Do both batteries charge with the motor on?

[sid007]

Will the alternator charge both batteries? I have a dial to switch between battery 1, battery 2, and both.

Will it charge both if I have it on battery 2? Will shoreline charge both if I have it on 'off'?

I'm a battery n00b with boats and having some, hopefully battery issues, and hope you can help!

 

[z28se]

Re: Do both batteries charge with the motor on?

both will charge only when the switch is on ALL. If the swittch is on battery #2 only that battery will charge, same apply for #1

 

[Splat]

Re: Do both batteries charge with the motor on?

Unless you have a on board alternator charger, or a battery isolator installed......

BIll

 

[Silvertip]

Re: Do both batteries charge with the motor on?

Splat -- He doesn't need an isolator and he does have an alternator. He has a conventional battery switch. Z28SE's response is accurate. I would add though, that if all of the accessories and other loads are connected to the COM terminal on the switch, then those devices will be powered by BAT 1 if that's what you have selected. BAT 2 if that's what you have selected. Or BOTH if that's what you have selected. So the idea is to designate either BAT 1 or BAT 2 as your starting battery. Switch to BAT 1 to start the engine. Then switch to BAT 2. When running both batteries will be charged. If you anchor for a period of time save the start battery (BAT 1) by switching to BAT 2. Your tunes and other stuff will draw only off BAT 2. When you are ready to go, switch to BAT 1 and away you go. Switch to BOTH to top off both batteries again. Or if you really sucked BAT 2, start on BAT 1 and then switch back to BAT 2 so it can be charged up. If your engine is properly tuned it should not suck down BAT 1 much when starting.

 

[John_S]

Re: Do both batteries charge with the motor on?

Maybe it is a misconception, but I thought "both" was reserved for when both batteries are run down, and you need both just to get the engine to turn over/start. More of a emergency use only. The premise was based on that it is generally not a good thing to have batteries in parrellel. ie if one was fully charged and the other was very weak, putting them in "both" would cause significant current flow as the two equalized. Is this wrong?

 

[Silvertip]

Re: Do both batteries charge with the motor on?

Think about this! If both batteries are run down, why on earth would BOTH help start the engine. The object of a battery switch, ACR, or Isolator is to AVOID running down the start battery. Only the battery switch provides the most versatility. I will explain it again. BAT 1 provides starting, charging and running all accessories ONLY on BAT 1. BAT 2 provides starting, charging and running all accessories ONLY on BAT 2. BOTH provides starting, charging and running all accessories on BOTH batteries. When you are parked/anchored and away from the boat, OFF obviously turns everything off to avoid killing BOTH batteries because you left something on. Now then, when you arrive at your boat from wherever you are, you do not select BOTH to start the engine. You can of course, but why. BOTH does not spin the engine faster if both batteries are charged. You start the engine on BAT 1 (if you've selected that as your start battery). You can then leave it on BAT 1, set it to BOTH or set it to BAT 2. It all depends on where you want the charging voltage to go. The obvious place is BOTH. The exception to that would be if you neglect the engine and it needs to crank for long periods before it starts, then you would leave it on BAT 1 since that's the battery you ran down trying to start the engine. It would get the full alternator output and BAT 2 would just go along for the ride. When you anchor, you want to save the start battery so the logical move would be to set the switch to BAT 2 would it not. When you are ready to go, you know you have a fully charged BAT 1. As for paralleling batteries, you will note in my example that BOTH batteries are being charged when the engine is running and the BOTH setting is used. You do not park with the switch in BOTH which is what you are confusing. If current is going TO the batteries they are being charged. When parked you are taking current FROM them so they are discharging and will equalize but only after an extended period.

 

[John_S]

Re: Do both batteries charge with the motor on?

Think about this! If both batteries are run down, why on earth would BOTH help start the engine.

Two equal batteries in parrellel = same voltage but double the current, correct? Cranking amps is what you need to get the engine turning. If you ran one battery down to the point where it just grunts, then do the same for the second, putting it in "both" you should get your last shot at starting the engine.

I don't dissagree with any of the Bat1 and Bat2 and your recommendation for switching. My concern is just the "both" condition, ie batteries in parrellel, and my question was not pointed at what your specific suggestion for switching.

As for paralleling batteries, you will note in my example that BOTH batteries are being charged when the engine is running and the BOTH setting is used. You do not park with the switch in BOTH which is what you are confusing. If current is going TO the batteries they are being charged. When parked you are taking current FROM them so they are discharging and will equalize but only after an extended period.

After thinking more about this and in particular when I have used other batteries to jump start a weak one, it was not instantaneous. It would take time to equalize, as you stated.

I am still not comfortable about charging two batteries in parrellel, but will have to think some more on the details of why.

 

[Silvertip]

Re: Do both batteries charge with the motor on?

