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How to Charge 2 Batteries?

Old 06-08-2009, 07:31 PM
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Default How to Charge 2 Batteries?

Two spare batteries in the storeroom get trickle charged about once/month.
It would be simpler if I could hook them up for Charging Both at the same time.

Can you battery (or charging experts) tell me:
a. Is it possible to charge both at once?
b. Is there a reason why I wouldn't want to do it?
c. If it is possible, how would the wiring go and would I be better off disconnecting that wiring after a charge, or could I leave it there for the next time?
d. If the charging is a 2Amp trickle, would wiring have to be much more than 12 gauge?

Thanks!
Old 06-08-2009, 07:34 PM
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I charge two all the time. First you need to link the two batteries together. Positive to Positive and negative to negative. Then put one charger clamp on the Positive of one battery and then the neg on the neg of the other battery.

Turn the charger on and you are all set. In fact if you get a float trickle charger, you can just leave it hooked up all the time. First charge the batteries to bring them all the way up. Hook up the float trickle charger and you should be good to go.

I would use regular battery wire to link the batteries togther, I would use at least 6 gauge. If you don't leave the trickle charger on then don't leave the batteries hooked up. If one is weak it will drain the other one.
Old 06-08-2009, 07:48 PM
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Thanks for the education, Tony M.

Just curious about the cable size, though. Starting loads up to 850Amps...I can see the reason for large cable. For a charging load of 2Amps, why the need for large cable?
What's the worst that could happen if using 12Gauge wire to jumper the batteries?

No, it isn't about cost or availability; just the reasoning behind a process.
Old 06-09-2009, 04:22 AM
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Lighter gauge wire is fine.
Old 06-09-2009, 04:40 AM
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If you charge both at the same time you risk having one at full charge and the other not. The reason is that the charger will sense the fullest charged battery and cut back on the charge level when it is fully charged.
That's why you see multi-bank chargers. On my boat I have 4 batts and a 4 bank charger so each battery receives it's own, full charge.
Old 06-09-2009, 10:08 AM
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yandina, what about a combiner for that situation..

jan
Old 06-09-2009, 10:45 AM
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Originally Posted by Tony M View Post
I charge two all the time. First you need to link the two batteries together. Positive to Positive and negative to negative. Then put one charger clamp on the Positive of one battery and then the neg on the neg of the other battery. ........
No need to put the negative lead on the "other" battery. They are connected together in parallel.

The wire gauge only has to be large enough to carry the charging current. Look at the maximum output of the charger and use that to figure wire size.
Old 06-09-2009, 10:48 AM
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Originally Posted by gsniffen View Post
If you charge both at the same time you risk having one at full charge and the other not. The reason is that the charger will sense the fullest charged battery and cut back on the charge level when it is fully charged.
That's why you see multi-bank chargers. On my boat I have 4 batts and a 4 bank charger so each battery receives it's own, full charge.
Nope, if you connect a fully charged battery in parallel with a partially discharged battery, the fully charged battery will attempt to charge the partially discharged battery. In time, they will equalize and both will be charged by the charger.

The reason you have a four bank charger is that the batteries are not connected together so one charger cannot charge them all at the same time.
Old 06-09-2009, 10:52 AM
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Originally Posted by bamaboy473 View Post
What's the worst that could happen if using 12Gauge wire to jumper the batteries?
The worst? If one battery shorts (unlikely, but it can happen), the 12 gauge wire will melt as the current from the good battery tries to charge the shorted battery. To be safe, use an inline fuse in the wire connecting the positive terminals together. 20 amp should be just fine for #12 wire.
Old 06-09-2009, 11:10 AM
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....so the worst that could happen would be that I might lose a short piece of 12Ga wire?

Looks like a couple pieces of spare wire, alligator clips for Pos/Pos and Neg/Neg connections, then one lead to battery 1 and the other lead to battery 2 and plug in the charger.

Seems like a simple way to do the batteries rather than doing two separate charging cycles.

Thanks, guys.
Old 06-09-2009, 11:52 AM
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Originally Posted by rwidman View Post
Nope, if you connect a fully charged battery in parallel with a partially discharged battery, the fully charged battery will attempt to charge the partially discharged battery. In time, they will equalize and both will be charged by the charger.

The reason you have a four bank charger is that the batteries are not connected together so one charger cannot charge them all at the same time.
You are absolutely incorrect.
Old 06-09-2009, 12:08 PM
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Originally Posted by bamaboy473 View Post
....so the worst that could happen would be that I might lose a short piece of 12Ga wire?

Looks like a couple pieces of spare wire, alligator clips for Pos/Pos and Neg/Neg connections, then one lead to battery 1 and the other lead to battery 2 and plug in the charger.

Seems like a simple way to do the batteries rather than doing two separate charging cycles.

Thanks, guys.
No, the worst that could happen would be a fire caused by the melting wire. A couple $$ for a fuse & holder will let you sleep at night.

And while you can connect the charger to both batteries, there's no need to. Connect it to both terminals of one battery. It's less confusing that way.
Old 06-09-2009, 06:10 PM
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Originally Posted by gsniffen View Post
You are absolutely incorrect.
gs

Not clear what he's "absolutely incorrect" about. He is absolutely correct about the fact that if two batteries are parallelled and on a charger, eventually they will end up with an equal charge, no matter how far apart (one with high charge, one with low) they were at the start. Of course, it may take while. (Assuming neither battery is shorted or otherwise defective.)

