How to check discharge rate

[freejuice]

Hi Everyone,

 I have 4 new golf cart batteries, ( 6 volt and if not mistaken about 105 amps each) I have them wired in series and parallel for 12 volts. I have read here and elsewhere that batteries should not be discharged over 50% of their charge.

My question is: How do I check this? Do I need an ampmeter in line with the battery cables, volt meter etc? (The batteries came fully charged and read about 12.5 volts.)

 I don't want to ruin these things right off the "get-go"

My thinking is (no electrical guru here, so bare with me) that if I have an ampmeter between the rectifier and batteries and for example the batteries are fully charged and the ampmerter reads "50 amps" and I run the batteries down to where the meter says "25 amps" I have essentially used up 50% of the charge, and it time to recharge the batteries again....is this correct?

 Thanks,

 Gavin

[bob g]

number one, 12.5 is not fully charged, maybe 75% or so

number two, your batteries will not take 50amps when they are fully charged

if they are you have the voltage set way to high,, at full charge i would expect

maybe 3-5 amps max.

your method of using a amp meter to determine state of charge is also flawed

it will tell you what the load is taking from the batteries or if connected correctly it will tell you how much power is being put back into the batteries but

that is all.

you can use a good voltmeter to get a rough idea of state of charge, but

you will need a couple of things, one being a good hydrometer and the other is

what the specific gravity is on a fully charged cell in your battery and you get that from your batteries manufacture.

while you are talking to the manufacture, ask him for a chart of state of charge

and its correlation to specific gravity,

it should read something like   100% state of charge = 1.270 (or possibly higher)

                                75% state of charge = 1.250

                                50% state of charge = 1.230

then use your batteries down to whatever the specific gravity should be according to the manufacture (at 50% = 1.230)and take a voltage reading right there,, this will be approx where 50% state of charge will be as measured in volts.

that will get you close,,, but temperature compensation comes into play as does

whether or not you let the cells rest for 24 hours or not before you measure.

i think it will get you close enough to learn how your batteries act, keep a good chart of specific gravity, volts, and time resting and after a while you can tailor

your voltage to mean closer to your goal.

just remember specific gravity on a rested cell at 77F is the most accurate method of determining state of charge.

hope  that helps, i am sure others will chime in and maybe point out what i have missed or misstated.

bob g

[OuttaSight]

Hi,

By "rectifier", I assume you mean the charger? Even in that case, the answer is "not really"

A 50% discharged battery will draw the same current when charging as a 90% discharged one.  Large banks will often draw as much current as you can throw at them and the charger will be the limiting factor (not the batteries).  Once a bank gets to about 70% full, it will start to draw less current (provided it's on a proper charger that limits the charge voltage to the appropriate absorption voltage for your type of battery - it varies from 13.8V to 14.4V depending on battery type).  When the current stops dropping for a time (usually 2-3 hours) then the charger stops or goes to float mode as the battery can be considered as "full".

There are only two ways to accurately determine the state of charge (SoC) of lead acid batteries.  The first is to measure the specific gravity of the acid and compare it to the reading when the battery was fully charged.  The second is to measure the voltage of the battery after it has been at rest (not charged or discharged) for more than 6 hours (maybe more, depending on how fast it was being charged / discharged prior to resting).  A fully charged battery will read about 12.8V and a dead one about 11.5V.

The trouble is, you usually want to know how dead your battery is while you're using it (discharge cycle).  Depending on how fast you are discharging it the thing will read anywhere from 10V and 12.5V, so reading the voltage during discharge is mostly meaningless and lots of people kill their batteries that way.

If your battery is sealed then you've got no option but to use a Amp-hour counter or something like a SmartGauge to estimate the state of charge.

Amp-hour counters work by measuring the charge / discharge current over time and adding it up to guess how much capacity has been used or put back.  They get out of sync with the battery after a while so now and then you have to fully charge the battery and reset the Ah counter.

The SmartGauge works by watching the voltage behaviour over time and estimating the SoC directly.  It reads in percent (not volts).  I've got one for my 24V solar bank and it seems to work pretty well (never needs to be reset) and works with 12 or 24V batteries of various lead-acid flavours (wet antimony, wet calcium, gel, a couple of AGM types). It learns the specific behaviour of your battery bank over time so works with a bank of any size but the bank has to be wired up right to keep it balanced as it can't tell if one pack is out of balance and getting over/under charged.

