Charging batteries in parallel-12v

Greetings: I have done some browsing here and can not find a answer to my specific question. I have a fishing boat with a blue Optima battery. I want to buy another battery, dedicate it to my trolling motor and charge them in parallel. It is my understanding that it is best to have the same brand and class of batteries when doing this.

Question...Can I add a new Optima blue battery to the front of the boat and charge it in parallel with the older battery that I already have without creating any significant problems? What else do I need to know about doing this? If someone would like to recommend an on-board charger for this task please do so.

Please advise.

Thanks

Re: Charging batteries in parallel-12v

You are way better charging them separate especially if they are different battery age/type/brand. Since yours will be the same brand/type, i would be worried more about the age difference. What I would do is charge them both up and check their fully charged voltages. If they are nearly the same, you may be ok charging them in parallel.

Re: Charging batteries in parallel-12v

Since you have a trolling motor battery that will routinely be discharged far deeper than the starting battery, those two batteries need to be isolated from each other while in use. If you leave the batteries in parallel during use, the troller will suck current from the starting battery as well the battery for the troller. That leaves you with the potential for a very discharged starting battery. There are a number of scenarios for charging but you cannot ignore discharging in the process. So here are a couple of options:

1) Leave the two batteries isolated. Use a single output 6 amp or higher on-board charger to charge the troller battery. You plug the on-board charger into 120 volts AC power at the dock. The engine will keep the starting battery charged. If you intend to fully charge the troller battery from the engine that is not a practical process unless the engine has a very high output charging system.
2) Add a dual battery switch (OFF-BAT 1-BOTH-BAT2). You can then use either battery for any purpose and you can charge either or both batteries separately or together. If you are switch challenged and don't want to bother, or again if the engine does not have a high output charging system, this is a waste of money. You would likely still need an on-board charger as the engine would need to run wide open for hours to fully charge the troller battery.
3) Add an ACR which will automatically switch between the starting and troller battery as needed. Again, the on-board charger will be needed.

For your puposes I highly recommend ignoring the dual battery switch and an ACR and simply install a single or dual output on-board charger. One output goes to the starting battery and the other to the troller battery. Each is charged and maintained automatically. Simple and the most trouble free system. If you don't have power at the dock and an extension cord is out of the question because of the long distance from AC power, other methods need to be employed -- such as a generator. Forget solar panels due to cost, size and relatively low output.

Re: Charging batteries in parallel-12v

Silvertip said:

install....a dual output on-board charger. One output goes to the starting battery and the other to the troller battery. Each is charged and maintained automatically. Simple and the most trouble free system.

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I think that's the best solution, keep the batteries isolated and charge them with a dual output onboard charger.

You could add a battery switch, but I'm not sure it's warranted. If the start battery dies, it wouldn't be all that hard to swap the batteries around.

What does the boat have for charging, outboard or I/O, make and HP?

Re: Charging batteries in parallel-12v

Putting one battery in back and one up front makes it less than optimum when charging with one charger whether you charge in parallel or with a two bank charger. Extending charger leads will reduce voltage charge levels at all current levels for that battery. If you are going to separate the batteries, it would be better to have two chargers.

Re: Charging batteries in parallel-12v

dwparker99 said:

Extending charger leads will reduce voltage charge levels at all current levels for that battery.

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Please explain???

Battery charger manufacturers sell extension cables/wires just for that purpose.

Re: Charging batteries in parallel-12v

Yes, I agree with Silvertip and fishrdan. Thinking about it more, since you are discharging one battery a lot more than the other, it would be best to keep them isolated from eachother during charge.

The length of the cables is pretty much immaterial because as the battery reaches full charge, the voltage drop is minimal so the battery will always receive its full charge in the end.

Re: Charging batteries in parallel-12v

My motor is a 2006 40hp ETEC. The distance between the two batteries is about 10 feet. When I bought the boat it had two different brand batteries, one of which was the Optima/blue. They were connected in parallel and the seller indicated that the ETEC kept both batteries charged. I did not like that concept, so I separated the batteries and currently use two chargers, or simply move one charger from battery to battery.

Re: Charging batteries in parallel-12v

fishrdan said:

Please explain???

