No room for a 3rd battery!

Was thinking of buying a new boat. I have a 24v troll mtr that I use now and need because of the river currents where I fish. The new boat only has room for 2 batteries. It's a 2006 duracrAft 1650 crappie stick with a 2006 mercury 60hp 4 stroke. Is it a bad idea, or even possible, to hook both batteries together to run my tm as a 24v(hardwired to the batteries, not using a plug) but also keep all other 12v accessories and electrical components hooked to one of the two battery. Essentially not leaving a battery separate for just cranking and other electrical needs.

Unless someone is familiar with this boat design and give me an idea how to get 3 batteries in there so I can have completely different power sources for the tm and cranking.

Many thanks

Re: No room for a 3rd battery!

Using 2 batteries to run a 24 volt system and a 12 volt system is somewhat difficult, but it can be done. You will also need a 24 volt alternator instead of a 12 volt alternator or you will not receive a charge (or what I linked to below). I would suggest seeing if the trolling motor will run on 12 volts, most will. If not pick up something like this http://www.powerstream.com/dc24.htm. Chances are, your trolling motor will run on 12 volts or 24 volts.

Re: No room for a 3rd battery!

While you can run some accessories off one of the trolling batteries, I wouldn't have the outboard on a trolling battery, it's only time before you the OB won't start (forget to charge the batteries, bad battery, trolling too long, etc.) and you are stuck. I'm not sure how the boat is laid out, but there has to be somewhere to put another battery.

maproy99 said:

You will also need a 24 volt alternator instead of a 12 volt alternator or you will not receive a charge

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That 60HP outboard won't do much to charge depleted trolling batteries anyway, so it's not that much of a consideration. 15-20 amps, isn't going to tickle a discharged battery for the short time the OB will be running. If the OP is going to do this anyway, a MinnKota DC ("alternator") charger would work since they are limited to 10amps/bank, that's the most they will output/bank. I wouldn't invest in one of those unless running hard for a hour or more off the OB, so the OB is actually putting something back in the the batteries. Even at that, it's going to take for ever for the OB to charge trolling batteries, best to have an onboard charger and rely on that for charging the batteries..

Re: No room for a 3rd battery!

Bad idea -- but if it is the ONLY option you can easily do what you want to do. What you need to be careful of is WHICH battery you decide to to pull 12 volts off of. You do NOT want to use the battery that has the POSITIVE connection from the troller as the 12 volt source. While it doesn't make any difference - there is an accidental grounding issue that can destroy electrical devices. So for simplicity, the starting battery should be used to power all 12 volt stuff. You then add the second battery by connecting the POS from the starting battery to the NEG of the second battery. The troller then hooks to the POS of the second battery and the NEG of the starting battery to provide it with 24 volts. Be aware that the engine will charge ONLY the starting battery so you still need a dual bank on-board charger. Also be aware that the starting battery will discharge at a greater rate than the second battery so run time for the troller will be cut short. How short depends on the size (capacity) of the batteries and what you are running from the starting battery. Both batteries should be deep cycles. Yes -- it is ok to use a deep cycle to start your 60 HP engine.

Re: No room for a 3rd battery!

Silvertip said:

Bad idea -- but if it is the ONLY option you can easily do what you want to do.......

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Yes easily done as ST posted up, and if I were to do that I would run a big group 31 battery as the start battery and a group 24 or 27 as the second (24V) TM battery. When the smaller battery is depleted and the TM doesn't run anymore, "hopefully" you still have enough juice in the bigger battery to start the OB. Though, you are "not supposed to" use batteries of different sizes in the 24V bank. And, your run time on the TM will be reduced.

Either way, I think it's a bad idea.

Oh yeah, you don't have to put all the batteries next to each other. For my 24V bow TM I have 1 battery at the bow and the other at the transom. Heavier wire/cable needs to be used for this type of configuration.

Re: No room for a 3rd battery!

fishrdan said:

Yes easily done as ST posted up, and if I were to do that I would run a big group 31 battery as the start battery and a group 24 or 27 as the second (24V) TM battery. When the smaller battery is depleted and the TM doesn't run anymore, "hopefully" you still have enough juice in the bigger battery to start the OB. Though, you are "not supposed to" use batteries of different sizes in the 24V bank. And, your run time on the TM will be reduced.

Either way, I think it's a bad idea.

Oh yeah, you don't have to put all the batteries next to each other. For my 24V bow TM I have 1 battery at the bow and the other at the transom. Heavier wire/cable needs to be used for this type of configuration.

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Using 2 different batteries is a bad idea. The smaller capacity battery always pulls down the power in the higher capacity battery, but this will never stop. So a bigger battery is really like putting a smaller set up on then 2 equal sized batteries. Plus it is hard on the batteries. Try hooking up 1aaa battery with a c or d and play around with a multimeter each day. (Hook up to double voltage) You can see that the larger one will have less power then the smaller one after being hooked up awhile.

Re: No room for a 3rd battery!

maproy99 said:

Using 2 different batteries is a bad idea. The smaller capacity battery always pulls down the power in the higher capacity battery, but this will never stop. So a bigger battery is really like putting a smaller set up on then 2 equal sized batteries. Plus it is hard on the batteries. Try hooking up 1aaa battery with a c or d and play around with a multimeter each day. (Hook up to double voltage) You can see that the larger one will have less power then the smaller one after being hooked up awhile.

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Not true. In a parallel system there is truth in what you say But batteries in series do not draw each other down. The smaller capacity will obviously run out of power before the larger one but it will not suck the life out of the other one. If the starting battery in this example is the larger of the two it will discharge at a qicker rate than the smaller one because it is also powering other items like the locator, radio and gps. So in effect, they will likely reach discharged state at the same time.

