11 minutes ago, The Terrible Triplett said:

No, I never said that.

Taking your analogy, what I have said is if you have bought a 4 door sedan and after years of using you realise one day it is always just yourself driving in it, you drive 2 km every day with the 4 door sedan, never over 60km/h IF the robots are green, then the scooter makes a lot more sense.

So then you may say you will get wet once in a while ... I say, get a raincoat. 

You just need to decide, you want the 4 door sedan with all the associated costs or can the scooter do the job as good at a 100th of the cost of said sedan? 

That is what I said and am saying. Check your data, does it justify your system or can you get really clever 2nd time around.

It is far more efficient to run a solar system at max all the time, with spare watts on inverter (!), than oversize it all just in case, having to replace it in a few years not having used it to its fullest potential.

I quoted your words, you said "Pro's: 12v is cheaper than 48v by a whopping large margin."

 

Out of context, I think my exaple fits the bill. With more context to your original quote, confusion like this could be spared. 

But, again, it's NOT necessarily better to any "solar system at max all the time". Most batteries drain quicker, at higher current drawn, but not in a linear scale, due to pheukert's laws. 

The most optimal setup would be to find out what your battery's pheukert's law is, and do some calulations with that in mind. Start with what your needs are, get the peukert's constant and calculate how many hours your battery will last at the desired current and see if it's optimal. 

Simply put, the same battery won't discharge double the energy at 10 Amps, than at 5 Amps. The higher the current drawn, the shorter the time period that it can be withdrawn. 

 

Look here for an example: http://batteryuniversity.com/learn/article/bu_503_how_to_calculate_battery_runtime

Think we are coming to an agreement, you said "The most optimal setup would be ..." I agree.

I see that as the next level of speccing a optimal solar system, used to its full potential for a maximum ROI. A level I think is unheard of in the solar industry. For we don't all have the same batts, yet we always work on the batts ah's to give a guidance to what is needed, now knowing how accurate the marketing material is.

As a matter of fact, you are the first person to go down to ".. your battery's pheukert's law is ... ".

I need a a lekker dop to contemplate this some more.

2 minutes ago, The Terrible Triplett said:

Think we are coming to an agreement, you said "The most optimal setup would be ..." I agree.

I see that as the next level of speccing a optimal solar system, used to its full potential for a maximum ROI. A level I think is unheard of in the solar industry. For we don't all have the same batts, yet we always work on the batts ah's to give a guidance to what is needed, now knowing how accurate the marketing material is.

As a matter of fact, you are the first person to go down to ".. your battery's pheukert's law is ... ".

I need a a lekker dop to contemplate this some more.

well, no offence, but this is a forum aimed at enthusiasts who want to learn more about solar, not a training platform. I do believe though that @plonkster has explained peukert's law before in-depth. And I think @Chris Hobson also touched on the subject a bit. 

1 minute ago, SilverNodashi said:

... forum aimed at enthusiasts who want to learn more about solar, not a training platform

It is both. 

No offense taken. Yes they have explained the law, also in the BMV's boekies, it is just a new take on getting ones own bank's setting, for you can adjust it on the BMV. 

The BMV boekie says: Don't touch the peukert's setting unless you know what you are doing.

That can bring us to the next level of what our batts can do, more accurate data to estimate more accurately how long they are supposed to last in real live, not marketing material.

1 minute ago, The Terrible Triplett said:

It is both. 

 

I couldn't word it properly, but meant that this isn't a professional training platform (i.e. college) so, what is learned here, is learned purely how it's shared. 

I think it is time to chat about how cost effective it is between a bank to run a load safely over an "at best guestimated" time - with all the unknown factors involved - versus taking a battery bank, run it per manufacturers specs down to 50% every day and use the claimed cycles, say 1600 cycles at 50%, knowing full well in 1600 days you need a new bank.

I know with Trojan, because of forklift and golf cart owners, the specs should be close as damn.

2 minutes ago, The Terrible Triplett said:

I think it is time to chat about how cost effective it is between a bank to run a load safely over an "at best guestimated" time - with all the unknown factors involved - versus taking a battery bank, run it per manufacturers specs down to 50% every day and use the claimed cycles, say 1600 cycles at 50%, knowing full well in 1600 days you need a new bank.

