Charging batteries in parallel?

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Originally Posted by Corker

Let's back up to your basic plan before solving the charger puzzle. Parallel hookups are not the best way to use your two batteries. As ReelMan points out, batteries charge and discharge at different rates--even a pair of identical batteries purchased at the same time.

It's true that hooking a pair of batteries in parallel lengthens your TM's running time but that's all you get. A two-battery parallel bank doesn't increase the TM's power. What you don't see happening (but will eventually learn from experience) is that the stronger of your two batteries will charge the weaker one. On the water, that means a bank will deliver less run time than the sum of the individual batteries.

A better plan is to hook your batteries to an A/B/Off switch. Start your day switched to the weaker of the two batteries and run it down to a safe voltage--a level that will not do permanent damage to the battery's plates, probably 12.00v. Then switch to your other (fully-charged) battery for the second half of the day. Doing it this way, you can squeeze every useable amp out of your batteries & protect them in the process.

Now, we come back to your original question. With an A/B/Off switch in place, two banks of your charger can be hooked to the batteries without concern. Should you choose an A/B/A+B/Off switch, you'll want to set the switch to any setting except A+B while you charge.

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This is very good info. I had wondered if my run time would actually double or not. My initial thoughts were that I can usually make it all day on one battery. So, by adding the extra, it would be a sure bet. So, let's talk about maximizing!

I know that the extra battery can't deliver more power, but what happens when you start to lose to battery power? For instance, on days I pull cranks or when I'm facing a strong wind, I'll have to increase the speed setting to account for the draining battery as the day goes on. If it were in parallel, I would be carrying a higher voltage level over a longer period of time. Shouldn't that increase the overall run time? I've got no problem installing a switch or disconnect, or disconnecting the parallel connection. I'm just wanting the most effecient, and safe, way to this situation.

Your run time should double if both batteries are in the same condition and deliver the same number of amp hours. Even if you have one new battery and another that's on its last leg, you can get the most out of two batteries by using them independently. What you want to avoid is having a strong battery depleted in its attempt to maintain the charge in the weaker battery. You want all the amps delivered to your TM.

Most of my trips are TMO lakes, so I'm totally dependent on my electrical system. I run my older (weaker) battery until lunchtime and then switch batteries for the afternoon. Yes, top speed drops as the battery delivers its amps. If it's important to maintain a constant trolling speed, I advance the power setting (actually, my iPilot does that automatically with the Cruise Control feature.)

When the morning battery is drawn down close to the danger level or at lunchtime (whichever comes first) I switch to battery #2 (the newer one.) If conditions are really tough in the afternoon I can always reconnect the morning battery and draw it down deeper in order to continue fishing and make it back to the dock.

Since my batteries are drawn down to the same level (more or less) I get the shortest recharge cycle. If I were to run one battery all day and draw it WAY down, recharge would take longer and the battery might be damaged--shortening its service lifetime.

An issue that hasn't been mentioned is the Peukert Effect.

The slower you draw power out of a battery, the larger the capacity of the battery is. That's why all the capacity measurements are made at defined currents.

Why is this relevant? Because if you place two equal batteries in parallel, the combination will last longer than the sum of the two batteries individually since you are drawing half the current from each.

There are several reasons not to connect in parallel as well. Discharging one battery into the other has been mentioned. If one goes bad, the other will probably go too unless you notice right away. The requirement that they be matched is important too.

One of my favorite solutions is to add a second TM connector to the bow of the boat for the second battery. It's cheaper than a switch, avoids some of the issues of parallel batteries, and is easy to change. Fish until the first battery slows down, swap the plug and you know you are halfway done with the day.

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Originally Posted by CatFan

swap the plug

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Nice! You gotta love solutions that are simple, effective, and cheap!

So, add another TM receptacle. If I use most of the juice out of the first battery, then switch to the second. If I happen to run that one down, just keep some wire handy and then hook my beat up batteries to run parallel and continue on...sounds like that would be optimum! :fish :cheers2

I wouldn't worry about carrying a jumper--just reconnect to the morning battery and draw it down deeper.