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Author Topic: Off gird battery voltage  (Read 7633 times)
nominous
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« on: August 13, 2015, 10:44:45 AM »

I'm curious what voltages people who are off gird run their battery packs at?

24V, 48V, higher ?

What are you running at and what is the determining factor for that ?
Inverter input ? Safety ? Charging controllers, etc ?
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biff
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« Reply #1 on: August 13, 2015, 11:33:05 AM »

Good question,Nominous,
                      The simple answer is,that everyone has their own ideas and they pick a voltage that suits them. Most folks start off at 12v on small scale installations, just to get to see how it works and then they realise that higher voltage travels better and the cables don,t have to be near as thick.
  Then they try and match inverters,,You do get 36v  and 90v inverters but not many. The most popular are 12v, 24v and 48v, all in, 48v seems to be the most popular with off-gridders
  With 48v you can buy a large 48v forklift pack to suit and there is a hell of a lot of power in that.
 But lets say you want a system that can dump big dc volatge into immersions, without using complicated boxs of tricks, say you go up to 120v like Roger and AG, The cables are even thinner again and the fuses are down to 20amps. However the voltage is at a dangerous dc level,,but knowing what you know and you are going to be extremely carefull even with lower 48 voltage,you forge ahead and go the 120v route. Then you would need 60 forklift cells in series, !
  So really, there are advantages and disadvantages but 12v is not really advisable for serious off grid work. All your finance could be taken up in jump lead thickness copper leads and really heavy connectors and the jump from 12v dc to 230vac is hard work for any electrical circuit.
   I would say, most off-gridders run 48v and even I have worked a 48v system in our shed for years. However going up to 120volt and the 60 forklift cells means that you grab the energy quicker and can run your system with very little discharge rate. We go for weeks without dropping below 125volts and our power demand is only a tickle on the bank.
  The other big advantage is that you can buy pv panels and configure them almost any old way to hit the 140v bracket in the strings without the nuisance of having to use a mppt controller so the system is a lot more reliable. Now these are my own conclusions and I know that other opinions differ but that is the beauty of the forum, somewhere in between lies the truth.
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jonesy
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« Reply #2 on: August 13, 2015, 12:22:43 PM »

I suspect you'll get a lot of answers on this thread.
I ran 24V/800Ah upto 1kva, then 96V/50Ah upto 3kva.  I also had 200V/7ah DC just to run CFL lights as this saved a huge amount of battery power over night. The ADSL router likewise was straight off the 24VDC.  The lighting battery was only 7Ah, but could run the house for a few weeks without charging, as the currents are around 10mA/lamp.  It came about after about a week of no sun in December, and the inverter quiescent + lights, router took too much, it all adds up.  I came within a few hours of needing a genny, then the sun came out.
My choice was based partly what I had to hand + spares.  I also had 48V/2kva gear, but as I had no spare that only lasted a week after the fan failed and it went pop very noisily.
My 1kva inverter wasted ie quiescent about 25W. The 3kVA about 100W. I ran the 1kva most of the time as our base load was trivial and was often less than the quiescent.  I switched over to the 3kva for the washing machine, big tools etc. No inverters ran over night as that was simply a waste of 25W x 8 hours - batteries are expensive so I used to molly coddle them.
I'd never go lower than 24V.  It is always the battery amps that scare me.  A 2.4kW load needs 100A @ 24V.  That's a thick, expensive cable to keep the volts drop down.  Equally, you need a good battery to delivery it. Get to 96V and it's only 25A, and a 4mm cable is plenty adequate, or 2 x 2.5mm, which is often lying around the house.
I havent looked for a couple of years, but 48V inverters were tricky to source. The newest sunny island is 48V up to 6kva, with an impressive quiescent of 30W (IIRC) Equally charge controllers above 48V are hard to find.  I used to use 2 x tristar stacked to get 96V. I did try direct panel connection like Biff, but as the batteries approached full charge, the panel open circuit voltage pushed me too high and the inverter tripped out on high volts.
I'd say there is no real extra safety issues with higher battery voltages.  You need to cover all joints either with a nice box, or individual covers at any voltage. Whilst there are regulations for touch voltage (50V), these tend to be based around adults.  Pets/animals and toddlers are inquisitive and can be injured on much lower voltages/currents.
Where some people fail on off-grid is incorrect (or non existent) neutral earthing and protection on the AC side. A 4kVA sunny island SI4.4M-11 (for example, but its likely all inverters are like this) won't clear a 10A type B MCB (6A max), but plenty install 32A MCBs for the ring main etc, so straight away a live to earth fault on say a washing machine case won't clear the breaker in the required 0.4s.
Protection and safety are tricky at any voltage.
As Biff says, each to their own.
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billi
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« Reply #3 on: August 13, 2015, 12:38:21 PM »

