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Author Topic: My new ‘old cells’, AGM charging advice, how much current?  (Read 3157 times)
Dyslexicbloke
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Blue sky thinking ...


« on: May 06, 2011, 03:39:20 AM »

Hi folks, I’v been a bit busy lately just keeping up with day to day stuff so not much progress on the CHP although the engine and alt are working splendidly.

Anyway I digress …. Batteries

I just picked up some ex-UPS cells, probably a bit long in the tooth but swapped out as scheduled maintenance so good when removed.

They are GNB Absolyte 400 Ah AGM’s, although I expect they are an earlier incarnation than the current Absolyte IIP spec. The ones I have are 90G09’s I think which would make them Absolyte GP.

Anyway I only paid £30 / cell which is probably about 25% over the scrap value so it seemed worth a punt, AGM or not, and the comments I found on the web suggested that they should be robust and long lived.  Cool

I picked cells from the available stack in the yard that all had consistent open circuit voltages, within 0.01v, slightly less than I would have liked but still above nominal at 2.08 which I felt wasn’t low enough to have caused a problem even if they had stood for a while.

I spent some considerable time searching the web for data and have found what I believe are valid float and EQ voltages with temp compensation data and to be honest it seems fairly consistent across the range of this cell type be them I or IIP’s.

What I haven’t been able to find is much on charging current, I get that charging must always be CV and kept below the gassing voltage even when performing EQ but what I need to know is weather I need to limit the current.  banghead

The maintenance data, several pages of it, contains a single line in the section on initial charge from new, which suggests that that charge should be at no more than 5% of  C but it doesn’t specify which C, 20 HR / 100 Hr, and then goes on to contradict its self by stating that the minimum float necessary to prevent damage is 2.2VPC which into a discharged cell would result in a huge current.

The rest of the data states unequivocally that any charging should be CV and no mention is made of current limited chargers.

After I hooked up the cells into a 12V pack I took a hundred what’s or so out of them for an hour until the pack was reading 12.05V and then tentatively offered a charge.

Now my charger is actually a reworked DC welder, so is a current limited supply, that can now be set to maintain a given voltage into any load that draws less then 140 A.

When it reached 120A, 10 seconds later, and still hadn’t achieved the target float voltage I backed it off to about 60A (C/8) and watched. There was no sound or perceivable temperature increase and within about ¾ Hr the charger started slowing reducing the current to maintain the 2.26vpc I had asked for.

I charged the pack for a further 3 hours or so at which point I had to stop because I needed less then 20A which I currently cant achieve. (New charger needed for finishing and maintenance charge)

SO … after all that history what I need to know is:-

(Apart from the gassing threshold all VPC’s mentioned would, in practice need compensating for temp)

1. What maximum charge rate should I apply, given that I couldn’t charge at more than 80A even if I wanted to?

2.  Can I use a bulk / float strategy taking a VPC just below gassing, 2.35VPC, as stage 1 whilst managing the current, so still CV once I am below 80A.
 When stage 1 approached the gassing threshold at C/20 stage 2 would start.
 Stage 2 would then be a float at 2.25 VPC which would continue until the current stabilised.

Feel free to berate my stupidity if that is what is required, In would rather look silly than turn what look like god cells into scrap overnight. Shocked

Alistair
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Off Grid - Big Caravan and huge enclosed gazzebo.
300W PV 12V system.
400Ah of AGM Absolyte GP cells. (Second hand)
600W Inverter (Maplin's finest :-) )
CHP in the works - Chinese Horisontal Diesel [S195 Generic - Kukje]
VAWT testbed flying - Back to that when its warmer I think.
Dyslexicbloke
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« Reply #1 on: May 06, 2011, 06:30:08 AM »

I found an answer to question 1 here .....
www.battcon.com/PapersFinal2006/GettisPaper2006.pdf

" Limit recharge current to 25-30 amps per 100-amp hour rating at the 8-hour rate."

The author go's on to say this data is not published but was provided by GNB technical support.

Asuming the newer technology is fundamentally simmilar to the older stuff, and I know the chemestry and basic design are,
then may cells will be fine with :-
(C@8) 344 / 100 * 25 = 86A provided that the battery is accepting that without exceeding the desired VPC

I am still intrested in any opinions or first hand experiance on this or my second question.

Alistair
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Off Grid - Big Caravan and huge enclosed gazzebo.
300W PV 12V system.
400Ah of AGM Absolyte GP cells. (Second hand)
600W Inverter (Maplin's finest :-) )
CHP in the works - Chinese Horisontal Diesel [S195 Generic - Kukje]
VAWT testbed flying - Back to that when its warmer I think.
EccentricAnomaly
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« Reply #2 on: May 06, 2011, 11:02:45 AM »

Sorry I can't help with the details but I think there's one misconception here worth discussing:

...the minimum float necessary to prevent damage is 2.2VPC which into a discharged cell would result in a huge current.