One more shot! Charging two batteries in parallel is not an issue provided: 1) you have an alternator with enough capacity to replensh them in a reasonable amount of time. 2) One of the batteries does not have a bad cell or is simply a bad battery. Starting an engine takes just a short burst of power from the start battery so it rarely gets deeply discharged. Trolling or house batteries are another story. They regularly get deeply discharged. So the switch provides you the opportunity to charge 1, 2, or BOTH depending on the situation and to keep the house loads away from the start battery. Put it this way, if you only used BOTH to start the engine, I agree that you could crank longer but if the engine doesn't start, you are now dead in the water. And contrary to popular belief, the engine does not spin faster on a parallel system than it does on a single battery. The starter draws only a specific amount of current. You can't force it to spin faster except by feeding it more voltage. If the engine doesn't start on BAT 1 (or 2 if that's your battery of choice) you obviously stand a pretty good chance of running down battery 2 if you switch to that one. Therefore if the engine doesn't start on one battery it very likely won't start on two in parallel either. The key is always make sure the batteries you do have are serviceable. Now then, if you are old enough to have been around diesel powered equipment running dual batteries (GM diesels of the late 70's and 80's for example), those were parallel systems and they worked just fine being charged in parallel and there was no battery switch so the batteries were permanently in parallel. Yes - if you had one bad battery it would pull down the other. But again -- this system is not to compensate for bad batteries. It is protection for the start battery. A voltage regulator senses SYSTEM voltage - not individual battery voltage so there is no problem with charging parallel systems -- it's been done for decades.

 

[Splat]

Re: Do both batteries charge with the motor on?

My post was in response to if he is using the alternator to charge his batteries, and has them hooked to the alternator through a isolator his batteries will still remain seperate circuits until the alternator is supplying current, it will then travel to both batteries providing a charge to both batteries.

The switch would allow him to draw load from either battery bank he would like, then charge both without having to touch a thing when the engine starts.

Bill

 

[Silvertip]

Re: Do both batteries charge with the motor on?

Double the complexity. The isolator is not necessary. Nothing wrong with using it but the more you add, the more chances there are for issues. There is nothing simpler and more reliable than a basic switch. If an isolator is desired by all means use it. But the flexibility is lost unless you also use a switch. The simplest solution is to keep the blasted engine tuned and learn how to start it so the start battery doesn't get run down in the first place. It drives me nuts to watch people grind away at a ramp trying to get an engine started. 50% of the time the problem is operator error and the other 50% is poorly tuned equipment.

 

[schoonerboy]

Re: Do both batteries charge with the motor on?

I would like to agree the battery switch is the most basic way to go. Charging with the Alt. or and AC battery charger on both will charge both.

Having a dedicated house battery bank that can parallel to the Start bank should there be an issue with the Start Battery is wise.

I also have to say that the ACR (Automatic charging relay) is a brilliant Idea. It connects when either battery (or Bank) has reached a full state and disconnects when either falls below a set point. So you don't accidentally leave your switch on both (paralleled) and drain your Start battery while using your DC.

They (acr's) can also be installed with a toggle switch for emergency combining of the batteries (or banks)

On charging on both-it is a good idea to have the same type and aged batteries and cycle them on a regimented schedule so you have similar charge and discharged cycles- all batteries have a limited life. Connecting two dissimilar batteries will shorten the life span of all batteries involved.

 

[Splat]

Re: Do both batteries charge with the motor on?

You both perfectly correct, and I totally agree. Its one more thing to go wrong. And while out on a boat in the middle of a lake the only thing I want to have to worry about is how the fish are biting and if there's enough ice left to keep the beer cold.

I was stating in the OP's original post there wasn't really enough information to answer the question effectively.

Bill

 

[John_S]

Re: Do both batteries charge with the motor on?

Silvertip,

The few factory parallel battery set-ups I have come across, have always been the same identical batteries wired in parallel, with the same load applied and charging circuit. ie the batteries are always close to the same charge. While I didn't own one of the GM trucks you describe, I believe that is what they had.

My main diesel experience comes from my 28hp 3cyl tractor. It has one large cca battery that is significantly larger than the one in my 350 Mercruiser. Most tractors I've seen have one large 12V battery, with two 6V in series, being the next common. Most of the time, that appears to have been done due to space reasons. I have not seen any parallel set-ups on them.

For most factory recreational runabout boats, they don?t come stock with a second battery or switch. This is usually an owner installed or maybe a dealer option. On the former, I doubt that the owner replaced with identical batteries. Along with possible battery difference, you are now combining two different charge state batteries to be charged together.

Lets say running on Bat1 you moored for most of the day with auto-bilge pump stuck running, and voltage dropped to 9V. It couldn?t turn the motor over to start. You switch to Bat2 but now you have flooded engine. You open the throttle and you eventually get it to start, but voltage has dropped to 11V. You are going for a short cruise and turn the switch to both. The combined voltage equals 10V. The alternator will put out the higher charging voltage and supply any available current to charge a 10V. In the process to get the lower battery (Bat1) to a full charge, doesn?t that expose Bat2 to an overcharging condition? I don?t think it is major but repeated times will lower the life of the battery.

Lets say bat1 didn't get to full charge and now you dock, and go chat with some people you haven?t seen since last season. Opps, forgot to go back in the engine compartment and take the switch out of both. Now you have a decent amount of current flow, while no one is present in boat. It is typically not fused either. More charge/discharge cycles, reducing battery life.

To me, Bat1, Bat2, Off settings are all pretty much idiot proof. Both can be problematic.

With over 40 years on a small lake, the people I have towed have had a dead battery from trying to start or just a motor that won?t start. Never had one from running the battery down on radio or other accessory. Most of the time it has been user error with flooding a hot engine. I still smelled the gas when I stopped to help. No way I am going to jump start these idiots. Most of them have to be reminded to turn the blower on! Sorry, only can offer you a slow tow on a long rope.