Need help with AA runtime (capacity)

[021411]

Please move this thread if it's not appropriate here. It's battery related though.

I'm going to rig up a fish finder on my fishing kayak and the specs on it says it draws 250mA. The unit itself requires 10-17v to operate. Let's say I use 10 AA Eneloops in a holder. I believe they are 1.2v each and have a capacity of 1900mAh. I'm assuming I should be good with the voltage requirement as 10 of the batteries together will be 12v. What about capacity? Is it safe to assume that the 10 batteries will give me 19Ah total? I've never really dealt with the capacity part of them.
The batteries should theoretically give me 76 hrs of usage, right? Or am I WAYYYYY off the mark.
19Ah / .250mA = 76 hrs
In all honesty I just need the unit to be powered on for at least 8-10 hours. My trips don't last that long. They'll get recharged before every trip.

Thanks steering me in the right direction.

 

[s.urfer]

The cells supposedly last 1900mAh / 250mA = 7.6 hours

Capacity doesn't grow when in series, it does when in parallel...

 

[45/70]

To put it another way, when cells are in series the voltage will add up, ie, 10 eneloops will yield 12 volts nominally, but the capacity will still be 1900mAh.

Ten eneloops in parallel will remain at 1.2 Volts nominal, but yield 19,000mAh, or 19 Amps.

Your fish finder specs don't specify at what voltage it draws 250mA. Usually the current draw on the "battery" of such devices will be less at higher voltage, and more at lower voltage. So, to make a conservative estimate, 1900mAh/250mA=7.6hrs, I'd figure around 6hrs.

Also, keep in mind that eneloops are durable cells, but I would try to avoid totally discharging 10 cells in series. The likelihood of reverse charging one, or more cells with this many cells in series, is rather high, if you drain the "pack" too low. Maybe take an extra set of cells and change them out halfway through?

The other option, if your making your own battery holder setup, is to make a 10s2p pack (12V/3800mAh) with 20 eneloops. While this setup should run the finder for ~12hrs, you'd still have to be careful not to fully discharge the pack, but it should give you your required 8-10hrs, no problem.

Dave

 

[021411]

Sorry about my short reply as I'm away from my computer. I checked the specs again and it draws 13.5v 250ma.

 

[45/70]

OK, just figuring that rate in my head, the 10 eneloops would probably go for 6+hrs then, but probably not quite 7hrs.

Dave

 

[021411]

OK, just figuring that rate in my head, the 10 eneloops would probably go for 6+hrs then, but probably not quite 7hrs.

Dave

Thanks Dave. Is this in series?

 

[45/70]

Yes, series.

Considering that the AA eneloop is rated for capacity at a 380mA current rate, you might do better than my estimate. Figuring Wh, the unit uses 3.375 W/hr @13.5 Volts and 10 eneloops are capable of 22.8 Wh @ a 380mA rate, at a nominal voltage of 1.2 Volt, so 22.8/3.375=6.75hrs. In actuality, AA eneloop cells average a bit better than 1.2 Volts (from Tom's NiMh shootout thread) during a 500mA discharge, so they may do a bit better than this. Still, a "fudge factor" is a good idea, so figure 6.5-7hrs.

Dave