48 volt would be my choice  today ,   but my 24 volt setup  is performing fine  , since years
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Billy
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« Reply #4 on: August 13, 2015, 07:16:22 PM »

Ditto.  I would go 48V but like billi says my 24V is fine and dandy after nine years.   Grin
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marcus
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« Reply #5 on: August 13, 2015, 07:58:36 PM »

if you get a UPS/inverter at a good price then that defines your operating voltage.

otherwise, as a rule of thumb i'd say:

small system running most things at battery voltage with occasional/small inverter: 12v as there's lot's of stuff that'll run direct off the battery.

medium system with inverter, where you tend to keep loads to match available power coming in, and only run one 'big' load at a time then 24v is better than 12v and you can still get lots of stuff to run direct off the battery for optimum efficiency, and you won't need specialist d.c. breakers/switches.

full house system - if you want to run your house like you're on grid so OH/kids can plug anything in anytime - then 48v or more is probably best. Although there's nothing actually stopping you making such a system work at a lower voltage - it's just the currents & cables start getting heavy.

I'm running 24v and not planning to change - I live by myself and tend to be a bit minimalist when it comes to energy consumption.
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Justme
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« Reply #6 on: August 13, 2015, 08:46:38 PM »

Whilst I agree with the higher is better advice you can do it lower.

We started off with a small 12v system. Due to budget we could never change enough of it in one go to be able to change the voltage. So we now have a whole house 3kVa system run on 12v.

If the inverter/charger went now I would switch to 48v.

It would mean a reduction in battery capacity as we have 30 x 2v cells. So would need to take 6 cells out & rewire the rest to 48v.

Keep the cable short between inverter & battery.

Ours are still 120mm2 CSA. Recommended is 95mm2.
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« Reply #7 on: August 13, 2015, 11:13:14 PM »

I am running 24v, slightly less efficient than a 48v system but I am on and off grid so I donít need a huge amount of power or capacity from the off grid part.

24v means I only need half the number of batteries, but gives sufficient capacity for my needs.
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billi
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« Reply #8 on: August 14, 2015, 03:37:48 AM »

Quote
full house system - if you want to run your house like you're on grid so OH/kids can plug anything in anytime - then 48v or more is probably best. Although there's nothing actually stopping you making such a system work at a lower voltage - it's just the currents & cables start getting heavy.

hmm ....  i need/have  2 outback mppt  chargecontrollers for my 4000 watt pv  and 24 volt setup  .... in a 48 volt  system i only would need 1     , this is a big difference 
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« Reply #9 on: August 14, 2015, 06:44:32 AM »

I would say, 'off grid' caravan, garden shed or boat then 12 or 24v. On the other if you want to run a house and family and you want any kind of 'normal' life then get the highest voltage you're comfortable with. Me I'd go 60, 72 or even 120V if manufacturers started to make inverter/chargers of those voltages. if your comfortable with using a UPS then I see nothing wrong with going 400v if you're happy working with lethal voltages. For now I'll stick with 48v, one advantage of which seems to be that it's easier to pick up a cheap 48v inverter than a 24v one, seen a few large Victron's and Studer's go on eBlag for much less than the price of a 24v one. Indeed just bought a BNIB Outback GVFX 3048 for less than £700 delivered, almost £1000 below new price  Shocked

Cheers, Paul 
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nominous
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« Reply #10 on: August 14, 2015, 10:34:28 AM »

Thanks for the replies. It makes for very interesting reading.
What made me ask was because I've been reading the specs on older solar inverter units.
They tend to have very high minimum input voltages. But they can produce rock steady sine waves, grid isolated and lots of power.
Designed for long term use.

