Marine Electronics Forum - Battery questions

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firstbase
03-27-2012, 07:10 PM
A couple of questions concerning upcoming purchase of 2 new batteries:

1. I know not to mix types of batteries i.e. one wet cell, one AGM, etc. Is it ok to mix Group Size? Have one more powerful than the other such as a 1000 CA and a 550 CA?

2) I am replacing two Group 24, 550 CA hybrid batteries. I would like to have as much capacity as possible so I would like to get 2 1000CA batteries if I can fit them in the space available. Can you overpower a motor? My Merc 225EFI requires 450 minimum CA. I know it is all 12V but can you go too big?

Thanks in advance for comments.


dreamin-on
03-27-2012, 07:26 PM
A couple of questions concerning upcoming purchase of 2 new batteries:

1. I know not to mix types of batteries i.e. one wet cell, one AGM, etc. Is it ok to mix Group Size? Have one more powerful than the other such as a 1000 CA and a 550 CA?

2) I am replacing two Group 24, 550 CA hybrid batteries. I would like to have as much capacity as possible so I would like to get 2 1000CA batteries if I can fit them in the space available. Can you overpower a motor? My Merc 225EFI requires 450 minimum CA. I know it is all 12V but can you go too big?

Thanks in advance for comments.

Answers;

1) No, do not do this. When you discharge the two, the smaller battery will be completely discharged first and then draw down the larger battery in the circuit. You risk physical damage to the batteries if you do this.

2) Yes 2 bigger batteries are fine and you wont damage your motor.

I don't know your specific application but for most, 1 big AGM battery will do just fine.

firstbase
03-27-2012, 08:16 PM
Thanks dreamin. I have a 25' CC with a 225EFI. Run a Raymarine E90W chartplotter/FF, Fusion stereo w/no amp and anchor light. The normal stuff, not excessive draw on the batteries. We do like to sit out fishing w/stereo, fishfinder and VHF on. I am replacing 2 Marine Master Group 24 batteries w/550 CA each. I guess I could go to one battery but do like switching to 1 when floating around so I have a fresh #2 when needed.


dreamin-on
03-27-2012, 08:33 PM
Ahhh. I misread your post. Thought you only had 1 battery and was wanting to go with two wired in parallel.

In that case I'd go with two of the biggest AGM batteries that will fit. Sears Platinum AGM's are the hot ticket right now and should make you a happy camper.

firstbase
03-28-2012, 03:52 AM
Exactly the ones I am looking at, the PM-1 or PM-2....heopfully I can get the PM-1 to fit. A little longer by a couple of inches.

firstbase
03-28-2012, 04:32 AM
One other issue with my replacements would be weight. WIth the Group 31's I would be almost doubling the battery weight to 150lbs. Extra 75lbs in the aft starboard corner. Not sure how much effect that have on boat balance, cause a slight list to starboard. Boat is a Pursuit 2470. 75lbs enough to make a difference that matters? No way to move them inboard or split the install to either side.

yandina
03-28-2012, 09:03 AM
Answers;

1) No, do not do this. When you discharge the two, the smaller battery will be completely discharged first and then draw down the larger battery in the circuit. You risk physical damage to the batteries if you do this.

2) Yes 2 bigger batteries are fine and you wont damage your motor.

I don't know your specific application but for most, 1 big AGM battery will do just fine.

OMG this is so wrong but a common mistake.

If you discharge the two they discharge uniformly, you cannot discharge the smaller one first. It cannot draw down the larger battery. It does not risk damage if you do this.

You CANNOT discharge the smaller battery faster because they are in PARALLEL. This means they have to be at the SAME voltage. To discharge the smaller one more than the larger, it would have to be at a LOWER voltage but this is not possible, they are connected together and at the SAME voltage.

This common mistake is created by assuming that each battery supplies equal current which would run the smaller battery down first but in fact each battery supplies current IN PROPORTION to its capacity and they share the load (and charging) current exactly in proportion to their size.

In situations where you just have 2 batteries in parallel without any isolation, it is not a bad idea to have one a high CCA to provide current for starting and the other a deep cycle to supply capacity for house loads.

Better still is use a Combiner100 to isolate a starting battery from a deep cycle house battery so you will always have sufficient power to start the engine.

dreamin-on
03-28-2012, 10:52 AM
OMG this is so wrong but a common mistake.

If you discharge the two they discharge uniformly, you cannot discharge the smaller one first. It cannot draw down the larger battery. It does not risk damage if you do this.

