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wiring trolling motor batteries in parallel


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14 replies to this topic

#1 triglet

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Posted March 01 2011 - 01:53 AM

Several times last year at the end of the day we found ourselves positioning the boat with the wind because the trolling motor battery was dead.  Granted we were on the water for 14 hours or so.  But several times it was so bad that the fish finder would cut off.  So it just hit me to wire 2 batteries in parallel to make it all day long?  I'm thinking i should buy two identical batteries for this?  Also when charging should i charge one battery at a time to prevent overcharging of one battery?  anyone else running this setup?

#2 Fishes in trees

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Posted March 01 2011 - 03:34 AM

Back when I had my previous boat (Buster Boats - Trophy) I ran 2 and sometimes 3 batteries wired parallel all the time.  I extends your running time.

I bought 3 identical batteries.  To solve the charging issue, I bought a 3 bank charger and just wired it up to the battery posts using wing nuts.   I left it plugged in all the time.

With the 3 bank charger I had, 2 banks were 10 amps and one was less.  I tried to vary which battery was wired to the middle bank, but it didn't seem to make much difference.  Overall the system worked great.
north of Richmond, MO

#3 Jig Man

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Posted March 01 2011 - 04:27 AM

I used to and when I charged I left them hooked up and put the charger on one battery so both charged at the same time.

#4 Ann-Marie

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Posted March 01 2011 - 09:47 AM

There is no requrement to have similar, same size or matched batteries when put in parallel, that only applies to batteries in series to get 24 or 36 volts.

The biggest killer of batteries is over discharging so increasing capacity with another in parallel will not only extend running time but battery life.

#5 TimJ

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Posted March 01 2011 - 03:48 PM

yes you should buy 2 identical batteries if starting a new parallel system. any multiple items in parallel have the same voltage across them. (batteries in series do NOT have to match as your total voltage to the motor is equal to the sum of the battery voltages).
try to charge both batteries identically to maintain an ideal situation.
over charging causes battery decay. full discharging is good for deep cycle batteries as it eliminates memory problems at less than fully charged voltages AS LONG AS the battery is fully charged immediately after the full discharge occurs.
TJ

#6 StatosDadRI

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Posted March 01 2011 - 08:20 PM

[quote author (batteries in series do NOT have to match as your total voltage to the motor is equal to the sum of the battery voltages).
[/quote]

INCORRECT Batteries in Series should definetly use the same amp/hr rating, ie the SAME BATTERY Size. When the bank is being discharged, the 150 AH battery will go dead and its voltage will fall well before the 200 AH battery.
A 150 AH is a commom 24 Series and a 200 AH is a common 31 Series battery.
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#7 TimJ

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Posted March 01 2011 - 09:44 PM

I was looking at the voltages. Your AH statement is correct.
My point was that a fully charged battery may be 13v or 12.5v or 12v and still have the same AH rating.
thanks
TJ

#8 triglet

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Posted March 02 2011 - 01:42 AM

thanks for all the help guys.  it's been about 10 years since i had my electronics courses so im a little rusty and just wanted to make sure!

#9 Way2slow

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Posted March 02 2011 - 05:53 PM

TimJ, you are so far off with you facts, please get educated on batteries before posting that crap.

One, You should NEVER, NEVER, NEVER fully discharge a deep cycle battery, or almost any battery for that fact. That is very damaging to the battery. A deep cycle battery should not be discharge below 30% and will give you a lot longer service life (cycle count) if they are not discharged below 50%.

Two. Lead acid batteries DO NOT develope a memory.

Three. With series connected batteries it is very critical they be the same "identical" batteries. You should not even replace one battery in a series circuit of identical batteries that has more than twenty five cycles on it without replace all the batteries in the circuit.

For parallel connected batteries, while it's ideal they be the same, identical batteries, it's not necessary. You can parallel a 120 Ah battery with a 80 Ah battery and still have 200 Ah, without damaging either battery. Where you run into problems is when you connect on battery that will only hold a voltage of say 12.4 volts with a battery that will hold 12.8 volts. The 12.4 volt battery will keep the 12.8 volt battery pulled down to 12.4 and it will sulphate before long, giving you two batteries that will only hold 12.4 volts.

Also, no fully charge 12v battery is going to be fully charged at 12.0 volts, That's as far as you should ever discharge a battery and it's not going to have the same Ah capacity at that level as a like battery with 12.8 volts.

#10 StatosDadRI

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Posted March 02 2011 - 06:26 PM

Way2Go !!!

Voltage State of Charge
12.6+ 100%
12.5  90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.9  40%
11.75 30%
11.58 20%
11.31 10%
10.5  0%


Some good battery facts
http://www.marxrv.co...volt/12volt.htm
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#11 Way2slow

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Posted March 02 2011 - 07:00 PM

Most manufactors commonly use two basic electrolites, depending on the type battery they are making, one will show a 12.84 VDC and the other a 12.6 VDC. The electrolite used in most deep cycle flooded cell batteries will show 12.84 VDC when fully charged. A few might use the electrolite that shows 12.6 but I find it more common in other batteries.

The two will also use a different voltage chart for states of discharge.

A good, quick rule of thumb with the 12.84 is 0.1 VDC drop is a 10% drop.  That's not exact before someone gets too critical, but gives you a quick idea.

#12 TimJ

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Posted March 03 2011 - 09:11 AM

ok...I thought I was educated on batteries. apparently, I have read the wrong information over the years. how could it have been so far off? (dont answer...its all there) I would not have given what I thought was good advice if I didnt believe in it. could explain what I thought was the occasional shorter-than-normal batt life.
I have learned much here. thank you.
At least I had one issue correct...parallel batteries must be the same vtg! little solace there.
tins...nice link. thanks
TJ

#13 StatosDadRI

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Posted March 03 2011 - 10:43 AM

NP We all are in here to learn. 
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#14 Way2slow

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Posted March 03 2011 - 04:56 PM

Don't feel bad, I don't know how many times I've seen people that thought you should run a battery completely down, before charging it.  It just makes the hair stand up on my back when I see that. 

You will also have some that will say a batteries cycle count is a almost a set number so it's better to run them down to get full use out of that charge before charging it again.  Again, totally wrong.  If for example you run  a battery down completely and you get 120 cycles, if you only run it down to 30% you may get 300 cycles, and if you only take it down to 50%, you may get 500 cycles.

#15 StatosDadRI

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Posted March 03 2011 - 07:02 PM

And to add to WAY2GO if you let them stay at Dead for a couple days the 120 cycles  becomes less than 100  and the 500 becomes 300.  Charge them immediately.

These 2 topics are part of the reason why my batteries last so long.  I had a TrollBridge 24 charge on the run that  my have not charged them 100% but instead of being  30-40 % after a long day of fishing mine where 60-80% Also when towing it home it would charge from the  Truck.
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