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Amp draw #5196237 08/25/19 01:36 PM
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mjk Offline OP
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70lb Powerdrive. On medium to low speed, what is the amp draw ? Also, at same speed, would you get about 3 hrs on a calm day, using medium as the max setting

Thx

Re: Amp draw [Re: mjk] #5196252 08/25/19 02:00 PM
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Rand Offline
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a minn-kota 70lb thrust trolling motor pulls a max of 42amps at 24 volts.

This story is incomplete. this is like saying can I go 500miles on my gas tank.

Without listing some kind of battery(s) a good answer isnt possible.


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Re: Amp draw [Re: mjk] #5196312 08/25/19 03:31 PM
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Quattro Pete Offline
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At full throttle, with a 105 Ah battery, you'd get 2.5 hours of runtime at best.
If you assume that medium speed is half the amp draw (21), then you should get about 5 hours of runtime out of the same battery.


2002 530i   2015 Q5 3.0T   2018 Charger SRT
Re: Amp draw [Re: mjk] #5196345 08/25/19 04:19 PM
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wrcsixeight Offline
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A fully charged and healthy 100 amp hour battery can provide 5 amps for 20 hours before voltage falls to 10.5 volts, which is considered 100% discharged. This 20 hour rate is a standard. a 90 amp hour battery can provide 4.5 amps for 20 hours. there are other rates some batteries publish instead of the standard 20 hr rate.

Yes one can drain a battery further down than 10.5v, but there is little energy stored once its voltage drops bbelow 10.5v

The same fully charged 100 amp hour healthy fully charged battery, however cannot provide 20 amps for 5 hours.

The peukert effect basically says the bigger the load on the battery, the less overall capacity the battery has to give.

Different batteries will have different their peukert numbers
All lead acid batteries numbers will change as they age.

The peukert calculator linked below says a 100Ah battery with a peukert number of 1.15 under a 40 amp load, will effectively be a 57.5 amp hour battery.

Not many batteries publish their peukert number, but Odyssey AGM does, on some of them, and 1.12 is, if I remember correctly, the number for their pc2150, a 100Ah group 31 battery.

To answer the 'how long' question, one would need to know the amp draw as the motor is going to be used.

Without knowing the amperage it is like saying I am going there, how long will it take? without knowing how fast one is going, it is impossible to answer.

Assumptions can be made, of course.

http://www.csgnetwork.com/batterylifecalc.html

When one gets into trying to achieve the run times expected from a battery, they will often find their battery is coming up well short of expectations, even if it and their charger are new and well marketed.

with more tools, like an ammeter and a voltmeter and a hydrometer to test specific gravity, one will see their charging source is not getting the battery fully charged and many cases will show it stopping well short of full charge.

Not only does the 100Ah battery charged to 90% only have 90AH total to give, it will degrade much faster if it is only ever recharged to 90% each time.

Each time it is only recharged to 90% it then becomes harder and will take longer to then reach a true 100% state of charge should a true 100% charge be attempted.

Lead acid batteries need to be returned to full charge often and quickly in deep cycle duty to retain their storage capacity for as long as possible.

Achieving a true full charge requires the voltage be held high enough for long enough. Almost no Automatic charging source holds the voltage high enough for long enough on a deeply cycled battery. Getting to 95% charged is good, but only half as good as a true 100% state of charge.

The Lead acid battery quickly returned to a true 100% state of charge after any level of depletion, will live a long life and perform well.

in general, taking a battery to below 50% charged is more abusive to it, and when they are discharged this low it is even more important to recharge them to full promptly and the regular 'garage' chargers become even more inadequate.


http://www.csgnetwork.com/batterylifecalc.html


Last edited by wrcsixeight; 08/25/19 04:29 PM. Reason: forgot to add link to peukert calculator
Re: Amp draw [Re: mjk] #5196513 08/25/19 07:37 PM
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mjk Offline OP
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I should have added some more information....sorry.

I think I made a mistake! In trying to save some money (trying to sell boat), I replaced my shot 27 batteries with 24 series. Yes, I had them load tested at 2 different stores.

The 24's have a RC of 130. I sparingly use the trolling motor - for example, a few times a year, and wouldn't use it for more than 2 hours at medium speed.,as I troll crank baits mostly on the river.

I have the option of returning them this week, and getting group 27 batteries. These have a RC of 175.

Thoughts ? I am not installing them till this weekend.

