Battery amp hours (Ah) and actual runtime

Is there an equation I can use for calculating how long a battery of specific voltage and Ah capacity will power a specific piece of equipment with constant draw?

Here's my situation. I have two 12V AGM 35Ah batteries connected in series to provide a 24V source to a DC-DC converter (75% efficient) that supplies 15VDC to equipment that draws a constant 23W of power. My calculations tell me that that equates to about a 1.3A draw across my batteries (at 24V). If the equation was as simple as taking 35Ah/1.3A = 27 hours, I'd be done, but I know that it cannot be that easy. As the batteries are depleted, they will not be able to supply 24V of power. The converter I've spec'd can convert any incoming power from 19V-36V, so it should continue to provide the needed 15V to my equipment. In this situation, without buying all the equipment and running the test, how would I calculate how long I can expect my batteries to last. In other words, how long will they last until they drop below the 19V that the converter needs for its input?

[JohnS]

Originally Posted by ShaneA

Is there an equation I can use for calculating how long a battery of specific voltage and Ah capacity will power a specific piece of equipment with constant draw?

Here's my situation. I have two 12V AGM 35Ah batteries connected in series to provide a 24V source to a DC-DC converter (75% efficient) that supplies 15VDC to equipment that draws a constant 23W of power. My calculations tell me that that equates to about a 1.3A draw across my batteries (at 24V). If the equation was as simple as taking 35Ah/1.3A = 27 hours, I'd be done, but I know that it cannot be that easy. As the batteries are depleted, they will not be able to supply 24V of power. The converter I've spec'd can convert any incoming power from 19V-36V, so it should continue to provide the needed 15V to my equipment. In this situation, without buying all the equipment and running the test, how would I calculate how long I can expect my batteries to last. In other words, how long will they last until they drop below the 19V that the converter needs for its input?

Well, several points:
*Battery Ah capacity is usually rated with a "10 hour" load. For loads below that, capacity is a little more, for heavier loads a little less. So, with a constant load, you will do a little better than your 27 hours would indicate.

*As battery discharges, you will draw a little more current. At 22 v, you'll draw about 1.42 A. (You might consider using a simple average.

*For good battery life, I STRONGLY recommend not operating below 22 V (11 V per battery). The rest of the charge will go pretty fast and it is hard on the battery as far as number of recharge cycles and future life.
(I'm assuming it is some kind of lead-acid technology, including flooded or gell cell)

OK, so instead of the 19V limit on the batteries I should be looking at 22V as my "bottom line" when computing burn time. That being said, does anyone know how to compute how much time it will take to drop my system from a full charge at 24V to 22V?

[JohnS]

My suggestion was to average the 1.42 A at 22 V and the 1.3 A at 24 V. (1.36 A). Divide that into the 35 Ah.

Given that you are drawing less than 35 Ah/10 h = 3.5 A, I would expect the capacity to be a little greater than 35 Ah, but I hesitate to guess by how much. Batteries are certainly a YMMV situation.

how long should I charge a 14.8v @2.2ah battery

[JohnS]

Originally Posted by Unregistered

how long should I charge a 14.8v @2.2ah battery

That depends on the charger current and also whether it "taper charges."

For a simple, constant current charger, put in about 20% more than the rated capacity, 2.2 Ah x 1.2 = 2.64 Ah, so a 1 A charger would need 2.64 h (2 h 39 min).

A taper charger starts faster and tpers off as it approaches full charge. Read the instructions to tell how to determine whether it has reached full charge. If the charger has an ammeter, there should be an appreciable drop from the initial current, around 5:1 or so.

I want to provide a 24V DC supply to one equipment and the power consumption of the equipment is 24w.So my plan is connected 2 batteries in series.how would I calculate how long I can expect my batteries to last. In other words, i need atleast two weeks.so advise me the capacity of battery that i have to use

[JohnS]

Originally Posted by Unregistered

I want to provide a 24V DC supply to one equipment and the power consumption of the equipment is 24w.So my plan is connected 2 batteries in series.how would I calculate how long I can expect my batteries to last. In other words, i need atleast two weeks.so advise me the capacity of battery that i have to use

24 W/24 V = 1 A
2 weeks = 336 hr.
So you will draw 336 Ah.

If this will occur regularly, you shouldn't discharge a battery more than 50% if you want good life, so you should have around 670 Ah of capacity.

If it will normally be recharged sooner, and "worst case" only happens occasional, 66% might be OK, or about 505 Ah.

Over two weeks, you may have some self-discharge too. I would round up liberally, say 600 Ah or 800 Ah for the two cases I outlined.

How can you use energy density (units are in Wh/kg) to tell how long the battery can store energy?

how can you use energy density (units of Wh/kg) of a battery to get how long that battery can store energy for?? Or in other words the capacity of the battery in hours??