Charge Question - Fiberglass RV


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Old 12-07-2014, 03:08 PM   #1
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Charge Question

Hi,
Can I charge two batteries at one time using a Battery Tender Trickle Charger?
If so, should I link the batteries in series or parallel?
Thanks!


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Old 12-07-2014, 03:22 PM   #2
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No, definitely not is series! The instructions should be clear about whether you can charge two at a time, but if I had to guess, I would say one.
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Old 12-07-2014, 04:27 PM   #3
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Josh, it depends what you have the charger set for. If you have two 6 volt batteries wired in series (12v) you should be able to charge them both at 12v. But if you just want to charge one at a time you need to set your charger for 6v. and batteries not connected at all.
If you have two 12v batteries connected in parallel then you still just have 12v so you can charge them both at once.

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Old 12-07-2014, 06:12 PM   #4
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Batteries in series add their voltages.
Batteries is parallel the voltage is the same as the lowest battery. DON"T CONNECT DIFFERENT VOLTAGE RATED BATTERIES IN PARALLEL.
Batteries of the same voltage is parallel the current capabilities are the added.
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Old 12-07-2014, 06:18 PM   #5
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I have a group 24 12V and a group 29 12V. I have one Battery Tender I use on my bike. I want to buy a 2nd battery tender to charge the two trailer batteries in unison. I don't want to have to buy 2 more battery tenders.
Sounds like I can buy two connector cables and link the trailer batteries in parallel and then tend them both for the winter with one charger?


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Old 12-07-2014, 06:36 PM   #6
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Quote:
Originally Posted by JALEE View Post
I have a group 24 12V and a group 29 12V. I have one Battery Tender I use on my bike. I want to buy a 2nd battery tender to charge the two trailer batteries in unison. I don't want to have to buy 2 more battery tenders.
Sounds like I can buy two connector cables and link the trailer batteries in parallel and then tend them both for the winter with one charger?


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Actually you don't need a battery tender for a shelf battery.
If you have a 12V lead acid battery in good shape which is taken out of service for storage you can simply bring it to a full charge, CLEAN the top of the battery, cover the top with something like a shower cap and store it on the shelf. In this state it will loose close to nothing over several months.
If you want to see why this works, do the following...
Get a VOM,
Check for voltage before cleaning by placing one terminal on the positive post and the other on the battery top (not the neg terminal) You will likely get a reading. Clean it and do it again, you will likely get no reading.
The shower cap will keep the battery top clean while in storage and prevent power drain.
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Old 12-07-2014, 08:28 PM   #7
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Originally Posted by Philsie View Post
Josh, it depends what you have the charger set for. If you have two 6 volt batteries wired in series (12v) you should be able to charge them both at 12v. But if you just want to charge one at a time you need to set your charger for 6v. and batteries not connected at all.
If you have two 12v batteries connected in parallel then you still just have 12v so you can charge them both at once.

Phil
First, there is no indication that there are any 6 volt batteries being used. That is purely an assumption based on nothing presented here by the OP.
I would advise against charging two 12 volt batteries at the same time. The main reason is that unless your batteries are exactly the same age, used the same way for the same loads and for the same duration (in parallel configuration,) and are not used independently, (as in one or the other, but not simultaneously,) then I'll guarantee you won't have two equally discharged batteries. In fact, this condition is just about impossible to achieve in the real world. Whichever battery is discharged more than the other will cause the demand to the charger for charging current. The charger doesn't know one battery from the other, so, being a dumb piece of equipment, it will continue to charge both batteries to whatever the lowest charged one demands. This will cause the "better" battery with a higher residual charge remaining to be overcharged. The charger only senses the lowest battery and will charge to its requirements. Overcharging a battery is one of the surest ways of destroying it. My advice would be to use one or the other, and charge only one or the other. Best put, use one and charge the other, then switch them out to use the second and charge the first.
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Old 12-07-2014, 08:38 PM   #8
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I stand corrected.
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Old 12-07-2014, 09:40 PM   #9
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Good information. I'll just have to remember to swap the batteries on the tender occasionally. Thanks guys.


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Old 12-08-2014, 12:19 AM   #10
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Originally Posted by Casita Greg View Post
First, there is no indication that there are any 6 volt batteries being used. That is purely an assumption based on nothing presented here by the OP.
I would advise against charging two 12 volt batteries at the same time. The main reason is that unless your batteries are exactly the same age, used the same way for the same loads and for the same duration (in parallel configuration,) and are not used independently, (as in one or the other, but not simultaneously,) then I'll guarantee you won't have two equally discharged batteries. In fact, this condition is just about impossible to achieve in the real world. Whichever battery is discharged more than the other will cause the demand to the charger for charging current. The charger doesn't know one battery from the other, so, being a dumb piece of equipment, it will continue to charge both batteries to whatever the lowest charged one demands. This will cause the "better" battery with a higher residual charge remaining to be overcharged. The charger only senses the lowest battery and will charge to its requirements. Overcharging a battery is one of the surest ways of destroying it. My advice would be to use one or the other, and charge only one or the other. Best put, use one and charge the other, then switch them out to use the second and charge the first.

This makes no sense at all.

Batteries of the same voltage that are used in parallel will equalize in either discharging or charging. It doesn't matter on the condition or current rating, they will charge and discharge in an equalized manner.
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Old 12-08-2014, 05:24 PM   #11
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Originally Posted by Byron Kinnaman View Post
This makes no sense at all.

Batteries of the same voltage that are used in parallel will equalize in either discharging or charging. It doesn't matter on the condition or current rating, they will charge and discharge in an equalized manner.
You would think so, and given enough time, it is so.
There is something I call a cascade effect... if the charger is wired to one battery and the charge time is consistently insufficient then the down stream battery will die a slow death.
Semi trucks which are wired this way and are run over the road do ok, but when they are used for yard work with frequent starts and very short run times, we would consistently loose the "down stream" battery.
A solution which works fairly well is to tap onto the positive post on one battery and the negative on the other when the two are attached in parallel. Experience alone seems to indicate a more even charge.
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Old 12-08-2014, 06:08 PM   #12
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Originally Posted by floyd View Post
You would think so, and given enough time, it is so.
There is something I call a cascade effect... if the charger is wired to one battery and the charge time is consistently insufficient then the down stream battery will die a slow death.
Semi trucks which are wired this way and are run over the road do ok, but when they are used for yard work with frequent starts and very short run times, we would consistently loose the "down stream" battery.
A solution which works fairly well is to tap onto the positive post on one battery and the negative on the other when the two are attached in parallel. Experience alone seems to indicate a more even charge.
Good point.

The parallel connection I think you're describing reduces the chances of connections between the batteries being defective. The whole system fails if the connection fails. For that reason it's a good idea to connect the charging and load with one connection to each battery for 2 batteries for more the connections to load and charging at the corners.
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Old 12-10-2014, 11:14 AM   #13
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If you go into the marine world (as in expensive) you can get dual or more output chargers to charge and maintain individual batterys. I use one on my Boston Whaler.

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Old 12-10-2014, 05:37 PM   #14
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Casita Greg is correct in saying that 2 different 12v batteries in parallel will not charge 100% completely as the weaker battery will pull most of the load. Also when charging in parallel be sure and connect the charger to the negative on one and the positive on the other, do not just connect to the first battery and expect good results. See here http://www.gearseds.com/files/twobat_onechgr2.pdf
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