Basic Battery Amp-Hour capacity question

[CmeBREW]

Hello guys:

Hate to ask such a basic question, but I have been looking for answer for days but no luck. Here's the problem:

I finally bought two brand new "cheap" (Walmart) 12v "deep-cycle" marine/RV batteries to start experimenting with my basic beginner system. They are 115 Amp-hour each hooked in parallel of course. (=230 AH@12v)  I have read that a 100 AH battery should, in rough estimate, put out 1 amp for 100 hours or 100 amps for 1 hour, or 50 amps for 2 hours, etc, etc,etc.  My problem is, I am only getting about 1/10th of that storage capacity!! I have had 2 years in electricity/electronics so i do know the basics of electricity, But I'm still a beginner with this stuff. They are full of distilled water and were fully charged when I bought them both at the same time. I then immediately hooked them both together to equalize. I've been reading many persons many times using a couple of these or golf cart batts to run a Tv or computer (150 or so watts)for 4-6 hours on a charge. My 100 watt TV(including 4 watt inverter) runs the full charge (12.72) down to the 20% safe discharge point (12.46v) in only 15 minutes! It's not my volt meter or new inverter either. It does the same thing with my campfire 5Watt 12v fan hooked up directly. These batteries act more like 7 Ah each! I wonder if a Walmart employee was secretly using these batts to go out fishing with, over the last few months or something! Oh yeah, the sticker on the batts say they are only 3 month old. It least there garanteed for a year or two.

Has anyone had similar problems with this type battery. I've only had them for a week. I was hoping to at least watch a 2 hour movie on a full charge. Then Recharge each day.   -Thanks "experienced ones".

[Flux]

"My 100 watt TV(including 4 watt inverter) runs the full charge (12.72) down to the 20% safe discharge point (12.46v) in only 15 minutes! "

I think this is where your answer lies.

Firstly a battery will normally only give its rated AH at a low discharge rate.

You would certainly get over 100 hrs from a 100AH battery at 1 A, but you would get nowhere near 1hr at 100A. There is a thing called the Peukert effect that reduces the capacity drastically at high rates of discharge. The normal rating is at the C/20 hr discharge.

Now to your problem, when a battery is left standing for hours with no load or charge, it will be at about 12.8V when fully charged and your 12.46 figure would be for one discharged 20% or so as you say.

This figure has no meaning during charge or discharge. On a significant load during discharge you will be down below 12v fairly quickly and most inverters and similar are set to cut off at about 10.5v. For your 4 A load that is a bit low and perhaps 11 to 11.5v would make more sense.

Similarly once you start charging the volts will rise above 12.6 almost immediately but you will not reach full charge until about 14v.

You should be able to track down the charge and discharge curves for your battery or one similar.

Measuring state of charge from volts is a major pain and little use as you need to let the battery sit off load for several hours.

Flux

[RogerAS]

Brew,

Without the ability to look at your batteries it's hard to say, but here's my suggestion.

Buy a hydrometer. You know, the thing with floating balls that measure the specific gravity of the acid in the battery. Test each cell when you think you have them equalized. They should test out to have 13.8 volts per battery or 2.3 volts per cell after resting for an hour or so post equalization (or better). There should be very little variance in this hydrometer reading between cells in a new battery. Just because they're new doesn't mean they are good.

You state that these are full of distilled water. That's what one replaces low electrolyte, which is not with what they should be filled. The hydrometer will tell you exactly how concentrated the acid is. Also there is a point where adding more water than needed dilutes the acid too much. If this is the case careful extended equlaization can correct the problem by evaporating some of that water and bringing the specific gravity higher.

If in doubt have Wally World replace them under warranty.

How are you equlaizing these now? Is it a grid tied charger and if so by what method are you checking the state of charge? A simple Wally World digital voltage meter should get you close enough when combined with the hydrometer readings. Is that what you have? To truely equalize one needs to hold the voltage on a 12V system at 16V with about 2 amps of current for around 2 hours. During this time be careful not to allow the batteries to get hot. They will boil, and that's the point, but the low current will not damage them, it will just drive off the sulphate from the plates and mix the acid. This is not a result of heat. Be sure to keep an eye on the acid level and NEVER let them get the paltes dry. Most off the shelf automotive chargers will not do a good job of equalizing.

I don't think you have a 4 watt inverter. I assume you meant 400 watt. Depending on the type of inverter this 400 watts, if run at full load, will be 33.33... amps drain on the 12V batteries. So 230 AH batteries, if really healthy, will run the 400 watt inverter at fulll load for about 7 hours before going to the dead point 100% discharged or aprox 10.5V. Assuming 100% efficent inverter which none are. I would bet a healthy battery in this amp rating range should supply that inverters full load for more like 4 hours. Your inverter may have a high cutoff point for the battery voltage, test that too.

