More battery charger questions

[BOWMAKER1]

The other thread got me to thinking about battery chargers. I want to get a 68 pound 24 volt Minn Kota Powerdrive for my pontoon. I have a compartment up front on a corner that should hold both batteries. My question is in order to charge both batteries with just a portable charger, do I need to disconnect the series connections and charge one battery at a time or can I leave them connected and hook the charger to a positive post on one and the neg on the other battery? I know this sounds like a silly question, but I have never had anything that is 24 volts before. Eventually I want to have an on-board charger and understand that you can get one with 2 or even three charging banks, but for now how would I do it? For anyone that might already have this trolling motor, what would you do about the remote co-pilot feature, not the I pilot.

 

[Silvertip]

Re: More battery charger questions

You do NOT need to disconnect the jumper between the two batteries. You MUST however, charge each battery separately and that means connecting the charger (both leads) to one battery. when that battery is charged, you remove BOTH leads and connect them to the other battery. It doesn't matter which one you charge first. You must NEVER connect the POS lead of the charger to the POS post on one battery and the NEG lead from the charger to the NEG post on the other battery because the charger is a 12 volt device and connecting across both batteries means you put the charger across a 24 volt source.

 

[cemcrae53]

Re: More battery charger questions

Your other option is to swap the cables on one of the batteries, making them one large 12 volt system. Now just hook the charger to one battery.

 

[Silvertip]

Re: More battery charger questions

Your other option is to swap the cables on one of the batteries, making them one large 12 volt system. Now just hook the charger to one battery.

Still takes the same amount of time. The only difference is charging each individually means you only have to switch the charger. Charging both simultaneously means you need to disconnect the jumper, create another jumper to get pos to pos on the batteries and then connect the charger. Too much fuss for no advantage.

 

[UncleWillie]

Re: More battery charger questions

Or, get a 24volt charger, and charge them as one battery.
You are draining them as one.

http://www.batterystuff.com/battery-chargers/brands/battery-tender/PT24v022-0158.html

 

[M9.9]

Re: More battery charger questions

Hi OP and all,
Noticed you mentioned you have a spot for the batterys on the 'Front' on the corner of your Pontoon. If your considering doing a long heavy gauge cable run between your intended trolling motor 30+ amps and the batterys, suggest you don't. Battery efficiency will go down the head. You might consider keeping the trolling motor batterys as close to the trolling motor as possible.

 

[Silvertip]

Re: More battery charger questions

There is a reason that device is called a "power tender"! At 2.5 amps output it would take forever to actually "charge" a pair of deeply discharged batteries. Running a troller for 4 or 5 hours at a 30 amp discharge rate sucks 150 amps out of the pair. That would take in excess of 60 hours to charge the pair. That's not good for the batteries and if one is using the boat every day it would simply not be able to keep up with the needs. That device is intended to "Maintain" a fully charged or slightly discharged system during periods of non-use..

 

[Monterey10]

Re: More battery charger questions

Either, you get a 24 volt charger, or you have two separate 12volt chargers, one on each battery. Deep cycle batteries will need larger, charger if you expect to use the boat each day.

 

[dwparker99]

Re: More battery charger questions

There is a reason that device is called a "power tender"! At 2.5 amps output it would take forever to actually "charge" a pair of deeply discharged batteries. Running a troller for 4 or 5 hours at a 30 amp discharge rate sucks 150 amps out of the pair. That would take in excess of 60 hours to charge the pair. That's not good for the batteries and if one is using the boat every day it would simply not be able to keep up with the needs. That device is intended to "Maintain" a fully charged or slightly discharged system during periods of non-use..

I agree that the recommended "tender" is more of a maintainer than a charger. However, it does raise a question. Does a 24v 2.5A charger for a 24v system charge like a 12v 5A charger for a 12v system time wise? I have a 12@7.2A/24v@3.7A charger that I use for my 24v system on my 16' Alumacraft. My initial thought was to charge each battery individually at 7.2A. Drawing from my Navy electronics training from more than 30+ years ago makes me believe that pushing 3.7A @ 24v thru two 12v batteries would be the same as pushing 7.2v @ 12v thru one battery. If I were to recommend a 24v charger to the OP for charging Grp 27 or larger batteries, what current would I recommend?

 

[sschefer]

Re: More battery charger questions

If the batteries are identical then in theory you could charge them both by connecting your charger to the first battery. The problem is that you're batteries might not be exactly the same even though they look the same and are the same age. You could end up overcharging one if one battery if one is slightly weaker than the other. That will eventually result in premature failure. There is a solution and that is a battery switch and automatic charging relay. Here's a pretty simple one that works well. http://www.iboats.com/Blue-Sea-Syst...9835447--**********.641464550--view_id.729810 This will let you hook up your charger and not risk battery damage.

When you get around to it, Pro-Mariner makes nice 20 and 40 amp onboard tripple battery charges. two of the charging circuits are designed for deep cycle batteries and one is for starting or dual purpose batteries. From past experience I would go with the 40amp charger on group 27 batteries. The 20amp takes about 10 hours to bring 27's back to full from 60% charge.

