Battery life math

[macgregor]

I've never been good with batteries and power.

If I have a battery with the following specs:

Li-Ion, Lithium-Ion
Output Voltage 14.8V
93Wh
6.3Ah

And a camera with the following power requirements:

DC 10.5 V to 17 V


Can someone tell me how do I do the math to know how long will the battery last?

[capt chuck]

Not enough information. You need to know the power draw of the camera not just the voltage. Once you have that just divide the batteries watt hours by the cameras power draw in watts. For example you 93 wh battery for a ten watt cam should last aprox 9.3 hours. Lots of other things come into play.. Temperature, battery age, etc. but this should give you a ballpark idea.

[macgregor]

oops, I forgot it. Here it goes:

Power requirements
DC 10.5 to 17 V
Power consumption
Approx. 56 W

So what is the voltage and amps for then?

[Cassius]

Volts x Amps = Watts. Volts and amps do different things, which I'm not going to go into now. Wattage tells you the capacity over time, but volts and amps tell you the amount needed for any given moment.

93/56 = 1.6, so about an hour and a half at a minimum. Realistically 56 watts is going to be the maximum draw, so two or three hours probably.

[John Ritter]

Hello

A few things to keep in mind before you even do the math, All Li-Ion battery packs are rated two ways (you sometimes have to read the small print closely) first the selling point,
this is the capacity maximum rating given such as; 90 watts, now look for the continuous maximum draw rating (for a 90 watt battery it can be between 40-55 watts max only)
use the maximum continuous rating to determine your power consumption requirements.

Keep this in mind as this will limit the applications for a specific battery, using a pack that has a 50 watt max continuous rating at higher power draws will dramatically reduce your run-time and the life expectancy of the battery pack.

The second factor to consider is the overall life expectancy of Li-Ion battery packs, this is only 28-36 months on average use them or not, so always get the freshest pack available.
and yes there are exceptions and some will last longer, keep in mind that manufacturers consider 60% of original capacity rating or better to be a good battery pack (for warranty purpose).

Your battery may already be overloaded with the 56 watt load and if so will experience reduced run-time and life.

Now for the calculation part:
In general the wattage rating is determined using the industry standard of 14.4 volts multiplied by the battery packs rated amperage (your battery would have cells; 3qty - 2150mAh or 2.15 Amp x 3 in a parallel bank with 4 in series, 12 cells total, resulting in an amp rating of 6.45 x 14.4 = 92.88 or rounded up to 93 watts, voltage of 14.8 nominal) this would be the manufacturers watt hour specification

Now to determine camera run time just take your actual rating (it will be on the camera ID plate or in the manual, expressed as watts or amps) and divide it into the battery rating.
i.e. your 93 watt battery and your 56 watt camera, 93/56 = 1.66 hour expected run-time.

Now the reality is that you should only expect about an hour of true run-time as most battery packs will not perform to full exact/original specification.
The variation from original spec is due to many factors; Age, Temperature, charger used (even exact same chargers will produce slight variations in final charge),
continuous or intermittent use, time it sat on a shelf before sold, etc.

I if you only need an hour of run-time then a 93 watt battery pack will work just fine for you, if you need a true 2 hour run-time you should look for a 120 watt battery
(the extra cushion of wattage will insure this).

Voltage is a variable with batteries as it will decline as the battery is exhausted, while amp draw will increase as voltage drops assuming wattage/load stays the same.
i.e. 50 watts @ 14.4 volts is 3.47 amps and 50 watts @ 12.5 volts is 4 amps
Note #1 just about all Li-Ion packs will auto shut down between 12-13 volts ( the actual usable power range is in the 14-13 volt stage)
Note #2 chargers will usually charge the pack from 16.8 max to 15.6 volts (this upper voltage range declines very rapidly when a load is applied and the pack will drop to 14.8-14.6 quickly)

This is why the power consumption is not linear but rather a curve as power is drawn from the pack, the load is increased and as the voltage drops and the battery exhaust's
itself faster at the end of discharge cycle.

Hope this explanation helps....

Regards John Ritter
www.ritterbattery.com

[macgregor]

Very useful! thank you.

So now looking at the specs of a random battery I get the following:
Specifications:
Capacity: 10Ah/150Wh
MAX Load: 95Watts
Weight: 2lb -- 720 grams
Size: 6"H x 4"W x 1.5"D

So it´s a 150Wh but will deliver only 95W at max output, so I could expect about 90mins of battery life with such a battery then?


Any advantages of NiMH over li-on for camera batteries?

[Amr Rahmy]

if you are only drawing 60, then it should last 2 hours. at max it would not last more than 1 hour, or you should not account for that. li-on usually lasts from 1 to 5 years depending on the draw level and the manufacturing, is a bit newer than nimh and is usually much lighter.

batteries don't appreciate loosing their juice too quickly. if you get a brand name battery and bigger capacity, it should last you more than the off brand, give you a few hours of use and last longer.

[John Ritter]

Very useful! thank you.

So now looking at the specs of a random battery I get the following:
Specifications:
Capacity: 10Ah/150Wh
MAX Load: 95Watts
Weight: 2lb -- 720 grams
Size: 6"H x 4"W x 1.5"D

So it´s a 150Wh but will deliver only 95W at max output, so I could expect about 90mins of battery life with such a battery then?


Any advantages of NiMH over li-on for camera batteries?

Hi macgregor

If it is rated at 95 watts continuous and you are only drawing 56 watts your load is well below the operating threshold so you will be able to enjoy the full
150 watt capacity of this particular battery pack.
The expected "real" run-time will be very close to the full rating of 150 watts so just under 3 hours run-time will be realistic. (150/56 = 2.67 hours) it will decrease with age/time.
As long as you stay below the rated continuous capacity the full output of the battery pack can be realized, it is only when it is exceeded that everything goes wacky.

The major advantage of NiMh over Li-Ion is durability and load handling capacity, i.e all cells are rated in capacity or C there are very few Li-Ion cells that are
rated at more than 2.5C in the higher capacity range That is why multiple cells are ganged together in banks (3 cells parallel in a bank at 1c would equal 3C as a total).
Also if Li-Ion cells are drained below a certain voltage usually 2 volts per cell they are trash and will not recover so the pack will be shot (even if only 1 out of 12).
Upside is much lighter battery pack.

NiMh cells are available in 10 C which means that they can be drained at 10 times their rating, this means that a single cell rated at 1 amp can take a 10 amp load (or 1 x 10 =100 watts)
(run-time will be much less but the load will not destroy the cell) so a battery like an Anton Bauer Hytron 140 could handle up to a 1000 watt load for a short duration and survive.
NiMh can also handle over discharge to a certain extent and can be rejuvenated so that it will continue working (pack will usually still work if 1 cell goes bad at reduced voltage & capacity).
Downside is the weight issue and charge time required
NiMh can also be recovered from a certain amount of accidental discharge (low voltage) and still work well.
If you work with consistent high power draws especially lighting loads NiMh/NiCd is still the best bang for the money.

BTW: I am an avid & staunch supporter of the Anton Bauer Battery line but there are some good imports (in the Li-Ion chemistry line) at very good dollar value, just ask
for referrals from other users and most important check for the manufacture date on the battery before buying it you always want the freshest pack possible.
And read all the fine print......

Regards John Ritter
www.ritterbattery.com