Electronics maths help!

[Anderoo]

Some help with what should be fairly basic maths would be appreciated...!

 

I have a fishfinder that has a power draw of 615 mA. I have a 12v 7a battery to power it. How long will the battery last?

 

(Also, a practical consideration, these batteries shouldn't really be drained more than about 50% of their capacity before recharging.)

 

 

EDIT my old fishfinder only had a power draw of 100 mA, and even after a 12-hour day the battery capacity had hardly been reduced at all. But I never actually measured it properly, as I didn't need to...

Edited August 11, 2015 by Anderoo

[Errrm]

Hiya,

 

I think (although someone who know more might be able to give a better answer) you need to find out the storage capacity of the Battery. There should be a Ah rating (possibly specified mAh) for the battery and I think you need to know that before knowing how long the batter will last...... I mgiht be wrong though

 

https://en.wikipedia.org/wiki/Ampere-hour

[Anderoo]

The battery is 7A.

 

I think the below is right but would be good to confirm:

 

7A = 7000mA [battery capacity]

615mA per hour [power draw]

 

7000mA [battery capacity] divided by 615mA [power draw] = 11.3 hours until it is totally drained.

 

Does this look right?

[Steve Walker]

Conclusion is right, I think, units are wrong

 

I suspect that the 7 figure is Ah (amps x hours) rather than A. So when you divide (amps x hours) by (amps) you get (hours).

[Anderoo]

Conclusion is right, I think, units are wrong

 

I suspect that the 7 figure is Ah (amps x hours) rather than A. So when you divide (amps x hours) by (amps) you get (hours).

 

So is the 11 hours of battery use right Steve, even if I got the maths wrong...?

 

If so, I'll need to get a bigger battery.

[Rusty]

Here you go;

 

http://www.digikey.co.uk/en/resources/conversion-calculators/conversion-calculator-battery-life

 

This site suggests that it's not a straight calculation so only gives you 0.7 of the calculated battery life duration. If you don't want to drain it more than 50% then the most you can hope for is 4 hours use....you need a much bigger battery.

 

Edit: Your new fishfinder is drawing over six times more current than the old one, @ 100mA your old device would last 24 hours before the battery was half drained. I think you'll also need a bigger boat.

Edited August 11, 2015 by Rusty

[Anderoo]

Perfect, thanks Rusty!

[Phone]

Anderoo, Rusty

 

That's no fun - a calculator.

 

In the manual of your fishfinder you can find how much mAh your device needs per hour although who ever reads a manual.

 

The "standard is a 2200mAH "lipo" battery. About 8 bucks in the US. ( www.hobbyking.com ). Although most guides carry 4.

 

Everything you need

Phone

[chesters1]

Its not so clear cut ,at what voltage / current does the device stop working

Then theres the factor a 12v battery is not 12v

If the battery is completely drained what is the voltage? ,its not 0 volts a battery drained to zero will not recharge in many cases ,perhaps drained in battery terms is 12.5v?

I have just gone through my collection of rechargeables and measured their real capacity ,half were half the stated capacity and chucked and only a few were the stated capacity on the lable and all those were less than 3 months old ,the older they got the less capacity there was even in slightly used backup batteries .

They were all charged on reputable smart chargers not your typical timed ones so were treated well but none reached the full charge ,only when i bought a charger that stated the actual capacity and a refresh cycle did they get to their fullest capacity

My biggest disapointment was chucking 2 unused 9.6v batteries that were religiously charged every month but failed miserably when the actual capacity was read ,they refused to get anywhere near the stated capacity wheres their replacements did ,so they decline used or not

 

So you have batteries that are not the stated voltage and batteries not the stated capacity as the charger or battery is not up to scratch

Best advice i can give is running the device from a fully charged battery and recording it ,when the device stops thats what your going to get

Edited August 11, 2015 by chesters1

[Anderoo]

I have a trickle charger that recharges the 7ah batteries back up to full capacity. Tested with a multimeter.

 

At 50% capacity it should give a readout of 12.2v, so I would set the battery alarm on the fishfinder to that and then swap batteries if necessary.

 

I have now come up again a load of additional issues, if I get a single battery big enough to run it all day, I'll need a new bigger box to keep it in and will have to reattach the unit head to it. Also the higher batteries have larger terminals, so the connectors I currently have on the power lead would need replacing. Not the worst thing but additional hassle and makes everything bigger and heavier.

 

I'm now leaning more towards getting a couple more 7ah batteries and just switching over when they drop to 50%...

Edited August 11, 2015 by Anderoo