How Long To Charge 24v System...Is My Math Right?

Re: How Long To Charge 24v System...Is My Math Right?

Yes and no... The numbers and math are correct--but there are losses and amount of sun per day... For me, I would use:

• 35 AH / (0.61 amps * 0.77 derating) amps = 74.5 Hours

I used the panel Imp rating because you would be using a PWM controller--which really uses the rated current to charge the battery, not the watts at 17.5 volts (MPPT controller could use watt rating).

Second, assume 4 hours of full sun equivalent per non-winter day:

• 74.5 Hours / 4 hour of sun per day = 18.6 days

Next, you are charging at a rate of:

• 0.61 amps * 0.77 derating / 35 AH =0.0134 = 1.34% rate of charge

Which is very near a trickle charge--or just barely keeping up with the self discharge rate--so depending on battery (flooded cell has worse self discharge, and older batteries has worse self discharge)--your actual time to recharge is even much worse (may take 2-4x longer?).

Next, batteries should not spend a long time below 75% state of charge as this is the range that batteries sulphate... So spending more than one day below 75% state of charge (technically even a few hours sufphating begins) is taking life away from your battery--so you would never want to take days/weeks/months to recharge the battery back above 75% state of charge.

One of the many reasons why we use the 5% to 13% rule of thumb for charge rate and take into account controller type+deratings when planning out a system.

When new, your panels may output more power, battery will have less self discharge, etc...--But we want the system to work for you many years from now for you too.

So we (I) like to be fairly conservative in our design guidelines/recommendations such that the system will still meet your needs for years to come.

Sorry for the long post about a "simple" question.

-Bill

Re: How Long To Charge 24v System...Is My Math Right?

Yep... You have the math correct.

As batteries age, they have self discharge (AGM/Sealed tend to have lower self discharge, "industrial" forklift type batteries tend to have higher self discharge)--So things can take longer.

Also, batteries have recharging losses too... AGM are ~90-98% efficient recharging, and Flooded Cell are ~80-90% efficient. Also, depending on how you cycle/recharge the batteries affects efficiency too (deeper cycling tends to be more efficient, shallow discharging to ~85% state of charge and recharging back to >90% state of charge is less efficient).

Again, being conservative, I like to use 90% efficiency for AGM and 80% efficiency for Flooded cell. So, we should add (for flooded cell in this example):

• 35 AH / (1.17 amps * 0.77 derating * 0.80 battery efficiency) amps = 48.6 Hours

Because this battery is being recharged so slowly (small solar array in this example), it would be better if the battery is not discharged much below ~75% state of charge--The longer the battery spends below ~75% state of charge, the more the plates sulfate (and reduce battery capacity). Sulfation starts in a few hours and the battery should not spend more than 1 day below 75% state of charge to reduce the amount of sulfation.

And--you should never plan on discharging the battery below 20% state of charge--Below that point, it is possible to take a "weak cell" too zero volts and actually begin "reverse charging" of the cell... That will usually kill the cell/battery outright (or within a week or two--from my experience with automotive batteries and people who have cranked-or left headlights on-their batteries to "dead").

For small battery systems (say remote monitoring site)--There are better choices for batteries (such as NiCad) which can be taken to "dead" without damage (or even be stored without charge).

-Bill