"Evgenij Barsukov" wrote in message
...
I am looking to buy a deep-cycle battery to run a 55 lb trolling
motor (45A max/12V) which will be used as the only motor for an
inflatable boat for fishing.

Any idea how long you would need to run the motor at full thrust?


I am converging so far on Optima D34/78 (gives you 120 min reserve
at 25A, so it should run motor for 1 hrs at highest power).

No, not likely.

Forget the "reserve" rating, it is meaningless for this discussion. What
you need is the amp-hour rating, which on this battery is 55 amp-hours.

Now, here is the rub: The amp-hour rating is specified at a low draw rate,
typically a 20 hour rate. Thus, this battery would be able to provide 2.75
amps for 20 hours. The available amp-hours drops dramatically as the
discharge rate goes up. I suspect that at a 45 amp rate you would probably
get around 1/2 of the 20 hour rate, or only 27.5 am-hours, which would
provide you with around 36 minutes of operation.

You may also find that as the battery voltage drops, the current the motor
draws will increase. This could seriously degrade your run time even
further.


It can be had for 144 $

Seems like a fair price.

- Can I get a better battery or similar one for
better price?

Define "better".

As another person said, the best price/performance will be obtained from a
pair of golf cart batteries, referred to as either T-105 or 2200. Here is
a link to Trojan: http://www.trojanbattery.com/GolfDC.asp?Product=51

I have bought a number of the 2200 series batteries that I use in a deep
cycle application. These things are rugged, you can expect 6 years or more
of life from them. I pay around $55 each for them.

You need two to get 12 volts, but you will have 220 amp-hours of battery
when you are done. Since your battery is so much larger, the 45 amp draw
will not represent nearly as large of a load percentage wise and the impact
on discharge rate won't be quite so bad. So, for less money ($110) you get
four times the battery capacity compared to the Optima.

The draw back is that these batteries weigh over 60 pounds apiece, so you
will be adding 120 pounds to your little boat. They also need to be kept
upright or they will spill. The spill factor is a major advantage of the
Optima battery.

- Is the battery sized properly for this motor/usage (its my first one,
so I dont know).

It all depends on how much run time you need/expect.

Rod