Understanding AH ratings

[mwallred]

I read your article on the solar powered fishing trailer and really enjoyed it. However, in it you talked about the AH ratings on the batteries as if that was the total capacity of the battery and to factor that into your calculations for how much battery you really need. I was under the understanding that the listed AH rating takes into consideration the fact that you don't want to drain the battery too far.

At least I'm pretty sure that's what it said on the Trojan web site. I think it also gave different AH ratings depending on what the current draw was off the battery. How much does the total AH rating change as the draw changes?

Can you give a quick overview of just what the AH rating means? I think it would be a good thing to add to your FAQ as well.

Thanks!!

[Madscientist267]

Amp hour ratings are kind of like the mile per gallon ratings of cars. Everybody uses a slightly different means to come up with the calculation.

Aside from having actual datasheets, a good rule of thumb for me seems to be comparison of the costs. Two batteries rated at 50AH, and all other things claimed equal, yet one is significantly cheaper, beware the 'free' capacity.

This becomes a real pain in the @$$ when youre dealing with a used battery too; trying to determine what it's actual capacity should have been isn't always discovered accurately by reading the label.

A good rule is to never design with an equipment requirement:battery capacity ratio of more than 50% of what the label on the battery indicates you can expect. This way, regardless, you stay inside the safe range of discharge level.

Not exactly a direct answer, but hopefully helped?

Steve

 

[dnix71]

The rated AH of a 12v deep cycle battery is usually what you can pull for 20 hours before the battery voltage drops to 10.5

If you actually do that to a lead acid battery more than once or twice, it will ruin the battery.

Half of the rated capacity is reasonable real world use assuming you recharge as soon as possible (next day at the latest). If the voltage stays below 12.3 for long you will get sulfation.

[bob g]

a bit more hopefully not to confuse

from many sources

a flooded lead acid battery will return the most for your investment in watt/hrs

if not allowed to cycle below 50% (although a few deeper discharges along its life is not going to drastically reduce its life), and then the question becomes what is the most efficient charging regime for this type of battery and service.

again from many sources

charge daily or every other day from 50-80% of capacity, State of Charge

the lead acid battery can take alot of current between 50 and about 80% before it

starts to gas and the rate must be reduced.

so,,

charge daily from 50-80% state for charge, but what about sulfation?

sulfation is a normal function of the discharge cycle, you cannot escape sulfation

but you can do something so as not to allow it to crystallize and harden. That being

topping off the battery from 80% to full charge once every week to 10 days.

so where does all this lead you?

if you figure on charging once per day and your needs are 300amp/hrs daily

you will need a battery bank of approx 1000amp hours, that way you can work the 300amp/hrs from the 50-80% state of charge daily without getting into much deeper discharge.

if you are only going to recharge once every other day, your needs double to 600amp/hrs and so does the bank overall size, it goes up to 2000amp/hrs.

the next issue is charging capacity, to do it economically with an engine driven genset, you will want a capacity of approx 25% of battery capacity in amp/hrs

therefore a 1000amp hour battery bank will need approx 250amps of capacity, a larger 2000amp/hr bank will require 500amps of capacity to do it efficiently and

keep run time to a minimum,, however these are very large charging capacities

which most folks don't have... so you end up running proportionately longer to get the same amount of amp/hrs back in.

basically don't put in a 2000amp hour battery bank that you are taking more than 30% out of on a daily basis if you have only a 50 or 100amp generator,, it will be running all night just to get you ready for the next day. (actually it is likely it will be running virtually 24/7 trying to keep up.

clearly the lesson here is conservation, if you don't burn it, you don't have to provide storage capacity for it, or in turn provide charging capacity to keep the battery bank in shape.

hope that makes sense?

there are a couple of very good books commonly available on ebay

"wiring for 12 volts" and

"living on 12 volts"

even if you plan for higher voltages such as 24/48 the info is very good.

bob g

[scoraigwind]

bob said

"basically don't put in a 2000amp hour battery bank that you are taking more than 30% out of on a daily basis if you have only a 50 or 100amp generator,, it will be running all night just to get you ready for the next day."

