Charging dual batteries

[beckoning]

I have 2 batteries with a Perko switch between them. I can switch to #1 or #2 or ALL. I use #1 (starting battery) for starting, #2 for electronics while trolling and the big motor is off, and switch to ALL when running on large motor in order to charge both batteries from the Mercury's alternator. I have a small, automatic charger I keep attached while the boat is in the hoist. For the last few years, I have connect charger's positive and negative to the positive and negatives on one of the batteries, then switch Perko to ALL assuming both batteries will be receiving a charge. Am I correct, or should I connect differently? Does anyone have a different approach to connecting the charger's + and - connectors so both batteries will be charging at the same time?

 

[NYBo]

You are correct.

 

[ihearth2o]

You are correct but when you have the switch set to ALL, you risk overcharging the battery that gets a full charged first. The charger will combine the charges from both batteries and keep charging until the charger gets a sufficient combined reading. For example, if one battery is at 10v and the other is at 12v, the charger will read a combined 11v. The charger will keep charging until it reads a ~12.6v so the first battery might get overcharged. Of course if you have two batteries that are similarly discharged, it shouldn't be an issue. Seems like it's working fine for you though since you've been doing it for a few years.

 

[bruceb58]

You are correct but when you have the switch set to ALL, you risk overcharging the battery that gets a full charged first. The charger will combine the charges from both batteries and keep charging until the charger gets a sufficient combined reading. For example, if one battery is at 10v and the other is at 12v, the charger will read a combined 11v. The charger will keep charging until it reads a ~12.6v so the first battery might get overcharged. Of course if you have two batteries that are similarly discharged, it shouldn't be an issue. Seems like it's working fine for you though since you've been doing it for a few years.

That's not how it works. If you put the switch to all, the more charged battery will feed current to the discharged battery until both batteries are at the same voltage.

Put a charger on there and each battery will accept an amount of current so that the batteries are at equal voltage.

A much better way to charge the batteries is to keep the switch off and use a dual bank charger.

 

[Chris1956]

As long as the charger is relatively small, say 6A or so, you are perfectly fine charging both batteries at once, with a single charger. As both batteries charge up, the charging current will taper down. So if they taper to 2 or 3 A, spread across two large batteries, the charge is essentially a trickle.

Bruce's call for dual bank chargers is overkill for your application, IMHO.

 

[oldjeep]

Personally there is no way I would be charging dual batteries in the manner you are doing, all you need is one bad battery and you ruin the other one. Dual bank chargers are cheap, the one I put in my boat costs about $80. I'd post a link where to buy it but it would get deleted or altered - so just google it.

This way you also get the benefit of a smart battery maintainer for each battery rather than just a dumb charger.

 

[ihearth2o]

That's not how it works. If you put the switch to all, the more charged battery will feed current to the discharged battery until both batteries are at the same voltage.

Put a charger on there and each battery will accept an amount of current so that the batteries are at equal voltage.

A much better way to charge the batteries is to keep the switch off and use a dual bank charger.

If I'm following correctly, I think we are saying the same thing (to a point). When set to ALL, the weaker battery is charged by the stronger battery, essentially, combining the charges. When hooked up to a charger, the charger reads the combined charge and will continue to charge until both batteries are charged to their combined voltage. Where you might run into problems is if one battery is discharged to a point where the stronger battery cannot feed the weaker battery efficiently enough and the stronger battery, even though it may be fully charged, will continue to receive a charge until the weaker battery starts accepting a charge from the stronger battery.

As previously mentioned, it doesn't seem like an issue for the OP probably because he's been good about tending to his batteries and not allowing one to discharge enough to a point where the batteries have vastly different loads.

 

[Chris1956]

Hey Jeep, you have got to be kidding. If you have a bad battery, replace it. His engine charges both batteries at the same time, as they are hooked in parallel.

His system of isolating the trolling motor load to a separate dedicated battery is a good one. If his motor starts at launch, it proves his starting battery is good. If his trolling motor battery suddenly goes bad,he should postpone his fishing trip and replace the battery. I am not sure where your fear comes from.

 

[oldjeep]

Hey Jeep, you have got to be kidding. If you have a bad battery, replace it. His engine charges both batteries at the same time, as they are hooked in parallel.

His system of isolating the trolling motor load to a separate dedicated battery is a good one. If his motor starts at launch, it proves his starting battery is good. If his trolling motor battery suddenly goes bad,he should postpone his fishing trip and replace the battery. I am not sure where your fear comes from.

My fear comes from having (presumably) a starting battery and a deep cycle battery being charged in parallel. From a personal standpoint I would never use a "both" setting to run the boat/charge the batteries unless it was an emergency and it was the only way to get enough power to start the boat.

 

[bruceb58]

When running your boat and having the alternator charge the batteries, you are way better off using an ACR that will not allow the batteries to parallel when one battery is below a certain voltage which could be caused by a bad cell.