Re: No room for a 3rd battery!

Bubba, it can be done with certain reservations. Hook the 24 volt trolling motor up in the conventional manner, hook everything 12v to the same single battery. The 12v stuff will run off the motor and that battery may even charge a little (operative word, a little), but nothing will be shunted to the other battery. The danger, obviously, is the trolling motor drawing down both batteries to a point you cannot start the big motor with either. A jump box might be your salvation in that happenstance ...

Re: No room for a 3rd battery!

Silvertip said:

Not true. In a parallel system there is truth in what you say But batteries in series do not draw each other down.

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100% correct!

In series, one of the batteries could be completely dead, seriously discharged and not affect the other battery at all, the other battery would stay charged up as normal. The 24V bank would be ka'put, but the good battery wouldn't be ruined by the bad battery (as would happen in a parallel configuration).

Re: No room for a 3rd battery!

Keep in mind that when you put two 12v batteries in series, you are actually putting twelve 2v batteries in series. If you put a lower capacity or older/weaker battery in series with a large capacity/newer battery, as the 2v cells of the smaller/weaker battery drop out, you will be reverse charging the dropped out cells. This results in reduced cycle life for that battery and instead of the dropped out cells providing power, they become a load on the system robbing power from the good battery.

If you have to go with two batteries, buy two deep cycle batteries with the most reserve capacity you can find. And like Outsider said, carry a jump box on those days you are going to be heavy on the TM.

Re: No room for a 3rd battery!

dwparker99 said:

Keep in mind that when you put two 12v batteries in series, you are actually putting twelve 2v batteries in series. If you put a lower capacity or older/weaker battery in series with a large capacity/newer battery, as the 2v cells of the smaller/weaker battery drop out, you will be reverse charging the dropped out cells. This results in reduced cycle life for that battery and instead of the dropped out cells providing power, they become a load on the system robbing power from the good battery.

If you have to go with two batteries, buy two deep cycle batteries with the most reserve capacity you can find. And like Outsider said, carry a jump box on those days you are going to be heavy on the TM.

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Sorry -- we already discussed this and what you say is not true. Reverse charge? Don't think so! I have a perfect example. I currently have a 12 volt battery that has one bad cell. If what you say is true, that one bad cell would pull down the other five which is definitely not happening. You cannot charge a bad cell and therefore it cannot be reverse charged. I repeat, put a dead cell in your flashlight along with a good one. Let it sit. In a couple days you will see no difference in the voltage of the good cell. The same process goes on inside a 12 volt battery. I guess it is time for me to prove this as I just happen to have another good battery that I can rig up with the bad to prove there is no "reverse charge" in a series system. Stay tuned.

Re: No room for a 3rd battery!

Thanks for all the great ideas. i guess if i decide to go with this one ill just create a small aluminum single battery"compartment" to set on the floor just rear of back seat, and fasten it to the floor somehow so it would be stable. This way it will be outta the way. This boat has livewells under both seats so i cant put it there, but the entire rear of the boat where the gas tank, cranking battery etc is located is in a "compartment" with a lid that opens.

Im very interested in finding a newer boat than i have now that has a more dependable motor on it, therefore this mercury 4 storke 60 is very appealing. Believe it or not its hard to find fairly slightly used boats with that type of motor.

Unless yall have comments on this particular boat and motor and why it may not make a good purchase.

Re: No room for a 3rd battery!

Silvertip said:

Sorry -- we already discussed this and what you say is not true. Reverse charge? Don't think so! I have a perfect example. I currently have a 12 volt battery that has one bad cell. If what you say is true, that one bad cell would pull down the other five which is definitely not happening. You cannot charge a bad cell and therefore it cannot be reverse charged. I repeat, put a dead cell in your flashlight along with a good one. Let it sit. In a couple days you will see no difference in the voltage of the good cell. The same process goes on inside a 12 volt battery. I guess it is time for me to prove this as I just happen to have another good battery that I can rig up with the bad to prove there is no "reverse charge" in a series system. Stay tuned.

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You are talking a static condition (TM off). I am talking a dynamic condition (TM on). All batteries have internal resistance. As cells drop out, the other cells in series with them have to push the current through the bad cells internal resistance. This is the same as putting a resistive load in the circuit robbing power from the power source. As for reverse charging, Google it..

Re: No room for a 3rd battery!

dwparker99 said:

You are talking a static condition (TM off). I am talking a dynamic condition (TM on). All batteries have internal resistance. As cells drop out, the other cells in series with them have to push the current through the bad cells internal resistance. This is the same as putting a resistive load in the circuit robbing power from the power source. As for reverse charging, Google it..

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I did. You can reverse charge a battery or a cell in two ways: 1) on purpose, or 2) by accident. Both of those are due to human error. Show us where this actually happens in practice when things are wired correctly. There are way too many series connected batteries of unlike capacities that do not, nor have I ever heard or read of an incident like this happening. Mind you I've been on this planet a long time but even so I've not heard or read about every possible event. If it was an issue there would be lots of 24 and 48 volt systems in regular need of battery replacement (especially golf carts). And you still haven't explained why this is doesn't happen with a single 12 volt battery. A single battery with a dead/dying cell simply powers its load until it can't. Recharge it to the capacity it can accept and it will do it again, and again, and again, maintaining its proper polarity all the while. Even a brand new battery does not have absolutely identical cells in it. For that matter, two identical batteries will not have exactly the same capacity. The whole point here is that in a trolling motor application a bad battery is immediately obvious and trying to force it to do what it cannot is a waste of time and most folks would simply replace the battery.