Batteries, trolling motors and bad math?

[Mr.J.]

Ok here's how I see it.

Battery is 130 minutes reserve at 75 amp hours

Trolling motor max amp draw is 38.

So my math tells me at full throttle I can run at 260 minutes or at half throttle I can run for 520 minutes.

Correct or bad thinking?

Oh I am running it on a 16 foot canoe.

[jsdx]

Amp hours and reserve minutes are two ways for computing capacity.

Amp hours are measured over 20 hours so 75 Amp hours = 3.7 amps per hour for 20 hours....

For your 38 amp draw that mean 75/38*60 = 116 minutes full out. At 1/2 speed, 236 minutes.

The reserve how long a batter will run at 25 amps draw. This means if you were running your motor at 2/3 of the 38 amps (25 amps) you'd get 130 minutes...

We'd get a full day on an electric in a 14 footer but we don't run the thing full time. Usually just to position the boat and move from spot to spot...

[MichaelVandenberg]

Excellent post guys. Good description of reserve minutes and amp hours.

Cheers,

Mike

[Mr.J.]

I fired off my math to Minn_Kota and this is their reply.

No, that is not correct. You are not reading the information correctly.
What that battery says it has is 120 minutes reserve and 70 amp hours.
When it says 120 minutes reserve, it means that if you have a device
that draws 25 amps, it will power it for 120 minutes. You can take that
to mean that there are 50 usable amp hours in that battery. Running
with the Endura 38, you can expect around 1.32 hours running time at
full speed.

Dave h.

[MichaelVandenberg]

Got this from a web site

Amp Hours

The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

Reserve Minutes

Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)

Simply math using reserve minutes:

Battery will last 120 min (reserve) @ 25 Amps
3000 min (50 hour) @ 1 Amps
78.94 min (1.32 hour) @ 38 Amps

Simlpy math using amp hours:

Battery will last 75 hours @ 1 amps
118 minutes (1.97 hour) @ 38 amps

I am not certain but I think the Amp hours are for low current draw and Reserve minutes is for high current draw. Keep in mind the behaviour of a battery will not be the same @ 3.8 amps as it is @ 38 amps. Probably not a linear relationship and hence the difference between amp hours and reserve minutes.

Therefore, since trolling motors typically have a high current draw you want a deep cycle battery with a high battery reserve. High amp hour would also be good but chances are if the battery reserve is high so is the amp hours.

Cheers,

Mike

[Seaweed]

Simplest Math for the rest of us is:

Electric Motor will Run as long as the battery has something left in it... then I have to recharge it.

Sorry the rest of it just hurt my brain

[fishhit]

Seaweed,
Funny as crap that is.! Gotta agree totally.! Too much crap to remember. What is an amp anyway! Who drew what and where did they draw it! Amp hours are for Imp whores! When she's fully charged you run it till she's not and then you run out of juice. The ability of a battery to hold a charge and then the rate it discharges at is relative to the state of the batttery and its condition at the time of its use. A properly charged unit at the peak of its condition will greatly outlast an old battery that hasen't been maintained properly.I.E. low distilled water level and improper discharge and recharge cycling. Voltage readings will not indicate the general condition of a battery nor will they give an indication of the time life of that battery.

Fishhit
There's a reason they call it fishin' and not catchin'!

[Stealth1Technology]

There are things about batteries that most people don't ever take the time to learn until they really need too. Then it is to late because they are usually dead in the water. First I will go on record saying voltage will tell the tail of the battery condition and the time left in the batteries cycle for use. The key is to know what the voltage is doing and understanding the numbers your looking at. Stealth has been doing this for 4 years now for this very reason most people don't know. So we are trying to change that. No one likes to mess with those heavy, acid filled, destroying things (Yes even me). So we brought a smart maintenance system to help the consumer to take care of their batteries. Put a gauge that reads 9 to 44 volts with low and high voltage alarms and monitors the trolling motor batteries at all times. Education, awesome technology, and service at it's best was and is the goals. Charging from any power source AC 110, 40 amps Towing the boat, 40 amps Running the engine, Solar, and Wind, And do it with technology that would enhance the physics of the battery even extend battery life 2 to 3 times. 12.65 VDC is a full state of charge or above. So 12.65 on 12 VDC… 25.3 VDC on 24… or 37.9 VDC on 36 this in a voltmeter tells the tail. The key is to know this, and being able to see it. Then to understand that the volts will fall after the charging has been disengaged identifies a battery condition. (The time that the surface charge will dissipate if it is not consumed). All of this is what Stealth I has made easy by giving you the tools to charge them as you go. Check out the technology it has been honored by various organizations. Such as Innovation winner from the National Marine Manufactures Association at Boat Builders in 2002, Best in Show at the BassMasters Classic. The latest being The Oak Ridge National Laboratory Spirit of Innovation Technology. You can read about it in this months (May 04) BassMasters Magazine page 12. It’s on power provision with Kevin VanDam and Larry Nixon or you can checkout the web site. And if you have any questions I will be glad to assist you.

Danny