This does not make sense to me.  If you plan to use a genset for your power then using one that roughly matches your needs and running it for long hours will be more efficient than running a huge one for an hour or two each day.  and spending a fortune on batteries.  Only use batteries at times when the power draw is too low to make engine use worthwhile.  If you use 300 amphours per day and you run your generator during the four hours of maximum power usage then it will probably only have to (also) charge the battery at 50 - 70 amps to keep the battery charged for the remaining 20 hours of the day.

Just because you have a large battery capacity, that does not mean that you need a huge generator.  Hopefully you will be keeping the battery charged with wind and solar energy anyway but there are always times when these sources fall a bit short of the mark, and then a generator is a good way to save the battery.  Again, a small genset that meets your demands and puts a bit into the battery at the same time will keep from over-discharging it until such time as the wind/sun come back to help you get it fully charged.

my 2 pence

[bob g]

respectfully i disagree

the problem with small gensets is they get horrible fuel economy

in gr/kwatt/hr

larger ones get better economy "if" they are run at or near full capacity, but

not many folks have need for this amount of power on a continous basis, or are willing to schedule around or startup a genset to provide for a load.

batteries therefore become a reality in most cases, and will allow a much more efficient larger genset to run fewer hours to recharge with, at which times those heavy loads that can be scheduled can also be provided for... such as washing clothes, power tools, and a few other heavy loads that can also be scheduled or controlled with a bit of thought, such as refrigeration and domestic water heating.

certainly it would be most desirable from an efficiency standpoint to use the power as generated without another conversion link in the chain.

bottom line my point should have been, don't buy a huge battery bank if you don't have adequate charging means to maintain it properly. it certainly does not make sense to put in a 1000amp/hr battery bank if all you have to maintain it is either

a harbor freight solar panel set (~50-60 watts) or an 8 foot windgenerator that is sited in a catagory 1 wind area.  if one finds themselves with this setup, he probably ought to start seriously thinking about some serious generator capacity to maintain those batteries while he is working on a taller tower or many more solar panels.

i have spent the last several months, (nearly a year now) running endless tests on a diesel powered trigenerator, S195 changfa (rated 12hp at 2000rpm) driving directly

an ST7.5kva generator and, a pair of prestolite 110-555jho alternators and, a sanden rotary refrigeration compressor. the results are generally as follows

1. the engine gets better and better fuel economy based on gr/kwatt/hr consumption

the more load that is applied, up to approx 40% over rated output where the head gskts fail. (i mention this to show the engine is not smoke limited, but gskt limited)

2. the alternators 110-555jho get vastly better efficiency running at higher output
   
3. the same goes for the st7.5
   
   

the bottom line is an a gr/kwatt/hr generated this setup has proven to be a contender with any high end genset in its class and anywhere from 20-50% more efficient than smaller class gensets (most especially if they are running at low loading)

a better recommendation might have been to

1. determine your amp/hr needs per day
   
2. size the battery bank to operate within the 50/80 regime (flooded lead acid cells)
   
3. provide a genset that can deliver upwards of 25% of the battery banks amp/hr rating
   
4. then add in wind and/or solar, or hydro or homebuilt nuke reactor (ok the last one is a joke just to see if anyone is reading this), so as to reduce the amount of run time further on the genset. the goal of course would be to get to the point you need rarely start the genset, but
   
   

you got the added capacity of the genset should the need arrise, such as after a natural calamity where neighbors are bring over extention cords.

i just read about too many folks spending a buttload of cash on a large set of batteries without any forethought into how to manage their charging. big batteries come with big responsibilities. the time to find out you are deficient in the charging arena is not after you buy/install/and the rundown the set, then be in a panic trying to figure out how to get them recharged before the sulfation starts to harden and crystallize rendering the set injured or worse damaged or murdered by means of a slow death.

