Electrical question. NoFear, feel free to jump in

[MississippiBoy]

I have a 24v trolling motor on the new boat. The deep cycle batteries for it are pretty old, but one of them still holds a charge, while the other one doesn't. I know the "good" one is probably not in the best condition, but it's still useable. My question is, if I get one new battery and hook it up in series with the old one, am I running the risk of damaging it, shortening its life, or draining it too fast?
If your advice consists of "Don't risk it, just get two new batteries and be done with it," that's fine, I just want to know if using a new battery and an old battery in the same circuit will cause problems.

[Jim80]

I don't think it will we have a few deep cycle batteries at the shop that we use for lifts and what not , any way one of the lifts has two batteries in it run in series and we only replace on battery at a time not that that happens often but every few years . It has been running like that for at least a year and a half that i remember and they are deep cycle batteries . Not exactly what you asked but I hope it helps


Jim

[bcklash]

I think a good battery and a not so good battery shortens the life of both batteries.

[bassboogieman]

I think a good battery and a not so good battery shortens the life of both batteries.

Agree. Everything I've heard says when it's time to replace the t/m batteries - it's all of them. And that's what I've done a couple times. Wet cell deep cycle batteries are not that expensive, but if you can afford it, check AGM batteries. They are more expensive but far superior to a wet cell.

[nofearengineer]

Unless there is something wrong with one of the batteries such as a dead short between the terminals, there is no harm that one can do to the other in a series connection. In a parallel, the same can be said as long as they are at the same voltage when initially connected.

In series, the problem is, the good battery will tend to keep the combination working past the point where the weaker battery is deeply discharged, which will often push it over the edge into obliviion. However, the weaker battery can not hurt the stronger battery. This is an old wives' tale, probably cooked up by a battery salesman.

However, don't let me get in the way of you paying tribute to the Monkey God. Fresh batteries are always a good thing. I wouldn't mind a nice new set of Optima D27M's for my boat.

[Abbeysdad]

When there are two batteries in series you're creating a larger electrical storage bank. If one battery is less capable than another, it only means that the bank is not as powerful as would be the case, say, with two more powerful batteries. As Nofear says, no way one can harm another unless there is a short.
However, I'm not sure about the data that a stronger battery will push a weaker battery to failure - in series, I believe a balancing occurs unless you push the discharge to below 20% total remaining.
In this case, with the two wired in series, I'd hook the TM to the newest, stronger battery.

So unless you have extra cash burnin a hole in your pocket, I'd suggest trading in the bad battery on a new one and play it out. If it works for you, keep going and replace the other older battery next season (if you feel the need). If it doesn't work, then replace the other old battery too.

Hmm, two TM batteries in series (monkey see, monkey do?)

[MississippiBoy]

Originally Posted by Abbeysdad

In this case, with the two wired in series, I'd hook the TM to the newest, stronger battery.

If the batteries are in series, the T/M is getting power from both of them at the same time...24v, instead of 12v. So it's hooked to both of them.

Originally Posted by Abbeysdad

Hmm, two TM batteries in series (monkey see, monkey do?)

Just make sure you have a 24v T/M before you do it. I don't know exactly what would happen if you ran 24v to a 12v motor, but I betcha it wouldn't be good...

[MississippiBoy]

Originally Posted by nofearengineer

Unless there is something wrong with one of the batteries such as a dead short between the terminals, there is no harm that one can do to the other in a series connection. In a parallel, the same can be said as long as they are at the same voltage when initially connected.

Ok, that was the way I was leaning. I didn't THINK any harm could come from using an old maybe-not-so-good battery with a new "perfect" battery, but I didn't want to spend the money then screw something up and end up forking over more cash for my stupidity. I think I'll try it with the old/new combination and see what happens. Thanks for the advice.

[Abbeysdad]

Originally Posted by MississippiBoy

If the batteries are in series, the T/M is getting power from both of them at the same time...24v, instead of 12v. So it's hooked to both of them.

Actually the positive lead from the TM is on + side of battery 1 and the negative lead is on - side of battery 2, so I was saying hook the TM + lead to the best of the two batteries.

Originally Posted by MississippiBoy

Just make sure you have a 24v T/M before you do it. I don't know exactly what would happen if you ran 24v to a 12v motor, but I betcha it wouldn't be good...

