Solar Expert Posts: 230 ✭✭
Ok so I suffer from chronic "capacity anxiety" when it comes to my batteries. Without wanting to run them down to their cut-off voltages over an extended time period I decided to pick a very sunny day (like today) when the batteries reached their full charge by 13:00 so I could run a "maximum load test" for 35mins at a constant-power discharge and log the data to see if I could deduce anything......

Battery bank under test: 24v 300AH,C20 (7200 w/h)
Constant Power load: 2400watts at AC side of inverter, 2850watts approx at DC side.

Results (voltage at battery terminals):

Open Circuit voltage after disconnecting PV: 26.5v
Temperature of batteries: 23 Degrees C or 73 Degrees F
0 mins: 24.5v
5 mins: 23.9v
10 mins: 23.4v
15 mins: 22.9v
20 mins: 22.8v
25 mins: 22.6v
30 mins: 22.3v
35 mins: 22.1v

PV reconnected, input current 25 Amps, voltage after 5mins: 25.2v

Could not run test for much longer as inverter LV alarm sounds at 21.6volts.

Battery manufacturer says a constant power discharge of 113 watts per cell down to 1.7vpc over 60mins, There are 36 cells in this bank so - 4068watts down to 20.4v in 60 mins. All data is for a fully charged battery at 25 Degrees C. Not really sure how to do the maths for this...

That is a lot of current from a 300 AH battery bank.

2,400 Watts * 1/0.85 inverter eff * 1/24 volt DC bus = 118 Amps
118 amps / 300 AH battery bank = 0.4 = 40% rate of discharge (aprox 2.55 hour discharge rate)

That is a very high discharge rate for a flooded cell deep cycle battery bank... C/2.5 or 40% discharge rate is what I would recommend for a few minutes (starting well pump, etc.).

In the end, you are really interested in "does the system meet your needs". So, what is your average load (watts) and how many hours per day do you expect to run it. For a 300 AH @ 24 volt battery bank, discharged to 50% capacity:

300 AH * 24 volts * 0.85 Inverter eff * 1/2 days of no sun * 0.50 maximum discharge = 1,530 WH per day (120 VAC)
1,530 WH / 5 hour of discharge = 306 Watt average AC load

So--I would be suggesting the average load that your system should be designed to support (rules of thumbs, nominal numbers) would be 306 Watt AC load for 10 hours with 50% battery discharge.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,183 ✭✭✭✭
CALLD wrote: »
There are 36 cells in this bank so - 4068watts down to 20.4v in 60 mins. All data is for a fully charged battery at 25 Degrees C. Not really sure how to do the maths for this...

So is your bank 3 strings of 12 cells?

How long did you wait before reconnecting the PV? In flooded batteries, you might see the voltage rise for a couple hours. Not sure about AGM's.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
• Solar Expert Posts: 230 ✭✭

Thanks for the replies guys! Yes it is a huge load, and is not the sort the system ever needs to support for longer than it takes to boil a kettle in a power failure with the minimum amount of water required.

The point of this test was to estimate battery health in a similar but more prolonged way to what mechanics use to determine the health of your car battery. Obviously it is nowhere near the current vs capacity ratio of an SLI battery cranking an engine - that would been over a thousand amps for a 300AH battery! This test was just to check how they cope under an extreme discharge test. I also checked for any hotspots in the wiring, and if the battery temperature changed significantly during the test - thankfully it hardly changed at all.

I also tested my smaller 100AH back-up bank at a lighter 18amp load. I stopped that test after 52mins with a terminal voltage of 23.3volts.

Under normal usage the system supplies 420watts for a few hours in the evening, 110watts overnight, 500watts in the morning for an hour, then 32 during the day while charging.

The two banks in question:

• Solar Expert Posts: 1,218 ✭✭✭✭

Some pointers for improving your install.

- i dont like to see a lot of flamable materials around battery banks.
- some kind of cover to prevent accidental bank shorts, cats, children etc
- battery cables look on the small side
- wire managment, conduit to protect wire from abrasion, sharp object contact
- cheap chinese inverter, not recomended nor UL listed for fixed wire use
- fusing not visible, dc disconnect required to protect unfused elements.

Off grid we like to be obsessive about safety.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar