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Thread: any problem with two batteries in parallel

  1. #1
    Registered User f14rio's Avatar
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    any problem with two batteries in parallel

    on one charger?



    thx,
    ed

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  2. #2
    Cam Killer marchyman's Avatar
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    It may be done. The proper way to connect the charger is to connect to the positive terminal of one battery and the negative terminal of the other battery. That balances the voltage drop seen by the charger across both batteries. You can get away without doing that when charging at low current. It becomes an issue at higher charging currents. Easy enough to do it right.

  3. #3
    Small road corner junkie pffog's Avatar
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    Quote Originally Posted by marchyman View Post
    The proper way to connect the charger is to connect to the positive terminal of one battery and the negative terminal of the other battery. That balances the voltage drop seen by the charger across both batteries. .
    How do you figure?? If the batteries are connected the voltage readings will be the same regardless.

    But back to the question, both batteries should start at about the same charge and voltage levels. Otherwise when you hook them together there could be a high current flow into the weaker battery, over heating it.
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  4. #4
    Lucky motorradmike's Avatar
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    Why can't you just switch the charger to the second battery every other week?

    If it's because your F-111 and your bike are both stored in a cave in the desert then I say you can buy a second charger.

    If you're really bent on doing it you can but I'd use diodes to isolate the batteries. This causes other issues but they can be mitigated.
    Mike Marr
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  5. #5
    neanderssance man sedanman's Avatar
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    Quote Originally Posted by marchyman View Post
    It may be done. The proper way to connect the charger is to connect to the positive terminal of one battery and the negative terminal of the other battery. That balances the voltage drop seen by the charger across both batteries. You can get away without doing that when charging at low current. It becomes an issue at higher charging currents. Easy enough to do it right.
    Assuming we are talking about 12 volt batteries. Connected in parallel the batteries are seen as a 12 volt load with double the amperage capacity. Connected in series the amp load stays the same and the voltage doubles, Don't connect batteries in series to charge them, unless there is a 24 volt setting on your charger.
    Paul
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  6. #6
    Unfunded content provider tommcgee's Avatar
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    Battery Tenders are cheap. You can usually find the Junior on sale for $20 or less:

    http://www.jafrum.com/Accessories/Ba...FUOK4AodA0YARA
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  7. #7
    Registered User kioolt's Avatar
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    The following is from the battery tender website.

    "Can the Battery Tender?« Plus battery charger be used to charge more than 1 battery simultaneously if the batteries are connected in parallel?

    Yes, but with restrictions. A parallel connection means that positive posts of each battery are electrically connected together and the negative posts of each battery are electrically connected together. The voltage of a parallel connected battery pack is exactly the same as the voltage of each battery in that pack.

    If the nominal battery voltages (i.e. 12V, 8V, 6V) are the same on each battery, and if the batteries are the same lead acid type (flooded, AGM, or Gel Cell), then yes, the Battery Tender?« Plus battery charger can be used to charge more than 1 battery simultaneously when those batteries are connected in parallel. Just remember that 2 batteries in parallel behave like one large battery. The charge storage capacity of each battery simply adds together. Two 12 volt batteries, each with 25 amp hour capacities, will look like one 12 volt battery with a 50 amp hour capacity. You may be able to charge more than 1 battery simultaneously, but it will take longer to do it. "
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  8. #8
    Cam Killer marchyman's Avatar
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    Quote Originally Posted by pffog View Post
    How do you figure?? If the batteries are connected the voltage readings will be the same regardless.
    The voltages will be ALMOST the same. To the charger the batteries are a lump of resistance. Different batteries may have a different internal resistance. The current going through the batteries may be different. See Kirchhoff's laws.

    The Battery Tender folks say the same thing. Look at the Two Batteries in Parallel, One Charger diagram on this page: http://batterytender.com/resources/c...s-chargers.htm and note the blue, W1 wire.

    Again, is is unlikely to be an issue at the low currents that tenders use.

  9. #9
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    Batteries of the same construction, flooded, or Gel, or agm, or oddesy type,, at the same temperature, can be paralleled. after all, each pair of plates is a battery, big batteries have many pairs of plates. No diff.

    A bad battery will kill all the batteries, cost of battery compared to cost of tenders suggests this is unwise.

    Rod

  10. #10
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    When I was the caddy master in H.S. , part of my job was to plug in the new invention,ala, elec. golf carts, each evening @ the country club. Never seemed to make one batt. go bad. Later in factory I saw a guy plug in many elec. lifts and sameo/OK there too...

  11. #11
    Small road corner junkie pffog's Avatar
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    Quote Originally Posted by marchyman View Post
    The voltages will be ALMOST the same. To the charger the batteries are a lump of resistance. Different batteries may have a different internal resistance. The current going through the batteries may be different. See Kirchhoff laws.

    The Battery Tender folks say the same thing. Look at the Two Batteries in Parallel, One Charger diagram on this page: http://batterytender.com/resources/c...s-chargers.htm and note the blue, W1 wire.

    Again, is is unlikely to be an issue at the low currents that tenders use.
    Unless the wire connecting the 2 batteries is VERY small gauge and cannot handle the current, or resistance wire their diagram and you explanation holds no water, as soon as you inter connect the 2 it essentially becomes 1 large battery in 2 separate cases.

    So anything over 14 ga, would handle 99% of the home car chargers @ 15 amps. So unless he is connecting the 2 batteries with wire he recycled from a set of ear buds, it makes no difference.
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  12. #12
    Still plays with trains. tinytrains's Avatar
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    At first I thought the split charger connections were non-sense. But thinking about it;

    At slow charge rates like 1 Amp. There would be no difference where you connect the charger to parallel batteries. The voltage drop in the cables will be almost zero.

    Now if you are rapid charging at 10 or more Amps, then balancing the applied voltage may make a difference due to the voltage drop through the cables being 10 times higher than at 1 Amp. Then the blue wire connections to opposite batteries are a good idea.

    The bottom line is connecting the charger to the first battery alone will work fine in most cases. Balancing the charge voltage connection as shown in the link, is just as easy and theoretically better over the long run.

    Scott
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  13. #13
    Ed Kilner #176066
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    Are you going to charge two batteries regularly? Will one need more than a trickle charge? If yes to both, you should Google "battery isolator" and maybe purchase one.

    On a boat, one battery might be used for staring, the other for "stuff". You don't want to be in the middle of a lake with a storm coming and have both batteries flat. A good isolator will allow proper charging of a good staring battery and a mostly discharged "stuff" battery.

    So, what are you trying to do?
    Ed
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