Battery Amp Hour Question

[ClydesdaleKevin]

So I've been fascinated reading Eureka, the website set up by Andy Baird, and after all your guy's help and his stuff, I'm finally getting a handle again on electrical stuff and systems, and of course amps, volts, and watts.

Something he wrote got me thinking though, and I can't seem to find a good solid answer to this one.

This is what got me thinking:  Amps at 120 volts are not the same as amps at 12 volts, so if you are running a 120 volt appliance that draws 3.3 amps at 120 volts through your inverter, you have to multiply that by 10, so in reality the appliance is drawing 33.3 volts from your battery bank!  Okay, I understand that.  If an appliance draws 400 watts at 120 volts, and Amps X Watts = Volts, then 400 watts divided by 120 volts = 3.3 amps.  But if you are using your inverter its actually 400 watts divided by 12 volts, which is 33.3 amps!  Easy enough math since its simply a multiplication of 10, but it got me thinking about how to determine the amp hours in my battery bank.

I have six Interstate GC2-XHD-UTL 6 volt golf cart batteries, essentially 3 banks of batteries, 2 each, wired in series to each other, then all 3 banks wired in parellel to produce 12 volts.

Each battery is rated at 232 amp hours each.  Does that mean I multiply that by 6 and really have a grand total of 1392 amp hours in my bank?  Or is that at 6 volts, and would I calculate it as 3 12 volt batteries producing half that many amp hours, at 696 amp hours of storage?

Or is the math even more complicated than that and I have even more or less than the above figures of battery amp hours available?

I know you shouldn't draw your batteries down past half, ever, so if the figure is 1392 amp hours max, then I'd really only have 696 amp hours of use before drawing the batteries down to 50%, and if the figure is 696 total amp hours, then I'd only have 348 amp hours of use before drawing the batteries down to 50%.  Still respectable figures, all of them, but I'm not sure if I understand the math when it comes to amp hours, and I'm not sure if the 232 amp hours each battery holds adds up to a grand total, or if they share the amp hours, or how that all works for multiple batteries and the way they are wired together.

Thanks guys!

Kev

[Oz]

By my calculations, you should have 5 pies in 4 days if you smoke a rain jacket before the Queen arrives in Milan.

[ClydesdaleKevin]

LOL Mark!  Some of the math associated with this is pretty darn easy, but when it comes to calculating the total amp hours I'm stumped.

Kev

[ClydesdaleKevin]

From some more research, I think I'm starting to understand this, and please, anyone correct me if I'm wrong!

When connecting batteries in series, the voltage is additive.  In other words, 2 six volt batteries wired in series equals 12 volts.  However, the amps stay the same.

When connecting batteries in parellel, the amps are additive, and voltage stays the same. 

So in the case of my battery bank, 6 batteries divided into 3 banks, each bank, since they are wired in series to produce 12 volts, still produces the same number of amp hours as a single battery, in my case 232 amp hours. 

But because the three banks are then wired in parellel to produce 12 volts, NOW that number becomes additive, and my total amp hours for the whole battery system is 3 X 232 amp hours, which equals 696 amp hours of storage.

Now I wanted to know exactly what this means, so first lets define what an amp hour is.

An amp hour is a way of describing a battery's capacity, how long it takes to fully discharge a battery.  The amp hour rating for a given battery is the maximum amperage that can be drawn continuously until that battery is fully discharged.

Battery manufacturers usually test their batteries to give them an amp hour rating, and the most common test is to put an amperage load on the battery for 20 hours, varying the amperage until they figure out how many sustained amps it takes to completely drain a battery over the course of 20 hours, and that figure is what the battery manufacturer stamps on your battery as its total amp hour rating. 

So to keep the math simple, lets say you have a 100 amp hour battery.  100 amp hours divided by 20 hours equals 5 amps, so your battery can sustain a 5 amp load for 20 hours before being completely drained, dead, kaput.

With our battery bank, theoretically we could run a 34.8 amp load for 20 hours before completely discharging the battery bank.  (696 amp hours divided by 20 hours = 34.8 amps)

But you NEVER want to completely drain your batteries.  So now some more simple math.

The most you ever want to draw your batteries down to is 40%, so that gives you 60% left over to play with.  So 20 hours X 60% = 12 hours, and THAT is as long as you want to go at a continuous 5 amp draw on a 100 amp hour battery, or in our case we could run a 34.8 amp load for 12 hours before drawing our batteries down to 40% and have to recharge them.

Now if you step up the amp draw, you reduce the amount of time you can draw from the batteries before have to recharge them, and this time the math is NOT simple, based on something called Peukert's Equation, which I won't get into except to throw out an approximate number, which is 90%.