i also read about too many folks having read some article in some magazine or webpage thinking that they can take a scrap lawnmower engine and cob together junkyard 10 dollar alternator, using cheap harware store drive componenents such as aluminum or potmetal pulleys and a fractional hp belt, and have something that will maintain their expensive batteries as they need. the reality is generally not very good to say the least, what happens more times than not is the battery owner now thinks he has the problem solved, but in building and tinkering with the substandard genset he has allowed even more time for the batteries to harden. So

by the time he finally finds out the genset he has just put together does not meet the need more unnecessary injury or damage has taken place.

for most folks that need batteries, their batteries represent the largest outlay of cash invested. most can cut costs building their own windgens, gensets, maybe hydro, salvage solarpanels, or buy them used. most folks understand how they can work around most other parts of their systems, but these same folks are stuck with buying batteries. my belief is the focus should be centered on the batteries first after all reasonable conservation has been considered.

the central focus being on the batteries would naturally extend to how and by what means am i going to efficiently charge these babies?

only after i have figured that one out, would i feel comfortable starting to tap their power in any meaningful way.

don't know if this makes sense or not?

bob g

[wpowokal]

Bob, I would generally agree with your comments on handyman gen sets but many people on this board are in that category, and it goes to the learning curve.

It is in the long term cheaper to run with the minimum size battery bank possible, as defined by a correctly designed system. Therefore running a back up generator as required, a battery bank that can supply your needs for 7 days of little input is of no use if you can not recharge it from renewable sources in short order and in a controlled manner.

Also my understanding is that most lead acid batteries respond best to a recharge rate of 10% of amp hour rating, correct me if I am wrong but that is my experience.

It is a complicated path to find and install a reliable Re system, not that the theory is hard but the client is!

I don't disagree with your general comments just need to make my point.

allan down under

[bob g]

Allan:

your points are well taken

having had some time to sleep on this topic, i think i am on solid ground here.

as you stated

"Also my understanding is that most lead acid batteries respond best to a recharge rate of 10% of amp hour rating, correct me if I am wrong but that is my experience."

in general i would expect this statement to be true, but

a battery that is 50% to approx 80% SOC, can accept upwards of 25% of its amp/hr rating without issues, before you must taper back (when the battery starts gassing)

we all know the part about tapering back when things start to boil.

what is not apparent at first glance is overall life costs of the batteries, generally speaking it does little good to charge at 10% even if it extends the life

50% (which is an exageration and likely would never happen in any event) but costs you twice what the batteries first cost were in fuel/repairs/maintenance of the genset (which is a very conservative estimate given fuel prices propensity to sky rocket)

i haven't done the math lately, but off the top of my head one could cut the engine time in half, along with fuel consumption if he has the 25% capacity charge capability over that of having a 10% charge capacity.

otherwise you are absolutely correct, this isn't rocket science, but

it is a multivariable arithmetic equation one must work through to get an optimized system.

believe me when i say, i argue these topics not so much to teach what i know, but rather learn what i don't know (which turns out to be much more than i thought going into this thing), so i can rethink my position, go back and do more research and study, and wear out a bunch more pencils

i learn a lot from you guys that have tons of experience in the field, where else is there as much practical experience on this topic but here?

thanks

bob g

[ghurd]

Seems all this is past what Mwallred wanted to know.

Seems everyone is looking at it from the opposite extremes.

Since 80% of us are on grid, when the battery is too low, we move the extension cord from the inverter outlet to the grid outlet.  I hope.

I personally do not see a problem if a $10 handyman gen set is used to fill in the occasional RE gap.  And I figure that is how most are used.

The RE controller will still be doing a majority of the 'fancy control'.

And what are the other options?  Candles and kerosene?  Gas powered AC gen for a reading lamp?  Run the battery down to 10.5V and hope the wind or sun is abundant tomorrow?

Maybe the gen is run for a pint or quart of gas 3 times a month for 2.5 months during the poor RE times of the year?  Most people are not going to be overly concerned about the fuel efficiency of something using 1 or 2 gallons a year.  Certainly the handyman-types will not spend 100X more money for double the fuel efficiency in the hope of saving 2 to 4 quarts of gas a year.

It would be a different story if the $10 gen was the only charging system, or a major percentage of the charging power, but I doubt many are.

