Batteries have a self discharge of percentage. So the more amphours you store the more you will also lose and have to make up. Forgot what the rate of discharge is, like 1% a day or so I think. May vary with battery types also. Figuring 1% per day for normall deepcycle lead acids below, I hope I was correct after typing all that, it got longer than I thought.
So 25x12=300amp hours day, X 8 days= 2400ah hours (used), at 25% your bank would be 9600amphours, 1% self discharge is like 96amps per day? That's almost another 4 hours of load for you!
Not sure if that is exactly correct, might be abit more or less? Point is you have to charge up that loss also with your wind or propane genny. Figuring maintanance, loses, wiring, etc.. Sometimes it may be better to use a smaller battery bank, and even deeper discharge (if rarley needed) also. If I was correct above, that is 768amps or 9216watts at 12v in 8 days just for loses?
It is said that the less discharge the longer batteries will last, but at what cost is that? You need far more batteries and have much more loses when only using the top 25% than if you used about 35% or 45%. I geuss what I am getting at here is how much does it cost you to make a battery last 12years instead of 10 years, and is that extra cost actaully worth it. 20% or 25% is good, sometimes I see people wanting to only use 10% though.
300amps per day is 3600watts at 12V. Just curious what your charging conditons are. If you have solar and wind both, it would seem very unlikely you will ever go 8 days without any RE charging. Any charging you are getting means the battery bank could be smaller.
I think most systems are figured for 20% discharge and about 3 days storage in case no charging. 300amp per day at 20% would be 1500 amps X 3 days= 4500amphours of batteries.
Or at 25% discharge 300 X 4= 1200amps X 3 days= 3600amphours of batteries.
Either way you have about 1/2 or 1/3 the batteries to worry about and that much less self discharge you will have to replace. Less self discharge also means your RE system has a better chance of keeping up with the loads so you have to run the propane genny less often. I mean figure if your losing 96 amps per day with a giant battery bank, that is almost 1/3 of what you are actually using (300amps). If you have 3 days storage at 25% use and a selfdischarge of 1% thats 36amps per day, the extra 60 amps difference would actaully be charging up the power you really used, not just making up for selfdischarge. 60 amps is allot of power, every 5 days you save 1 entire days use (300 amps).
Or with solar pannels 60 amps is 720 watts loss, very expensive! 150watt pannels in 5 hours of great sun just barely covers that extra loss.
Every project and location is different and you know yours better than me of course but the above is the way I look at battery storage basically. The more losses you have then whenever you need to run the propane genny it will also have to run longer (and use more fuel) to make up that extra selfdischarge loss. So say you have to run the genny every 8 days to catch up, 60amps extra loss for 8 days is 480amps (5760 watts) more that the gennie has to produce just to cover that extra self discharge.
That extra loss is also 1.6 days of power figuring your usage at only 300 amps per day.
Looking at this I would geuss if your system could cover 8 days of use without running the genny to catch up, then it probably could cover 3 days of use with far less losses using a smaller battery bank. Figuring all the extra losses even if your system could not cover 8 days, it still might cover 3 days just fine with a smaller bank since you have far less losses it has to make up. 36amps of self discharge for a 3 day system will be far easier for your RE to make up than 96amps for an 8 day system. You could actually find that with the same RE power a 3day system may never need the propane genny ran to keep up, but an 8 day system may need it often!
If you found yourself needing to run the genny every 4 days or so even, you would most likely be saving money on propane since it would not have to make up the extra lost amps. It may need run more often but for a total of less time since it has to make up less total lost amps.
If that turned out to be the case I would be watching the charging of the RE sources close and the dump loads, if you are never dumping excess power then more batteries would do you no good other than maybe extend your time between running the genny from 4 days to 8 days, but increase your feul usage at the same time. If your often away from home 5-7 days maybe you need to do this? Course if not home, probably not the 300amp loads on those days?
If you found your dumpload does kick in sometimes because you made more power than you could store at that time, but a 3 day battery bank is not storring enough average to run the full 3 days, then I would add more batteries to catch and store what was being dumped. Maybe think of a bumpy supply, maybe Sundays your at full charge and lots of wind so some power is dumped, but then winds die out and you have to run a genny on Wendsday, just before the winds pick back up on Thursday again. In such a case adding a few more batteries to catch the Sundays dumped power would be good if there is much of it. Course it don't work by days like that, but averaging it is what I mean.
You can always add more batteries if you have the RE to charge them and will use the power from them, hard to take back extras you don't need and get a refund on them though! They say not to mix old batteries and new ones, replace them all at one time etc.., but it does not hurt to add more new ones after 6 months or so if needed. Actaully I don't think it matters after 1 year even, the discharge cycle is small and the batteries still pretty new. Only thing it seems to do mixing old batteries and new anyway is drag down the new ones to the old ones level. Weakest link type thing, the entire battery system would only be as strong as the weakest battery basically.
I use allot of batteries I salvaged from a scrap yard, have to mix those up, never had a problem yet. Not like they explode or anything, just weakens any new ones added but they all work till one dies then it pulls down the entire chain untill removed. I do try to match up the batteries in banks of the same types, L16P together, T105 togther in seperate banks for different uses, but age does not matter as long as they work I mix them just fine as I get them.
So mixing New 1year old, and NEW NEW, should not make any reall difference. Mixing 9 year old and NEW just drags down the new ones if the old ones are weak, so that's not good.
So myself, figuring the losses of self discharge and all the extra costs of extra batteries, wires, space, maintanance (cleaning cables, filling with water, etc) I would start with a 3 day storage battery bank and see how that works. If at times I ran out of power but other times I was dumping excess I could not store, then I would add more batteries of the same type and size as needed to even it out. If never dumping excess power then I would not really need more batteries, no way to charge them, so run the genny as needed, half as long every 4 days instead of once in 8 days twice as long, which also as I mentioned uses more fuel because of the extra amps lost in the larger bank that it also has to replace.
If I never ran out of power but was dumping alot of excess I still would not need anymore batteries since I would not be using the extra storage, unless I just wanted a less level of discharge, like if I was running 25% and always dumping power I could add more batteries and run at 10%. In this case the batteries might last a bit longer and if I was always dumping excess power the self discharge would not matter to me, it would only be a matter of cost and maintanance time. Most likely I would just find a use for the excess as a dumpload and forget the batteries though.
I mention always dumping excess power part because I have heard people thinking if power is being dumped they need more batteries to store it. That is not true, if the power is not being used then more batteries would just get full and then the excess would start dumping again! The only thing this might do is reduce the discharge level of the bank from 20% to only 10% and encrease self discharge amps also. Still if a system is making average of 500amps but only 400 amps is being used at the max, and the system is never without power, there is no reason to add more batteries to store the extra 100amps that gets dumped.
I heard a salemans pitch claiming a person needed to add more batteries because they were dumping excess power often (but they never run the batteries low). Don't know if he really beleaved that or just trying to sell extra batteries the person did not need! Never beleave anything a sales person says, if you don't know yourself already and it involves buying something get a second opion or more! The sales person actually got mad at me when I asked him what they should do when the extra new batteries reach full charge and the power started dumping again because it was not being used. He got mad but did not have an answer either.
