Theory questions?

Solar Expert Posts: 327 ✭✭✭
I wanted to see if my thinking was correct:
Usually my AC load, per the Xantrex inverter meter, is about 2 amps; on a sunny day my array generates 15+- amps, per the Prostar-30. Does that mean that I am using 2 X 120 = 240 watts and generating 15 X 24 = 360 watts? This is just a snapshot in time, just wanted to see if my calculations were okay.
If they are correct then theoretically I could disconnect the grid during the day with no impact on the battery bank or usage? I don't know if there would be any advantage to doing that since the inverter does everything automatically but it is good to know.
What voltage do you drop your battery bank to before you use a generator and do you charge up to the bulk (28.8) or float (26.8)? I have not had a chance to use the gen but did not want to make a mistake when I do.

Comments

• Super Moderators, Administrators Posts: 29,482 admin
Re: Theory questions?

Yep--You got it--Of course, your batteries are probably charging 24 volts--So the wattage going into the battery bank is actually higher.

But if you charge at 29 volts and discharge at 25 volts--That is, more or less, your battery charging losses.

Interestingly, lead acid batteries are almost 100% efficient in Amp*Hours (charging/discharging) except for the last where the batteries are gassing (bubbling). So--Doing your battery capacity analysis via Amp*Hours is actually pretty accurate.

Lead Acid battery bank voltage is a poor estimate of battery state of charge... Amount of current, battery temperature, state of charge all affect battery output:
New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
leaf wrote: »
Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

[note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

vtMaps
]

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

-Bill

So--What to do... If you don't want to discharge your battery--Don't let the voltage fall below ~12.7 volts (25.4 volts) or battery "resting voltage".

If you don't want to kill your battery, you would want to set a generator start after a few minutes of ~11.5 volts (23.0 volts) or after 10-60 minutes or so of ~12.0 volts (24.0 volts). Assuming the batteries are at ~70-77F (room temperature).

Otherwise, you are looking at temperature compensation too (hot battery, lower set point voltages; colder battery, higher set point voltages).

Or, you can use a Battery Monitor with a programmable alarm output like the Victron family. For example, set the generator to turn on at 50% state of charge and turn off at 80% state of charge--Efficient and minimum generator run-time/fuel usage.

Then again, automation is a problem in itself... The more automation you have, the more that can go wrong, and the later you will probably spot the problem(s), and more likely you are to suffer damaged (killed battery bank, etc.).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 327 ✭✭✭
Re: Theory questions?

I put 24 volts in the equation; 15 amps X 24 volts = 360 watts from the solar array. What do you mean that the wattage going into the battery bank is higher?
Thanks for the parameters on when to use the generator. It will be a manual operation, no automatic start, so I will monitor the voltage and when it drops below Float I will recharge before it hits 25.4 volts. When you say 80% SOC, is that the Bulk/Absorb voltage? Mine would be 28.8 volts.
• Super Moderators, Administrators Posts: 29,482 admin
Re: Theory questions?
stmar wrote: »
I put 24 volts in the equation; 15 amps X 24 volts = 360 watts from the solar array. What do you mean that the wattage going into the battery bank is higher?

Because a battery bank charging generally is around 13.5 to 14.5 volts (or 27-29 volts for your battery bank):

15 amps * 24 volts nominal = 360 Watts (not really charging voltage)
15 amps * 28 volts charging = 420 Watts (what is really going into the battery bank)
Thanks for the parameters on when to use the generator. It will be a manual operation, no automatic start, so I will monitor the voltage and when it drops below Float I will recharge before it hits 25.4 volts. When you say 80% SOC, is that the Bulk/Absorb voltage? Mine would be 28.8 volts.

Looking at my post above, you can see a pretty good "knee" in the charging curve between 80% and 90% state of charge (depending on rate of charge somewhere around 28 to 29 volts)... So--More or less, that knee is where charging is very efficient and when it becomes inefficient (when batteries start to bubble/out gas).

Have the genset do the "fast charge" on dark days... And let the solar array finish charge when the sunny weather returns.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 327 ✭✭✭
Re: Theory questions?

Definitely a "Duh" moment, Homer Simpson would be proud of me, lol. Thanks for explaining it to me. I keep forgetting that voltage is not constant!!
So a rule of thumb would be to not let batteries drop too much below Float and quit charging near Bulk.
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