G-

[bob g]

Ghurd:

you are likely correct in that it all comes down to perspective

if you are ongrid, and have a small battery bank, a small budget homebrew gas power genset likely will suffice in some cases.

now my reality

most of the folks i am dealing with are offgrid, no chance of grid connection

and wouldn't connect if it were given to them free.

a few of which have 2400amp/hr @48 volts, several of which have a couple thousand amp/hrs at 24 and even more that have over 2000 amp/hrs at 12 volts.

none of which could even recharge their battery banks even one time with 2-4 quarts of gasoline. to suggest otherwise would lead to disappointment in my opinion.

if one were to have a large battery installation and even has a suitable sized windgen in a good wind area, or sufficient solar panel density, or hydro of sufficient size.. its not so much what happens when the time comes when as is said the wind don't blow, sun don't shine, the streem run's dry,, but

what happens if there is some other major failure? an unusually high wind, hail, lightening, or other calamity that takes out your AE generating capacity for a protracted length of time?

here is the thing, i am not suggesting what size battery bank one should put in, or that one need spend a small fortune on charging capacity, what i am suggesting is

the most efficient use of whatever size battery one decides he must have and what the most efficient means of charging is likely to be.

like i said, i am not hear to be arguementative

just relating my reality as i see it, and trying to support my assertions.

btw, if one were to have a large installation of AGM batteries, even higher capacities would be beneficial in reduction of runtime

bob g

[Madscientist267]

I was going to stay out of the latter part of this since it all seems a tangent.

BUT -

I agree that a larger generator is more efficient when heavily loaded, and that batteries can accept heavier charge rates in the lower SOC window. The way I see it, everyone has made very valid points, but they need to be combined into something that makes more sense as a whole... so here goes:

The way I see it, use the genset to augment RE in recovering from deeper discharges after extended periods without renewable input, and maybe somewhat during, to the extent that one is able to forecast such things.

Start it up at say 60% or less SOC. Forget the idea of trying to just use it as the charger. Make it big, load it hard, and run it short; 'emergency' recovery and peak loading only.

Kill the genset above 70%, and let the RE do the lighter, less critical charging that unpredictability won't have such a negative effect on when it's not there (sulfation).

You're killing two birds with one stone here - Your batteries aren't as 'desperate' to get out of the danger zone at that level, and the transition happens at about the same time the genset/batteries will collectively begin to really start losing efficiency as a system anyway.

Load begins to reduce, and the acceptance rate is decreased - so thats a double hit if you just kept running the generator by itself. Since RE is flaky anyway, and comes and goes in spurts, it makes sense to have it as your primary source when the bank level is higher, because you won't 'feel' the efficiency losses as much as with gasoline or diesel.

And yes, run the genset during peak load times to help keep the batteries from dipping in the first place. I'm sure this is a trade off that requires some additional thought, depending on if the collective load is more or less a steady drain, or transient in nature, and for how long, etc etc...

Oh, and before I forget - I prefer perpetual motion to home built nuclear - it is much safer, and has an infinitely more predictable failure rate... >;P~

[mwallred]

Thanks to everyone for all the great information. Lots to absorb...

Getting a little back on track, though, here's my situation which isn't your typical solar powered home setup.

Basically, I want to be able to run my ham radios off battery power during periods of blackout- emergencies or even just lightning hitting a tree in the neighborhood. I've got two radios that can be run directly off the battery with some optional equipment that would need AC, but nothing that my little 100W inverter can provide and even those items would only be used once in a while. So, for all practical purposes I can run the whole thing off the battery.

Now, I'm obviously on the grid for everything else in the house and even the radios can be switched to grid power by simply switching two power cables. I can run off the grid or battery as needed. What would be the best way to run in order to maintain the life of the battery? Only use it every so often or use it all the time, making sure to keep it from getting below some percentage?

The battery I have is a Trojan 31-AGM. It's listed as a 110AH unit, but when you look at the spec sheet for it that is the peak AH rating. It says it's only good for 83AH if you're running it at a 5 hr rate and 110 if you're using it at a 20 hr rate. They don't even list 100 hr ratings for this one like they do for some of their other models, but their capacity ratings also say I can draw 25A for 3:10, which pretty well matches up with the other numbers. So, if I do my math correctly, I can draw about 5A off it for 20 hrs or about 17A off of it for 5 hours before it gets drawn down to 1.75V per cell. That's the spec they give as for how they determine the AH rating.