Say you want to draw 10 amps continuously...you would get 6 hours from the battery bank, since it seems logical, right?  Nope, it doesn't quite work that way.  Based on Peukert's Equation, when you raise the amp draw, you will lower the capacity of the battery bank by about 10%, or 90% remaining.  So, 100 amp hours X 90% = 90 amp hours.  Or in our case 626.4 amp hours.  So take your 90 amp hours and divide it by 10 amps = 9 hours of use before you completely drain your batteries, but you don't want to completely drain your batteries, so 9 hours X 60% (what you have in your batteries that is usable before they drop below 40% state of charge) = 5.4 hours, significantly less than what you thought you had at 6 hours.  In our case, 626.4 amp hours divided by 10 amps = 62.64 hours that we draw a 10 amp load continuously before completely draining the batteries to nada, which is a lot!  But again, you don't want to completely drain the batteries, so you have to calculate it at 62.64 X 60% = 37.584 hours that we could draw a 10 amp load before having to completely recharge the battery banks.

Fortunately Peukert's Equation is pretty constant at any amperage draw over 5 amps, so if you wanted to draw 20 amps continuously, that magic 90% number stays the same.  So lets say you want to draw 20 amps continuously.  100 amp hours X 90% = 90 amp hours.  90 amp hours divided by 20 amps = 4.5 hours.  4.5 hours X 60% = 2.7 hours.  You could run a continuous 20 amp draw on your 100 amp hour battery for 2.7 hours before drawing them down to a 40% state of charge.  So in our case, the same holds true.  696 amp hours X 90% = 626.4 amp hours.  626.4 amp hours divided by 20 amps = 31.32 hours.  31.32 hours x 60% = 18.792 hours before drawing our batteries down to a 40% state of charge.

The math is actually pretty simple if I'm understanding it all correctly.  I think the most difficult part would be to figure out how many total amps your RV systems are drawing and then plugging that figure into the equations. 

And then you have to remember that the bigger your battery bank, the more amps have to be forced back into them to fully recharge the system, and the longer it takes to do so.  So even with a huge battery bank, you still have to be careful with power usage, or else even a huge 705 watt solar array and a 60 amp MPPT charge controller in full sun might not have enough daylight hours to produce the necessary amperage to bring a 696 amp hour battery bank back from a 40% state of charge all the way back up to 100%!

LEDs, only using enough lights as is necessary, turning off the inverter when not in use, even turning off the antenna power when not in use...these are still practices we will have to implement when boondocking if we don't want to have to run our generator at all. 

Kev

[gadgetman]

Perfect explanation   My rule is to try and not take more than 30% from the batteries.

[ClydesdaleKevin]

Yay!  I tried to simplify it and write it out as Amp Hours for Dummies...lol!

Once I sorted through all the complex equations and simplified it all down to layman's terms it all made perfect sense to me.

Kev

[Oz]

Well, I must be less than a dummy then, 'cause all I read is yaddidy, blah-banks, diddy, 696 amps, blah, diddy-battery-blah-blah - LOL!

So, is this topic solved then?

BTW - anyone can go to their original question post and change the Question Mark to a Topic Solved, Green Check Mark.  That tells everyone you got your issue solved!


   

[ClydesdaleKevin]

Topic solved and checked! 

As I come up with any more electrical and battery questions and pare it down into simple words I can understand...which means almost anyone else can understand them...I'll post them as simply as I can.

Kev

[Froggy1936]

It may be solved But there is a mistake .  Mark was incorrect with 5 pies in 4 days   It should be 4 pies in 5 days for a desert compilation of 20 deserts per rain jacket !! Frank

[moonlitcoyote]

Well, I must be less than a dummy then, 'cause all I read is yaddidy, blah-banks, diddy, 696 amps, blah, diddy-battery-blah-blah - LOL!

I'm in the same club as you Mark, no matter how slow I read it, it still looks just like what you said.  i??

[ClydesdaleKevin]

Okay, maybe this will make it easier to understand, starting at the beginning.

Electricity flows in wires like water flows in a hose.  So for the sake of simplicity, consider the two very similar.  Electricity and water both can do Work for us.

So just like a hose, the Amount of Work the water is capable of depends on how much water is flowing, and how much pressure is pushing the water.

Now apply that simple image to electricity.  The same principle holds true.  The amount of electrons is called Amps, and the pressure pushing them is called Volts.  The amount of work generated by either is called Watts.  The amount of work of both combined is also called Watts.  Simple as that.

So now you have the simple equation below, and know what each one means by comparing it to the water hose example:

Amps x Volts = Watts

And that's it.  Amount x Pressure = Work.