I know with Trojan, because of forklift and golf cart owners, the specs should be close as damn.

I guess now isn't a good time to tell you, running batteries to 50% isn't necessarily the most optimal. Many batteries, run to 30% is as cost effective as running to 50%. i.e. the cost per cycle is the same at 50% and 30%

And then, how sure are you that you're running your batteries to 50%, every night? What happens if you only run it to 70%? How much money do you loose this way? Or do those few shorter cycles cost the same (i.e. Rand/Kwh) as the longer cycles?

23 minutes ago, SilverNodashi said:

Many batteries, run to 30% is as cost effective as running to 50%. i.e. the cost per cycle is the same at 50% and 30%

I think that depends on the make and type of batt?

Thinking of Trojans as a benchmark seeing as they are designed to be used and have been in SA for many many years.

When I have cash to use for more experimenting, I would run my T105RE's to 50% every night and see if I get 1600 cycles out of them.

By then Eskom could be more than my current R2.28 per kWh and then it makes sense to go for another set for 1600 cycles if the T105RE's are cheaper then than Eskom. More so if CoCT implements this new charge of R250 pm connection fee if your house is valued over wot, I THINK, 1 mil, and you are still on the old meter.

How sure would I be they are at 50% - well, as best as the BMV (tuned to the bank) can tell me with gravity measurements to compare against as often as possible?

It's an optimisation problem. Higher voltage is almost always better. If you go high enough, you can get away without a boost stage in your inverter. There is currently a Tesla+Solar Edge setup that works like that, high voltage battery. Take away the boost stage and efficiency jumps from an average 92% to above 95%. Downside? Seriously long strings of batteries that needs to be balanced and such.

Conversely, low voltage has advantages, lower initial outlay, less cells to take care of, lower quiescent current draw, just generally simpler. But limited power, thick cables.

24V aims to hit the middle. If you had two linear equations representing the constraints, the 24V system will sit on the intersecetion of these lines. My personal opinion is that it succeeds in doing this best-of-both-worls thing up to around 3kva. Combined with Lithium Ion batteries it might have a slight edge even, the thicker cabling likely costs less than a larger battery. When running in a grid-tied setup such as mine, designed specifically to eliminate the <1kva loads, it becomes a lot more sensible to go for the lower voltage.

But this question is about off-grid batteries, and off-grid implies carrying high peak loads, or at least more so than in the city. So 12V is definitely out, and even 24V is for the weekend place, not the primary home. All my personal opinion of course :-)

Jip, it all comes back to loads and run times. I am tossing and turning on a 12v 1200w - even 12v 800W - Multiplus with CCGX and do what @edmundp did.

 

Any case, matter on hand, to haul out 10's of thousands of rands at a time is hurting no matter how much you plan for it or how much it saves you on gennie fuel. It just grinds.

Seeing as we have been left around the tree, lets go right this time.

What cost the most ito batteries? It is the evening loads right.

So, looking at the constant loads of below say 500w or less, loads that drain the batteries every night, lights, fridge, warm blankets, seeing that @Chris Hobson is on the way to replace his bank, I am wondering if it is not an option to split the loads.

One system runs the 24/7 loads with new batts - specced to never need a generator. Now 12v and huge 2v batts can be installed for the load is small.
Other system handles the high end loads, needing a generator (as at present) for early evening, with the peak use daytime and some intermittent evening loads like microwave.This could mean that the existing batts last another number of few months, as they are lightly cycled. And when they finally die, one can get a cheaper 48v bank for this system as it is not drained every night.

My system works like that, Eskom is my gennie. so in my case the computers are the heavy evening loads (too expensive to run on batts) with the fridge, lights, routers the light loads that can easily run on a 12v system as the watts are very low and evenings is the non-negotiable time they have to be on.

I think the whole voltage thing is rather futile. In the end there is roughly a price per kwh, you can always compensate by taking smaller batteries and using more of them to get the higher voltage. Well... that isn't 100% true, below 100Ah there is mostly just rubbish on the market, so there is a slight case to be made for a 24V system with 200Ah (or larger) batteries, but only a slight case.

With lithium batteries, I noted that you can buy them as small as 60Ah (at 12.8V), the use case being exactly for higher voltage setups. (They are 15k each though!)