Same reason you want a dual charger so that if one battery has a bad cell, it doesn't bring down both batteries. This is especially important if the charger is on the batteries in the off season unattended.

 

[bruceb58]

Where you might run into problems is if one battery is discharged to a point where the stronger battery cannot feed the weaker battery efficiently enough and the stronger battery, even though it may be fully charged, will continue to receive a charge until the weaker battery starts accepting a charge from the stronger battery.

That's not going to happen. Once the charger sees a certain voltage, it tapers off the current so that nothing but a trickle is output. It has no idea what is hooked up to it. It just looks at voltages at its output.

 

[ihearth2o]

That's not going to happen. Once the charger sees a certain voltage, it tapers off the current so that nothing but a trickle is output. It has no idea what is hooked up to it. It just looks at voltages at its output.

I guess it just depends on how risk aversed you are. When it comes to batteries, I tend to err on the side of caution so I'm not as sure as you that this possibility is not going to happen. All I know is that it only took one time for me to be towed in to the dock due to dead batteries for me to pay closer attention to them.

 

[bruceb58]

I guess it just depends on how risk aversed you are. When it comes to batteries, I tend to err on the side of caution so I'm not as sure as you that this possibility is not going to happen. All I know is that it only took one time for me to be towed in to the dock due to dead batteries for me to pay closer attention to them.

I keep mine separated at all times except when my ACR is hooking them together. I never use the "both" selection on the switch unless it was an emergency start situation. If a battery is really bad, I will disconnect it before using the "both" selection. My EE background makes me sure it won't overcharge.

 

[ihearth2o]

Gotcha. I will put another vote in for "it won't happen" and give it due consideration in light of the "it is a possibility" and "it will happen" votes when I previously discussed the issue over the years with other electrical engineers, marine shops, mechanics, other boat owners, other forums, etc.

 

[Brian 26]

From a personal standpoint I would never use a "both" setting to run the boat/charge the batteries unless it was an emergency and it was the only way to get enough power to start the boat.

+1

I had a fishing boat for about a year that had a perko switch and I never used the both function.

 

[Bondo]

Ayuh,..... I guess I've been doin' it wrong all these years,.....
Probably gonna continue to do it the wrong way too,.....

My tinbarge needs it's bilge pumps, as it's berth is 'tween my 2 riverhouses, 'n the eve-fall sorta hits the boat, though any hittin' the roof, mostly runs off the gunwales,.....
One of those fairly common Tarantula Downpours, puts Alota water there,....

Even with 2 good fresh batteries, I've Learned(Right Ted) to leave my auto-charger plugged in, 'n clipped to the trunk wirin'(buss) from the house battery,...
In the cabin my Napa plastic box auto-charger stays nice, 'n dry,....
The battery switch is usually on All,...

When boostin'/ chargin' the batteries in the Macks, I hook the charger to only 1 battery, or battery group, if twin 6v batteries,...
They're All 4 batteries each,....

Some of the Equipment is 24v, 2 batteries,....
I hook up to each battery's posts, charge 1 battery, then switch to the other battery,.... Both take a charge,....
'n if I leave the hook up on 1 battery, it "Helps" in a jump-start situation,....
So long as my portable Toolbox(pickup), don't touch metal to metal(earth to earth), it don't hurt a thing,...

 

[beckoning]

Thanks all. I feel much more comfortable now.

First, a bit of clarification: I keep the automatic trickle charger (max 1.6amp) installed in large part because the automatic bilge pump (cycles on every 3 minutes) will drain the battery(s) if they are not being charged. Hence, either the starting battery or deep cycle will be at low charge when I head out onto Lake Michigan for salmon....not a good thing.

I think my take-aways are the following:

1. What has worked for the past few years without damaging the batteries should continue to work.
2. The motor or charger will charge either or both batteries based on how the Perko is set.
3. Keep a close eye on battery condition. A bad cell in #1 battery could force a bad cell or failed #2 battery.
4. A dual-bank charger is probably the ultimate solution.

One final question: I understand what is meant by hooking batteries in parallel. But I would feel more confident if someone would explain or diagram 2 batteries hooked in parallel with a Perko switch.

Thank you in advance for your response.

 

[smokeonthewater]

the ground is always in parallel... the perko switch has 3 terminals... batt 1 in, batt 2 in, and power out.... In parallel (1+2, both, emergency, etc) all 3 terminals are connected to each other... in 1, batt 1 in and power out are connected and in 2, batt 2 and power out are connected... obviously in off none are connected.....


In order to make your life better do 2 out of 3 of the following:
1: GET RID of the automatic cycling bilge pump and get a normal pump with a float switch or sensor
2: get an ACR and stop constantly messing with that darn battery switch.... life is too short for that.
3: send me $50 for the great advice.......

 

[bruceb58]

Yep. Both my boats have ACRs on them and both have dual bank chargers. Never have to worry where the switch is sitting. All I know is that it is off when I am not using the boat.

 

[beckoning]

Thank you for taking time to address my situation.

Pardon my ignorance. What is "ACR"?