I also feel your range of useful voltage is way too narrow. The difference between 12.72V and 12.46V is only .26V. I typically range my bank of 12V batteries from 14V to 11.75V, a difference of 2.25V. I have taken them much lower, but try not to do so, especially this type of battery as they are not true deep cycle batteries.

So provide us with some updated info and we'll be happy to try to help.

[kurt]

12.46v is 80% discharged with the batteries at rest for 3 hours. voltage will be much lower with a load on them. try this run your inverter/tv till you get a voltage reading of 12.46v then turn everything off for 3 hours and check voltage again i bet the batteries will read almost fully charged. most people pick a lower voltage that they stop discharging at under load like 12v or 11.75v.  most inverters cut out at 10.5v that is way to low and will damage your batteries so you need to pick a higher number.

[CmeBREW]

Thanks guys for responding. I guess i should've clarified it better. I am just beginning and don't have a Hydrometer yet. Ive been letting my batteries 'rest' overnight to take the voltage reading with my (i hope 'accurate') voltmeter to get a percentage of charge. I can see that confused everyone. Sorry about that. I follow the "battery metering" chart here on this website to determine state of charge at this time. Also when I said i have a 4 watt power inverter, I mean that, the inverter by ITSELF uses up 4 watts. (.3amps) It is actually a 600watt rated  inverter.(1200 surge/low batt cuttoff 10v &high volt cutout 15v)  But I don't want to put much more than 100watts thru it until I know what I can get away (safely)with using these two 115 AH batteries. It is this amp-hour stuff that is confusing. I plan on buying one of those Hydrometers so I can get more 'active' readings though. It could be my voltmeter is not accurate enough, even though it is suppose to be a faily accurate meter and get consistent readings each morning. Presently, I only have a 5 watt (wow!)solar panel hooked up and I am testing/charging a small 20watt wind mill on and off. And I use a 2amp 120vac charger.  Nevertheless, for now, what is your guess as to what is 'average' and how long I can run 10 amps at 12 volts (which is about my 100w TV) by using my fully charged 230 Amp-hour battery capacity, down to the safe 20% discharge? 15 minutes seems very small with what I've been reading on the archives.  Bottom line,I will get the hydrometer and see if something is screwwy with my voltmeter.   -Thanks for yo' input!

[Ungrounded Lightning Rod]

One possibility is that the batteries sat a while in the store before you got them and some cells leaked more than others, so the individual batteries aren't in equalization.

I'd give them an equalizing charge and top off the cells before proceeding further.

Ball-type hydrometers have pretty low resolution and are mainly for checking for defective cells in automotive starting batteries.  You can tell if things are significantly out of equalization with one.  But since you're going in for serious RE experimentation, try to get one of the kind with the glass float, and preferably a large one rather than something like an oversize eyedropper.  That will give you finer readings than counting floating balls.  (Best are the ones with a thermometer inside and a compensation chart.  B-)  )

When buying batteries at a store that doesn't keep them on a float charger, take a digital voltmeter with you and check the batteries for voltage.  Once you've identified the ones with the highest voltage (which will eliminate the ones that sat around the store for 6 months and any high-leakage defectives), try to pick a set with the same date code stamped in the case.  That will give you batteries from the same manufacturing lot, which will give you the best match for parallel connection.

[Ungrounded Lightning Rod]

I'd give them an equalizing charge and top off the cells with distilled water before proceeding further.

[SamoaPower]

Okay, now we have a horse of a different color. It really does help if you supply all of the information on the first go to get good answers.

It's pretty clear that your batteries simply are not charged in spite of what the voltmeter tells you. As pointed out, terminal voltage is not a very good way to determine state-of-charge (SOC). That, along with a questionable voltmeter, can lead you down the garden path. Get the hydrometer.

The culprit here are your charge sources - they're just too light for that size battery. I would guess that you don't see any bubbling of the electrolyte during what you think is a full charge. For that size bank, you need a charge source capable of at least 10 amps to get them up in a reasonable length of time. Even better would be 15-20 amps.

Your 600 watt inverter probably isn't very efficient with a light 100 watt load. Say it's 70%, which with a 100 watt load means your input power will be about 140 watts at 12.6 volts, or 11.3 amps. Have you actually measured the current?