The only thing you win in the long run by splitting the system, is lower quiescent draw at night, eg a 20W draw overnight compared to a 35W (48V multi) or 50W (48V Axpert) draw. I think everything else evens out.

Also, when you run two systems, you have more batteries. More things that need to be replaced, more things that are wearing down over time. Reminds me of the time my dad bought a fancy SLR camera and wanted to keep the old 2MP camera around for taking the odd photo. My response was: When you spend this kind of money on a camera, you take ALL your photos with it! :-)

You are spot on Plonk, but I am just getting to a point where one looks at the replacement costs of a once size fits all bank versus spending smaller increments over time.

Very rough analogy, it is one thing to spend R40k every 10 years (If you are lucky) versus R20k every 5 years. Just easier on the cash flow.

Re. more stuff that can break, not so sure. Adding a additional small off-grid inverter, split some panels off onto a small controller. cabling and fuses are negligible.

All I am saying is are there more options? Not, fine. If there are, which one is the best.

20 hours ago, Chris Hobson said:

Thanks for the tips LivSol. Nice to have practical experience.  

I don't know whether I will  find a 400+Ah battery reasonably priced. My 108 Ah draw is due to the gennie still running in the evening for my staff. The next expansion will bring them onto solar too and therefore I know 260Ah is too little. In my ideal world there would be a 24 x 2V bank the same capacity and for the same sort of money as two strings of T105 REs.

One bank of L16RE-B would be marginally cheaper than two banks of T105REs but I am after Ah. Unfortunately one you get to that sort of size you better like your batteries a lot. The Trojan industrial line is going to be with you for a while. I am not set on any particular battery yet but one string is preferable over two strings but 400-500Ah is probably not negotiable for me being completely off grid.I see DB9 has 150Ah of batteries. Somehow we just do not seem to be that energy efficient. Probably (700W for 5 hours and 350W for 7 hours). Plus I know the more power you have the more you use. Creature comforts are great especially in winter. A bigger bank would also help with losing the Axpert's Solar Balance capability.

My batteries are behaving themselves at the moment. I had a good talking to them and said any funny business they will be shipped to Bellville for an unknown fate that includes sparks and smoke.

Hmmm that might just swing it into a single string's favour.

 

Another thing to consider - 

Design life - 

As an example, the Trojan RE-B will give you around 2500 cycles (i.e 7 years odd) at around 32% DOD (i.e 118ah). The design life of this battery is 7-8 years, so there is little point in designing for a much lower DOD as the battery will likely die from old age (or at least have significantly reduced capacity) before it has delivered it cycles. 

The Signature line 435AH L16 battery will give you (according to Trojan DOD chart) 2400 cycles odd at around 27% DOD (i.e 118ah),  so a slightly shallower DOD due to increased capacity form 370AH to 435AH, BUT, the battery will likely fail from old age before it reaches 2400-2500 cycles. 

Of course there are other factors that will effect the above numbers, such as accelerated corrosion due to heat in summer and the very occasional 70% DOD cycle, and the fact that most of use use our batteries over a 13 hour period in winter and even fewer hours in summer, so right there one needs to adjust C/20 rating a bit to get more realistic numbers. 

If you can install the batteries in a temp controlled environment, thats a big win, and will go a long way to helping you reach manufactures spec cycle life. But this is not always possible.

Being off grid myself for many years, I certainly understand the sentiment of wanting more capacity being able to let the belt out a bit, and the comforting thought of having 5 days autonomy, but it comes at a huge cost. Staring a gennie on the third day of cloudy weather while not convenient, is far far cheaper! 

From a cost perspective, sizing your battery bank to around 30% daily DOD seems best, with the examples given above. But there are other opinions on this, I just speak from my experience.

Now, if you want more capacity, then you should look at a battery with a design life long enough to use all the cycles is has to offer at the DOD you use. Select a battery with a longer design life, like the M solar 2v cells, or Trojan industrial range, to name two I can think off off the top pf my head. 

If you decide to use two banks for budget reasons (which is totally understandable), use copper bus bars to parallel the two banks and ensure ALL cables are equal lengths and terminated correctly, and check the batteries more often, and install mid point monitoring!