So with this simple equation you can figure out how many amps something is drawing in your RV.

If Amps X Volts = Watts, then Watts / Volts = Amps and Watts / Amps = Volts.
Easy peasy lemon sqeezy.

How do you figure out the amp draw of an appliance?

Say you have a 30 watt 12 volt florescent bulb...30 divided by 12 = 2.5 amps.  So that is how many amps that 30 watt bulb is drawing from your batteries, the amount of electrons its drawing, which is what your batteries are storing for you to do work with later, just like your freshwater tank holds water for you to use later.Running 120 volt appliances from your inverter makes it a little more complicated to figure out, but again, its pretty simple if you just think of it like water.Say your coffee maker uses 400 Watts (work) to make your coffee, and its a 120 Volt household coffee maker.  If you were plugged into shore power, and the coffee maker is plugged into an outlet, to figure out the amp draw you simply use the equation, Watts / Volts = Amps, so 400 Watts divided by 120 Volts equals 3.3 Amps, the amount of electrons your are drawing from shore power, just like drawing water from the tank...but in this case your holding tank isn't your batteries, but your shore power coming in.But if you run it off your inverter, the Volts changes, the Pressure used to do the work.  Now you are using lower pressure (Volts), so you have to use more water (Amps) to perform the same task (Watts).Its the exact same equation though.  Watts / Volts = Amps.  400 Watts divided by 12 Volts equals 33.3 Amps.  Which is now coming out of your batteries!  And that's a lot! So if you have a calculator, its easy to figure out how many Amps, or stored electrons, you are drawing from your batteries with any given appliance or light, be it a 12 Volt item or a 120 Volt item.The lower the Amps that a light or appliance draws when being used, the less Amps you are taking out of your batteries.  And just like your water tank, you have to refill the batteries with electrons when they start to get too low.  And remember, a 120 Volt appliance running off your inverter is drawing 10 times the amount of Amps out of your batteries than if it was running off of shore power.  120 divided by 12 equals 10, so if you figure out the Amps of your appliance for 120 Volts, you can either recalculate using 12 volts, or even simpler, just multiply the Amps by 10.That is just one of the reasons that getting 12 Volt appliances tends to mean that they are drawing less Amps at a given moment than if the appliance was a 120 Volts being run off of an inverter.  In the case of the coffee pot, we make our coffee with a vintage Corningware stovetop perk pot...which draws NO Amps, doesn't use much propane, and happens to make the best darn coffee on the planet.

So lets talk about recharging your batteries.  When you recharge your batteries, think of it to being similar to filling your water tank, but with some major differences.The first difference of course is that instead of filling it with water (although you need to keep them topped off with distilled water...lol!), you are filling them with Electrons, or Amps.  The more you took out, the more you have to put back in.The second difference is that, unlike your water tank which has a vent, think of a battery as an unvented water tank, or even an air tank.  You start to refill it, but as you get closer and closer to maximum capacity, it gets harder and harder to fill, so now you have to use more Pressure, or Volts, to get her filled to the top.This is why 3 stage chargers, be it from your converter or your solar controller, have 3 stages.  A Bulk Voltage stage, which can be anywhere from 10.5 to 15 volts, when your batteries are low, and they stay in this stage until the batteries are about 80-90% full.  Then it goes to an Absorption Voltage stage, the highest voltage stage of up to 15.5 Volts, sometimes more, to finish filling the batteries...the Pressure, or Volts, has to increase to completely refill the electrons, or Amps, your batteries can hold.  Once full, they drop to the Float Voltage stage, around 13.2 Volts, to keep your batteries topped off and fresh.The final difference between your batteries and your freshwater tank is that, aside from being inconvenient, you won't hurt your freshwater tank if you drain it dry.  You NEVER want to drain your batteries down to less than 40% of their capacity, or you will way shorten their life.  For best life and performance of your batteries, shoot for only 30-40% of draining them before recharging them back up, either from solar panels, or running your generator and letting your converter do the work.And this is why, if you are going to be a fulltimer and boondock a lot, you need to plan your system well, and make it as powerful as you can afford.The more battery Amp Hours you have, the more Amps your batteries are holding for you, just like the bigger your freshwater tank, the more water you have available for your use.But the more battery Amp Hours you have, the bigger and better your charging system needs to be in order to recharge those batteries in the shortest time possible without overcharging and damaging the batteries.

