Ampere hour capacity

[King]

When the battery spec provides an ampere hour capacity,  does that mean until the battery voltage reaches 10.5V?  The various websites specify that voltage for reserve capacity, but nowhere can I find the definition of ampere hour capacity.
Art

 

[JohnSandyWhite]

Hi King.
What is an ampere-hour (AHr) capacity?
Product of amps multiplied by time. It refers to the volume of electricity that can be drawn from a power cell. Other things being equal, higher ampere hour capacity power cells will run onger than those with lower ampere hour capacities. AHr capacity of a power cell depends on the number and design of plates per cell.
Here is how they work in practice:- >> HERE <<. 

 

[Ned]

From http://www.nlectc.org/txtfiles/batteryguide/ba-glos.htm :

Ampere-Hour Capacity -- The number of ampere-hours which can be delivered by a storage battery on a single discharge. The ampere-hour capacity of a battery on discharge is determined by a number of factors, of which the following are the most important: final limiting voltage; quantity of electrolyte; discharge rate; density of electrolyte; design of separators; temperature, age, and life history of the battery; and number, design, and dimensions of electrodes.

A Google search on ampere hour capacity will turn up numerous variations of this definition.

 

[Gary RV_Wizard]

There is no  standard methodology for amp-hour measurements - each battery manufacturer can set his own measurement spec and conducts the tests.  The rationale for this is that different battery types can have different usage profiles and that the amp-hour measurement can reflect the "typical" use of the battery.  For example, a motorcycle battery has a different usage profile than a golf cart batery and the measurement methodology can reflect that. The  generic definition is to simply count the amps used until the battery is totally discharged, but the rate of use (amps/hour) and the definition of "totally discharged" can vary. Thus the number is mostly for comparative purposes and not something to count on. A typical amp-hour measurement spec would be to discharge at 20 amps/hour until the voltage reaches 10.5 volts, but 15 amps/hour is also used.

More than you wanted to know...
A battery will delivery many more amp-hours if discharged at a rate of 5 amps/hour than it will at 20 amps/hour. Thus the way in which you use a battery is a major factor in the number of hours available.

For the purpose of powering 12v lighting, a battery is not fully discharged until it reaches 10V or less, but for powering a large  inverter under heavy load it is fully dischaged when it reaches about 11V. Ditto for larger pumps and other higher-amp loads.

The amp-hour spec does not address the fact that a total discharge is not good for a battery's health and that actually using all those amp-hours can shorten the batery life.  In essence, actually draing the rated amp-hours one time will probably result in a battery that can no longer deliver that many amp-hours ever again.  Likewise, the spec applies only to a new battery - any older battery will not have that capacity.

RC - Reserve Capacity - is a number derived by a standardized measurement method and is intended to be usable across different brands of batteries.

 

[King]

Thanks for the responses.  The only time we ever reached "discharged to 1/3" (the charge indicator lights 4 leds for full, 3 leds for 2/3, 2 leds for 1/3 and 1 led for 0) The CO detector started with a beep every minute or so, indicating low voltage.  We went from 8 hrs on the road, which should have charged the batteries fully.  We stopped for the night at temperatures in the 10s and 20s using the furnace (6A) and the fridge (4A) and watched a videotape (TV 4A VTP 2A).  By morning we were still warm and cozy, but the CO det. was complaining.  I've been trying to figure out what the real limits were since then.
Art

 

[Gary RV_Wizard]

Those "charge level" lights aren't of much value, since you have no idea what voltage level each indicates and they probably aren't all that accurate to begin with. But 1/3 charge would be pretty low on any scale and I'm not surprised your Co detecor beeps - they are widely known to be voltage sensitive and will often beep long before the lights dim and other symproms show up. The LPG alarm may be sesnitive also. 

You can buy a digital gauge that shows the actual voltage - I have one that plugs into  a standard 12v outllet and it was under $15.  My coach doesn't have a direct voltage read-out either, so I wired an outlet direct to the battery just for this plug-in gauge and placed it where I could easily see it, so I can always read the true voltage right at the battery.