Like you wanted, those batteries should give you about 4 hours at a 20% draw-down. That's about 46 Ah. Charge efficiency will be about 85%, so you will need to replace about 54 Ah to bring them back up again. With a two amp charger, that would take 27 hours. At that low rate, it would probably take even longer (it's not really linear).

I wouldn't leave them in this low SOC very long. Chronic undercharge is the battery killer.

[SamoaPower]

Another point, is it getting up to over 14 volts under charge? Probably won't at that low charge current.

[CmeBREW]

Thanks SamoaPower:

Actually, i just measursed my batt bank again with my brother's digital voltmeter and it shows almost the same as my digital voltmeter; 12.74 volts. It still doesn't make sense. According to DanB's State of Charge chart, that would be a FULL set of batteries. They have been 'resting' all night and day. (no charge/no drain) I also check car batteries charge after resting all night and it always shows 12.7 volts. Thats why i bought this accurate digital voltmeter, to show me state of charge.

My inverter says that it is 90% efficiency. Are you saying that the lesser power i have going thru it, the lesser the efficiency? You mentioned 70%. Furthermore,These batteries are only 3 months old from manufacting and they are the same make. They have been sitting on display there at Wak mart for only 2 months. I only have a cheap 10amp ammeter right now so I can't measure the Tv. But the rating on back says 110Watts. So your figures have to be close. I am current working on my Dual rotor Axial wind alternator right now. It should do  50-500 watts depending on wind. But it won't be done for a few weeks though. I got impatient and bought the batteries just to begin experimenting. I'll post it when done. But i still don't understand why I can't run my 100w TV more than 15 minutes.(at 20% discharge)  I don't know much about 'equalizing' batts yet. I don't have the power suggested anyway to do it. I need to learn alot more before attempting 'equalizing'. There sure is alot more to batteries than i thought. I will still get the hydrometer for a second opinion though.      -I'll figure it out sooner than later. Thanks for the info.--i know these batt AH questions can be a real pain in the ACE!

[SamoaPower]

There's three possible explanations. The battery is not fully charged.  The load is much greater than you think it is. The battery has a lot less than rated capacity.

I still think number one is the most likely. If the battery sat, with electrolyte, on the shelf with no charge for two months, it definately needs an equalizing charge but you can't do it with your current charge sources. Two months is a LONG time with no charge. Take them to a service station and have them give them a charge.

Measure the DC current between the battery and the inverter. Since your meter is only 10 amps, plug in a smaller load. A 60 watt light bulb perhaps.

If number three, exchange your batteries because they're badly sulfated.

Are you sure you are measuring the voltage to determine draw-down with the load removed? You don't have to wait overnight for that.

Your inverter may very well be 90% efficient at optimum load which is probably around 400-500 watts. Above or below that point the efficiency typically decreases, being the worse at very small loads.

I strongly suggest you don't wait weeks to get a full charge. It sounds like damage may have already been done.

You may want to look at:

http://www.fieldlines.com/story/2006/1/23/8428/49319

[Flux]

Yes, now that you have explained what you are doing it helps.

I agree with Samoapower that the batteries need equalising and you can't do that with a 5 watt panel. To be perfectly honest, when they are equalised, I dont think the 5 W panel will charge them anyway.

You might have got away with it with AGM but the self discharge of flooded batteries will be more than you panel can handle long term.

If that panel can't drag them up to 14v it will not fully charge them and they will progressively discharge and sulphate.

Flux

[CmeBREW]

Heres alittle more info:

I topped off the batteries with distilled water, they were alittle over a half a quart low each battery. Is that bad news?I then "jump-start" charged them with my truck. However, I can't seem to get more than 5 amp (60W) going into them for whatever reason. (I even unhooked the cable from the truck battery, still only 5 amps. What a wimy truck alternator. Anyway, when i begin charging, voltage was 14.1 volts and after about 45 minutes the voltage actually went down some to 13.8 volts. Batts did not even get warm. Alittle water came out of one of the batts. I can't afford the gas so i stopped charging and went to test them on a small load. I know 5 watts solar is nowhere near enough to charge batts with. Right now I can't afford a bigger solor panel. I plan on getting a 45watt to start with. Right now i'm triing to get my windmill working which i think should be adaquate if i monitor batts daily. By the way, where i live, its been sunny 90% of the days and the wind is pretty regular too. I live in a vally w/no trees. I can see I need to immediately buy a 6amp 120vac (yes i have grid) in order to keep these charged and to do experiments with until my windmill is up. (I can't afford the Gas for the truck!) Anyway, here are some pictures.

I plan on selling the extra power from this puppy!




