Taking Cycle life into Charging considerations

HadesOmega

Quick question before I submit my proposal, I'm a bit confused. If I have an 800AH battery bank and I calculated the 800 AH for my 50% cyclelife calculations already. That means if I drain the battery to 50% for the night I will only have to recover 400AH in the daytime? This is assuming I do drain it to exactly 50%.

I've already calculated how many panels I will need for 800AH so... if I what I said above is true and I only need 400AH to charge then I've doubled my calculation and have twice as many panels than I really need.

BB.

Re: Taking Cycle life into Charging considerations

The 5%-13% * the 20 Hour Battery rating--I.e.:

800 AH * 5% = 40 Amps typical minimum recommended charge rate
800 AH * 13% = 104 Amps typical maximum recommended charge rate

Has some battery physics reasons behind those numbers... Below 5% charge rate--for large (and tall) flooded cell batteries, the amount of gas (hydrogen/oxygen) produced during "equalization" is not very great--and as the batteries are used, the electrolyte (sulphuric acid) tends to stratify (dense acid at bottom of cell, "light" water near top of cell)--and the 5% minimum current is recommended to "stir" / mix the electrolyte back to a uniform mixture.

The 13% maximum current is normally about the rough maximum current you would want to pump into your battery bank as higher currents can cause the battery bank to overheat--and is also a waste of energy...

There are other battery technologies (such as AGM) which can allow you to go below 5% (as there is no "free electrolyte") or above 13% (battery tends to limit maximum current anyway--and lower internal resistance?).

And that gets back to another recommended rule of thumb--provide 3 days of "no sun" power and a maximum discharge of 50%--Or:

3 days * 1/50% = 6x daily battery bank load...

This recommendation is based on several recommendations--One is that the flooded cell lead acid battery tends to suffer sulfate hardening if below 75% state of charge for more than a few hours.

The first day of no-sun, the battery stays well above 75% state of charge--the second day of no sun, you fall into the 66% state of charge by morning--and you want to consider firing up the generator if continued cloudy weather is in the forecast...

By day three of no-sun--you will need to fire of the generator to ensure long battery life--as below 50% state of charge--most batteries tend to have less cycle life (although--this is even up for debate--fewer batteries mean deeper discharge and shorter life--but fewer batteries to replace--so you end up with, as an example, 8 batteries that last ten years, vs 4 batteries that need to be replaced every 4-5 years--which is better for you).

Time to Question the 3 day Rule ?

When you look at how many panels you need--you are really sizing them for your daily load (average sun, plus a little bit extra to recharge your battery bank from a few days of clouds).

Then, we are suggesting the 3 day/50% rule to roughly size an optimum battery bank size.

Lastly, going back to the 5-13% charge rate of a battery bank to ensure that we don't have too few solar panels to properly charge the battery bank (and possibly shorten the battery bank's life)--or too many panels which tends to be a waste of money spent on too many solar panels (battery bank will never be able to absorb all of the power from the solar panels).

But--rules are meant to be broken... If you have lots of cloudy weather and don't want to use a generator--your battery bank and solar panels will tend to be larger than recommended.

If your loads are variable and you can delay them based on weather (such as no clothes washing during stormy weather)--then you can size the bank/panels for a minimum load--and you just run your heavy loads only when the sun is up (or you are running your generator).

I will stop typing here--I am probably not really answering your question by this time--but just causing more confusion.

Your thoughts?

-Bill

icarus

Re: Taking Cycle life into Charging considerations

Also keep in mind that battery charging efficiency is not 100%. For example,, in rough numbers if you take out say 100 ah,,, it will take somewhere in the neighbourhood of 120 am to get back to even. (The Perket effect?)

Tonyt

niel

Re: Taking Cycle life into Charging considerations

HadesOmega wrote: »

Quick question before I submit my proposal, I'm a bit confused. If I have an 800AH battery bank and I calculated the 800 AH for my 50% cyclelife calculations already. That means if I drain the battery to 50% for the night I will only have to recover 400AH in the daytime? This is assuming I do drain it to exactly 50%.

I've already calculated how many panels I will need for 800AH so... if I what I said above is true and I only need 400AH to charge then I've doubled my calculation and have twice as many panels than I really need.

if what i had bolded means you have enough pvs to replace 800ah within a typical day then you may have too many pvs as you will only need to replace 400ah if it is 400ah you will deplete per day. that would mean you could double your battery bank right now (especially if you are draining 800ah per day from the batteries) or send back half of the pvs you bought and keep in mind if you have excess pvs that you could keep some of the excess pvs for just in case and losses or future use rather than sending all of the excess pvs back.
without going through your posts to see your history i will generalize here and say that with an 800ah battery bank and 800ah in per day from pvs that this is usually over about 5hrs of time and possibly less hours of production in winter and cloudy/rainy days. this 5hrs would represent a 20% rate of charge at the most. for some batteries this is too much and at the very least would cause extra maintenance in standard lead acid batteries even if the manufacturer oks the higher charge rate. agms shouldn't have a problem with a 20% rate of charge btw.
the 13% rate was selected because of some trojan and generic batteries not handling more than this very well, but many batteries can handle more. if wishing to stick to the 13% rate and keeping the pvs you have, this can be done by having a large enough load on the pvs at the times of the excess and know that even smaller loads on the system at the time of charge will subtract from the overall charge and its %. example-100ah battery and 10a from pv. load at 6a will net a charge to the battery of 10-6=4a and is a 4% rate of charge because 4a/100ah=.04 for the record, in replacing 400ah of an 800ah battery bank this would represent a 10% rate of charge from the pvs to the batteries because 400ah/5hrs=80ah and dividing that into the full 800ah battery bank rating is 80ah/800ah=.10
as i had said if you did overestimate your pv needs then do not send all of the excess pvs back as you will find that you could use some of the excess pvs

hillbilly

Re: Taking Cycle life into Charging considerations

Just a note on the 13% max charge rate; you may also want to verify this with the specific battery manufacturer, as some of them may allow (or even suggest) higher charging rates. I know this was the case with my Surrette batteries, where they felt that "ideal" charging rates were more like 15%-20%. (If I recall correctly, the "TOP" limit they recommended was 26%). Not that this is a real issue for a lot of us... can't see having THAT many PV's on my roof... but for generator charging it might be worth double checking what they recommend.

niel

Re: Taking Cycle life into Charging considerations

i looked back and your initial inquiry stated a battery bank of 1575ah and is about right for allowing to replace up to 800ah or near 50%. if you changed the bank to 800ah then the pv requirement should've been refigured.
what do you actually have or want for this battery bank? number of series/parallel batteries of what brand and model?