You didn't mention how many batteries you have or what size & type, but the furnace is a notorious consumer of amp-hours. The fridge is not and does not, I believe, draw 4 amps when in LPG mode, where only the circuit board is powered. That 4A fridge fuse you referenced is probably for AC power to the heater and not powered when in LPG mode.  A videotape player (using an inverter? DC powered?) is probably not  a large consumer either, but any use that lasts 90-140 minutes of continuous operation will have a noticeable effect. Then there are the 12V lights and such as well.

As a rule of thumb, we often estimate that you have about half the rated amp-hours available for actual use.

 

[King]

A pair of 4 year old interstate SRM 24s.  The fridge is electric only, either 12V or 120V as available.  The TV and video player are 12V.  The currents are as measured with a digital multimeter.  Sadly, I think the batteries may need to be replaced soon.  The plug in multimeter is a good idea though.
Art

 

[Gary RV_Wizard]

A pair of 4 year old interstate SRM 24s.

Not much capacity to start with (about 170 amp-hours) and probably only 1/2 - 2/3 of that left after 4 years. Those aren't great batteries either, so it's definitely time to replace them.

 

[Len and Jo]

Gary,
I don?t Really need to but?.I think it would interesting to build a battery well in my ?B? van and install two Lifeline GPL-31T AGM batteries in it.  They do appear to be pricy (Ebay?$229 each w free shipping).  In your expert opinion is the Lifeline AGM a good choise?  At the present we are using a single Delco marine/RV size 31 house battery that is recharged off of the alternator.  My plan would be to recharge the double batteries also off the engine alternator.  I do have a digital readout on the house battery voltage and do not go below 50% (12.05v).

 

[AlGriefer]

Hi Gary,

Maybe you have a rule of thumb for this one.  I'm pretty familiar with battery and inverter systems from my boating experience, but I used a Xantrex Link to measure amp hours used rather than using voltage.

All the voltage vs capacity charts assume that there is no load on the batteries and that there is no surface charge.  My coach with a few lights on, the inverter and fridge on, and the electronics on standby draws about 20A.  Do you have any idea how that will affect the voltage measured?

Al

 

[John From Detroit]

Uh, what is the Xantrex Link you speak of?

 

[Tom]

John,

I believe Al is talking about a shunt used in conjunction with the Link 2000 panel to monitor current and voltage, in addition to keeping track of amp-hours consumed. This was what he used previously in his boat.

 

[John From Detroit]

Well.. after I posted the question I did the logical thing... www.xantrex.com and looked it up.  Now I know what it is.

Kind of an interesting device, monitors a bunch of different factors about the battery..... Don't know if it's worth the cost but I'm having to think about it

 

[AlGriefer]

The pointer to some of the Links is here

Al

 

[Gary RV_Wizard]

All the voltage vs capacity charts assume that there is no load on the batteries and that there is no surface charge.  My coach with a few lights on, the inverter and fridge on, and the electronics on standby draws about 20A.  Do you have any idea how that will affect the voltage measured?

Al,
The short answer is "no, I don't", but I'm not quite sure what you are asking.  Do you have a Link 1000/2000 in the motorhome or was it only on the boat?  Are you trying to estimate capacity from voltage in the motorhome? If so, the voltage reading with your 20A load is probably giving you a fair estimate of how much longer it can continue with that 20A load.  It's less than the no-load voltage in the tables, but it reflects the 20A rate of discharge and that provides a compemnsation factor of sorts.  The numbers in the tables are very much a "broad brush" and based on a new batery anyway, so the estimate is not going to be real accurate.

Incandescent lights wil continue to work (but dimmer) at most any voltage.  Pumps will also continue to work with fairly low voltage, though they draw higher current and geta bit warm.  Flourescent lights and your inverter are voltage sensitive, so your main concern is approaching the low limits for the inverter (11.0 DCV? - check your manual). DC flourescent lights have an inverter built in and will quit about the same time.