 

[Silvertip]

Re: More battery charger questions

I agree that the recommended "tender" is more of a maintainer than a charger. However, it does raise a question. Does a 24v 2.5A charger for a 24v system charge like a 12v 5A charger for a 12v system time wise? I have a 12@7.2A/24v@3.7A charger that I use for my 24v system on my 16' Alumacraft. My initial thought was to charge each battery individually at 7.2A. Drawing from my Navy electronics training from more than 30+ years ago makes me believe that pushing 3.7A @ 24v thru two 12v batteries would be the same as pushing 7.2v @ 12v thru one battery. If I were to recommend a 24v charger to the OP for charging Grp 27 or larger batteries, what current would I recommend?

No - It doesn't matter what the "output voltage" of the charger is in relation to the "output current". To charge a 24 volt system simultantously with a single output charger you obviously require a 24 volt charger. How long it takes to charge that system depends on the "output current" of the charger. If you suck 30 amps out of two 12 volt batteries in series you need to replace 15 amps in each battery. Therefore a 2.5 amp 24 volt charger would take about 15 hours to do the job. A 7.5 amp 12 volt charger would take just 2 hours per battery or a total of 4 hours although you need to charge them individually. If you used a single battery and sucked 30 amps from it, that same 7.5 amp 12 volt charger would charge the battery in about the same 4 hours. Remember, batteries in series look like a "one" big battery with double the voltage, but not double the capacity (AHr) rating to a charger. Batteries in parallel have the same voltage as a single battery but 2, 3, or 4 times the AHr capacity of a single battery depending on how many there are in the system. This entire concept is no different than analyzing a single 12 volt battery which is nothing more than a group of six 2.1 volt batteries in series but contained in single box to create a 12.6 volt output.

When selecting a charger you need to determine the number of outputs you need and then consider that flooded, lead-acid batteriy can be charged at a rate up to 20% of the AHr rating of the battery. A 100 AHr battery can be charged at up to a 20 amp rate. You can use a lesser charge rate but the charge rate is directly to the time it takes to charge the battery. higher rate = less time and lower rate = more time. Batteries that are routinely deeply discharged -- such as trolling motor batteries, should be recharged as quickly as possible which dictates use of a charger with a higher capacity. My suggestion is that for trolling motor batteries one should have a charger with absolutely no less than 6 amps output per bank. 8 or 10 amp would be prefered. Starting batteries will live nicely during storage or periods of non-use with a smart 1.5 - 2 amp battery tender plugged in continuously.

 

[UncleWillie]

Re: More battery charger questions

The purpose of suggesting the "Tender" is that the OP is looking for a Temporary solution.
This was a "Food for Thought" suggestion, not a "This is your Best Choice" decision.

He may not have considered the 24 Volt Charger option, and would factor in his cost Vs. performance consideration in his final decision.
He may only go out once a week, so taking 3 days to topoff the batteries may not be a big factor.
If he wants to go out three times on a weekend then a larger ($$$) system would be justified.

One 12volt battery is really six 2volt batteries (Cells) in series.
A Single 24volt battery would just be twelve 2 volt cells in series in a single case.
Two 12volt batteries in series is the same thing as one 24volt battery.

Ideally the two 12 volt batteries should be purchased as a pair.
Even identical twins have differences. Same with batteries.
When you use discharge the pair, the (Slightly) weaker one will discharge more.
If you charge them as a pair the weaker one will require more charge.
The same thing is going on in each individual cell of the batteries.
Every cell in the battery has a slightly different capacity.
That is why you always slightly overcharge, you need to be sure you have fully charged the weakest link (Cell).

A long slow charge is prefer over a quick charge any day.
Less heat. Less water boil off. Less outgassing.

Silvertip,
It is all about power!
7.2A @ 12v = 86.4 Watts
3.7A @ 24v = 88.8 Watts (Essentially the same!)

The 7A,12 volt charge will take half the time.
But now you still need to charge the other battery.
And you are back to where you started. It's a wash!

 

[Silvertip]

Re: More battery charger questions

No -- it's not about power (watts). Your forumulas are correct. But the appplication of those formulas is not. In my example of sucking 30 amps from a 24 volt system I used a 2.5 amp 24 volt charger as the baseline. A 2.5 amp, 24 volt single output charger will take 12 hours (30/2,5) to recharge those two batteries. It will actually take much longer than that since the last 10% or so is at a much lower charge rate. So now let's use a single output, 12 volt, 7.5 amp charger to do the same job. You didn't suck 30 amps from both batteries. You sucked 15 amps from each. So at a 7.5 amp charge rate, battery #1 is charged in about 2 hours. Switch to battery #2 and you have that one done in another two hours. Therefore, total time = 4 hours. Geez -- a 7.5 amp charger has three times the output of a 2.5 amp charger. How can you even argue that. Do the same experiment. At a 3.7 amp charge rate at 24 volts it takes 8.1 hours (30/3.7 = 8.1) hours to charge the two. I still get the job done twice as fast using a 7.5 amp 12 volt charger.

