Which way to connect battery charger?

[Roger Long]

Which would be the better way to connect my new Guest battery charger?

Later: This is a pretty fine point. Both will work but it's a question of whether both leads going to a fused connection is worth an occasional imbalance in charging. Here's a more practical and serious question:

I just got the charger installed and looked at the output cables. Wow, these were never intended to be cut apart and separated enough to accommodate either arrangement shown above. I'm skeptical of separating the black and red wires without cutting the insulation of one of them.

Just how bad would it be to connect the positive leads to the battery switch post as I did before? The switch and the ground buss are close enough to do this without having to do major surgery on the leads. The only problem would be a slight difference in voltage sense by the charger due to the drop in the battery cable. It's big and I'll only be using this charger occasionally since marinas are not a big part of my cruising.

Should I try stripping back the leads or just wire it the way my old charger was?

 

[Chuckbear]

Roger, since it appears that you have both batteries connected together, neither of your diagrams would be correct. If that is the case, the diagram on the left with only one connection to the positive on your bank would be correct. It won't matter if you have both negatives connected but it is not necessary. With either of the other connections you will have issues if either of the batteries develop a problem, the other will wind up overcharging. Chuck

 

[Roger Long]

neither of your diagrams would be correct.

The charger manual shows that the outputs can be combined for two batteries tied together in a bank. It has a diagram similar to the left hand option except that each negative lead goes to a negative battery post.

With a battery monitor, the negative leads have to go to the shunt so that monitor will know how much is being put back into the batteries.

 

[Chuckbear]

Roger, If that is the case then that would be your preferred connection. Chuck

 

[Stu Jackson]

The charger manual shows that the outputs can be combined for two batteries tied together in a bank. It has a diagram similar to the left hand option except that each negative lead goes to a negative battery post.

With a battery monitor, the negative leads have to go to the shunt so that monitor will know how much is being put back into the batteries.

Yes, Roger, the manual neglects to recognize that some boat have inboard engines. Boats with outboards use the negative battery posts fro their ground.

You're right about the monitor needing the negatives run as you show.

Maine Sail ahs noted, many times, that some chargers split their charge on the two outputs, while others allow only one connection with full charging from it. Don't know specifically with your charger, might ask Guest.

 

[Roger Long]

It get's weirder. The Guest manual shows that the charger can be hooked up this way with the outputs combined when two batteries are tied into a single bank as mine are:

After a brief phone call with Maine Sail who was just boarding a launch to go work on a boat, I decided it was OK to connect this charger the same way my old one was with the positive leads going to the battery switch. There is no temperature sensing through the leads on this charger and connecting directly to the batteries would require gruesome surgery on the leads to separate the positive and negative wires enough to span the distance from the battery posts to the Victron shunt.

This is how I decided to do it:

The only possible problem is a slight voltage drop through the large, fat battery cable causing the charger to slightly overcharge. I won't be using this charger a lot so it isn't a big issue.

However, there is a really, really big issue. I connected the positive leads to the battery switch as show and went to connect the negative leads to the shunt. There was a spark. I checked and there is full battery voltage between the charger negative leads and ground. The charger has never been plugged into AC and the boat isn't connected to shore power. The charger unit is basically a dead short.

I'm stumped. A defective unit with the blocking diodes failed closed right out of the box?

Later:

I called Maine Sail and he said to check the amps. Doh! Full voltage but almost no amperage. This, including the spark, is normal. I probably over reacted but, when you are curled up in a tight space lying on top of a battery box and there is a spark a few inches in front of your face, it tends to look larger than it actually is.

The Guest charger draws a lot of current when you first connect the 12 volt leads. It turns on all it's LED's, goes into diagnostic mode, and starts waiting for button press instructions. It looks like a rather large parasitic load on the Victron monitor and I was thinking I would need to remember pull the fuses if leaving the boat for any period of time. However, after about 20 minutes, it gives up and goes to sleep showing virtually no amp draw.

I plugged in the AC and the charger worked perfectly following exactly the output profile in the manual without the wild excursions up into AGM cooking equalizing territory of the Schumacher.