# Battery capacity

Discussion in 'The Projects Forum' started by nareshp, Jul 1, 2013.

1. ### nareshp Thread Starter New Member

Jun 11, 2013
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How to determine the battery capacity for 10 hours..I mean discharging time of battery.
This calculation required for solar inverter design

My rated power outputs are 750w with 3.2A..which works on A.C 230V supply

2. ### LDC3 Active Member

Apr 27, 2013
920
160
If you have the number of amp-hours for your battery, you would divide that by the number of hours and this would be the amount of amps you could draw from the battery for 10 hours. A 40 amp-hour battery should supply 4A for 10 hours.
A high current draw from a battery would heat the battery, which would cause a voltage increase and an increase in current.

3. ### nareshp Thread Starter New Member

Jun 11, 2013
7
0
thank you sir

4. ### nareshp Thread Starter New Member

Jun 11, 2013
7
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can you elaborate high current..what will be the life time of battery....
Can somebody tell me how to choose the solar module

5. ### WBahn Moderator

Mar 31, 2012
18,093
4,919
Your rated power outputs from WHAT?

Is that the actual power level that your battery has to support for 10 hours?

Keeping in mind that the "amp-hour" rating is not something that is engraved in stone, meaning that the effective capacity is dependent on a number of different factors, a very crude way to put you in the ball park would be to look at total energy needs.

If you need to supply a power P (750W) for a time T (10 hours), then the total energy E1 you need to deliver is

E1 = PT

A battery with a terminal voltage V and an amp-hour capacity C has, on paper, an amount of energy E2 available equal to

E2 = VC

If the process of getting energy from the battery to the load has an efficiency of η, then you have

E1 = ηE2

so

PT = ηVC

So the amp-hour capacity you need would be, at a minimum,

C = (PT)/(ηV)

For example, if you want to deliver 750W for 10h from a 12V battery and your total conversion efficiency is 80%, then you need

C = (750W*10h)/(0.80*12V) = (7500/9.6) Wh/V = 781 Ah

Note that powering something that consumes about 1hp for 10 hours off of a battery is not something that is going to be done with a small battery.

The following site might be useful:

The energy density of rechargeable batteries, according to the site, lies in the range of about 30 Wh/kg and 200 Wh/kg. Unless you are dealing with lithium technologies, you will be more like the 30-70 Wh/kg range. Let's say 50Wh/kg. That means that your battery above would upwards of 200kg (between 400lb and 450lb).

6. ### LDC3 Active Member

Apr 27, 2013
920
160
The heat is from the internal resistance of the battery and the current (P=IR). As the battery heats up, the chemical reaction to produce the power becomes easier, so the voltage increases. How much heat the battery can withstand and the lifetime depends on the composition of the battery.
A car battery can supply 10A (or more) of current when you start your engine. But you wouldn't have this current for more than 15 seconds, so the battery barely heats.

7. ### WBahn Moderator

Mar 31, 2012
18,093
4,919
Uhm... generall the current supplied by the battery to start a car is a LOT more than 10A -- typically several hundred amps and potentially well over a thousand amps for larger motors. Diesel motors without automatic compression release also tend to draw more because of the high compression.

Just turning on your headlights, particularly on older cars, may draw over 20A.

Interestingly, on a very cold day (like -20°F type cold), if you try to start your car your battery may not have enough power available to crank the engine and you drain the battery very quickly. But if you turn on the headlights for a while you will notice that they start out dim and then brighten up in a few minutes. This is precisely due to the effect you describe. By warming up the battery with the modest draw from the headlights, you are often able to get enough power out of it to the starter to light off the engine. A better use of that power, if you've equipped the car appropriately, is to use it to power an oil heater rather than the headlights. That way, not only do you warm up the battery, but you thin out the oil which will reduce the cranking resistance - and possibly prevent blowing out an oil seal or two.

8. ### nareshp Thread Starter New Member

Jun 11, 2013
7
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thanks you WBahn very good explanation.Useful for me to proceed with other circuitary...I will keep posting for doubts for my solar inverter design

9. ### wayneh Expert

Sep 9, 2010
12,403
3,251
Do you have personal experience with this? I've heard it before but I think it was offered as a myth. It sounds plausible but I've never tried it.

10. ### strantor AAC Fanatic!

Oct 3, 2010
4,302
1,989
I've never heard it offered as a myth or an actual suggestion, but I'm from Texas . When I read this, I started thinking other ways to warm the battery that didn't involve draining it, and I came up with enough material to fill several episodes of the SpikeTV show "1000 Ways to Die."

11. ### wayneh Expert

Sep 9, 2010
12,403
3,251
I've had the pleasure of seeing "-30°F" on the car thermometer up here where I live (northern IL). Thankfully I have a garage or I'd have never made it out driving.

But anyway, in my experience, if the engine doesn't fire in the first few seconds when bitterly cold, you're done. Give up and go get warm. If you really need to get going, take the battery into the house for a while. I've screwed around with hair dryers and 100W bulbs under the hood, but with no luck I can recall. Do you have any idea how hard it is to remove a battery at -20°F?

12. ### LDC3 Active Member

Apr 27, 2013
920
160
When I was young, my parents always had a block heater in the car which would be plugged in during the coldest nights. There are also heaters available for the battery.