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Author Topic: Measuring / Estimating DOD whilst discharging .. How?  (Read 7190 times)
Dyslexicbloke
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« on: May 07, 2011, 04:15:25 PM »

The question says it all really …
How do I establish depth of discharge whilst I am using the batteries?

I have a few 24Hr loads, telephone system, egg incubator …. The upshot is that I do not completely rest the batteries over night so I don’t get to use rested open circuit voltage as a guide in the morning.

I have herd tell of AH counting systems but I expect that these will suffer inaccuracies as the batteries age, particularly if there are any periods of abuse which, in a catch 22 situation, might result from inaccurate Ah counting.

I am looking for any suggestions here but I cant help wondering if it is possible to estimate DOD by comparing the terminal voltage with the current discharge rate.

A formula would be nice, but I expect even if its possible the relationship isn’t going to be linear.
Anyone tried this or know if it is even practical.

As always I will welcome any and all comments.

Thanks,
Alistair
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« Reply #1 on: May 07, 2011, 04:47:08 PM »

The 'Smartgauage' is the beast Alistair but if you thought the 'Watts up' was expensive you'll have a coronary at the http://www.smartgauge.co.uk/ though it is supposed to be the 'dogs'

I did see graphs somewhere and I'm sure it was based on what you said but I've a memory like a hen  wackoold, hopefully Justme or someone who knows will be along shortly  Grin

Good luck, Paul
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« Reply #2 on: May 07, 2011, 04:55:46 PM »

I agree the smartgauge is the one you want.

Unless you want to get technical & measure the specific gravity of each cell & temperature compensate it every day.

AH counters get worse every cycle & the smartgauge gets better. Yes it costs but it can save you more in charging costs. It also has a built in controlable  relay that you can use for genny start or load control via a suitable external relay / system.
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« Reply #3 on: May 07, 2011, 05:03:15 PM »

I'm still waiting for the mythical 48v one  flyingpig

Cheers, Paul
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Dyslexicbloke
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« Reply #4 on: May 08, 2011, 12:30:34 AM »

Thanks folks …
Well based on the writeup I can see why you would recomend the smartguage but even though it wasnt as expensive as I first feered its still too rich for me at £150.

To put it into context I only paid £180 for the whole 12V 400AHr pack and it took me a week to perswade her with the purse string to stump up for that.

Fortunatly the used bats are wildly outdoing my expectations, despite looking a litle mis shapen.

I am going to have to hunt down that graph and see if I cant come up with some aproximation of DOD that isnt going to cost as much as the cells to fit!
Given that my asumptions about AH counting, being a poor solution I mean, seems to be the mainstay of the marketing of the smartmeter it is clear that I shouldnt go that way.

So I am left with measuring current and voltage ..... Questions:-
I know the internal resistance of a battery is linked to age, condition, type and capacity but is it linked to anything else:-
State of charge?
Rate of discharge / charge?
Temperature?
And the big unknown … Surface charge?

I am asking because when the load on a battery changes by a known amount if the internal resistance changes by a predictable amount or stays the same it could be calculated relatively easily.

That would in tern make calculating the no load terminal voltage a sinch based on the loaded terminal voltage and the calculated internal voltage drop.
The fly in the ointment is surface charge ….
When you charge a battery it needs to rest for at least 4 hrs, as far as I know, allowing the surface charge to dissipate or be absorbed before the terminal voltage is of any use in determining its current state of charge.

What I don’t know is if there is any such lag when discharging…..
If a battery is discharging for some time and is then disconnected will the terminal voltage continue to drift over time or is it immediately stabile?, not withstanding self discharge, I am talking short periods here.
If it does drift, then is the drift predictable based on the magnitude of the previous discharge?

My thinking is to look at current and predict how the battery is behaving in respect of its apparent internal resistance so that I can calculate its resting terminal voltage without resting it!

I am beginning to see why the Smartmeter is the price it is …..

Anyone up for a DIY project to bring smart meter type algorithms to the poor among us?
How hard can it be it only took the 3 years  Tongue

My head hurts .... 'O' for a Smart meter genuflect

Alistair
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« Reply #5 on: May 08, 2011, 10:25:37 AM »

I'm still waiting for the mythical 48v one  flyingpig

Cheers, Paul

I'll ask him again. I know they were working on a "super" system so that might have taken over the 48v one. He does say that you could monitor a 12 or 24v string within the 48v bank & be almost as accurate.



DB they do come up second hand occasionally on the forum he is on. If I see one I'll let you know.
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« Reply #6 on: May 11, 2011, 01:13:36 AM »

DB:  You can get a quite good feel for the state of charge just from a Voltmeter.  It's how the Smartgauge does it but you have to use a bunch of experience to do the same (and watch the battery and loads).