The other thing to bear in mind is that as a battery gets used over and over and ages, it's capacity goes down.  So a brand new 100Ah battery has a capacity of about 95Ah.  After about 15-50 cycles it peaks at maybe 105Ah and then it declines to an end-of-life value of about 80Ah.  You can use it longer than this but the capacity starts to drop off much faster from the 80% limit and old batteries charge more slowly and gas a lot so need more water maintenance.  Old AGM and gel batteries just die of dehydration!

When you consider discharging the pack to 50%, you have to remember that it is 50% of the current capacity.  So for a new one it means you can extract 47.5Ah, for a mid-life one it's about 52.5Ah and for an old one it means 40Ah.  If you discharge an old battery by 50% of 100Ah, you will have actually discharged it by 62.5% (50Ah out of 80Ah real capacity) and will hasten it's demise even faster than you thought!

[freejuice]

Hi Folks,

Thanks for the information...looks like a good hydrometer is needed!

 The information you folks gave is priceless!

 Thanks again,

 Gavin

[wooferhound]

Here is a chart that shows State of Charge for Lead Acid wet batteries. It does not take into account temperature and other factors but it's a fairly good guideline to let you know what's going on with your batteries.

State of Charge (SOC) Chart:

% of Charge                Charging               At Rest       Discharging

100. - - - - - - - - - - - - - 14.75 - - - - - - - - 12.70 - - - - - - 12.50
     
90. - - - - - - - - - - - - - 13.75 - - - - - - - - 12.58 - - - - - - 12.40
    
80. - - - - - - - - - - - - - 13.45 - - - - - - - - 12.46 - - - - - - 12.30
    
70. - - - - - - - - - - - - - 13.30 - - - - - - - - 12.36 - - - - - - 12.25
    
60. - - - - - - - - - - - - - 13.20 - - - - - - - - 12.28 - - - - - - 12.15
    
50. - - - - - - - - - - - - - 13.10 - - - - - - - - 12.20 - - - - - - 12.00
    
40. - - - - - - - - - - - - - 12.95 - - - - - - - - 12.12 - - - - - - 11.90
    
30. - - - - - - - - - - - - - 12.75 - - - - - - - - 12.02 - - - - - - 11.70
    
20. - - - - - - - - - - - - - 12.55 - - - - - - - - 11.88 - - - - - - 11.50
    
10. - - - - - - - - - - - - - 12.25 - - - - - - - - 11.72 - - - - - - 11.25
    
    

http://solarjohn.blogspot.com/2007/03/measuring-battery-state-of-charge.html

[mjby]

Thanks for the info. I am wondering if I am charging my AGM battery system properly?

 I have 4 Enersys (Yuasa) UPS back up batteries(12 volt, 135 amp hr@20 hr rate, each weigh approx 98# ) from an old telephone switching system reportedly very lightly used. They were put into use in Dec 2005 and have worked great so far. I monitor via trimetric 2020 and  have 4 solar panels panels, total of approx 400watts. I try not to run the SOC to < 80%, occasionally it is lower. Company recommends a float charge of 13.5 volts  @ 77 degrees F. I am float charging a 13.6 volts, temperature compensated. I do every 6 months check the SOC of each battery after 6-8 hours of no charge at rest. The last time I checked they were 12.56V, 12.68V, 12.68V, 12.68 V, so one of them was slightly less. Should I be concerned about the weak one?

I do not equalize these batteries as I am under the assumption AGM batteries generally should not be equalized. Is that correct?

Any info from you battery experts will be much appreciated. Thanks-- MJBY

[SparWeb]

Being the batteries from a UPS, it may not be fatal to discharge them a bit more than you're doing.  Being AGM batteries, a bit of exercise would do them good because you can't shake them up by equalizing.  Equalizing as you know may release gas that you cannot replace in the "sealed" battery cells.

At least, that's my point of view.  I have a set of old AGM backup-batteries and I keep them fit by draining them to nearly flat!  Okay, I'm exaggerating, but not as much as you might think.  Their data-plate capacity is 880 amp-hours and I got over 700 AH out of them last time I cycled them.  They're over 10 years old.  I'm just lucky, I guess.

I'm not suggesting you run yours flat, but you could put a relatively large load (C/5) on them and take them down to about 50% charge, then re-charge "immediately".  If you're going to make a big deal of doing this (like I do), then you could use a fixed resistance load, and ammeter, and measure the current and voltage during the process, then determine just how close to the rated capacity you are getting out of them.  This all depends on how much trouble you want to go to.

(BTW You would get more responses to your question if you posted it in a separate thread)

[OuttaSight]

Correct. AGM batteries cannot be EQ charged above their limit voltage (usually less than 14.2V). The exact voltage requirements depend on the manufacturer and Yuasa actually say that some of their NPC AGM batteries can cyclic charged to 14.6V as they have efficient gas recombination.  Other AGM types may be destroyed through emergency gas venting if charged to this voltage!