See, that's where my initial question came in. If they rate it as 110AH (assuming I'm drawing 5A off it it) they are basing that rating on the assumption that it will put out that much until it gets to a voltage that's 25% below full voltage. Therefore, I should be able to use the full 110AH rating in my calculations, right? It sounds to me like they are already accounting for the fact that it shouldn't be drawn down too far.

For charging it, I have a small solar panel now (with plans for a bigger one), but mostly I will be using a fancy grid/generator powered battery charger that is designed to properly charge and maintain AGM batteries.

My estimate is for an average draw during power out situations would be somewhere in the neighborhood of 2-3A, factoring in a typical draw of 1-2A with occasional draws of 10-15A while transmitting on the big radio. That ought to give me a good day and a half without any charging at all off of just the one battery.

Can I do the math that way because of what the specs are on the battery, or do I need to do some other factoring to make it all work?

[mwallred]

You know, you just can't proofread your own posts.

I meant to say that the few AC powered items were nothing that my little 100W inverter couldn't handle. Sorry about the confusion.

[Airstream]

Absolutely false. They provide depth-of-discharge points (1.75, 1.80, 1.85V per cell) that ARE damaging to battery chemistry as a courtesy to corporate (911 repeater station) or others (fishermen to shore before a storm) where shortened service life of back-up system battery is cost effective if it prevents a process or activity from complete shut down.

The 20 hour rate is to standardize across manufacturers, the extra capacity by lower discharge rates is known as "Peukert's Law"

"...Peukert's Law is well (if inversely) demonstrated by pouring beer into glasses! If you pour it in quickly, only a small amount of liquid is transferred: the rest is foam. The slower you pour the beer, the more beer and the less foam enters the glass. You may feel you need to test this a few times." http://www.caravanandmotorhomebooks.com/articles/peukerts_law.htm

I flinch at your willingness to run the battery down to 1.75V/cell.

"The depth of discharge (D.O.D.) has a major effect on the life expectancy of a battery - discharging only 80% of the total capacity of the battery will typically get you 25% more cycles than total discharges, and discharging to only 20% will make the battery last essentially forever." (http://www.gizmology.net/batteries.htm)

Required reading:

http://www.ccis.com/home/mnemeth/12volt/12volt.htm

http://www.ccis.com/home/mnemeth/12volt/12volta.htm

http://www.phrannie.org/battery.html

[RandomJoe]

I run my ham bench (and a few other things) solely off a solar / battery array.  There is a major item you need to keep in mind, aside from what the battery can handle.

Your station is likely to shut down LONG before you get to 1.75V per cell.  That's 10.5V battery voltage, and most all 12V ham gear I've seen is specced at 13.8V +/- 15%, so the low end is around 11.73V.  Even if the radios do keep running, they may (at best) act squirrely, not put out full power, or just not put out anything at all.  At worst, they may start generating garbage all over the band(s) as various circuits start operating outside their design parameters, or resetting the microprocessor and losing your settings, or even potentially damage themselves.  (I've seen forum posts on a few highly microprocessor-controlled rigs that can wipe service settings in hidden menus that effectively leave the unit useless when the power drops too low.)

Also keep in mind, the voltage will sag some when you start pulling current.  The larger your current draw is a percentage of the battery's total AH rating, the larger that sag will be.  Different chemistries and designs will also affect that.

So I'd suggest you at least size up the battery so you don't fall below 11.7V or so (or be optimistic and go with 11.5V) for the desired "outage duration" you want to target.

Each person is different, of course, with differing motivations, but in my case I figured just getting a "backup battery" to standby on float all the time would be rather boring...  I rarely have power outage, even short ones.  And I wanted to be able to continue operating at full capacity even during an outage, so I just installed a solar system capable of doing that and am now off-grid full-time for the ham bench.  I sized my battery bank primarily to handle the refrigerator as well during an outage, but that just means I draw it down even less during normal times.  Or I can opt to run something else off of it "just because"!