You CANNOT discharge the smaller battery faster because they are in PARALLEL. This means they have to be at the SAME voltage. To discharge the smaller one more than the larger, it would have to be at a LOWER voltage but this is not possible, they are connected together and at the SAME voltage.

Not true my friend. Yes they are at the same voltage because they are in parallel, but batteries of different sizes have a different internal resistance.

This common mistake is created by assuming that each battery supplies equal current which would run the smaller battery down first but in fact each battery supplies current IN PROPORTION to its capacity and they share the load (and charging) current exactly in proportion to their size.

At some point, the smaller battery will lose voltage faster in this instance, and when it's voltage is lower then the larger one it becomes a load.

In situations where you just have 2 batteries in parallel without any isolation, it is not a bad idea to have one a high CCA to provide current for starting and the other a deep cycle to supply capacity for house loads.

Better still is use a Combiner100 to isolate a starting battery from a deep cycle house battery so you will always have sufficient power to start the engine.

When you connect two batteries in parallel they should be the EXACTLY the same. A physically larger battery has a larger CAPACITY than a smaller one therefore they ARE NOT THE SAME.

The current output for each battery connected in parallel will be in porportion to its internal resistance. Smaller batteries have a higher internal resistance.

The smaller one WILL run down faster. In the same respect the smaller one will charge faster. This is because the discharge characteristics for two different size batteries are different even though their construction may be exactly the same.

firstbase
03-28-2012, 06:38 PM
Well now. One vote for yes you can. One vote for no you can't. Guess that clears it up!

Lyle29464
03-28-2012, 06:51 PM
"If the voltage " was ever different the higher voltage battery would charge the lower volt battery to the same level. Ok I have done it now. Pissed off a neighbor.

firstbase
03-29-2012, 04:46 AM
"If the voltage " was ever different the higher voltage battery would charge the lower volt battery to the same level. Ok I have done it now. Pissed off a neighbor.

Is that a vote for yes I can or no I cant?!? Being a complete amateru on batteries I am surprised that it isn't an exact science. It would seem that the questions regarding using two different capacity batteries would be easy to test and sort out....seems not!

Glen E
03-29-2012, 04:49 AM
You hook two healthy batteries together that are the same type in parallel and you have one big battery...end of story...the others here are right but making it too complicated. I have rigged boats for years and this is all you need to know if you want a simple, safe answer....

firstbase
03-29-2012, 05:39 AM
Thanks Glen. It would seem like the best set up would be a good size deep cycle AGM and a good size starter AGM, both with as much capacity that room allows, would be the best for my 25' CC. Use both for starting, flip to battery 2 the deep cycle when sitting fishing and listening to music, lights, etc. Anything wrong with that thinking?

yarcraft91
03-29-2012, 06:26 AM
Thanks Glen. It would seem like the best set up would be a good size deep cycle AGM and a good size starter AGM, both with as much capacity that room allows, would be the best for my 25' CC. Use both for starting, flip to battery 2 the deep cycle when sitting fishing and listening to music, lights, etc. Anything wrong with that thinking?

Wrong, no. Unnecessary- if your motor only needs a 450 CCA starting battery, either one of those batteries alone is more than enough to do the job. No need to use both for starting. Use the starting battery for starting- it's designed to better tolerate the high, short-term current demands of starting. Use the deep-cycle for starting only if the starting battery goes dead.

dreamin-on
03-29-2012, 06:47 AM
We got a little sidetracked. Sorry about that.

Anyhow, AGM batteries are neither starting nor deep cycle. They can be used for both purposes.

firstbase
03-29-2012, 07:41 AM
Thanks for all the comments. I believe the rule is 150% of minimum CCA on the engine, correct? That would put me at needing 675 CA for the motor. Diehard Platinum 31M comes in at 1150 CA. Seems like two of these would be approaching overkill (if having too much in the battery area is possible). My concern about these is really the additional weight in the aft corner of the boat. I am doubling the weight, going up 75 lbs. Wonder if that is going to be a problem as I can't move them inboard at all.

dreamin-on
03-29-2012, 09:30 AM
Thanks for all the comments. I believe the rule is 150% of minimum CCA on the engine, correct? That would put me at needing 675 CA for the motor. Diehard Platinum 31M comes in at 1150 CA. Seems like two of these would be approaching overkill (if having too much in the battery area is possible). My concern about these is really the additional weight in the aft corner of the boat. I am doubling the weight, going up 75 lbs. Wonder if that is going to be a problem as I can't move them inboard at all.

I dont know on that one. Maybe put some extra weight back there and see how she does.



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