new system..24v or 48v?

jimbob01 · October 2013

hi guys..im revamping my system..i have existing 12v fridge,toilet and water pump ...and led lights
i have 6x 235 sharp panels @38v
i have 8x200ah 12v powersonic sealed lead acid
2x mstar 45 mppts
i need to buy a new inverter... thinking the outback vfx what are the ups and downs of 24 vs 48 v?
thanks jim

bill von novak · October 2013

Re: new system..24v or 48v?

48 volts - cheaper cables, can run them longer distances with the same loss, cheaper breakers, fewer losses overall, higher power
24 volts - more battery flexibility

In general the higher the voltage, the better.

niel · October 2013

Re: new system..24v or 48v?

a 48v battery bank would also allow you to need only 1 of the mstar 45a mppt ccs as 45a x 48v = 2160w. in either 24v or 48v setups you will need a converter to run your 12v loads.

the larger setup mandates more batteries and pvs when expanding. batteries would be 4 12v or 8 6v for 48v banks (rather than 1/2 that for 24v setups) and 3 of your pvs in series rather than 2 for 24v setups.

Surfpath · October 2013

Re: new system..24v or 48v?

jimbob01 wrote: »

hi guys..im revamping my system..i have existing 12v fridge,toilet and water pump ...and led lights
i have 6x 235 sharp panels @38v
i have 8x200ah 12v powersonic sealed lead acid
2x mstar 45 mppts
i need to buy a new inverter... thinking the outback vfx what are the ups and downs of 24 vs 48 v?
thanks jim

Howdy Jimbob.....
If you have 1410 watts of panels and plan to charge all eight 12v batteries (400 amp hrs), you may find yourself a little low on charging amps.

Again, this is not a hard and fast rule, but the general recommendation for a balanced system is ~ 10% peak charging current.

I have 2000w of panels on my 48v system and am probably OK for most of the year (~8/9% peak charging current)

I'll use the Coot's formula:

"When checking the array size against its ability to recharge a given battery bank the simplest thing to do is look for the peak charge current using this rule-of-thumb formula: Watts * 0.77 (typical efficiency) / nominal system Voltage = current."

So, in your case 1410 Watts * 0.77 = 1085.7 / 48 = 22.6 Amps.

Which is only 5.6% of the battery bank capacity at the "20 hour rate" (standard for RE use). That's on the low end I would imagine. Again, I am presuming you want to use all 8 of your batteries in your new 48v config.

Working backwards a 400amp hr bank's 10% charge current translates into about a 2,360 watt array.

I believe I have the math right. Hope that's helpful.
-SP

jimbob01 · October 2013

Re: new system..24v or 48v?

thanks guys..i do have another 800w of panels kicking around which i could configure to supplement the 1410w..just the roof of my boat is only so big...
but i would like to use my tig welder when the sun is out in the summer from the inverter
i have a westerbeke 15k gen set on board with a 4.5k yanmar as back up..
keeping the batteries charged over the time ive had solar has always been a matter of care.
im bending towards the 24v system as its easier to add batts...
great forum this for learning about stuff..thanks guys

mike95490 · October 2013

Re: new system..24v or 48v?

I believe it's MUCH more difficult to expand a 24v system, than a 48V system. I would go to 48V now, and get the required new equipment, if you think you will need to expand in the future.

Use your 4.5KW genset to bulk charge your batteries in the AM, and let the sun top them off the rest of the day. At least once a week, get them charged up fully.

Blackcherry04 · October 2013

Re: new system..24v or 48v?

I am a boater, I just never could get comfortable having 48v DC between me and the aluminum hull. I guess I have been lit up to many times with 24 V no matter how careful I am, everything is always wet. Any thoughts from the experts ??

BB. · October 2013

Re: new system..24v or 48v?

Over ~30 Volts is considered a shock hazard (60 Volts is the maximum "touchable" limit I have seen in any code--obviously wet conditions make things worse). And DC is "worse" than AC (causes your muscles to contract and can be difficult to pry loose if your hands are wrapped around a cable/metal structure/etc.).

And with AC, it is much easier to install a Ground Fault Circuit Interrupter (GFCI) outlet or breaker to reduce the chances of shock (there is no such DC device available).