As to whether he's incorrect about how you have your boat's banks and charger set up, I've of course got no idea whatsoever; you could even say "absolutely" no idea.
Old 06-09-2009, 07:33 PM
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The reason for putting one clip on the positive of one battery and on the negative of the other is so that they do equalize and if you are using a float charger it does not top one off early never bringing the second one up to full charge.

Why use 12 guage wire when you can go to any auto supply store and buy inexpensive short made up battery cable that you don't need to worry about. I wouldn't use it on the boat but it works for charging two batteries. I do it all the time.
Old 06-10-2009, 05:13 AM
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Originally Posted by Tony M View Post
The reason for putting one clip on the positive of one battery and on the negative of the other is so that they do equalize and if you are using a float charger it does not top one off early never bringing the second one up to full charge.
When you connect the batteries together, positive to positive and negative to negative, it doesn't matter where you connect the charger (or the load for that matter). You could even connect the charger leads to the middle of the wires. Makes no difference. Draw it out on paper and it should be clear.

We're not filling buckets of water, we are charging batteries. Electricity is fast!
Old 06-10-2009, 05:16 AM
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Originally Posted by KeyPineSavage View Post
....... if two batteries are parallelled and on a charger, eventually they will end up with an equal charge, no matter how far apart (one with high charge, one with low) they were at the start. ...............
They don't even have to be on a charger, they will still equalize.
Old 06-10-2009, 09:34 AM
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Originally Posted by rwidman View Post
They don't even have to be on a charger, they will still equalize.
So true!
Old 06-10-2009, 10:01 AM
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Originally Posted by gsniffen View Post
If you charge both at the same time you risk having one at full charge and the other not. The reason is that the charger will sense the fullest charged battery and cut back on the charge level when it is fully charged.
That's why you see multi-bank chargers. On my boat I have 4 batts and a 4 bank charger so each battery receives it's own, full charge.
A charger does not really sense battery voltage. What it does sense is its own oputput voltage and it tries to maintain that voltage. If a discharged battery is connected to the charge source the current flow will be heavy causing the voltage to drop. The charger will make an internal adjustment to try and maintain the setpoint of its output voltage by increasing current flow.

Current always flows from high voltage to low voltage. Connect two batteries in parallel and current will flow from the one with the higher voltage to the lesser voltage, until their voltages are the same in which case current will cease to flow. Connect a charge source, with a voltage that is higher than the two batteries combined, and current will then flow from the charge source to the batteries. The current flow to each of the batteries will then depend up the internal resistance of each battery.

The charge source voltage will be higher than either of the two batteries and current will flow to the two batteries, each according to its own needs. Connect an ammeter at the output of the charge source and you will see total current flow. Connect an ammeter to each battery and you will see that they draw different amounts of current. The differing amounts when added together will equal the current flow that you see at the first ammeter.

We need the Mythbusters to work on battery myths and misconceptions.
Old 06-10-2009, 10:31 AM
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Originally Posted by jethro1 View Post
A charger does not really sense battery voltage. What it does sense is its own oputput voltage and it tries to maintain that voltage. If a discharged battery is connected to the charge source the current flow will be heavy causing the voltage to drop. The charger will make an internal adjustment to try and maintain the setpoint of its output voltage by increasing current flow.

Current always flows from high voltage to low voltage. Connect two batteries in parallel and current will flow from the one with the higher voltage to the lesser voltage, until their voltages are the same in which case current will cease to flow. Connect a charge source, with a voltage that is higher than the two batteries combined, and current will then flow from the charge source to the batteries. The current flow to each of the batteries will then depend up the internal resistance of each battery.

The charge source voltage will be higher than either of the two batteries and current will flow to the two batteries, each according to its own needs. Connect an ammeter at the output of the charge source and you will see total current flow. Connect an ammeter to each battery and you will see that they draw different amounts of current. The differing amounts when added together will equal the current flow that you see at the first ammeter.

We need the Mythbusters to work on battery myths and misconceptions.
Jethro1, rwidman and KPS are correct. Without going into an epic dissertation, the charge state of any storage cell (including individual cells within a multi-cell battery) determines internal resistance, and thus current flow, into that individual cell. Differences in cell chemistry aside, the availibility of a potential (voltage) high enough to cause a net increase in bound electrons (charge) is the sole criteria for charging a cell.

Battery chargers connected to multiple batteries see the net voltage and detect the net current flow. The amount that "goes" to each battery is a function of internal resistance ("need") of each battery, and does not require anything other than "good" interconnections for the cell network to charge correctly. Note that not all cells will charge at the same rate (even within the same battery), but the net resistance of the cell network will ultimately determine charger behavior - the charger simply sees the battery network as one large battery.

In lay terms, provide enough voltage (with the ability to support the current flow) to a (partially) depleted cell and the cell will add charge until it is saturated. This is simple storage cell chemistry. Overcharging occurs when poorly-designed chargers do not properly detect saturation and reduce input voltage to the cell.

Look at is this way - charging multiple batteries is no different than charging one battery because boat batteries are themselves "mutliple batteries" - they're collections of individual cells! When was the last time you saw a charge balancing system (other than the cell interconnections) inside of a conventional starting or house battery?

Mike
Old 06-10-2009, 07:20 PM
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Ok I have an idea instead of going to the mythbusters, lets simplify this discussion and go right to the source from one of battery charger manufactures. Try this link.

http://www.batteryminders.com/batter..._batteries.php

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