If you can watch the Voltage and the net current in/out of the battery and the load is quite steady then the Voltage tells you quite a lot.

If you're running a small 5A load on a 400Ah bank, the Voltage will creep down slowly and by the time you get to 12.2V, the bank will be pretty dead (maybe 70% DoD).  Sticking a massive load on it of 30A will be misleading as the Volts will sag probably quite quickly to around 12V, but recover over an hour or so if you remove the load.  Probably a good rule of thumb is to not run the bank below 12.5V with a steady C/100 load on it; or below 12.0V with a steady C/20 load on it. 

You're right about the internal resistance.  It changes with almost everything!  Age (absolute calendar age and cycle age), state of charge, recent charge history (lots of partial charges are bad), temperature, acid density, electrolyte stratification...
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« Reply #7 on: May 11, 2011, 07:00:17 AM »

DB:  You can get a quite good feel for the state of charge just from a Voltmeter.  It's how the Smartgauge does it but you have to use a bunch of experience to do the same (and watch the battery and loads).

If you can watch the Voltage and the net current in/out of the battery and the load is quite steady then the Voltage tells you quite a lot.

Couldn't agree more Outtasight, after a couple of years experience you really can tell what your state of charge is and even when someone has left a light on upstairs. The secret is to have your voltmeter where you can see it all the time. Mine are in the kitchen where I spend most of my indoor time, apart from sleeping  Grin

Cheers, Paul
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Dyslexicbloke
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« Reply #8 on: May 12, 2011, 01:22:15 PM »

Interesting guys thanks ...
When you say the voltage will creep down with a light load do you mean that it changes relative to DOD or DOD and the period of discharge?

What happens when the discharge is removed? is the voltage immediately stable or will it then creep up over a period of time?

I was under the impression that surface charge had a big hand in this, skewing the voltage up or down over time depending on the rate of charge.
Isn’t that why the terminal voltage is only a reasonable indicator of DOD when the batteries have rested for a long period?

Voltages to discharge to ...... the 12.2 that you quoted seems very high, am I missing something?
The published data I have for these cells quotes constant current discharge curves from 1 Hr to 24 Hr's enabling the effective AH rating to be determined for a range of discharge rates.
The figures are quoted for several final voltages, I assume corresponding to different DOD's, but the highest voltage quoted is 11.64v with the lowest way down at 10.5v

Given that the cells are 400Ah nominal and that is achieved with a 24Hr discharge to 10.5v @ 17A it leaves me wondering about your assertion that I shouldn’t be discharging below 12.2

I have attached the data table, and would appreciate your thoughts RE what it actually means in practice. The bats are rated for 80% DOD over 1200 cycles although a shallower discharge will result in a longer calendar life. I think the figure was 5% design life reduction for every 10% DOD below 50%DOD assuming all cycles are simmilar.

Alistair


* CCDischarge.png (10.28 KB, 856x205 - viewed 423 times.)
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Dyslexicbloke
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« Reply #9 on: June 15, 2011, 02:33:51 AM »

I'v been running these batteries for a while now and havnt let them drop below 11.3, to be honest I am a little nervous given the comments earlier in this thread.

Recharge times are as expected, 55A current limmited then a taper to the termination voltage.
I am stoping at 15A because I dont want to run the genny with a minimal load but that is only 90% full which leaves somr headroom for the solar the following day
The recharge seems to be reasonably close to the data I have but obviously I dont have much idea of how full they realy are.

I am only having to run the genny every 2-4 days depending on how much sun there has been and a recharge is taking 6-8 Hrs depending on where they were when I started.
I am looking at the voltage and deciding late afternoon wheather I think I will get through the evning because I dont want the thing charging over night.

Id be interested in any further thoughts RE voltage .... see above.

Thanks
Al
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« Reply #10 on: June 15, 2011, 09:29:28 AM »

I'v been running these batteries for a while now and havnt let them drop below 11.3, to be honest I am a little nervous given the comments earlier in this thread.



I am assuming that the 11.3v is whilst under heavy load?

If not you could be killing your bats. I dont see 11.3v even under heavy (2-3kw) loads. I never see sub 12.2v at rest.

RE running genny.

The best time to run it is when you have other high loads to power. We try to add loads as the charge tails off to keep it within  50-80% of genny load capacity.

The next best time to run it is so that the genny run time stops just as the PV comes online to take over (so not ideal as thats late at night to early morning). Down side to that is you are not getting the best / most energy out of your PV. You need to weather watch & make a charging choice based on your estimation of will the PV doing it all unaided. Last week the forecast looked like we would have 2 bad days in a row so I stuck the genny on. In the end the second bad day was great & I have waisted the charging fuel as the PV could have done it. But as I was not going to be there on the second day and my auto system is off line as I am moving stuff about I had to make the call I did.


