Battery bank configuration for powering all Loads at the same time.

[fabieville]

I am designing my battery bank but I'm kinda confused so I need you guys to help me. I did some research online on how to calculate sizing the correct battery bank for your system and almost every website had similar steps/instructions, but then I stumbled upon a pdf file from xantrex saying that whatever amp hour is required for your daily load after you add inverter efficiency which is normally 85% and discharge limit on battery which is a recommended 50% max depth of discharge you should also multiply the daily amp hour requirement by 2 to compensate for the fact that in case you have a in-rush load(s) on the system, meaning that instead of running some loads at the preset time you normally run them this time you decide to run all your required loads at the same time, which normally is not the case in a average household. Doing this would put more stress on the system and when you multiply the daily amp hour requirement by 2, the battery bank will be able to can sustain all the loads at once running each of them at the required time  that they normally runs at.

Is this theory correct?
If the theory is correct does that mean that if I decide to have a 3 day or 5 day backup(days of autonomy), after I sized my battery bank would I still have to multiply the total batteries by 2 to compensate for the fact that in case I decide to run all loads at once running them at their required time daily and doing this method for each of the backup days?

Or this theory just applies to a one day backup? because when you configure the battery bank for a 3 day or more backup it would be quite large so it can withstand the in rush loads(stress) on the systems with each load running at the same time for each of the backup days?

[dnix71]

Most people here stagger their heavy loads. What do you plan to leave on all the time and what can you schedule for daylight when you will have excess solar that would otherwise go to waste?

[fabieville]

actually the battery bank I am designing is not for myself but for a customer and for me I know that it would be best not to power all devices at once but due to the fact that it is for a customer chances are they might run all loads at a given time even though I would explain to them that it is best not to operate the inverter in that manner.
I just want to know that I configure the battery bank correctly so that it don't create any hiccups down the line because that it would be bad on my reputation. So according to the theory that I wrote is it correct? what or your ideas/suggestions?

[dnix71]

If you really want to cover yourself, explain to the customer to two choices and the difference in cost (in the contract) and let the customer decide.

Every time you recharge a battery it takes something off it's lifespan, but I wouldn't let a battery discharge for 3 days before recharging it. That will lead to sulfation pretty quick.

How big is this setup and what does it power? If it's a whole house way off the grid you might consider using a flooded lead acid traction battery (forklift). Those are affordable in that size and forgiving of under/over charging.

[oztules]

Re the pdf..... without overtly saying so, I think they are referring to the peukert effect...... so if you run the loads concurrently you will need a larger starting capacity to make up for this effect... this table for L16 battery says it all really.



For a 48v system:
At light discharge we have 460AH available useful capacity if we keep under 250 watts......., and say with all the gadgets running together  ( eg 6kw or so), we have only 135AH capacity....and every where in between makes the 2:1 look about right ( wild average thing) that Xantrex alluded to.... or put another way, light discharge can do 3.5 times as much work as a heavy discharge can do... so if you can stagger the loads... good, if your victim of human nature, and ignore your advice and run everything when ever you want ( eg... wife )... then size accordingly.

Interestingly, my electric hot water uses 80ah/day@50v...2400w element... so 50 amps if I can use a lower wattage element ( arrived yesterday), compared to it's normal battery discharge, it will be less of a drag..... if I could get it down to say 240w... ie 5 amps....I get the water heated for near free ( comparatively)... need very good insulation around it too, and running near round the clock .... but will be an interesting thing to try.


................oztules