Estimating Battery Life

TinkersALot

For 9-Volt and/or 6 Volt "lantern batteries"....

1) If I needed to have a device keep working for 96 hours on one of these batteries (either a single 9-volt OR a single 6-volt latern battery) how much current could my device draw? Or, if my devices draws "n milli-amps" can that value be used to derive an estimated battery life?

2) How many "amp hours" are these kinds of batteries typically rated for?

3) If 2 9-volt batteries are connected in parallel across a load, does the time that they can drive a given load "double" ?

4) If 2 9-volt batteries are connected in series to a load, is that a better solution than connecting them in parallel? Or will the additional capacity of such an arrangement get·"wasted" in the·increased voltage drop across a regulator?

5) Are there any other considerations that come to bear with regard to battery life that I should bear in mind when defining a battery pack for a small device?

thanks, in advance, to all that help me understand more about the "black art" of properly sizing battery packs to a load.

Mike Green

It's less of a black art than you think. First thing you need to do is find the datasheet for the battery you plan to use. Go to the manufacturer's website and look up the battery. For Eveready Energizer 522, here's the link: data.energizer.com/PDFs/522.pdf. There are all sorts of graphs that show the service life (to a specified minimum voltage) for different amounts of current drawn or for different load resistances.

#3: No. As one battery drops in voltage, the fresher battery will actually discharge through the weaker battery, so you won't really get "double".

#4: No. The extra voltage will just get dissipated as heat in the regulator. If you have a switching regulator, this isn't true. A good switching regulator will allow a series connected pair of batteries to have close to double the life of a single battery.

Tracy Allen

The Energizer type 522 9 volt alkaline has a capacity of about 600 mAh at 25 mA drain, and 600/25 = 24 hours. At that point the battery voltage would be down from 9 volts to less than 5 volts, so realistically you might get 18 hours out of it. If you need 96 hours from a 600 mAh battery, then the average current could be as high as 600/96= 6.25 mA. Again you have to discount that if your project needs a certain voltage overhead in order to operate.

Another good online reference is battery university.

RE: question #5. If you have a BASIC Stamp project, make use of the SLEEP and NAP commands if possible. Those commands drop the current drain down to less than 0.1 mA and then your project would wake up periodically to take care of business. That strategy can extend the life of a 9V battery out to weeks, at best, 600 mAh / 0.1 mA = 6000 hours = 250 days.

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Tracy Allen
www.emesystems.com

vaclav_sal

Batteries 101
Batteries are electro – chemical devices (you know that)
“Lantern “ batteries are generally dry cell alkaline batteries.
(That is: technical term.)
Battery capacity is rated in Ampere-hours or miliampere hours.
·
In theory the battery will deliver rated current for so many hours.
Example – if your equipment requires 1 Ampere to operate and you have 10 Amperehour battery you can operate your equipment for 10 hours continually. Of course your mileage will vary – but that is how the battery capacity is rated.
If your equipment requires 2 Amperes to operate the operating time should be 5 hours.
But there is catch – the physical makeup of the battery is build for continuous current of 1 Amp, drawing more than rated current is no good for battery life.
Look at it at the extreme – if you draw 10 Amps from this battery it will not last for an hour and things will get uncomfortably warm.
·
It is not a good idea to directly connect batteries in parallel – they are not exactly equal (chemically and therefore electrically) and will discharge from higher voltage to lower voltage.
It can be done safely using isolation diodes. But that is the subject of next semester class!
·
Connecting same size (in Ampere hours) batteries in series does not change the capacity, only the output voltage. For example· - ordinary flashlight uses several D cells connected in series.
·
Car batteries are also rated by “cranking power” – ability to deliver high current for short period of time and at specific temperature.
·
Batteries are temperature sensitive and when rating battery for service you also need to consider equipment operating environment. In general – the colder it gets the less overall battery capacity.
·
Last word of caution – directly shorting battery terminals is not a good idea. Sparks will fly around outside and hot chemical reaction inside will result.
Using isolated tools is a good practice.
Modified Murphy’s Law – a tool dropped will fall into place where it can do most damage!
·
·Cheers Vaclav

TinkersALot

As always, the "regulars" here offer consistently useful gems of information. Thanks to all of you that replied to offer insights into these questions. I truly appreciate it!

Signed,
"LearnsALot" by TinkeringALot

Steve Andrues

Regarding Battery 101 ... there is a difference in the rating of primary batteries (non-rechargeable). They cannot give you a 'real" ampere-hour capacity. You get an equivalent ampere-hour capacity. By using the manufacturers load and runtime data you can infer what the starting capacity is. The reason for this is that the battery (lets say an Energizer D-cell rated at 10 ampere-hour) will not deliver its C/1 or 10 ampere-hour capacity in one hour. On secondary batteries (rechargeable) the problem is more confusing. Lead-acid batteries are particularly bad at ratings. This includes 'glass mat', 'advanced glass mat', sealed lead acid, and valve regulated sealed lead acid batteries. These are all marketing terms. These all refer to lead technology batteries with Jello to immobilize the electrolyte. The ratings for these batteries can be at the C/5, C/10, or C/20 rate. Remember that the battery looks like a series resistor in the circuit. The higher your discharge current, the more energy you will dissapate in the form of heat getting the energy out of the battery. That is why your 12v 7Ah battery (rated at C/20 rate) will only deliver a little over 4 ampere-hour at the one hour rate. All Ni-Cads and NiMh batteries are rated at C/1 rate so what you see is what you get. Pay attention to temperature too. Primary cells and lead-acid cells do not like cold.

Don't worry if it sounds confusing. I have been fixing aircraft batteries for 40 years and somtimes it still throws a curve-ball.

phil kenny

There is a way to operate the batteries in parallel and avoid the problem
that Mike mentioned of one battery discharging into the other one. The method
is called diode isolation.

If you connect a Schottky diode in series with each battery before you
tie them together, each diode will block reverse current from the other battery.
Schottky diodes have less forward drop than regular silicon diodes.

As an example the STPS1150RLCT-ND (Digi-Key part number) is a 1 amp
diode, but at a forward current of 100 ma, the drop is about 0.5 volts.

However, for maximizing the useful life of the batteries, the switching regulator
approach is the optimal solution

phil

Post Edited (phil kenny) : 7/11/2009 5:50:54 AM GMT