 

[UncleWillie]

Re: More battery charger questions

... If you suck 30 amps out of two 12 volt batteries in series you need to replace 15 amps in each battery.

You need to replace 30Amps in each battery. (Actually AmpHours but we'll let that go)
Two 12 volt batts in series = 24volts.
30A x 24Volts = 720 Watts
30A x 12Volts = 360 Watts

To replace the 720 Watts, you need to replace 360 Watts in each battery.

Therefore a 2.5 amp 24 volt charger would take about 15 hours to do the job.

Agree ...12 hours theoretically but about 15 in reality due to inefficiencys.

A 7.5 amp 12 volt charger would take just 2 hours per battery or a total of 4 hours

30amp / 7.5amps = 4 hours per battery. 8 Hours Total. And about 10-12 in reality.

Remember, batteries in series look like a "one" big battery with double the voltage, but not double the capacity (AHr)

Agree ... But when you split them back apart, do not halve the capacity.
30A@24v = 30@12v Twice ... Not 15A@12v Twice

Batteries in parallel have the same voltage as a single battery but 2, 3, or 4 times the AHr capacity of a single battery depending on how many there are in the system. This entire concept is no different than analyzing a single 12 volt battery which is nothing more than a group of six 2.1 volt batteries in series but contained in single box to create a 12.6 volt output.

Absolutely Agree.

 

[Silvertip]

Re: More battery charger questions

Stay tuned! I'll prove one of us wrong.

 

[tx1961whaler]

Re: More battery charger questions

Stay tuned! I'll prove one of us wrong.

Make sure you include power and time in the next set of calculations. Convert everything to watt-hours (Joules) and everything becomes clear.

 

[UncleWillie]

Re: More battery charger questions

Make sure you include power and time in the next set of calculations. Convert everything to watt-hours (Joules) and everything becomes clear.

+1

It IS always about power!
There is no free lumch!

 

[Silvertip]

Re: More battery charger questions

No need to. Here are the pictorial results and I am looking for a fork and spoon so I can eat some humble pie. I know better but had my head so wrapped up in the parallel scenario the series bit clouded my thinking. So -- that said, here is test best that hopefully will help others understand what this discussion was about. Placing an ammeter between the two batteries would show the current flow between the two batteries. The motor represents anything being powered from the system. That motor has an inrush current of about 15 amps and a steady state (no load) current of about 3 amps. Moving the meter to the second battery shows the total current draw from the system and in that position it too showed inrush of 15 amps and a steady state current draw of about 3 amps. It really doesn't matter where the meter is placed in the system since it would show the same current draw everywhere. Without going back to the point where this thread ran off the rails, the net result is one must be careful (as I wasn't) in how you look at the charging situation. To keep numbers simple, to put 30 amps back into a 24 volt system (lets use one hour as an example) you would need a 24 volt charger with a 30 amp output, 15 amps in two hours, etc. The same holds true for 12 volt systems. If you suck 30 amps from a 12 volt battery in one hour you need a 12 volt charger with a 30 amp output to do the job in an hour. 15 amps in two hours, etc. I apologize for leading anyone astray here and thanks Uncle Willie for keeping me honest. When I'm done with my humble pie I'll take 30 lashes with a wet noodle. One last little tidbit here is that some dual bank chargers permit bridging of outputs in which the negative lead from one output is tied to the Positive lead of the other. The remaining negative and positive pair then outputs 24 volts which can be used on a 24 volt system. A note of caution though is that not all chargers allow that configuration.

 

[UncleWillie]

Re: More battery charger questions

... When I'm done with my humble pie I'll take 30 lashes with a wet noodle.

Absolutely no need to. My hat is off to you Silvertip!! You are a Gentleman and a Scholar!
And I will eat the Humble pie if my gender bias proves to be wrong.

If anyone reading this thinks it was some kind of contest with a winner and a loser, you are incorrect.
This has been a search for the truth and an agreement among individuals.

My bigger concern is that we are preaching to the choir.
Whatever happened to the OP.

BOWMAKER1 ... Are you still out there?

 

[dwparker99]

Re: More battery charger questions

Uncle Willie, don't think that because you are not hearing a hearty "Amen" every now again that the congregation is not listening or learning. I agree there should be no losers in a debate. We all win by becoming more knowledgeable. It is unlikely we will even remember who was on either side, but hopefully we'll remember the facts that came out. I currently have 15 lead acid batteries to maintain (9 being marine) that represents a $1200-$1500 investment. I would like to get as much service out of them as possible. I would like to discuss charging methods (low & slow vs fast and furious) and advantages/disadvantages of charging with a 24v charger vs a 12v charger. However, instead of taking the risk of high-jacking this thread, I'll do a little more research and then start a new thread. Good debate, thanks.