If you are only using the power for AC loads--the amount of DC wiring is reduced. If you will be using a lot of DC loads, you may have to stick with 12 or 24 volt appliances/battery bank (48 volt native loads are not real common). AC loads are usually the best compromise for a home sized setup.

Look at your maximum loads and charging current... When you get much above 100 amps (~1,200 watts at 12 volts, ~2,400 watts at 24 volts), the size of the wiring and voltage drop becomes quite a difficult issue (lots of copper and $$$ to carry large amounts of current).

You may be best to stick with 24 volts maximum--But only if your AC (and DC) currents remain relatively small. If you are going to have high loads/charging currents--You may have to be go with 48 volts. High levels of currents are available from any lead acid battery bank--And dropping a battery cable against the aluminum hull is not my idea of fun either.

BB. wrote: »

I posted this link to 290 VDC on a standard AC breaker:
https://www.youtube.com/watch?v=HLX9cdB4TFQ

And here is what 300 amps of 12 VDC does to a nail/screw/bolt:
https://www.youtube.com/watch?v=Avg5mJw0qHc

Just to give an idea of what can happen with PV Power Systems--Very different than wiring your house for 120/240 VAC.

-Bill

With large power systems, you will have to be very careful in any case (large breakers to prevent excessive current flow and allow you to turn off power during servicing), well supported wiring (nothing that can flex/fail and short circuit), cover over bank to prevent tools from falling on bank, etc...

Don't let one fear drive you into a design that causes you more problems.

-Bill

Blackcherry04 · October 2013

Re: new system..24v or 48v?

I'v set a few 24V systems up and I always take a piece of Duct tape and put a warning somewhere around the battery bank. There always so many different type people that work on boats and have no clue what they are getting a hold of, 12 V is a mindset with most of them. You always end up leaning a hatch and grounded to something. I have a 24 V bank and have been doing something stupid ( like tightening a terminal ) bare foot and you can sure feel it.

Ozparker · October 2013

Re: new system..24v or 48v?

jimbob01 wrote: »

hi guys..im revamping my system..i have existing 12v fridge,toilet and water pump ...and led lights
i have 6x 235 sharp panels @38v
i have 8x200ah 12v powersonic sealed lead acid
2x mstar 45 mppts
i need to buy a new inverter... thinking the outback vfx what are the ups and downs of 24 vs 48 v?
thanks jim

Hi Jimbob, having a similar issue myself, tempted to go 48v but bit concerned about danger of such high voltage battery bank, also not so much choice in inverters
Great forum btw

jimbob01 · October 2013

Re: new system..24v or 48v?

thanks again,suitable for my needs is the 24v bank ...i think...im still open minded though.
i will wire everything in heavy cable,circuit breakers and fused panels...im on a boat so everything is close together,
just want to use my tig welder from the batterys(small jobs)in the summer...
this has never been a prob b4
looked at my roof again today and think ill ditch the 17v panels,buy another 2x 235w sharp 38vs
then ill end up with a better charge rate
so ill end up with 8x 235 sharp pv
2x mstar mppt 45s
i reckon the outback 3524 vfx (they are around £2000 in the uk) ahhh!
maybe i can get one shipped over from america...?
the mate..and 8x200ah12v powersonic sealed batts
...im getting close...anymore suggestions as to improving the set up gratefully recieved.jim.

westbranch · October 2013

Re: new system..24v or 48v?

Why not use the small gen set instead of PV/batteries when welding ? It will remove your heaviest demand from the calcs for the solar system.

jimbob01 · October 2013

Re: new system..24v or 48v?

thing is my loads arent heavy in the summer and im dumping power normally by 12 midday...
plus its a good feeling making stuff straight from the sun;-)
its the winter here in the uk where we dont get much sun...:-(

Cariboocoot · October 2013

Re: new system..24v or 48v?

Two questions:

1) What are the total Watt hours daily that need to be supplied?
2) What is the maximum Watt load at any given time?

In general you shift up to 48 Volts when you need the greater power it can provide. The downside is that there is a greater minimum battery expense and the circuit protection/disconnects can be more difficult to find/costly on account of having to handle the higher Voltage.

If you're looking at greater than 3kW steady use you would want to go to 48 Volts (125+ Amps @ 24 Volts, 63+ Amps @ 48 roughly speaking - it's always nice to keep the current below 100 Amps if possible). If you're looking at greater than 3kW hours stored capacity you may want to consider 48 Volt as well.