But reality says that you run it when you can / are there to start & stop it, till you automate the system.

But when automating it do remember a time clock lock out so it wont start during unsocial hours. I am using an adapted programable room stat that is also converted to run on 12vdc (idea from RECUK).
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Dyslexicbloke
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« Reply #11 on: June 16, 2011, 10:59:12 AM »

Mmmmmmmmm .........

I am getting increesingly concirned RE this battery voltage issue.
I know you folks generally know what you are talking about either form experiance of conventional wisdome which you are applying to your systems.

Given that my batteries are a nominal 400Ah and my inverter is only 600w constant 1200w peek they are never under anything like a high load.
In the evning, typically the telly and a couple of lights, the discharge rate is only about 6A, and that is when the 13.3 would be in play.

The other issue is that the pack is seldom if ever over 12.3, I'v seen 12.5 for a short period of use after a particulaly long soak but doing that with the genny just isnt echanomical.
The pack would want a charge rate that tapered from 15A to about 3A to fully charge and I just dont have any loads to keep the genny efficiant for that long.

It is clear that I need more PV but I am up against cost and will have to manage for the moment.

Is it possible that these AGM cells that I have are different to the cells that this conventional wisdome applys to?
If not how should I be interpreting the data that I have in order not to kill them?

I am confused, just ignoring what is being said seems silly and arrogent but believing it suggests that my pack is dead and shouldnt work at all which is obviously not the case.

Frustrated ...
Al
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« Reply #12 on: June 16, 2011, 12:54:56 PM »

The "end" voltages in your chart are to 100% DOD IE flatter than a flat flat thing.

You can kill a bank doing that in very very few cycles (like low single numbers or even once). You should for long & cost effective bat life aim for no more than 50% DOD / SOC with proper deep cycle bats & more like 70%SOC for leisure ones.

You say you get 13.3v when using the system yet you also say it never sees over 12.3-12.5v.

Can you explain further?

I am guessing that the 13.3 is when the genny is on & the 12.3 when its not. If so you are not charging your bats enough & are killing them. We start to recharge at 12.2-12.3v ish.

From memory jell bats need a more gentle charge at around 14.2- 14.4v ish after the bulk phase has finished.

You need to more accurately measure your total energy used per day, how much energy is going into the bats & how long they last till needing a recharge. I would guess that they are reduced to much less than 80% of new capacity already.

You will either end up changing your bank much more than should be needed or you will need to charge it to full much more often.

You can get away with just charging to say 80% as a daily charge & then say weekly do a full 100% charge plus a gentle (as its an AGM) EQ charge as often as is needed.
« Last Edit: June 16, 2011, 01:02:47 PM by Justme » Logged

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biff
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« Reply #13 on: June 16, 2011, 09:52:58 PM »

you would save money in the long run if you went for more pv, then the use a xantrex c40 and when you get a good bit of sun stick on the equalising charge for 20 mins at a time every 3 days, i found this to be an exellent way to rejuvenate batteries, however dont overdo it.
 as paul says, you can do a lot with the multimeter and experience but your batteries are being hammered at the moment, it would pay to try and get them to recover before you kill them off.it is possible you might have a sick cell.test them individually and if you find a dud there is every possibility you can find a replacement.
                                                                 biff
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« Reply #14 on: June 18, 2011, 01:00:22 AM »

The Marathon AGM telco batts and the gel Deka ones I have seem to like (according to their user guides) almost the same charge Voltages.  Some AGM ones like higher Voltages than gel.

Absorption Voltage is 13.95V @ 25C / 14.10V @ 20C (important to temperature compensate this).
Float Voltage is 13.65V @ 25C / 13.80V @ 20C

After a REALLY good charge, the bank sits at 12.70V  (25.4V as I have a 24V bank)

Some little 7Ah burglar alarm batts that I have sit for months out in the shed at 12.6V to 12.7V with no charging.

I might see my house bank sink to 11.90V (23.8V) at breakfast when we run the 900W kettle and a 400W toaster at the same time.  But the Voltage comes right back up after they stop.  Tonight with no heavy loads but a poor day of charging, it has gradually sunk to 24.6V and the Smartgauge reckons it's at 52% SoC.  That wasn't resting (my batts never get to rest Grin ).  They had like 4A load on a 495Ah bank.

My previous set of wet batts died after a little over a year, but I didn't have a Smartgauge and used to quite often run the bank down to 11.5V... BIG mistake.

The chart you has is for 100% DoD.  The 11.3V is DEAD at low load.  The 10.5V is DEAD at high load.  Most inverters have next to useless low voltage cut-outs set at 10.5V.  If you run your battery down to the inverter cut-off every day, you'll be lucky to get a few dozen cycles before they are murdered.

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