Actually in my case (12v 54# MinnKota) I was toying with two batteries wired in parallel, not series, but so far I've never depleted my big max more than 20-25% (75-80% charge remaining) so it would be overkill. I've only used it a couple of times for true serious trolling...most times I just use it to compensate for drift and lately I've been thinking I might better just get the anchor out

[nofearengineer]

Originally Posted by Abbeysdad

Actually the positive lead from the TM is on + side of battery 1 and the negative lead is on - side of battery 2, so I was saying hook the TM + lead to the best of the two batteries.

Abbey....just so you know, in a series connection, it won't matter which battery is "on top". It still just acts like one big battery.

[nmanley]

Originally Posted by nofearengineer

Abbey....just so you know, in a series connection, it won't matter which battery is "on top". It still just acts like one big battery.

Correct!!! It's all about Cell count and not all cells in each battery are equal. Like NFE said before only the weaker battery 's capacity may be pulled too low. If you have a digital pulse trolling motor even that is not an issue if you charge each night.

[Abbeysdad]

Originally Posted by nofearengineer

Abbey....just so you know, in a series connection, it won't matter which battery is "on top". It still just acts like one big battery.

Yes overall power consumption, but it seems to me that by connecting to the + TM wire to the + post on battery A (neg wire to - post on battery B), the 24v power draw from the TM is thru battery A

[nofearengineer]

Abbey, without getting too ridiculous here...

For DC systems, Power = Voltage X Current

I think you will agree that the same current will pass through both batteries, as they are in series. Any current that leaves the positive terminal has to re-enter the negative terminal.

However, the voltage across each battery might be slightly different, though they will both add to 24V. Example...12.1V + 11.9V = 24V. This will result in one battery providing slightly more power to the TM than the other. All is still okay.

Here comes the problem.

Assuming the batteries are similar, the voltage will drop at the same rate.

A lead-acid battery doesn't like to be discharged below 10.5V. And your TM might have a performance curve such that it won't run any more once the voltage drops to 21V (10.5V + 10.5V). But you might actually have discharged your batteries down to 10.6V and 10.4V...the TM has no way of knowing that one of the batteries has now slipped into the "permanent damage" range.

[nmanley]

I had this POUNDED into me during my 1st Semester of Electronics back in 1982. Oh the memories.

[nofearengineer]

Ooooooooh.....visual aids!!!!

That and good ol' ELI the ICE man, right?

[Abbeysdad]

Thanks for the clarification. I didn't know that discharging a battery in series below a certain voltage would result in 'permanent damage'.
I've seen many a dead battery recharged and go on to live a respectable life (sometimes in a better neighborhood ). I guess I'm glad just to have a 12v TM.
So I guess the advice here should be to install two new batteries, even if the one is still 'good'.

[nofearengineer]

Yes, Abbey, it can be done.

The lead-lead sulfate reversible reaction is how the battery operates. If you drain it completely, it can start producing lead sulfite (I think), and that reaction is not reversible.

[MississippiBoy]

Another question, on the same note...the guy I bought the boat from put an old battery out of his truck in the boat, beside the starting battery. It's not hooked to anything, it's just sitting in there. It took a charge the other day, so it might still be ok. I was thinking about hooking it up in parallel with the starting battery, which will still give me 12v, I'll just have a bigger "tank" of power. Of course, the CD player, live wells, fish finders, lights, etc all run off of this battery, and the big motor charges it while running. I'm thinking "Why not? It's already in there, might as well use it for something."
Now, if I do this, will the motor charge both batteries, or just the first one? Or will it charge the first one first, then start on the second one?
Thoughts?

[nofearengineer]

If you connect the batteries in parallel, with no switches, relays, etc. in between them, you essentially have a larger 12V battery.

The outboards alternator will charge both simultaneously, with the rate depending on the internal resistance of each battery. For instance, the stronger battery may provide a little more power when cranking. That battery would also get more of the alternator current when charging.

You need to make sure the charging amp rating of the batteries doesn't exceed the amp rating of the alternator though. When you hook them in parallel, the total resistance goes way down. If you overload the alternator, you will be an unhappy camper.

For instance...a battery with 1 Ohm internal resistance (it's actually much lower, but for sake of demonstration), in series with another identical battery will have a total resistance of 2 Ohms.

But in parallel, the two batteries will have a total resistance of 1/2 Ohm. This can cause the alternator to deliver far more amps than it is designed to handle. Smoke (and usually swearing) soon follows.

Also, all of your other equipment will also be running off of both batteries, so if you're not careful, you could possibly find yourself in a situation where you don't have enough juice left to crank the motor. Keep it in mind.

