(03-03-2012, 11:51 PM)subhas Wrote: Batteries have Ampere Hour (AH) rating. If you divide AH rating of the battery by the Amp rating your machine you will get the number of hours the battery will power your machine. A 12Volt 120AH battery will last for 40 hours powering a CPAP machine rated at 12V 3Amp.
Using that formula would assume that you're planning on depleting the battery completely down to a 0% charge, which is a big no-no with standard lead-acid type batteries that have liquid electrolytes sloshing around and lead fins (the "marine" or "deep cycle" car-sized type battery).
Roughly, 11.6 volts is considered fully discharged (0%). You never
want to do that to a standard lead-acid battery or you're ruin it.
Ideally, you want enough battery capacity so that you don't regularly discharge the battery much below 70-80% of capacity. That will preserve battery life to a much greater extent. Go below that 70% level too many times and battery lifespan decreases. Go below 40% a few times and you have a doorstop instead of a battery. In other words, in reality, you only have 60% of your battery capacity to work with (otherwise you risk ruining it).
The amp-hour rating for a battery is the maximum amps that can be drawn until the battery is completely discharged, over a specific time period. The effective
amp hours available drops as you draw more amps. So a battery rated at 100 amp-hours (for instance) doesn't really have the capacity that the manufacturers claim.
Battery manufacturers do tests on the battery to give them an Amp-Hour rating. A typical time period for a test is 20 hours, but it varies... batteries are tested over different periods, such as 24 hours, 75 hours, sometimes 100 hours. Let's use a 100 amp -hour battery as example, just to make the numbers easier.... A 100 amp-hour battery tested over a 100 hour period (i.e. 1 amp drawn for 100 hours) will not have the same capacity as a 100 Amp-hour battery tested over a 20 hour period (i.e.-- 5 amps drawn for 20 hours)... If you drew 1 amp continuously from a 20 hour test battery, it would last about 110-120% longer than on a 100 hour test battery.
Suppose you have a 100 amp-hour battery, tested over a 20 hour period. 100 Amp-hours divided by 20 hours = 5 amps. That means that the battery manufacturer claims the battery can sustain a 5 amp load for 20 hours until the battery is completely dead.
That's great, but wait... like I said, you don't want to drain a battery to it's completely dead 0%. On a lead-acid deep cycle battery, you should only drain a battery down to 40% of it's original capacity, absolute max. (in other words you've only got 60% of the battery to use).
On a 100 AH, 5 amp load, for instance... You have 20 hours of time at a 5 amp draw, but really you only have 60% of that time, so 20 hours x 60% = 12 hours.
Thus far, what I've said is true because I've used the same amperage over the same time period at which the battery was originally tested (20 hour rate). So what if you took your 100 amp-hour battery, but wanted to draw 10 amps from it until it was at the "safe" discharge level (say 60%)?
Do the math: 100 AH / 10 amps x 60% = 6 hours, right? Wrong
The More Amperage You Draw, the Lower the Battery Capacity.
So, the effective amp hours available drops the more amps you draw. Now, that 100 amp-hour battery is not what it says it is.
Ok, now, using a 100 AH, 20 hour test battery, at 5 amps draw, you get the full 100% of the battery rating (that's how the battery was tested in the first place, so you should get all 100%). But step up the amperage to 10 amps, and you will lower the capacity of the battery by about 10% (or 90% remaining).
That makes 100 AH x 90% = 90 AH / 10 amps draw = 9 hours x 60% (max safe discharge number) = 5.4 hours. And that's a difference from the 6 hours you thought you had.
Where did I get the 90%? It comes from Peukert's Equation
... that's where the approximate 90% comes from. The Peukert Equation explains this... using it, you can predict how much time you'll actually have on a battery given a specific discharge rate.
The whole point is that the more you allow your lead-acid deep cycle battery to be discharged, the more you reduce your battery's lifespan. Even though you can
discharge it to low levels, it's probably not wise to do so, so buy a battery that has a relatively high amp-hour rating to begin with so you won't have to worry about discharging it too low and keep your battery charged up if you can rather than allowing it to go below 70% of charge (which is about 12.3 volts). Keep in mind that I'm primarily talking about lead-acid deep cycle batteries. Other battery types can be discharged to a greater extent, depending upon their design.