# Discharging a battery

Discussion in 'General Electronics Chat' started by Shah_Key, May 4, 2013.

1. ### Shah_Key Thread Starter New Member

Jan 8, 2013
23
0
Hello

I am trying to work out how long it would take to discharge a lead acid GEL battery if the following loads were attached.

Q1.
1 Mobile Phone
@
V = 5v
I = 300-400mA

And
Q2.
8 Mobile Phones
@
V = 5v each
I = 300-400mA each

The battery I am trying to discharge is a 12V battery.
Model: BEG120120

As mentioned earlier, I am trying how to work out the time on how long it would take to discharge the battery.

2. ### timescope Member

Dec 14, 2011
298
44
A device rated at 5v will be damaged if it is connected to a 12v battery.
You would need a 12v to 5v converter.

It is recommended that batteries should not be discharged more than 50%.

Find out the Amp Hour capacity of the battery, divide it by 2 then divide the result by the total current (in Amps not mA) that you wish to obtain. That will give the number of hours available.

Timescope

3. ### wmodavis Well-Known Member

Oct 23, 2010
739
151
And what is the amp-hour rating of the battery?

4. ### GopherT AAC Fanatic!

Nov 23, 2012
8,025
6,788
You might find a number on a marine battery that says 460ah or 600aH. An RC airplane battery might be rated at 3000 to 7000 mAh (milliamperes hours). A motorcycle battery at 60 to 90 Ah.

In other words, it is the number of hours the battery can supply one amp (but not really). It is just a standard way of rating batteries. Some batteries cannot deliver one amp and some many more than one. In the end, it is the discharge rate (in amps or milliamperes) multiplied by the hours it can discharge at that rate (really). But, in general, you don't want do discharge more than 50%, or what ever is eprecommended for the specific battery chemistry you have (led ace, lead gel, lithium ion, lithium polymer, NiCd, ...

5. ### GopherT AAC Fanatic!

Nov 23, 2012
8,025
6,788
I didn't realize you posted a battery model number. On googling, I found it is a 12aH battery. Assume we can use 6 amp hours (6000 mA h), you can power a 300 mAmp phone for 20 hours. 8 phones would bring this battery to its max load (2.4 amps) so I would not recommend doing that (8 x 300 mA = 2400 mA = 2.4 amps). See the DATASHEET for details. Here

.

Again, that is if you have reduced the 12 volts down to 5 with some type of voltage regulator. The 12 volt drop down to 5 causes a power loss but you get to use most of the amps, just not all the potential.

6. ### Shah_Key Thread Starter New Member

Jan 8, 2013
23
0
Thank you everyone
I will look into your posts in more detail.

Regarding the 12V to 5V conversion, I forgot to mention I am using an energy efficiency regulator.

7. ### Brownout Well-Known Member

Jan 10, 2012
2,373
1,003
Regardless of the amp-hour rating, a 12V lead-acid battery will sustain permenent damage if it is discharged to a point that it outputs less than 10.5V. When discharging the battery to it's nondestructive level, the output voltage must be monitored.