[MississippiBoy]

Originally Posted by nofearengineer

If you connect the batteries in parallel, with no switches, relays, etc. in between them, you essentially have a larger 12V battery.

The outboards alternator will charge both simultaneously, with the rate depending on the internal resistance of each battery. For instance, the stronger battery may provide a little more power when cranking. That battery would also get more of the alternator current when charging.

Ok, I'm with you there.

You need to make sure the charging amp rating of the batteries doesn't exceed the amp rating of the alternator though. When you hook them in parallel, the total resistance goes way down. If you overload the alternator, you will be an unhappy camper.

For instance...a battery with 1 Ohm internal resistance (it's actually much lower, but for sake of demonstration), in series with another identical battery will have a total resistance of 2 Ohms.

But in parallel, the two batteries will have a total resistance of 1/2 Ohm. This can cause the alternator to deliver far more amps than it is designed to handle. Smoke (and usually swearing) soon follows.

Sounds like fun...I do have an ampere gauge on my instrument cluster. Normally, it's around 13-14 amps while running the big motor. Can I just keep an eye on that to see what the alternator's putting out?

Also, all of your other equipment will also be running off of both batteries, so if you're not careful, you could possibly find yourself in a situation where you don't have enough juice left to crank the motor. Keep it in mind.

Ok, but there's no more danger of that than there is now, without the second cranking battery being connected. In fact, there will be less, since I'll have, as you said, just a bigger 12v battery. It'll do the same amount of work, it'll just be able to do it for longer.

I have to type here for the message to be long enough...

[nofearengineer]

Originally Posted by MississippiBoy

Ok, but there's no more danger of that than there is now, without the second cranking battery being connected. In fact, there will be less, since I'll have, as you said, just a bigger 12v battery. It'll do the same amount of work, it'll just be able to do it for longer.

You're missing something here. If the batteries aren't connected, there's NO CHANCE your other gear will drain the cranking battery. True, the parallel combination will give more maximum cranking amps, but it also gives your other gear access to your cranking battery. Assuming you don't use a lot of power from your "house battery", in most cases you are correct, it is a good thing to connect them. But like I said, you should keep it in mind.

As for the gauge...it sounds like it is measuring your alternator current. Find the specs for your motor and see what the alternator is rated for.

[bamabassman]

daniel, don't hook the troller to your starting battery man. IF oyu ever run down from trolling all day, you won't have enough power to start the motor. just my lousy 2 cents.

[MississippiBoy]

Originally Posted by nofearengineer

You're missing something here. If the batteries aren't connected, there's NO CHANCE your other gear will drain the cranking battery. True, the parallel combination will give more maximum cranking amps, but it also gives your other gear access to your cranking battery. Assuming you don't use a lot of power from your "house battery", in most cases you are correct, it is a good thing to connect them. But like I said, you should keep it in mind.

As for the gauge...it sounds like it is measuring your alternator current. Find the specs for your motor and see what the alternator is rated for.

Let me back up to the beginning. We're both missing something.

There are 4 batteries physically in my boat now:

2 are deep cycle, connected to the trolling motor, and that's it...they're strictly T/M. They each have one red cable and one black cable, running up to the trolling motor. Nothing else is in that circuit.

1 of them is used to crank the big motor, and it also supplies power to the depth finders, CD player, bilge pump, lights, etc. All the accessories are powered by this one battery.

The last one isn't connected to anything at all. Like I said, the previous owner just set it in there when he took it out of his truck. I can understand his thought process, too...in case the other battery dies completely, he could swap the cables and pray it has enough juice to start the motor to get home.

I'm talking about connecting this "naked" battery in parallel to the starting/accessories battery, and not even mess with the T/M batteries. I want to leave them dedicated to the trolling motor and nothing else. If that battery wasn't in there already, I wouldn't go out and get another one to hook up, I'd just leave well enough alone. But since it's already in there, I might as well get some use out of it.

[MississippiBoy]

Originally Posted by bamabassman

daniel, don't hook the troller to your starting battery man. IF oyu ever run down from trolling all day, you won't have enough power to start the motor. just my lousy 2 cents.

Definitely not. In my old boat, there was only one battery, and it did everything. Cranking, trolling motor, lights, bilge pump, the whole nine yards. Once or twice, I had to crank the motor with the pull cord after running the T/M for too long. I'm not really wanting that to happen with the new boat...

[nofearengineer]

Okay, Daniel....I get you now. You're just wanting to add some extra oomph to your cranking/house battery. Yes, that all sounds good, except for the alternator part I mentioned.