Can I run 2 12v batteries in sequence for 12v motor?

[USA_boater]

I say I wanted to motor to last longer on a charge, can I buy a duplicate 12v battery and increase the run time of a 12v TM? Or is it best to just get a larger reserve single battery (bang for the buck)???

 

[Scott Danforth]

you run them parallel. its done all the time. you can install 100 of the same batteries in parallel and have about 100X the reserve power if you have the room.

in series they would be 24 volt.

 

[USA_boater]

Well so far so good but I think when the current batter gives up I'll go with a larger series...BUT there is only enough room to either go with a larger battery OR keep same size and add a second TM batt. I have a 2-bank onboard charger I added so I may as well stick to one battery unless I get desperate enough. I do not recall but I want to say the TM batt that came from factory was a 27 series...I could prob fit a group 31 or 34 in there if I continue to run only 1 TM battery and still increase capacity/longevity of the TM charge? To do two batteries I may have to go down to dual group 24's to fit but I'd have to measure it.

 

[harringtondav]

Two parallel batteries will last twice as long a single battery. But they'll go dead at the same time. Two separate batteries with a switch will last the same time, and give you a 1/2 empty warning.

 

[fishrdan]

Two parallel batteries will last twice as long a single battery.

Not being nit-picky, but just providing information. 2 parallel batteries will last "marginally" longer than 2 separate batteries drained individually. The slower a battery is discharged, the more AH it can deliver. I wouldn't expect a "LOT" more run time of the trolling motor, but it can provide more run time than draining the batteries individually.

Of course then comes the problem of running parallel batteries, as they will stabilize to the weakest battery (which could equal running them individually). And, 1 bad parallel battery can ruin the other, and ruin a fishing trip...

I have a battery switch on my jon boat, so I can run the TM batteries parallel, or single. If the batts are new or in the same condition I'll run them parallel. If I know one of the batteries is weak, I run them separate.

 

[harringtondav]

Not being nit-picky, but just providing information. 2 parallel batteries will last "marginally" longer than 2 separate batteries drained individually.

It's cool. When I wrote my comment I sensed a ringer somewhere. Normally when less that certain, I use a weasel word like "could" vs. "will". Thanks for the correction.

 

[gm280]

Just remember, if anything happens to either one of the parallel batteries, it takes out both. I would only use parallel battery setup with huge steering diodes for protection reasons.And such huge current diodes would be expensive at best. I would simple use a switch and use one or the other for a safety reason as well. But that is just me...

 

[fishrdan]

It's cool. When I wrote my comment I sensed a ringer somewhere. Normally when less that certain, I use a weasel word like "could" vs. "will". Thanks for the correction.

Not really a correction, just added information, as you are correct that 2 parallel batteries will run 2x a single.

That's a generic chart I posted just to show the discharge rate (wrong size of battery), but apply that to marine batteries and it could add 10+- AH of power to each battery, maybe 10%. Not earth shattering, but that 10% could be useful at the end of the day.

Agree with GM280 on the battery switch, to separate the batteries when needed. I killed a good battery last year with a bad one, while connected parallel, forgot to flip the switch......

 

[USA_boater]

Well I am lazy so I like the idea of a switch to just add a second battery when the first runs down. That is what I have in my run about although the switch is just for going to second battery if radio and starting runs down the first. Not sure the parallel risk is worth if for someone like me who'd never take the time do the diodes thing; but I never even though about a second TM battery with a switch like my bigger boat has for a different purpose! Thanks for the idea!

EDIT: so on my bigger boat, is it best to only let the alternator charge one at a time when running or is it better to let juice flow to both batteries and then switch to just one when the motor is off to run the radio...It has a Perko switch. I generally charge before a lake trip and then run all on one battery and only switch to the second if running on one uses too much juice despite the alternator charge while the motors is running.

 

[The Force power]

Just a note; no matter what battery is charged the most, the current from that battery will bleed into the lesser charged battery when they are connected/switched together regardless if the motor is producing power to both of them.
(they will equalize/level out)

Depending on what rectifier your motor has; it is always better to let the current that the motor produces to flow in both batteries at once instead of "charging" them individual a specially when it's not regulator (older motors)

 

[fishrdan]

EDIT: so on my bigger boat, is it best to only let the alternator charge one at a time when running or is it better to let juice flow to both batteries and then switch to just one when the motor is off to run the radio...It has a Perko switch. I generally charge before a lake trip and then run all on one battery and only switch to the second if running on one uses too much juice despite the alternator charge while the motors is running.

What type of engine in your "bigger boat"? If it's an I/O, the engine's alternator should have plenty of output to charge 2 batteries, so combine the batteries when the engine is running.

I charge 4 trolling motor batteries on the water with a 140HP Mercruser fitted with a 105amp alternator. Takes several hours charging that many batteries.

 

[USA_boater]

The larger boat is a 4.3GL Volvo Penta I/O...If I was to leave the switch charging both batteries while the motor was running, it seems like a lot of hassle to then have to open the hatch and turn the switch back to a single bank to run the radio every time we stop. Obviously the purpose of having 2 batteries is to be able to switch over to the other if one has been run down too much. If I leave the switch to run off both batteries while the motor is off, then I have the remote chance I could be stranded if we run them both down.

Keeping that in mind, where you suggesting I run on both banks when the motor was running and then switch to a single bank when stopped? Or did you mean leave it on both banks all the time?

Thanks