Battery charger manufacturers sell extension cables/wires just for that purpose.

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It is all about transmission loss. When a charger manufacturer designs a charge algorithm for a charger they do it with the resistance of the leads known. For instance, if during the bulk stage they want the current to be 15 amps until the voltage rises to 12.8v and then step down the current to 10 amps until the voltage rises to 13.2v, they want that voltage level at end of the leads/battery terminals. If you add extension cables those voltage levels at that current will never be seen at the battery. The charger is not smart enough to differentiate between the resistance of the battery taking a charge and the resistance of the transmission lines.

One thing to consider though is that transmission loss will be determine by wire size used. A run of 20 feet of ten gauge wire will drop 0.6 volts when 15 amps is flowing. For the algorithm mention above, the charger would drop from 15 amps to 10 amps when battery voltage reached 12.2v instead of the intended 12.8v. For all practical purposes, the battery would never see the 15 amp charge.

Although I agree with bruce58 that the battery will receive a full charge, I don't think the length of cable is immaterial. If we don't reverse the majority of the sulfation process that takes place when a battery is discharged, the capacity of a full charge will become less and less. To reverse the sulfation process, the charge algorithm needs to be felt at the battery terminals.

It is my understanding that the AGM batteries need a pretty aggressive charge to reverse the sulfation. Odyssey recommends a 0.4C minimum charge for their batteries. So, if I were the OP and was going to run the Blue Tops, I would not use extension cables to extend the leads to the front.

Re: Charging batteries in parallel-12v

You are correct, the bulk charge will turn off sooner than expected so the battery will not be receiving its 10-15A charge as long before the charger measures its limit point and goes into absorption phase.

Here is the Odyssey battery charging literature. No mention of sulfation in it. You have the specs that show what Odyssey says about the subject? I have always hear that a pulsed current was best for removing sulfation but that was for flooded cell designs.

http://www.odysseybattery.com/documents/US-ODY-TM-001_0411_000.pdf

dwparker99 said:

It is my understanding that the AGM batteries need a pretty aggressive charge to reverse the sulfation. Odyssey recommends a 0.4C minimum charge for their batteries.

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Guess my 10A Guest charger isn't going to cut it charging the Odyssey 34M-PC1500 battery. It would mean I would need a 27A charger.

Also, instead of buying the Odyssey battery, you can save money buying the Sears version that is made by Odyssey:
http://www.sears.com/diehard-platin...p-02850134000P?prdNo=2&blockNo=2&blockType=G2

Re: Charging batteries in parallel-12v

bruceb58 said:

Here is the Odyssey battery charging literature. No mention of sulfation in it. You have the specs that show what Odyssey says about the subject? I have always hear that a pulsed current was best for removing sulfation but that was for flooded cell designs.

http://www.odysseybattery.com/documents/US-ODY-TM-001_0411_000.pdf

Guess my 10A Guest charger isn't going to cut it charging the Odyssey 34M-PC1500 battery. It would mean I would need a 27A charger.

Click to expand...

The specs I'm looking at are on page 15 of the website you referenced. Figure 6 shows the recommended charge profile. It calls for a minimum bulk charge rate of 0.4C at the 10 hour rate. For the PC1500 battery that would be a 25 amp or higher charger. The only chargers Odyssey recommends for this battery are the 25 and 40 amp chargers. Also, if you look at page 15 of the referenced site there is a chart that shows the affect of charging at 14.2v (standard lead-acid rate) versus 14.7v (AGM rate), you will see that cycle life is doubled when using the higher rate.

What I'm pointing out is optimum vs something less than optimum when charging the AGM batteries. It seems that the deeper you discharge the AGM, the more critical the higher charge rate becomes.

Re: Charging batteries in parallel-12v

I agree with you. I might be changing out chargers because I am about to switch over to the Odyssey battery made for Sears.

Re: Charging batteries in parallel-12v

Thank you gentlemen. I think I will buy another Optima/blue to match the one I have, keep them isolated from each other and charge them separately. If I can scrape up some extra money I would like to install a dual port on-board charger. That should keep me powered-up on the water. Thanks again