So in the case of a converter, the higher its Amp rating, the maximum Amps it can put out in ideal conditions, the faster it can safely recharge your batteries, and the shorter a time you have to run your generator and use up expensive gasoline.  We went with a Powermax Boondocker 100 Amp unit, and we can recharge our bank from 60% back up to 100% in about an hour and a half.Now lets talk about the solar controller and solar panels.  The more Watts your panels gather from the sun, the more Work they can do.  The Charge Controller takes the Volts coming in, say 32 Volts, and converts it down to usable charging Voltage in 3 stages, very similar to your converter.  But if you only have 100 Watts being generated by your panels at 32 Volts, then your available Amps is only going to be Watts / Volts = Amps, or 3.125 Amps available for recharging, which would take too long with a big battery bank but would certainly be helpful with a small battery setup. 

Now lets say you have 400 Watts of panels at the same voltage.  Watts / Volts = Amps, so now you have 12.5 Amps of available electrons for your Charge Controller to dump into your batteries.  Now up it to a system like ours, 705 Watts of panels and a Voltage of 32 Volts.  Watts / Volts = Amps, so in ideal conditions we would have 22.03 Amps being generated by the panels for the Charge Controller to play with.  But that isn't all your Charge Controller does!  It plays with the current coming in and ups it!  It lowers the Volts to usable Voltage, and when that happens, remember, Amps X Volts = Watts!  So if its charging at 15 Volts, and your panels are producing 705 Watts, the Amps go up!  Watts / Volts = Amps!  705/15=47 Amps! 

The Controller can also Reduce the Watts!  Which ups the Amps to the maximum rate your Controller is rated for.  In our case we went with a Morningstar Tristar MPPT 60 Amp charge controller, so it will play with the numbers as it were to produce up to 60 Amps of charging voltage provided there is enough power coming into the panels from the sun for it to work with.  That's a lot, and for us should eliminate, or nearly eliminate, the need for a generator!  There is some loss of power when power conversions are made, but again, the bigger your needs for power, the better off you are getting the biggest and best and highest Amp converter you can afford, the highest amount of Amp Hours you can afford and/or fit in your battery compartment, the highest watts of solar panels you can afford and/or fit on your roof and still walk around them, and the highest Amp charge controller you can afford to go with the panels. 

Hope that simplified things even further for those of us, myself included, who really have a hard time with the math and just what certain terms like Watts and Amps and Volts mean in real time application.Kev

[ibdilbert01]

Thank Kev!!!!   Very well done!   

I actually run my batteries down to 20%, something everybody frowns on.    But I don't expect to get a long life out of my batteries compared to someone who drains them to 40 or 50%.   I'm also using golf cart batteries, I wouldn't do this with marine / RV deep cycle batteries.   Gadgetman posted his personal rule is to pull no more than 30% from the batteries, his batteries will last a very long time.   

My battery bank is now on year 5, because I abuse them like a golf cart does, I'm surprised they are still doing well.   My next set will be Gel Cells.   

[DanD2Soon]

I don't know what percentage of my bank is used.  It is a default setting within my old Freedom 3KV inverter/charger.  Seems like it is slightly over 50%.  I believe it's a voltage value below which the inverter will no longer draw on the battery bank.  Bank is 8 220AH 6V golf cart bats wired in 4 12V sets.  So each set is good for 220AH @ 12V x 4 = 880AH.  I'm pretty sure that bottom 400AH has never been tapped so the effective system output is around 480AH.  So slightly less than 55% - That's still a lot of livin'.
 
Tim,  20% may be pushing it but that's exactly what golf cart & forklift batteries are built for - just my opinion and not a challenge - I think you could probably get 10 years out of your golf cart batteries - if you're good to them - there's really not much in one to "wear" out. 
Three simple things can help & I'll bet you already do all three...

   
• Keep them clean: A sprinkle of baking soda and a light spray of fresh water will neutralize the conductive residue that develops on top of batteries. (Caution - Keep it out of the cells)
• Prevent corrosion: Wipe every connection in your system with Dielectric Grease - Battery posts and cable connectors; even the threads and washers of connector bolts and nuts; and the inside of crimp connector sleeves. It's cheap - pays for itself.
• Equalization: Most modern smart battery chargers and charge controllers have equalization settings or are set by default to periodically "blast" the accumulated sulfate from the plates and stir up the chemical mix of the electrolyte.  Most batteries die of sulfation (a lead sulfate build up on the plates that reduces their ability to hold a charge or create output.) If the condition gets bad enough, it actually bridges the gap between plates = bad cell/shot battery.

[ClydesdaleKevin]

Thanks Dan!  Great addition to the post!  I set my charge controller to manual for the equalization stage, so I could make sure sensitive equipment is unplugged and LED lights turned off, etc.  Once we get out to the desert this winter in full sun, I'll equalize them once a month or so manually, when the panels are actually producing enough power to do so.  That is the one setting the Boondocker Powermax 100 doesn't have, an equalize stage, or I'd do it more often while on shore power.