If the cell is discharged then the float voltage is irrelevant because you're not floating it. Float charging is the last phase of the charging cycle and serves just to keep the battery charged against its own self-discharge so doesn't apply to discharged cells. It's a slightly lower voltage than that used in the previous stage of charging.
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Dyslexicbloke
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Blue sky thinking ...


« Reply #3 on: May 06, 2011, 02:08:55 PM »

Yes I understand that, it was the lack of any published info on an apropriate initial current that had me wondering.
Perhaps I didnt explain well, sorry I thought it was too long already.
The docs state charge at a Min VPC of 2.2 and if you are recharging from a discharged state, after an incuident is the way its described because they are talking in terms of a standby power system, then do an EQ charge to bring it up quickly.

The instructions for an EQ say charge at 2.35 VPC, just below gassing I guess, untill all the cells have equlised then drop to float. It never even hints at any form of curent limmited phase which is just silly.

I guess you are supposed to be using their charger which would know what it is doing even if its not stated as a cell parameter.
I am going to stick with the third party info I think, because that seems to make sense, despite the fact that I just found some product brochures that state the cells will cope with 100A for every 100Ah@8 which in my case would be a litle over 340A  Shocked  I wont be going there, I'd need a bigger genny for a start. 25% of that sounds much safer to me and it still represents an initial charge rate of 1Kw which I think is still huge.

Ah well what would I know .... garden


Alistair
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Off Grid - Big Caravan and huge enclosed gazzebo.
300W PV 12V system.
400Ah of AGM Absolyte GP cells. (Second hand)
600W Inverter (Maplin's finest :-) )
CHP in the works - Chinese Horisontal Diesel [S195 Generic - Kukje]
VAWT testbed flying - Back to that when its warmer I think.
Outtasight
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« Reply #4 on: May 06, 2011, 04:32:32 PM »

Initial current limiting requirements depend mostly on the internal resistance of the cells.  If they have high internal resistance then they'll heat up with a large charge current.  It also depends on the construction of the plates, as some kinds can absorb charge faster than others.  If you charge them too fast, the energy doesn't get absorbed by the lead but just heats the cell up or causes other unwanted side reactions to speed up.

For most cells, the recommendation I've seen is to limit charge current (initial or otherwise) to C20/5.  In the case of my 180Ah rated batteries (at C20), this meant limiting charge current to 36A.  C8/4 doesn't sound unreasonable.

Some very low internal resistance AGM designs claim to not need current limiting (or at least can be charged at crazy rates like C/2). But this would be strictly for CV charging to the float level only (no absorption phase). 

For UPS batteries, there are two types of cell used: "normal" and "high rate" cells.  High rate cells are good at providing an enormous current for a UPS load for a short time only (the 30 seconds until the generator gets going).  Normal ones are for longer run time (10's of minutes to maybe an hour or so) at more moderate currents for systems without a generator backup.

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http://solarbodge.blogspot.com/
3.58kWp & 800Ah LiFeYPO4 off-grid(ish). See 'Cobbled together PV in W.Sussex' (in "Show Us Yours")
Dyslexicbloke
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Blue sky thinking ...


« Reply #5 on: May 06, 2011, 06:54:27 PM »

Good stuff thanks .... The gaps are begining to fill in now.

I am a little embarassed to say I found the current limit data yesterday, I just didnt understand it ...  facepalm

There ia agraph on page 17 (printed) / 22 (Pdf Page) here:-
http://industrialenergy.exide.com/exidepdfs/Section%2026.10%202010-12%20Absolyte%20GP%20Constant%20Current%20Specs.pdf

Now I have some idea what I am looking at, THANKS ALL, it makes perfect sense.
From 80% DOD curent limmiting to 74A is required and should take aproximatly 1 1/2 hrs to reach mean float (2.25 VPC @ 25C) at which point a conventional CV taper charge will take over.

At aproximatly 75% recharged, after 4 1/2 Hrs or so, the taper should have fallen to 20A at which point I will stop and either switch to a seperate finishing charger or wait for the sun to come up and finish the charge with the PV's over the next 6 hrs or so.
At 95% charged float current should be 4A or less.

The plan ...

