Calculating Battery Life

Does the 9V battery drive the 6V pump as well as the timer and the relay? If so, what is the rating of the timer and of the relay?

What is the amp-hour rating on the battery?

Is is rechargeable?

To get decent life you will need to run the battery right down to 6v.
So you need to put it in circuit and measure it's current consumption when new ~9v and when it's right down near 6v.
The important question is...
Is it driving any circuitry at all during the rest of the time? Even if it's only a few milliamps, that is what flattens batteries.

Disregarding all other answers, you don't say what size battery you intend to use.

The answer to your problem lies within a simple google search. Simply search for the brand of battery you wish to use & once at the correct site, you WILL be able to obtain discharge curves for various batteries. It doesn't take a genius to understand these curves.

If it is only driving 500mA for 180 secs a day, then that equals 25mAhours per day.
Varta quote 550maH life for an Alkaline PP3, but that is probably at low current delivery...so be pessimistic and call it 300mah.
That would give you a life of 300/25 days.
But like I said before it's the quiescent current that is leaking away 24hours a day which is the killer.

So you can see 12days life is pathetic... you need to use C or D cells... much higher capacity, and you may get away with just 4 of them, if the pump will run down at 4v.

PP3s are no good for driving high currents like 500mA.
Del

Hi Del,

Yeah, what u say make sense. The 9V battery is no match for the load i intend to run. Maybe a lead acid battery, 9V with 6AH maybe?

The timer too consumes the power from the same battery, but offhand i do not know th power of the timer. It has an internal battery to power up the lcd display.

If the timer draws 1mA, then over 24 hours thats 24maH which is almost as much as the pump!
Stick a meter on it and measure it...
Remember battery manufactureres are born optimists, take there figures with a large pinch of salt. Muliply their figures by 0.75 or 0.5 to be safe, as they always quote best case load etc.

Del

This may come as a surprise but lead acid batteries don't have such a good power density as Alkaline
(E.G Power for the ame size or weight.)
Del

Making a few assumptions, to get a feel for it:

Assume battery runs everything, and timer+relay load << pump. Neglect timer load when pump off.

Only alkaline 9V battery I could quickly find data on is a Duracell size 6LR61 (AKA "PP3" size). Datasheet gives horribly low amp-hour figure for high currents, so you may have to include a big capacitor, 'trickle-charged', to get good life out of the poor little cell(s).

Anyway, assume 0.75 Ah available from the battery, with your load being 0.5 x 180 / 3600 = 0.025 Ah per day, you get 30 days.

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