Going back to what someone said about wiring (in my car audio days I was running AWG4, a mate was running 0 if not 00 !!) that would make a lot of sense, but with the down side of more observance of protection and safety.
With more car battery packs likely to come on the market, these are held at higher voltage strings. Getting used to working on high DC voltage is going to become more common.


Myself, there is no chance I'll go off grid where I currently am. We don't have the space or the lifestyle to support it. In the future maybe, who knows...
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billi
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« Reply #11 on: August 14, 2015, 11:23:31 AM »


What made me ask was because I've been reading the specs on older solar inverter units.
They tend to have very high minimum input voltages. But they can produce rock steady sine waves, grid isolated and lots of power.
Designed for long term use.

can you  provide a link or a name of those inverters ? are you talking about battery-inverters ?
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heatherhopper
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« Reply #12 on: August 14, 2015, 12:06:48 PM »

24v system for coming up five years. Choice was dictated by budget (2x24v Inverters and 680ah bank were cheaper than 48v at the time) and original Turbine choice which was also a budget choice and intended as a stop gap. Given a larger budget 48v would have been the preference and when an upgrade is necessary (expect this to be when the batteries and/or inverters need replacing) this would still be the intention although it depends entirely on what is available at the time. All the reputable manufacturers have 48v inverters to choose from currently and there are plenty of s/h options but 24v is less well covered.
24v has been entirely adequate as things have turned out and there is a small bonus with the 180a charging capacity of the two inverters. A bigger bank would always be useful - at least 1000ah to reduce diesel use but this is very low anyway. All AC coupled makes a big difference of course. We now operate pretty much as a "normal" household although still weather dependent. This means all the white goods including tumble dryer and electric cooking as conditions dictate (these have been acquired retrospectively to take advantage of the excess power when available - which is 50%+ of the time).
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« Reply #13 on: August 14, 2015, 01:07:36 PM »

I don,t want to sound like I am blowing my trumpet here,
                                                   And I know that a lot of happy 48v off-gridders would scorn the 120v installations but you do get a real leg up with the higher 120v systems.
 There are the UPSs to match them and the heavy iron core low frequency inverters available as also.
 
. I regard this baby as the best thing going in inverters. This is built to not break down. It is designed to sort out its own problems and if something like lightening decides to knock it out, it will switch over to another brain and keep working. It will tell you exactly where the problem lies and when lightening did strike some years back and blew it,s brain, it was able to restart again with the Chinese inverter providing the boost. They are normally good cold starters, once it got up and running I could remove the boost and then it set about telling me what happened and what module I needed to leave it 100% again. The Brain Module cost £100.00 to replace s/h and took less than 2 minutes to do so while all the time the machine kept working. I kept it running so that it could tell me exactly what was going on.
  I grouse against the present day inverter builders simply because they are aware that the technology to design and built good machines already exists but they will not do so because they make more money out of all your misfortunes and every so many years you have to update, they add on all these totally unnecessary bells and whistles that set the model apart from the last one. They are money spinnners while your systems are lying crocked. To design a system based on modules would cut out the installers and the Buy 3 get one free lark ,join our club.
  It is a bit like bringing religion into politcs, it will impede advances in the long term.
   Off=gridders should only need to take a few minutes and replace a faulty module, then they can shop about at their leasure and find the replacement instead of getting clobbered for a ransom. Sure these inverters are guaranteed but you are paying dear for it and it don,t last forever. These companied would be better to take a leaf out of APC,s Symmetra rm and build better to last longer.
                                                                   Biff
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nominous
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« Reply #14 on: August 14, 2015, 02:00:16 PM »

can you  provide a link or a name of those inverters ? are you talking about battery-inverters ?

no. I'm talking about solar inverters. Or wind. The SMA stuff actually, but I suppose anything.
Unless I'm missing the key to the puzzle, they would be just as happy converting 300Vdc into 230Vac as they would from a solar panel.
But they wont do anything to manage your battery bank. You'll be doing that with something else.

The new old stock stuff which is not G83 and sells for buttons on ebay. Still worthy for off grid.
Cheap enough to have a spare or two lying around.
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