I used a dielectric grease spray from Interstate on all my connections, including open to the elements connections in the engine compartment and radiator compartment, and all the solar connections on the roof.

Wow!  2 more golf cart batteries than we have...that's a lot of amp hours!

Diet Pepsi or Diet Coke works great for cleaning the batteries as well...imagine what it does to your stomach lining...lol!  Glad I don't drink soda!

I dunno Dilbert...Gel cells are expensive and only very marginally better than flooded cell golf cart batteries.  I'll stick with the good old fashioned lead acid batteries...I don't mind checking the water.  We had gel cell batteries, 3 12 volters, in the Itasca, and they were great!  But they came with the rig and we didn't have to pay for them ourselves...lol.

Kev

[DaveVA78Chieftain]

I   :)rotflmao :)rotflmao   Love  :)rotflmao W% :)rotflmao   It!!!!!!

[PwrWgnWalt]

Great stuff, Kev!

As we continue to plan what to do with our 'new' MH, I've learned a whole lot from the forums. This thread in particular really got me thinking about what house batteries to consider... and resulted in me playing with numbers (Amps, Watts & Volts) to see what an "expected" day might use in terms of battery capacity.

As a result, using the data you came up, I made up this little automatic calculator that made it easy for me play with battery numbers... just plug in numbers in the three bright green areas, and the rest automatically shows.  Hope you don't mind me using your info 

Thought I'd share the Battery Life Estimator, in case someone else finds it helpful - it eliminated all the 'Greek' for me.  If you prefer to only deplete a battery bank down to 50% of capacity, that can be changed also.

Enjoy!
  - Walt

Here's a preview, and below it you can click the link to download the Microsoft Excel file:

[ClydesdaleKevin]

No problem Walt!  I can't wait to see the downloadable interactive version!  SWEET!



Kev

[PwrWgnWalt]

The file has been loaded to my previous post, just above. 

Knowing that actual loads usually vary and are rarely (if ever) constant, it should be interesting to see if there's any real-world application of this as people play with it.

Thanks again for the info!

- Walt

[ClydesdaleKevin]

Pretty neat Walt!  We will be boondocking for the better part of 3 weeks starting today, so we'll let you know how real life compares to calculations.

Kev

[DaveVA78Chieftain]

I am lazy, I prefer to just look at LCD display's and let them tell me whats going on.
I already monitor coach battery current and voltage which allows to see battery state / converter condition at a glance

Working on a panel that provides:
Coach Battery - Current Volts, Current Amps, Current AH usage, and Battery level in AH's.
Chassis Battery - Current Volts, Current Amps, Current AH usage, and Battery level in AH's.
Converter - Current Volts, +/-Current Amps
Coach load - Current Volts, +/- Current Amps
Inverter load - Current Volts, +/- Current Amps
Solar option - Current Volts, +/- Current Amps

I should have my new toy built, programmed and installed by June. Sort of a battery monitoring center.
AH's only apply to battery state so the rest of the stuff is just to monitor what the charge or discharge device is doing.  The output of a charging device (converter, alternator, solar array, etc.)  output is divided between the coach loads and the battery so unless you look at the big picture your really do not know what is happening.

Dave

[ClydesdaleKevin]

My Tristar monitor panel tells me a lot of that info, but not the amp draw from the batteries, so someday I'll add a battery monitor and shunt.  For now, I use the info the Tristar panel gives me, in conjunction with the digital volt meter that tells me at a glance my current battery state.

If you go to Andy Baird's Eureka website, he reviews a neat little gadget that you plug into the outlet at any appliance, and plug the appliance into it, and gives you the exact amp draw of that appliance in real time.  I might want to add one of those to my toolbox someday as well, since it would come in handy and aren't too expensive.

Kev

[JDxeper]

Just ordered this meter, have several things to check usage.
http://www.amazon.com/P3-International-P4460-Electricity-Monitor/dp/B000RGF29Q/ref=pd_cp_hi_0

[ClydesdaleKevin]

So it definitely appears that our battery and solar power system is up to the task!  Running the inverter from about 4pm until 2am...then putting it back on a 7am...and running the big flat screen TV and DVD player off of it all night with a couple of LED lights...and the furnace in the morning long enough to take the chill out of the air until the coffee is ready...that all draws the batteries down to 12.5...barely a dent in their capacity.  The solar panels completely recharge the battery bank to 12.7 on the volt meter...full recharge...by around 2pm!  And that is with the winter Arizona sun!  SWEET!

Kev