Here I am lighting a 14Watt efficiency light bulb. Together with the 4 watt (usage) inverter, it is 20 watts (1.54A)total. The voltage under this load is about 12.84v and a minute after removing all loads is 12.9 volts. Can you make anything out of this?

[AbyssUnderground]

I get the same thing with my 17.2Ah SLA which I use with my 6w PV. Its the amount of current that drops the voltage. When I remove my load which takes it down to 11v under load, it shoots back up and levels at 12.5v, which according to the chart is 75% charged still.

I know you can be a little bit harsher with SLA's compared to your batteries but Ive not had any issues with this yes. Im having to mains charge my battery today though as weve had no sun for 3 days and nothing has gone into the battery with my panels.

[Vtbsr]

Hello cmebrew, Can you give the size of the wire from the battieries to the inverter? It looks very light weight.  Say if you wanted to run a power saw it will have to carry 100 amps for a short time.

[CmeBREW]

You are right. The wire is only 12/2 Ga. romex. Its only temporary for these little initial experiments. The wire is only 2 feet long. I will use a much bigger wire when permenent. Also, I never plan on using anything over 20 amps anyway on this little system.

[CmeBREW]

Thanks guys for the info.- - I will try to equalize the batts when I purchase my 6 amp charger in a few days. If they are too far sulfated, i will simply wait till they break down completely and will replace them on the warrantee. Meanwhile i'll keep on learning more cause theres alot to all this battery sh*&@#%!.  -Thanks again!!

[asheets]

This looks almost exactly like my setup -- the problem is that I can't get more than 4 hours worth of light.

[CmeBREW]

It's a week later and probably nobody will see this but i got to the bottom of my battery problem. I recently read the new link on this website from the FAQ on deep-cycle batteries and understand more. My batteries were brand new and simply needed to be 'preconditioned' several times before getting a true reading on the voltmeter. Now I can easily run my 100 watt TV for 3 hours straight on a charge. What a relief. Thanks for all the advice it helps alot to know the 'active' voltage readings without waiting for the batteries to rest overnight. That new battery information link help tremendously!

[ghurd]

That is totally wrong!

We see everything.  

[CmeBREW]

What part is totally wrong? I Am a beginner, so i would appreciate knowing.

[ghurd]

The part where you said

"It's a week later and probably nobody will see this but..."

We WILL see it.  This is a very active group.

That 5W PV is still far too small.

Not to endorse these, but the info could give a better understanding.

http://www.bigfrogmountain.com/calculators.cfm

http://www.solar-power-answers.co.uk/solar_panel.html

I didn't intend to knock you.

The learning curve is steep.  Everone is a beginner in one way or another.

G-

[Brian H]

I've searched and searched but can't find any info on "preconditioning" a new battery. Could you kindly point me in the right direction?

Thanks,

BrianH

[TomW]

I searched too but no luck, please provide a link to that page.

Cheers.

TomW

[CmeBREW]

Sorry ,I never check these old postings I did---I just now seen your question. I don't know how to do a link response yet. I will try to learn this. I can try to get you there though: It is the 'Car and deep cycle batt FAQ'. Goto the IDEX. It is under sub-heading "How do I test a Batt?" (4.5.2) I'm still just a beginner with my first batteries and got misleading readings since they were new-- But it says:

" A battery with 80% or more of it's manufacturer's original rated capacity is considered to be good for most applications. Some new batteries can take up to 30 charge/discharge "preconditioning" cycles before they reach their rated capacity. If the battery passed the Capacity Load Test, then skip the next test, Section 4.6 Bounce Back Test and go to Section 4.7. Recharge below."

Sorry, I will keep learning how tto uuse these funtions better.

[CmeBREW]

Sorry ,I never check these old postings I did---I just now seen your question. I don't know how to do a link response yet. I will try to learn this. I can try to get you there though: It is the 'Car and deep cycle batt FAQ'. Goto the IDEX. It is under sub-heading "How do I test a Batt?" (4.5.2) I'm still just a beginner with my first batteries and got misleading readings since they were new-- But it says:

" A battery with 80% or more of it's manufacturer's original rated capacity is considered to be good for most applications. Some new batteries can take up to 30 charge/discharge "preconditioning" cycles before they reach their rated capacity. If the battery passed the Capacity Load Test, then skip the next test, Section 4.6 Bounce Back Test and go to Section 4.7. Recharge below."

Sorry, I will keep learning how tto uuse these funtions better.

[Brian H]

http://www.uuhome.de/william.darden/carfaq4.htm#load

I never realized that FAQ was so comprehensive! There's a couple days worth of reading there!

Cheers!

Brian