Am I wrong? Charging two batteries in parralel, my way or your way

[desertislander]

Here we go, a silly little problem but I don't want to blow up the boat. I am defo not a sparky so I really don't want to chance it with this. I want to charge these two 12 v batteries in situ so which terminals do I connect to? The layout is the two positives at the bottom and the negatives at the top. The left and right positives go to the change over switch, the negs loop across each other then on to the engine.
The way I see it is the postive charger clip goes to the far right positive terminal with the earth clip going to the top left earth terminal. Also, to complete the loop, I assume the change over switch is set to "both" ?
How did I do, thanks anyone
IMG_1214.jpg

[Triple]

I would only charge batteries in parallel if they are same type and age. And only then it would be on a trickle charge after I've bulk charged them individually.

[EdBerg]

I also would not charge the two batteries together, if they are different sizes types or capacities, not with your setup, you may overcharge one to it's detriment. I have two batteries on my boat but connected via a digital voltage controller, which isolates each battery from each other and I would leave the charger on, it would then charge up one battery and when fully charged, it would then switch over to the other battery. I didn't bother wiring it up myself and got an boat electrician to do it.

[Fed]

The way I see it is the postive charger clip goes to the far right positive terminal with the earth clip going to the top left earth terminal. Also, to complete the loop, I assume the change over switch is set to "both" ?
How did I do, thanks anyone

You did good.

[Crunchy]

If you have a smart charger and charge in parallel, once one battery is fully charged the smart charger will "think" both batteries are charged and switch to maintence mode, this could leave you with one under charged battery. Charge seperately then combine by switching to both (after disconnecting the charger!) and use the charger to keep a maintenance charge on both batts (assuming smart charger). If battery switch on "both" the charger can be connected to any negative and any positive terminal. Also clean up the terminals on your back battery, they need some maintenance by the looks.

[Fed]

No Crunchy, the charger will see the charged battery as additional load and continue charging.

[Crunchy]

No Crunchy, the charger will see the charged battery as additional load and continue charging.

True bro? Didn't know that, was going by what the bloke who sold me the UB said, i was new to it all so thought yep no worries

[Fed]

Guaranteed Crunchy, the more batteries (fully charged or discharged) you connect in parallel to a charger the more load it puts on the charger simply because the charger output Voltage is always higher than any battery Voltage.

[goanna1]

http://www.zbattery.com/Connecting-B...es-or-Parallel

[Dignity]

Guaranteed Crunchy, the more batteries (fully charged or discharged) you connect in parallel to a charger the more load it puts on the charger simply because the charger output Voltage is always higher than any battery Voltage.

So does this mean one of the batteries wil be over charged if the other won't fully charge due to s fault.

[Fed]

When the 2 batteries are connected in parallel the terminal Voltages will equalize resulting in neither fully charging unless the 'bad' battery comes up.

More importantly I just noticed desertislander has wingnuts, EEWWW!

[ranmar850]

All the charger can "see " is resistance. So when you parallel two of the same resistance, the charger will simply output more, up to it's limit. It is effectively seeing half the resistance. If they are different resistances, the voltage across them will be the same, but the current flowing through each will vary. This is basic electrical theory as it relates to lead/acid cells. Two batteries of different sizes will charge up OK in parallel if they are hard-wired---if you start thowing smart relays into the mix, it can be different, unless you have a bypass switch.

As to where to put the chargers connections, you can put the charger positive and negative leads on the appropriate terminals of just one of the battery's or split it over two, if they are paralleled, makes no difference whatsoever electrically, as long as your connections are good. And yes, lose the wingnuts, they make rather a small contact patch for connection, are easily loosened, and can be the cause of problems.

[TheRealAndy]

All the charger can "see " is resistance. So when you parallel two of the same resistance, the charger will simply output more, up to it's limit. It is effectively seeing half the resistance. If they are different resistances, the voltage across them will be the same, but the current flowing through each will vary. This is basic electrical theory as it relates to lead/acid cells. Two batteries of different sizes will charge up OK in parallel if they are hard-wired---if you start thowing smart relays into the mix, it can be different, unless you have a bypass switch.

As to where to put the chargers connections, you can put the charger positive and negative leads on the appropriate terminals of just one of the battery's or split it over two, if they are paralleled, makes no difference whatsoever electrically, as long as your connections are good. And yes, lose the wingnuts, they make rather a small contact patch for connection, are easily loosened, and can be the cause of problems.

Negative. Modern chargers sense both the battery voltage and the charge current and adjust accordingly. If you parallel 2 batteries that are not exactly the same there is a chance one will get neglected as the charger will only know about the overall voltage.

Also, with parallel batteries, there is always the chance that one can go bad and drag the other down with it. In extreme cases there can also go bang, seen it before a few times. If you rely on the batteries, its simply just bad practice to join them together without some smarts in between (ie, a voltage sensitive relay).

[ranmar850]

I was basing my argument on having two functional batteries. Of course, the big risk in paralleling is having one go bad and drag the other with it. The big trap with the cole-hersee type of 1/2/Both battery switches. I agree, a smart relay should always be used, at least bridging the Both position so that the second battery will recieve charge when the start battery attains the 12.7v or whatever. But then, they are acting exactly the same as when you have them paralleled on both.( for charging) So how can we tout that as different.? Apart from removing the risk of a dud second/or something left on scenario. They are still charging in parallel.
Your argument of them sensing charge current as well holds some water, but remember it is just seeing them as a total load--you do understand the theory of parallel resistance? The differing resistance of the two cells, unless they are identical, will always cause a difference in charge current through each leg. Hell, even two supposedly identical batteries will have slightly different resistance, so charge current will be different between the two. The voltage across them will be the same.

[TheRealAndy]

I was basing my argument on having two functional batteries. Of course, the big risk in paralleling is having one go bad and drag the other with it. The big trap with the cole-hersee type of 1/2/Both battery switches. I agree, a smart relay should always be used, at least bridging the Both position so that the second battery will recieve charge when the start battery attains the 12.7v or whatever. But then, they are acting exactly the same as when you have them paralleled on both.( for charging) So how can we tout that as different.? Apart from removing the risk of a dud second/or something left on scenario. They are still charging in parallel.
Your argument of them sensing charge current as well holds some water, but remember it is just seeing them as a total load--you do understand the theory of parallel resistance? The differing resistance of the two cells, unless they are identical, will always cause a difference in charge current through each leg. Hell, even two supposedly identical batteries will have slightly different resistance, so charge current will be different between the two. The voltage across them will be the same.

The problem with having 2 different batteries is one will charge faster than the other. Once it comes up to charge it sits there boiling away whilst the other one still charges because your charger has not gone into float mode, then finally the voltage comes up thanks to battery 1, and your charger goes into float mode leaven battery 2 in a incomplete state of charge. At the end of the day, all that means is the life of the battery is typically reduced. Catastrophic failures are fairly rare.

Remember a battery is not just a simple resistance. At its most simple model, its both a resistance, voltage and current source.

You do still get imbalanced charge conditions in same type/model/age batteries also, but its typically so little that its of no practical consequence.