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Author Topic: Battery Short-Cycling  (Read 2203 times)
Scruff
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« on: August 09, 2014, 01:12:50 AM »

Time to de-lurk methinks. I've been looking around and short-cycling seems to be the best kept secret in the lead acid industry.  Manufacturers omit the first 10% of their DOD vs cycle life graphs but few say why.

Have any off-gridders implemented any preventative measures to stop under-discharge?  I've two crown CR-235s in series and it's a bittov a job some Summer days to discharge them 10% before the sun comes up again and my PV has at it.
My system idle is < 70mA and I'm running everything on DC to forego inverter-transformer losses.  I'm pretty light on electron use most of the time and just decided on a large bank (by my standards Grin) for the Winter and Peukert reasons.
Much as I'd like to have the batteries going into float some days I'm concerned that the initial absorption charge before they do is less than healthy for them.
I've been thinking of using a relay driver or a micro-controller to selectively engage my PV after a certain threshold is reached (12.5v maybe) and disengage similarly post full charge.  In the meantime I just turn the PV off until my Ah counter tells me I'm 14% DOD....meaning ~10% @ C100.

Do you folks think it's worth doing, or have done but differently?  Or should I just leave the PV on and let it go to float as it pleases?
What is the minimum recommended DOD?  I see reports of 5% to 15% depending on sources but mostly it's not mentioned.
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billi
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« Reply #1 on: August 09, 2014, 08:09:47 AM »

 Hi

You have a strange problem  Cheesy

Personally , i would not worry too much about "not cycling" , but watch the water levels , and probably  do not set the absorbtion charge V too high

If you have demand for hot water then switch on a heater ?

CU
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1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
Tiff
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« Reply #2 on: August 09, 2014, 09:15:14 AM »


I've heard that too many shallow discharges can cause problems with 'clumping' on the plates. I don't know if this is correct nor do I have anything to back this up with. Could just be an old wives tale.

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billi
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« Reply #3 on: August 09, 2014, 09:45:37 AM »

....I think  i remember this as well, mentioned here on the forum (by a battery manufacture)  , that a frequently  discharge and -full-charge is healthy

Any way , my bank is now about 11 years old and 9 month  of the year  mostly only discharged by  10 %  or ca 160 ah or 4 kWh  over night ... surely lots of Microcycles in between the sunny hours ... when dishwasher or washingmashine or el cooker are on ....

But , i expect that my battery has not its full capacity any more @ 1600 ah (24 v) so the numbers  are just vague





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1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
clockmanFR
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« Reply #4 on: August 09, 2014, 10:15:14 AM »

I can confirm with my lot, 48v 1300ah, big Gel types, that after a few days of short Bulk and then normal float and a little discharge my battery holding voltage drops by about 00.10v.

But give the batteries some work, about 5% to 8% discharge for 8 hours in day light, like 1.5kw under floor heating just to get the sealed kitchen warm again and do the family heaps of clothes drying, when its raining. Then my batteries have bulk charge then Float the following day, my holding voltage is improved by 00.20v.

I know its not a lot, but definitely noticeable.    
« Last Edit: August 09, 2014, 10:16:48 AM by clockmanFR » Logged

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Scruff
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« Reply #5 on: August 09, 2014, 12:37:54 PM »

It could well be an old wives tale there's some information about it but generally it's a characteristic that's published as rare as hen's teeth. Here's one report stating 5% DOD. http://72.10.52.249/index.php?option=com_content&task=view&id=27&Itemid=32
Allegedly it causes the build up of hard lead dioxide clumps on the positive plates.
I can force a higher discharge rate if needs be but it seems a bit wasteful to me.

I've absorption set to 14.7v with temp compensation (higher than manufacturer recommendation because I use PV as a primary charger not mains).  Water levels are fine, they barely gas and SG is 1.275 per cell.  
« Last Edit: August 20, 2014, 09:19:18 PM by Scruff » Logged
billi
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« Reply #6 on: August 09, 2014, 01:08:19 PM »

Why not discharge them from time to time and do an equalizing charge each month  ?

To mix electrolyte and clean plates

12 deep cycles per year   and fully charged again , will probably  do a better job on the life of the battery
« Last Edit: August 09, 2014, 01:19:12 PM by billi » Logged

1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
Scruff
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« Reply #7 on: August 09, 2014, 03:27:06 PM »

I do.
Just picking people's brains here, and wondering if I should give it much credence.
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Scruff
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« Reply #8 on: August 20, 2014, 09:16:43 PM »

Had a look at the batteries today it's been a while since I tested them but they're only a few months old and they took about a hydrometer of water per cell. Last set went  chocolateteapot after 4.5 years 3.5 of which were on rare and rubbish chargers.

Electrolyte density is down about 1.260 per cell (fully charged, equalised 10 days ago, removed from float for maintenance) from 1.275 new. There's flakes of gubbins in the electrolyte mostly discharging to 10% per cycle the last while. Hrmm...I'm gonna drop them to 40% DOD and see if things improve.
« Last Edit: August 20, 2014, 09:18:54 PM by Scruff » Logged
Scruff
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« Reply #9 on: September 12, 2014, 04:25:14 PM »

The deep cycle got rid of the flakes.
I've had the batteries idle 4 days now, daily PV float and I've 4 cells back up to 1.275. The last two are being very stubborn, equalising isn't doing much for the SG either...just give them another 4 days so.
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