp 7.85 panels to the system,on one FM80, the other will have all 12 170 watt panels. Maybe have to run only 11 panels in the winter. Also changing inverters to 24 volt.

What do you think, knowing his daily usage of 4.5 kwh plus, a 24 volt battery bank of 1104 amp hr. and a total of 3920 watts of panels? Or will he have to double his battery bank?

Your help is very much appreciated.

Steven M

vtmaps

Re: Changing from 12 volt to 24 volt

firefly wrote: »

We have found the batteries are very sulphated, EQ correction is being implemented. There is a Generator bypass on the system, so he can run the house separately and have the solar do the EQ. The system has not been changed from 12v to 24v yet, felt it was more important to get batteries back in shape, if we can.

It may not be possible to get the batteries in shape while they are in three parallel strings. Does your DC clamp meter show equal currents in each string? When you turn off the charger, it is possible that one string is discharging into another string.

In this situation it may be best to disconnect two strings and try charging one string at a time.

from the OP:

What he wants to know is, if he changes his system over to 24 volt, (buying two 24 volt inverters)

When he goes 24 volts, does he really need two inverters? It is more efficient to be running one inverter.

Also, why are you even considering 24 volt? There are no DC loads that need 12 or 24 volts so he could go to 48 volts. That has a number of advantages, especially with regards to the batteries. He might be able to get down to one string.

--vtMaps

Cariboocoot

Re: Changing from 12 volt to 24 volt

firefly wrote: »

At present he has three FM80 Charge Controllers, two in use one as spare. Each FM80 has 6 170 watt Suntech Panels for a total of 12, 2040 watts.

Now Outback says, 6 of these panels are to much for their controller, max 850 watts or 5 panels. Don't understand since their manual says 1250 watt Max for FM80.

The problem here may not be the Wattage but the configuration, depending on what the Voltage of those panels is. A string of them may exceed the V-max in of the FM80. If they are Vmp 35/Voc 43 then three in series would have a Voc of 129 which could go over 150 in cold temps. How they'd figure on using five panels I have no idea. All in parallel I guess. In which case you could use six in parallel just as well. Sometimes computer tools don't work as well as they should.

After ice out, we plan on adding 8 Solarworld 235 watt Poly Voc 37, Vmp 30, Isc 8.35, Imp 7.85 panels to the system,on one FM80, the other will have all 12 170 watt panels. Maybe have to run only 11 panels in the winter. Also changing inverters to 24 volt.

What do you think, knowing his daily usage of 4.5 kwh plus, a 24 volt battery bank of 1104 amp hr. and a total of 3920 watts of panels? Or will he have to double his battery bank?

With a total of 3920 Watt of array you could reasonably expect over 8 kW AC of 'harvest'. It should also be more than sufficient to recharge 1104 Amp hours of battery (and how is that going to be configured, as it is a very large bank?) with a total peak output current around 125 Amps.

1104 Amp hours @ 24 Volts & 50% DOD would be approximately 12 kW hours AC capacity, so keeping the DOD down to 25% would yield roughly 6 kW hours.

It looks viable so far.

firefly

Re: Changing from 12 volt to 24 volt

Cariboocoot, thanks for your help.

The battery bank will consist of 6-4 volt 1104 amp hr., Surrette's wired @ 24 volt.

Steven M

Cariboocoot

Re: Changing from 12 volt to 24 volt

firefly wrote: »

Cariboocoot, thanks for your help.

The battery bank will consist of 6-4 volt 1104 amp hr., Surrette's wired @ 24 volt.

Steven M

Good; no dozens of parallel strings of very small batteries.

firefly

Re: Changing from 12 volt to 24 volt

Hi Guys

Moving on to another part of this make over. Grounding the entire system. I have read many posts and I hope I have got it right. The attached rough drawing is how I believe it should be wired. I will try to explain.

Both panel racks grounded to earth rod, each array of panels is connected to combiner box via one continuous #6 wire. Combiner boxes where attached to frame, but with 2x6 wood between frame and box. Then #6 wire run to DC disconnect.

Question, can I join both #6 wires from Combiner boxes and just run one to DC disconnect?

I have run grounds (bonds) from Inverters and Charge Controllers to DC disconnect.

Question, can I join via Burndy Servit the DC ground to the AC ground at a point after AC ground leaves Main AC panel?

The generator is frame grounded to grounding rod, also grounded (bonded) to AC disconnect. PSX 240 grounded (bonded) to AC disconnect also, the ground (bond) wire run to Main House breaker panel. At ground Buss point #6 wire runs to Ground Rod.

On another point, should we be using Surge Protectors on the combiner boxes?


This array is very close to the lake.

Attachment not found.

Cariboocoot

Re: Changing from 12 volt to 24 volt

With the caveats that I might not understand the questions and you can always run up against an idiot of an inspector ...

Yes, the ground wires for the two arrays can join at the combiner box ('permanent' splice required) and continue on as one.

Yes, the DC and AC ground can both clamp on to the same grounding rod.

Yes, surge protectors on arrays can be a very good idea. http://www.solar-electric.com/suprde.html Note the SPD300 model.