--- In Rob Danielson <> wrote:
The CCA figure of 240 is for engine starting [see a battery terminology
FAQ/glossary like http://www.payless-batteries.co.uk/Batterytip/tips.php\].=
With batteries rated for extended discharge (e.g., deep cycle) there shou=
ld be=20
another number which will be about a factor of ten less. This rating-- the =
one
relevant to your task -- will be something like the maximum number of Amp
hrs obtained discharging over 20 hrs to a final voltage of 10.5.
The deeper you discharge nearly any lead acid battery the fewer total
discharge-recharge cycles you get out of it before capacity begins to decli=
ne
rapidly. Deep cycle batteries are built to handle this much better than sta=
rting
batteries. Spec sheets at the web sites of Yuasa or other mfgrs have graphs=
of
number of cycles vs. proportion of capacity discharged. Gel cells are often=
rated as better than AGM in this respect, *but* they don't tolerate high ch=
arge
rates.
You want to extract ca 72 Ahr. An impractically conservative value (favorin=
g
long total battery life) would be to discharge to 50% of capacity, but thi=
s
would entail carrying 70-80 lbs of batteries. With lead acid to get the wei=
ght
down, you have to substantially shorten the life of the batteries. More =
expensive batteries with higher mass energy density and different response=
to deep discharge because of different chemistry are appealing if it is al=
l on
your back.
bill r.
>
> I'm cheapie at heart. Maybe to a fault! "240 Cranking Amps" is what
> they say on top. They're about 9"X 6" X7", come with carrying handle,
> cost $30 each and you are absolutely right, they can be spilled!
> Many times I've toted one for miles inside of a backpack wrapped
> inside of an 1/8" thick rubber sheet. Never spilled a drop so far but
> I wouldn't hike on loose gravel either. Gell cells would be
> fantastic and much safer. How many cells/packs would one need to pull
> down 3 amps at 12vdc for say 24 hours? How about bulk and weight?
> Cost? Too often I prove that my motto, "You never have enough power
> and disk space." is correct. I can stop at an AC outlet to put a 50
> amp quick charge on a garden tractor battery and be 95% sure I'm
> heading out with enough power to get me through two days if needed
> (no powering external hard drive though). Rob D.
>
> At 8:39 AM +0000 11/2/05, werainey wrote:
> >Rob,
> >
> >Is the 240AHr a typo? That would be a big lead acid battery. After
> >inconvenient accidents with tipping and draining of wet cell garden tra=
ctor
> >batteries, I'd suggest the more expensive option of ganging equivalent
> >capacity of smaller gel cell or AGM batteries. No orientation
> >issues, easier to
> >carry, more tolerance for deep discharge if the right models are
> >chosen/available..
> >Bill R.
> >> One thot here: Be prepared with more than one "solution" to the power
> >> challenges. For your fixed site recordings, I'd consider one or two
> >> "small" ~240AH 12 volt garden tractor size lead acid batteries.
> >> They'll run a 722 for days, but best of all, they can be fast-charged
> >> with car battery chargers you're likely to find even in remote places
> >> or with a small 1-2 amp trickle charger you can carry with you. The
> >> Sound Devices site has considerable discussion about power options
> >> too. For portability with long duration ability too, I recently added
> >> two more cells to one of my D-Cell NiMH packs and the MAHA charger
> >> seems happy charging it (while on AC!). The 722/744 runs much longer
> >> on 12 cells (than 10) because of the large voltage drop to 10.8
> >> volts, the cut-off point.
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