Battery Questions

You would likely be money/energy ahead buying a very small gas powered trolling motor or change to fly fishing/spin casting.
http://www.boats.net/outboard_motor/Honda/2HP/parts.html

To figure how long a battery will last you will use the following formula:
Battery amp hours X .85% divided by motor amp draw = approx. running time.


I agree, they mean 85% not .85% BUT I don't know what they mean by running time. Minutes? 34 minutes seems a bit low for a 40AH battery drawing 8 amps. but ???

I don't think they mean 85% discharge which will mortally wound a lead acid battery which for practical purposes should not be discharged more than 50%. And that is for a deep cycle or trolling battery. A starting battery will not do well with repeated deep discharges. Essentially that means that a deep cycle battery of 40AH capacity should not have more than 20 Amp hours taken from it.


So: At position 1, drawing 8 Amps, your 40 AH battery should theoretically give you 2.5 hours until it reaches 50%. It will run the motor longer but at the cost of battery lifetime.

Here it gets a bit complicated. There are various ways to rate Amp hours, which is just Amps drawn for H number of hours. ie 8 Amps over 5 Hours is 20 Amp Hours.

BUT (I told you it gets complicated) The lower the current (amps) drawn over a longer time results in more amp hours. Higher current draw will give you less amp hours available, even if the basic A times H math works.

The most common rating of AH is taken over 20 hours of current draw until the battery is DEAD. For your battery, that would be 4 Amps for 10 Hours = 40 AH

BUT you will be drawing more than double that current so you cannot expect to pull 40 AH from that battery. (And remember you should NEVER pull more than 50% or 20 AH anyway)

Yours is listed as a "dual purpose" which is to say the very commonly though not very accurately labeled "Marine Deep Cycle" battery. If you keep above the 50% mark it should do you just fine. How long? You will have to experiment.

So what is 50%? Roughly 12.1 to 12.2 volts with the battery at rest, ie no load or charging happening, and has been resting for a while. An inexpensive multimeter is needed to check this. What is the real AH capacity of your battery at 8A current draw? Less than 40 for sure, but not a huge lot less. (I don't have the math for that calculation.)

Since batteries don't like too long periods of heavy current draw, as someone mentioned earlier, not more than C20 (20% of AH rating) 8 amps in your case is right on the line for sustained draw. You can exceed it for short periods. (I'll pull 150 amps from my 480 AH of battery running the microwave via the inverter but just for few minutes at a time.)

Make sure you FULLY recharge the battery after each use as soon as possible. Storing a discharged lead acid battery will kill it. Note you shouldn't recharge really at more than C20 either but you can get away with it for a while.

How long will the battery last for you? You'll just have to experiment a bit. I would start with at an assumption of the motor maker's suggestion of about half an hour (85%AH or 85% of 40=34) and hope you'll do better. You might get a couple of hours out of it.

Enjoy.

Addendum: I have an electric bike with a 350 Watt 36V electric motor, drawing from a Lithium Ion battery pack and on the flat I can run about over an hour. If I can find the specs I'll post them here later.

I'd keep paddling. Better for you in the long run.

Hi,

I think an LI battery pack intended for bikes might be considered as an alternative to AGM. What ever battery bank is decided on, I would want it sealed for use in a marine situation.

Once you have arrived will there be a way to recharge the battery bank?

Ok, so I said I can do the math too. However, experience shows you rarely run it constantly. I cannot imagine a steady two hour run.

You can also use a couple of UPS batteries rated at 20 AH each. Run them in parallel. These are used in our mobility scooter. It is amazing how long it will run.

Just my four cents of course.

OP here, thanks everyone for your help. I have the bracket made for the kayak and I am going to try this battery. My thinking is this, I don't mind paddleing, the rivers we fish are what I would call pool and drop. There are class 1 and a few class 2 rapids with pools of water in between. Most of the pools are short with a little current flowing so we just drift along and fish (we use spinning rods, we do not troll). Some of the pools are longer and the current will not pull you along so paddleing is required, this is where I would be using the trolling motor, so 10 minutes at a time tops. If the battery goes dead, I would just have to finish up the trip with the paddle.

As far as turning over, I have done that one time in the last five years and it was my fault. I tried to paddle around a downed tree in a rapid and went into the tree and flipped (I now portage around those).

Another thought about the life of the battery. If I pull the battery down below 50% and shorten the life of the battery, that is no big deal to me. 8 trips on a $50 battery is $6.25 per trip. My fishing lures are $5 yo $8 each and I normally lose 2 or 3 on each trip so a new battery every 8 trips would not be an issue.

Thanks again everyone for your help.

I'm going to follow this thread because I'd really like to see how you make out and what sort of run time you actually get. If you never get the battery below 50%, recharge promptly and correctly and never let the electrolyte get below the top of the plates that battery should last for years. (If you do treat the battery like that you will be doing better than a lot of RVers. :B

If it works well you might want to consider a couple of 20AH AGM batteries. Lots more money but can be used in any orientation even up side down. And the do deep discharge really well.

Keep us posted. And let us know how the fishing was!

Canadian Rainbirds wrote:
I'm going to follow this thread because I'd really like to see how you make out and what sort of run time you actually get. If you never get the battery below 50%, recharge promptly and correctly and never let the electrolyte get below the top of the plates that battery should last for years. (If you do treat the battery like that you will be doing better than a lot of RVers. :B

If it works well you might want to consider a couple of 20AH AGM batteries. Lots more money but can be used in any orientation even up side down. And the do deep discharge really well.

Keep us posted. And let us know how the fishing was!

It may be a few weeks. The forcast high for Monday is 17 degrees with a low Monday night of 7 degrees. This is Arkansas, I thought we lived in the South.

Here is a decent Smallmouth from last summer.

Boy I hate to say it, but this is one application that almost screams for a decent absorbed glass mat battery. Here's why:

When my flooded lead acid battery tipped over on my in my Sevylor I did not notice it. But everything the acid touched noticed it a lot. A lot of stuff got ruined.

Trolling motors LOVE the higher overall voltage range of an AGM battery. That three to four tenths of a volt can be felt in pushing power.

An AGM battery does not get damaged nearly as much as a flooded battery if it is totally and completely discharged. The cycle life increase at 90% discharged is five to ten times as many cycles. Not good for the battery but a lot more utility when you need to suck the battery down a lot. What good is a high cycle count when a battery only gets a person halfway through a trip? Yes, lithium would be best and I'll bet the color match would be perfect for the trunk of a Rolls Royce.

A 60-70, 20-amp hour rate AGM will simply far-and-away outlive and outperform any flooded battery in this situation. The battery will store better in the off-season as well. Yeah yeah yeah, the AGM is pricey. But I'll bet a dollar to a doughnut an AGM will be far cheaper to own over a 5 year period, and oh yeah, it will do the work asked of it. There is a time and place for every style of battery and this one is tailor made for an AGM. Let's forget about lithium.

MEXICOWANDERER is THE battery guy here. I can never do other than agree with him when it comes to the care and feeding of batteries.