UPS and "telecoms" AGMs are thin plate float operation batteries that are unsuited to cyclic RE use.  I've seen used ones being sold very cheap on eBay but have avoided them.  Yuasa sell two classes of AGM sealed batteries.  The EN and NP series (and the extended life ENL and NPL) are thin plate float application batteries that are intended to almost never be cycled.  UPS systems can call on their batteries to be drained to 20% DoD before automatic shutdown but mostly they are used as a 30s buffer to allow a diesel genset to start and take over the load and immediately recharge the batteries.  In any event, it is expected that they would only be cycled no more than a handful of times in their 10-15 year lifespan (how often is there a major power cut these days?).  So they are essentially used in the same manner as car batteries - to deliver a high current for a short time and then immediately recharged for days at a float rate.  Only 5-10% of their capacity will be used in most cases.  Car batteries can sustain a full discharge cycle maybe 20-40 times before death and the NP/EN series batteries are a little better (maybe 100-150 cycles) but the NPC (the C denoting "cyclic use") are designed for repeated deep discharges when used for scooters, fork lifts or street cleaners, etc. that drain the cells daily. NPC are rated to 700 cycles for 50% DoD and 600 cycles for 75% DoD.

Just because a lead-acid battery "hasn't been used" in 4 years of service doesn't mean it's going to perform as new.  The day the acid was poured into the cells, they started to age.  If the NPL battery was stored at 20C it will reach end of life without discharges at 15 years. If stored at 30C it will half the lifespan.  Most UPS enclosures are not air conditioned as it costs more to air condition it than it does to replace the batteries at shorter intervals (4-5 years).  Batteries that are kept on float for years corrode their positive plates and fail. I have a UPS in my computer room and the batteries (if left inside the casing) die after just 2 years of float use because they are sat next to the transformer and get slow cooked at 35C.  If I extend the wires and keep the battery outside the box (at room temperature) they last about 4 years. Many charging systems were not closely temperature controlled and so lots of AGM batteries die of dehydration so telecoms companies replace them at shorter intervals just to be on the safe side as a set of batteries costs less than a service outage from a duff UPS.

"Exercising" your batteries does not "wake them up".  It shortens their life by 1 deep cycle.  If a battery hasn't been used for a while or has been short cycled and float charged, the acid may stratify in a wet cell (but this can't happen in a AGM so there's no need to EQ charge them). EQ charging "boils up" the acid and stirs it to mix the layers.  New batteries do have lower capacity than ones that have a few deep cycles and this is probably where the misconception of exercising cells came from.  After the first dozen or so deepish cycles though, the battery reaches peak capacity and thereafter, deep cycles degrade the capacity with each cycle.  So don't "exercise" your batteries! If they haven't been used for a while, give them a top-up charge above the long term float value.  Even sitting at float, they will develop a bit of sulphate (especially if the float voltage wasn't temperature compensated)

If you have a out of balance battery bank it could be because the battery is weak or it could be because the wiring is wrong and one battery has longer leads on its interconnects than the others.  The charge/discharge currents must pass through exactly the same lengths of wire of the same cross section area and the same contacts as all the other batteries in parallel or else the current will favour one path over another and the bank will become unbalanced.

[Simen]

Now, that was a good charge of information.

Thanks. )

[mjby]

Thank you for the great info, extremely helpful!

This may be a dumb question but what is the best way to tell if AGM batteries are beginning to fail? Just seeing how well they are holding a charge,  or periodically checking the voltage at rest? Sealed Valve Reg batteries can't be opened easily to check the spec. gravity. Thanks, MJBY

[SparWeb]

I think the only way to do that is exactly what I suggested above.  A controlled and metered discharge done in a way that can be compared to data from the manufacturer.

I've done so three times with various battery banks.  The tests take a long time to set up, and to conduct the discharge takes hours.  Crunch the numbers carefully afterward because falling voltage affects the current, but the manufacturer's data will assume constant current and their factory tests should have been set up to make it so.

[zeusmorg]

 One thing that will help the longevity of AGM's is to rotate their positions occasionally, that way, the same battery will not be getting more charge/discharge all the time.

 I do not recommend equalizing AGM's often, but if they start getting imbalanced it can help, however you will shorten their overall life by gassing out.

[wellusee]

What a reply !

This is simply GREAT, I'll have to read this over and over again and again.

I'll bw watching out for "OuttaSight" again.

Thanks.

Paddy.