Also, have a look at the Discover tubular range of batteries if you decide on two strings, they make 215ah 12v with a long design life from what I recall and have floaters to help with watering.

oh, and I would not convert to 24v, as this would be a penny wise and pound foolish move I feel. 48v is the correct way to do things.

Just my 2c.


 

42 minutes ago, LivSol said:

Being off grid myself for many years, I certainly understand the sentiment of wanting more capacity being able to let the belt out a bit, and the comforting thought of having 5 days autonomy, but it comes at a huge cost. Staring a gennie on the third day of cloudy weather while not convenient, is far far cheaper! 

I am looking at about 16 hours autonomy. I must make the fact I live in the middle of nowhere and get lots of sunshine work for me.

42 minutes ago, LivSol said:

If you decide to use two banks for budget reasons (which is totally understandable), use copper bus bars to parallel the two banks and ensure ALL cables are equal lengths and terminated correctly, and check the batteries more often, and install mid point monitoring!

I want one string budget may take me to two strings. Already have midpoint monitoring , Cables cut to the same length for my single string (done early on in anticipation of perhaps having a 2nd string and then deciding against it)   and I have 4-5 m of the same cable spare so should manage to keep the two strings balanced.

I like the look of the M-solar and Discovery batteries. Who are the agents? 

 

49 minutes ago, LivSol said:

Of course there are other factors that will effect the above numbers, such as accelerated corrosion due to heat in summer and the very occasional 70% DOD cycle, and the fact that most of use use our batteries over a 13 hour period in winter and even fewer hours in summer, so right there one needs to adjust C/20 rating a bit to get more realistic numbers. 

That summer heat is something to watch our living room reaches 32°C indoors. So two summers is nearly equivalent to about 9 months normal use.

My batteries have been as high as 36°c in summer. A hot day is only part of the reason, the temp gain while they are being bulk (absorption) charged amounts to several degrees.

Sent from my SM-G950F using Tapatalk

12 minutes ago, Chris Hobson said:

I am looking at about 16 hours autonomy. I must make the fact I live in the middle of nowhere and get lots of sunshine work for me.

I want one string budget may take me to two strings. Already have midpoint monitoring , Cables cut to the same length for my single string (done early on in anticipation of perhaps having a 2nd string and then deciding against it)   and I have 4-5 m of the same cable spare so should manage to keep the two strings balanced.

I like the look of the M-solar and Discovery batteries. Who are the agents? 

 

Enertec are the SA distributors for Discover. And M solar are First National battery.

If you do end up with two strings, look at the Discover TFL 3000 as an alternative to T105 RE. 8 batts vs 16 is less connections and thus points of failure. I am however a fan of the RE line but have had good result with Discover too.

Don't rule out the 435ah Trojan. they were well priced for a time, if you can't find them in stock in SA, let me know, I can source a set from USA if need be, but will be a couple of  month wait.

Having one string is a bigger difference to life span than most think. 

 

32 minutes ago, LivSol said:

Don't rule out the 435ah Trojan. they were well priced for a time, if you can't find them in stock in SA, let me know, I can source a set from USA if need be, but will be a couple of  month wait.

OK if it is going to be a Trojan it must be a L16H. Fits all the criteria.  Will see whether GEL,AGM or Lithium options are able to match.

32 minutes ago, Chris Hobson said:

OK if it is going to be a Trojan it must be a L16H. Fits all the criteria.  Will see whether GEL,AGM or Lithium options are able to match.

FLA will be much more economical than the others you mentioned.

 

Side note- Gel and AGM batteries that claim high cycle life in thier marketing materials should be approached with caution, as the reliable brands claim far lower figures for thier versions.

3 hours ago, LivSol said:

... due to heat in summer and the very occasional 70% DOD cycle ...

If batts temps can be regulated, 1st prize.

2nd best if it cannot, controller has a temp sensing cable, or installed in same area as the batts, so that it can adjust charging based on ambient temperature.

HADMD? (Hoe Anders Doen Mens Dit) 

And I am told, with Trojans, that it is a good idea to take them to 50% every once in a while properly stirring them. Doing it once in a while does not shorten their live expectancy so that you will notice, but it does stop calcification if they are lightly cycled, that you WILL notice.