[mwallred]

"The depth of discharge (D.O.D.) has a major effect on the life expectancy of a battery - discharging only 80% of the total capacity of the battery will typically get you 25% more cycles than total discharges, and discharging to only 20% will make the battery last essentially forever."

Since I've got people who actually know about batteries, I'll ask...

What is the "total capacity" of a battery? For this definition, would it be the AH rating? To my ignorant mind that doesn't make sense since by it's own admittance the AH rating factors in stopping before the battery is drained. Right?

[bob g]

the amp/hr rating given by the manufacture almost always is based on full charge

and then discharging it to something like 1.75volts per cell

which is basically fully depleting the poor thing.

this is actually not dishonest, because there is a huge industry that needs batteries to do just that a half dozen times or so over the lifespan of the battery. things like telecom's and even the little ups under your puter desk use this theory of operation.

about the only way you are going to know for sure, is hook up the battery and start using it, time how long it can carry your load till it hits something like 11.5 volts,  and then make appropriate sizing changes.

this would likely give you good service over a fairly long life

me i would quit at 12.2 or so, but i am talking hundreds of amp/hrs in capacity.

bob g

[mwallred]

I've read a bunch on running ham stations off-grid and nobody ever mentioned that you really can't go below 12V. At least not that I recall. It makes a whole lot of sense, though. Thanks for bringing it up.

Just thinking outside the battery box for a second here... would it maybe make sense that rather than boosting my 13.8V battery bank (currently a bank of one) up to where I don't ever go below 12V, instead I add an appropriate 6V battery in series to create an 18V (or 20.7V) source and put in a heavy enough 12V regulator to run everything? Wouldn't that give me a lot more run time before it dropped below 12V? Or would I lose too much in the regulator to make it worth it?

Either way, I'm even more convinced that I need to increase my solar output.

[ghurd]

Charging, controlling, and using an 18V bank will be a bit of a pain.

Have to build a charge controller, which would have to be a labor of love to build a decent unit comparable to a $50 12V unit.

Have to very inefficiently use expensive 24V solar panels, which will have an open voltage past 40V (see build a controller), unless you could find 6V panels with enough output to be useful.

Have to have a switching regulator to get much efficiency.

Brings up some series battery issues I would not care to have in a small system.

PowerSteam makes an off the shelf solution.

Input shown as 9 to 14V.  Output of 13.8V.  ~90% efficient.  Starting at $120.

There is a note about ham radio use at the bottom.

http://www.powerstream.com/dc2.htm

It may be a non issue.  Like Bob said, I would not run the batteries below about 12.2V if there was a way around it.  

The fancy charger on a generator, a $10 handyman generator, long jumper cables to an idling car...

G-

[Wheeler]

I am now using 16 T-125 batteries with a 240 ah each, they are is groups of 4 and 4 banks for a 24 v. system. I have 32 KC-120 panels with a rated power of 3,840 watts using an Outback MX-60 and a trace C-40 controller, how do I determine the proper panel to battery sizing?

Also, my neighbor has 8 L-16 batteries in a 12 volt system, how many watts of panels would he need to be a balanced system?  The houses are in the desert of Arizona.

Thanks for you help!

Wheeler

[ghurd]

A lot will depend on the loads, and when they are used.

For solar, anything more than capacity in AH divided by 10 for the charging amps is not very efficient.

Too many charging amps gets the battery bank to regulation voltage quite fast (well before noon?), then a small percentage of the available charging amps is actually being used, because the controller will limit the charging amps when the battery reaches regulation voltage.

hmmm... read that 3 times fast.  :/

I have pretty good luck with capacity in AH divided by 20 or 25 for the charging amps, if there is no load during charging.  A majority of what I do is for charging a battery in one place for use in another.

C/20 sounds like nowhere near enough to most people at first, but it is the same as a 85W panel into a 100AH battery (100AH/20=5A,=85W PV), and that will get to regulation voltage pretty darn fast.

G-