These are the Fleet Farm store brand - Resolute. In some Walleye forums, they get good remarks.

Last edited by mjk; 08/25/19 07:39 PM.
Re: Amp draw [Re: mjk] #5196596 08/25/19 09:06 PM
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wrcsixeight Offline
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More capacity is usually a good thing if one has the room, and can handle the extra weight.

Still, as far as how long they can power a trolling motor, one needs to know how much amperage draw that motor has, at whatever speeds they are using it at.

These linked 'wattmeters' will record:
amperage
voltage
wattage
minimum voltage
amphours
amp peak
watt peak
watthours.

While they will say 100 or 130 or 150 amp rating, these are maximum short burst ratings, not continuous
They are good for about 40 amps continuous, but the 12awg silicone insulated leads will get hot.

[Linked Image from images-na.ssl-images-amazon.com]
https://www.amazon.com/T-Trees-Mete...mp;psc=1&refRID=72TCXNGSX3K7QHPHWK0Y

There are MANY clones of this same device with differrent prices, the above is just the first example I clicked to link. they only count to 65 amp hours. They are not 100% accurate and are not accurate below 0.5 amps. They are imperfect but a million times better than guessing.

These have a source and a load side. The source is the battery the motor is the load.

When charging the charger is the source and the battery is the load.

If one removed 50 amp hours from a battery it needs no less than 52.5 amp hours returned, and likely much more than that to be returned to full charge. The best AGMs say 105 to 112% returned tin order to reach full charge. Flooded usually have a much lower charge efficiency factor. and this CEF gets worse as the battery ages.

Anyway instead of guessing, you can see exactly how much amperage the motor draws, how much amp hours it is pulling from the battery, how low the voltage falls, how much the absolute highest amperage provided is. If you then reverse this wattmeter on your charging source you can see how much amp hours are returned to the battery.

This is another way to prove that most smart chargers stop well short of true full charge. If the charger does not return ~110% or more of what was removed on a marine flooded battery it is not fully charged.

Collect actual data.

I use 45 amp anderson powerpoles as my connectors on these wattmeters and replace the 12awg with 8awg. They easily handle 40 amps continuous and a pair of them has passed 180+ amps to start my v8 engine.

They do have versions that come with 8awg leads.

https://www.ebay.com/itm/220-Amp-Watt-Meter-WITH-SPECIAL-HEAVY-AWG-WIRE-for-Solar-Panels-Wind-Turbines-O/392054284553?hash=item5b4841a109:g:XJQAAOSwzFxa33ZX

One could also hardwire an individual wattmeter into the load, and then one on the charging source wiring, rather than using connectors and just one unit..

So for ~30$ you can collect actual data. Once you realize that your charging sources are mocking you and prematurely killing your batteries, they will pay for themselves, if you restart the chargers over and over until the batteries reach closer to true full charge

Restarting automatic chargers successfully required dropping battery voltage to below 12.7 by applying a large load, then restarting charger, then removing the load that dropped voltage below 12.7v.

If you just unplug and reattach it it will see 13+ volts surface voltage, assume the battery is fully charged and goto float voltage.
Almost no charging occurs at float voltage, so one has to be smarter than their smart charger in order to get it to do its job.

I gave up on smart chargers for this reason and use an adjustable voltage power supply set to 14.7v and bring the battery to then hold it there there until I determine the battery is full, via amperage or with a hydrometer., at which point I lower voltage to float levels, or remove it completely.

One can also use a clamp on DC ammeter to determine how much amperage a motor is consuming under load. These are also wquiteuseful and about 32+$ or so, but they do not record amp hours or watt hours like the linked wattmeters above do. They are handy for other tasks though and good to have. Make sure it can do both AC and DC current. Many will say both but they mean it can read voltage ac or dc, but can only measure AC amperage, not DC.

The amperage the trolling motor draws in open air at a speed is not what it will draw when underwater pushing/pulling a boat so actual measurements in actual use are required. The wattmeters are not waterproof, but do not really get all that hot passing 40 amps continuous and could be put in a ziplock with some rubber bands.


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Re: Amp draw [Re: mjk] #5198278 08/27/19 07:09 PM
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mjk Offline OP
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Lots of good info. I just returned the group 24s, and picked up the 27s.


They say the best 2 days of having a boat is the day of purchase and the day it sells. Couldn't be truer, unless you have the ability to use it more than a handful of times a year.

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