I intend to work between circa 5% DOD and 70% DOD allowing the pack to fluctuate between these levels based on opertunity charging.
If DOD exceeds 70% then a charge cycle will be initiated.
Once per month a charge cycle at EQ level for the CV will be performed with a final soak at nominal float for an aditional 3 hrs or untill all cells are equal.

I am also considdering installing some aditional short term cyclical storage capacity to capture any exess solar capacity that cant be utilised during the low current soak period. This capacity will then be recycled and used to extend the solar day.
During periods of low solar availability this aditional capacity will be rapidly charged whilst the generator is running and used to finish the charge cycle to a deeper level without the need to run the generator for extended periods without any significant load applied.

As always any and all comments are welcome, particulaly if you think I am planning something silly.   stir

Alistair
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Off Grid - Big Caravan and huge enclosed gazzebo.
300W PV 12V system.
400Ah of AGM Absolyte GP cells. (Second hand)
600W Inverter (Maplin's finest :-) )
CHP in the works - Chinese Horisontal Diesel [S195 Generic - Kukje]
VAWT testbed flying - Back to that when its warmer I think.
Outtasight
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« Reply #6 on: May 06, 2011, 09:11:27 PM »

As you picked them up cheap, it might not matter that you'll hammer them so hard (70% DoD is pretty low).  I try to stay within 10%, down to 50% DoD and I've been cycling the ones I have now for about a year for the AGMs and 18 months for the gel ones.  I rarely get above 90% charged, as I only have solar charging.  It takes about 2 days of sunny weather with no night time loads to get there.  As I use the system every day, it never gets there.

UPS batts aren't generally very good at cyclic use (as they're designed for float standby use).  Most are good for about 250 cycles at 80% DoD. Traction / cyclic use AGM cells are good for about 400 cycles, and so are gel ones. Typical caravan wet batteries are good for about 100 cycles to 80% (if you're lucky).

Mine were similarly cheap but the problem is finding more when they die without spending lots of money...

I've thought about sacrificial small batteries for absorbing wasted energy and to "top-off" the main bank, but that's what they'd be... sacrificial.  Unless you use a lithium battery or a NiCd / NiMH pack, that does not mind sitting flat all night and the rest of the time at partial charge (so it's always got some empty capacity that needs filling) then whatever lead acid battery you use for that small battery will not live long.

Then there's the losses in charging it, and discharging it and converting the battery Voltage to something a bit higher that can charge the main bank.  Remember a 12V battery can't finish charging a 12V battery (as you need about 13.8V to do that).  So you'll have some loss in the DC-DC converter and some more loss in the charge controller...  Generally doesn't work out for the sake of the last 5% charge on the main bank.

If once in a while you decide to go without using the battery for a couple of nights, you can pump it up to 100% on the solar.  The other way is with some wind power to supplement the PV (your sig mentions getting round to this...  good plan).

I used to use a bit of grid power to do mine (before I joined the FIT scheme).  Not sure if the FIT payments balance out the cost of extra batteries over time, from not having a reliable source of power to do maintenance charges on the battery bank.
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http://solarbodge.blogspot.com/
3.58kWp & 800Ah LiFeYPO4 off-grid(ish). See 'Cobbled together PV in W.Sussex' (in "Show Us Yours")
Dyslexicbloke
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Blue sky thinking ...


« Reply #7 on: May 07, 2011, 03:27:22 PM »

Mmmmmmm all good points, It hadnt occured to me that not fully recharging them, last 5% or so, wouldnt be detrimantal.
I dont know where that mindset has come from now I think about it properly, I was just asuming that I would want to be topping them off regulaly rather than only after an QE charge.

After considdering all your points, and a few wrinkles I was pondering last night, that on ballance its not worth mucking about with.
I will concentrate on getting some wind resource I think.

On the 70% DOD point .... That is being conservative, at least I thougyht it was. The manufacturers data for these cels spaciffically states that cyclical operation to 80% DOD is within their design paramiters, all be it with a 5% reduction in life expectancy.

Having said that they are rated for some 1200 sycles over an expected operating life of 20 years and given that these are 6 - 10 years old and have only ever been held at float I am not expecting too many issues.

They also state that recovery from 100% DOD is possible and go so far as to issue expected charging curves from that condition !!!
( I dont think I will try that )

Thanks for the input another perspective is always good to rein in an oneractive mind.

Alistair
Logged

Off Grid - Big Caravan and huge enclosed gazzebo.
300W PV 12V system.
400Ah of AGM Absolyte GP cells. (Second hand)
600W Inverter (Maplin's finest :-) )
CHP in the works - Chinese Horisontal Diesel [S195 Generic - Kukje]
VAWT testbed flying - Back to that when its warmer I think.
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