1 hour ago, The Terrible Triplett said:

controller has a temp sensing cable,

 

3 hours ago, Gerald_db said:

temp gain while they are being bulk (absorption) charged amounts to several degrees

High end MPPTs will pull back during bulk to avoid unnecessary rises in temperature... but by high end I mean high end, Morningstar, Outback, and the top two BlueSolar models. I think the lower-end controllers can also do it, but only if you have a temperature sensor somewhere in your system and the full comms layer is in place... in other words, this is a feature that costs money :-)

High end MPPTs will pull back during bulk to avoid unnecessary rises in temperature... but by high end I mean high end, Morningstar, Outback, and the top two BlueSolar models. I think the lower-end controllers can also do it, but only if you have a temperature sensor somewhere in your system and the full comms layer is in place... in other words, this is a feature that costs money :-)

Yes, mine monitors temp and backs off the current however the batteries still will heat up just slower and to a lower temp compensated voltage.

Sent from my SM-G950F using Tapatalk

4 minutes ago, Gerald_db said:

Yes, mine monitors temp

You have decent equipment :-)

5 hours ago, LivSol said:

Another thing to consider - 

Design life - 

As an example, the Trojan RE-B will give you around 2500 cycles (i.e 7 years odd) at around 32% DOD (i.e 118ah). The design life of this battery is 7-8 years, so there is little point in designing for a much lower DOD as the battery will likely die from old age (or at least have significantly reduced capacity) before it has delivered it cycles. 

The Signature line 435AH L16 battery will give you (according to Trojan DOD chart) 2400 cycles odd at around 27% DOD (i.e 118ah),  so a slightly shallower DOD due to increased capacity form 370AH to 435AH, BUT, the battery will likely fail from old age before it reaches 2400-2500 cycles. 

Of course there are other factors that will effect the above numbers, such as accelerated corrosion due to heat in summer and the very occasional 70% DOD cycle, and the fact that most of use use our batteries over a 13 hour period in winter and even fewer hours in summer, so right there one needs to adjust C/20 rating a bit to get more realistic numbers. 

If you can install the batteries in a temp controlled environment, thats a big win, and will go a long way to helping you reach manufactures spec cycle life. But this is not always possible.

Being off grid myself for many years, I certainly understand the sentiment of wanting more capacity being able to let the belt out a bit, and the comforting thought of having 5 days autonomy, but it comes at a huge cost. Staring a gennie on the third day of cloudy weather while not convenient, is far far cheaper! 

From a cost perspective, sizing your battery bank to around 30% daily DOD seems best, with the examples given above. But there are other opinions on this, I just speak from my experience.

Now, if you want more capacity, then you should look at a battery with a design life long enough to use all the cycles is has to offer at the DOD you use. Select a battery with a longer design life, like the M solar 2v cells, or Trojan industrial range, to name two I can think off off the top pf my head. 

If you decide to use two banks for budget reasons (which is totally understandable), use copper bus bars to parallel the two banks and ensure ALL cables are equal lengths and terminated correctly, and check the batteries more often, and install mid point monitoring!

Also, have a look at the Discover tubular range of batteries if you decide on two strings, they make 215ah 12v with a long design life from what I recall and have floaters to help with watering.

oh, and I would not convert to 24v, as this would be a penny wise and pound foolish move I feel. 48v is the correct way to do things.

Just my 2c.
 

There we go! The sweet spot of good RIO

Either way, batteries are expensive, but following this advice you will get the best value for money. 

One thing I like about 2V cells, you can drain them to 0%.  One thing I don't like about 2V cells, is that they need maintenance and they need more wires - so it doesn't always work for everyone. 

 

2 hours ago, plonkster said:

High end MPPTs will pull back during bulk

My 100/30 and 150/35 both have temperature compensation in now, built into the controller, therein my suggestion to have the controllers in same area as batteries for the same ambient temperature.

Morningstar PWM's, old models from years ago, had it as standard. There you buy the temp cable for a few bob if you want, otherwise the controller uses the temp around the controller.

And the V controllers, a question that was lost in the restore of the forum, can reach Trojan and other higher equalizing volts like up to 68.4volts for the likes of PzS Tubular plate traction batteries. So the 64.8v for the T's are available.