But if you don't know how much power you need to supply you're going to have a hard time designing any sort of system at any Voltage.

westbranch · October 2013

Re: new system..24v or 48v?

And how big a boat are you living on? Must need a fair bit of room to have a welder on board... Plus to have room for all those panels..

jimbob01 · October 2013

Re: new system..24v or 48v?

hi cariboocoot..things change here so quick in the uk...in the summer;
im only running a fridge which draws 1ah @12v
there is intermittent use of a water pump and toilet and no lights go on so hardly any draw from these...
if i get a welding job say making a small tank in the summer then the tig runs from the inverter for an hour intermittently at around 2kw via the 240v inverter.
winter...led lights total 50w@12v x maybe 6hrs
1 ah from the fridge and a few chargers..laptop etc
i guess i use around 80-100ah @12v from the batts in the winter
maximum load would be the inverter running at 3500w jim

jimbob01 · October 2013

Re: new system..24v or 48v?

hi west branch,we live on a 70x 13 barge(self built) and feel very fortunate to do so...my partner is just telling me how beautifull b.c is...sounds great.. been off grid a few years but not really sorted the pv out till now

Cariboocoot · October 2013

Re: new system..24v or 48v?

jimbob01 wrote: »

hi cariboocoot..things change here so quick in the uk...in the summer;
im only running a fridge which draws 1ah @12v
there is intermittent use of a water pump and toilet and no lights go on so hardly any draw from these...
if i get a welding job say making a small tank in the summer then the tig runs from the inverter for an hour intermittently at around 2kw via the 240v inverter.
winter...led lights total 50w@12v x maybe 6hrs
1 ah from the fridge and a few chargers..laptop etc
i guess i use around 80-100ah @12v from the batts in the winter
maximum load would be the inverter running at 3500w jim

When you change your system will you be switching things like the refrigerator and water pump to run on 230 VAC? What about the lights?

Right now your 100 Amp hours @ 12 Volts is about 1200 Watt hours, which is pretty easy to come up with. Even easier at 24 Volts.

The inverter may be 3500 Watts but that does not mean it will produce 3.5kW all the time; it will produce and draw according to the demand placed on it. When not needed it can be run in standby (automatically - FX consumes about 6 Watts as opposed to 20 when running) and use less power or even shut down completely if not needed.

So much depends on whether or not you will be rearranging your loads. if you intend to stick with 12 VDC loads you will need to adapt the new higher Voltage power system to run them. There's a couple of different ways of doing this, including DC to DC converter (easier to find one for 24 to 12 than for 48 to 12) or using a 12 Volt battery bank and an MPPT charger from the 24 Volt bank to keep it up, or keeping the two systems entirely separate.

So many possibilities.

jimbob01 · October 2013

Re: new system..24v or 48v?

yes im going to check but im pretty sure the toilet and fridge run from 12/24 so shouldnt be a problem running straight from the batts which would mean say a 30amp 24-12 converter just for 12v sockets and led lights...can feel an outback vfx 3524 inverter purchase coming on...:-)

westbranch · October 2013

Re: new system..24v or 48v?

jimbob01 wrote: »

hi west branch,we live on a 70x 13 barge(self built) and feel very fortunate to do so...my partner is just telling me how beautifull b.c is...sounds great.. been off grid a few years but not really sorted the pv out till now

So it is safe to assume for the winter you are moored at a dock, so stationary? This can affect your PV harvest otherwise.
Are you planing on adjustable panel mounts?

Yes BC is smashing in places. see my avatar, taken in early November.

ps sounds like a double width canal barge...?

Cariboocoot · October 2013

Re: new system..24v or 48v?

Of course B.C. in Winter is just one solid wall of white.

jimbob01 · October 2013

Re: new system..24v or 48v?

yes its a double width sort of canal barge..built from the back of a fag packet..we are moored mid river most of the time ,most of the panels will have adjustable tilt but unfortunatly not swivel .. winter november to jan they are best here at around 80 deg tilt...

jimbob01 · October 2013

Re: new system..24v or 48v?

so you get a lot of reflection from that white wall i guess..

westbranch · October 2013

Re: new system..24v or 48v?

On my first system I had the panels vertical in winter and got great production, as the reflection added a good jag...

new system..24v or 48v?

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