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rgormley
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Joined: 22/02/2006
Location: Australia
Posts: 245

Posted: 01:20am 18 Jan 2014

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few questions on batt volts

have a 24v battery bank (4 x 12v batts) 75AH each batt
solar panels 700 watts (4 x 175Watt)

http://www.csb-battery.com/upfiles/dow01376019700.pdf

they say to float them at 13.65v each (so 27.3v for my 24v system)
i have the solar controller set to float at 27.3v during the day.

my pull from the batts is
5 amps 24/7 (computer+router etc)
5 amps chest freezer fridge cycling on/off

what i want to know is...
what is a safe voltage range for the batts? (more so the lower limit volts)?
what is a safe point to take them down to before my charger kicks in and maintain them till the sun is back on the panels.??

overnight my batt volt will sag and i have set my charger to 25v as a safe guard.
current limited to 10 amps.(the inverter will disconnect out at around 22volts if i don`t maintain the charger on them)

what is a safe lower level of discharge for these batts?

Thanks.....

Tinker

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Joined: 07/11/2007
Location: Australia
Posts: 1904

Posted: 03:06am 18 Jan 2014

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Your battery capacity is way too low to successfully run a fridge overnight. Your batteries won't last very long IMO.

"what is a safe lower level of discharge for these batts? " That depends how long you want to keep your batteries working for you. 25V lower level sounds right to me if your charger can supply the total load on the bank plus a bit extra to put into the battetries once the voltage drops to that level.

I started off with 200Ah/24V and quickly increased this to 300Ah to stop flattening the batteries. Even then the fridge was switched back to mains power if overcast conditions did not guarantee a fully charged battery bank by lunchtime.
Initially I had 420W solar power on a tracker to feed the MPPT charge controller.

Now I have 1.2Kw on 3 trackers and 600Ah of battery capacity, that powers most of the house & fridge and I only switch the fridge back to mains on rare winter days when the sun is hiding a few days in a row.

Klaus

yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166

Posted: 03:12am 18 Jan 2014

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that's not how I read it.

I would be looking at the bottom left graph on the PDF for cycle use not standby.

they are saying charge to 2.45/cell so that's 29.4 volts.

for a 50% discharged battery charged at 0.1C (15 amps in your case) dotted red line its 5 hours at 15 amps then 3 more hours at amps reducing to almost nothing.

max charging amps (45A) would reduce the charge time a lot.

I guess you would be looking at 90+ amps discharge overnight

about 53% discharge 90/14 hours 6.42amps/hour

6.42/150A = 0.04C discharge rate

If I look at the graph bottom right and read up from 14 hours and at a discharge rate less than 0.05 C it shows a terminal voltage of 24.4 volts.

the catch is that this number is resting volts so the 5 amp draw of the router will make that lower and when the freezer switches on it will be lower still.

I would take a different approach.

I would target a discharge point of between 35-40%, make sure that the batteries are fully charged at sunset.

35% of 150A is 52.5amps

router 5A/hr
freezer 1.6A/hr (based on 20 minutes running per hour)
total 6.6A
52.5/6.6 = 8 hours to 35% discharge point

So if the solar panels stop producing at 5 pm then around 12 - 1 am as the freezer gets towards the end of its 20 minute run cycle that will be your target voltage to set the charger cut in. Or put it on a 24 hour timer and run it for an 2 hours (20 amps ).

from there it is only 4 hours (26.4amps) till sunshine again.

I guess that is as clear as mud. My figures for the freezer could be a bit rubbery but I guess you can tweak it to suit yourself.

Cheers Yahoo

I'm confused, no wait... maybe I'm not...

rgormley
Senior Member

Joined: 22/02/2006
Location: Australia
Posts: 245

Posted: 03:36pm 18 Jan 2014

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yea the mud factor...hhmmmmm

have to go out, but when i return, will re-read closely, and see what i make of your information and how to apply it to my bank...

thank you for the feed back....

yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166

Posted: 06:46pm 18 Jan 2014

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I guess I am saying 3 things

1. charging to standby voltage is not high enough if you are cycling the batteries. you should be able to charge and float during the day if your panels are big enough.

2. draining 35% from the battery will give close to 1000 cycles a good target to aim for IMO

3. the target voltage you are looking for is at 35% discharge when the full 10 amps have been drawing on the batteries for at least 10 minutes.

I'm confused, no wait... maybe I'm not...

rgormley
Senior Member

Joined: 22/02/2006
Location: Australia
Posts: 245

Posted: 02:30am 19 Jan 2014

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ok had a read and a look on the web for more info....

what i read is i`m best to only run my batts down by 30%

(4 x 75AH) 2 in parallel =150AH 100% total (Yes??)
150AH less 30% is 105AH (yes??)

the draw on my bank working on 6pm till 6am (12hours no sun) will be
5 amps for the computer/router
2.5 amps for the deep freezer (allows for 50% on/off duty)
so 7.5amps x 12 hours is 90AH

as i figure i should be ok with the batt capacities i have.

now for the voltages....?????

i do have the ability to set the charger to 100% 24/7 float voltage 27.3v over night, but does this not defeat the idea of using the stored solar charged battery energy during the night?

hence why i have it currently set to 25V if they sag a little..

am i better to set the overnight "float" voltage to 27.3V (bats will last longer)
OR
"use" some of the stored energy and allow a little "sag" down to 25v

you thoughts very appreciated and welcomed...

yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166

Posted: 02:18pm 19 Jan 2014

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your maths is backwards.

If you remove 90 AmpH from a 150 AmpH battery? think about it a bit more.

the way you have thing set up now the batteries are probably only loosing 12% of their charge before the charger takes over and supplies power to the loads.

I am pretty busy for the next few days, I can look at it and offer some suggestions after that.

the big issue for me is your daytime charging. it works now because you are draining very little power from the batteries, like having an emergency backup, that is exactly what standby is all about.

As soon as you start to cycle the batteries more deeply every night they will not fully charge during the day at 27.3 volts there is just not enough hours in the day.

the solar controller needs to charge at 29.4 volts till the amps to the batteries stops reducing then switch to 27.3 for the rest of the day so the cells don't vent gas or form cavities in the gell or paste from overcharging when they are full.

My guess is 24.45 V is close to the point that you need to set the overnight charger at to remove 50 amps from the batteries BUT if you leave the daytime charging as it is then the batteries will not fully charge so you will be removing 50 amps from a battery that is 75-90% full not 100% full like it should be.

This is the number one killer of batteries regular and persistent undercharging.

I'm confused, no wait... maybe I'm not...

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