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Charging APC batteries off truck circuit?

  1. Aug 25, 2010 #1
    Hi Gents, new guy here. I'm an ME by degree ( 25 years ago.) but it's been a while since I've done any real number crunching. ( Got moved to management.) Especially in EE areas. So here is my question.

    I have a couple of small 12V lead acid batteries in the 2.5-7Ahr range that I use to power my electronics on my kayak. ( Commonly sold as emergency light/APC back-up batteries.). Occasionally, I'll go on extended camping trips to places where I may not have 115V shore power to charge them. SO I'm thinking that a pair of leads to my truck battery should do the job on these occasions.
    That said, I'm wondering:
    a) will the depleted yak battery charge too fast /get damaged if I jumper it to my trucks battery and 115A charging circuit? If so, would using 12 ga or smaller wire slow the process?. Maybe partially charge with truck/alternator off?
    b) How long would it take to charge a 7 ahr battery in this fashion? I'm thinking minutes vs hours?

    Any insight appreciated!
  2. jcsd
  3. Aug 25, 2010 #2
    I think what you are suggesting is to charge your boat batteries from your truck's battery, both nominally 12v. This is exactly the same as jump-starting your car or connecting a high-amp boost charger. Lead acid batteries are pretty forgiving so they shouldn't have a problem with just connecting them in parallel.

    Using small wire is not a good idea as a current limiter, except in the terminal case of burned out wire. If you really want to current limit put a high power low resistance resistor (1-5 ohms maybe) or a regular light bulb in series someplace. The charging time will depend on the voltage difference between the two batteries but I would expect something in the one hour range.

    Do make sure that you don't run down your truck battery and get stuck out in the wilderness in the dark...
  4. Aug 25, 2010 #3
    Thanks Much. Just to info I was looking for. As for killing the truck battery, I'll be careful and be sure to start it every 15 min or so. It's got a large reserve bat for the 800W stereo anyway.
  5. Aug 25, 2010 #4


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    You will either need to get an inverter to run a standard battery charger or run your car continuously to maintain the voltage necessary to charge the battery. If you just use another battery, they (the 2 batteries) will arrive at some average charge state with neither of the batteries fully charged.
  6. Aug 25, 2010 #5
    Good point about average charge state.... But the being-charged battery is much lower capacity than the source so it should be a net OK balance. Running the truck charging system would actually take care of the problem as the combined batteries would look like an only slightly larger main battery. Using an inverter to run another charger is probably an overkill situation, non?
  7. Aug 25, 2010 #6
    The Linear Technology LT1513 is a good SEPIC-mode (boost or buck) switching power supply suitable for sealed lead-acid battery charging. The LT1513 can be run off a car battery (12 volts) or an automobile alternator (13.8 volts), and provide a constant voltage output.



    The LT1513 output current for this application limited to ~ 1 amp. Using the pin-5 npn switching transistor collector output to drive an external switch should be able to extend the output current range to ~ 5 amps.

    Bob S
  8. Aug 26, 2010 #7
    That's what I am hoping. I'm going to give the simple jumper set-up a test in the safety of my driveway the next time I run the battery down. That will give me a good feel for it!
    Thanks for the valuable insight!
    Last edited: Aug 26, 2010
  9. Dec 1, 2010 #8
    DANGER, DANGER WILL ROBINSON !! Do *** NOT*** hook up jumper cables from your truck to your kayak battery. This will likely blow up your kayak battery like a giant fragmentation grenade with the addition of sulfuric acid !

    First, you need a charger designed specifically for the lower amp-hour capacity of the kayak battery so that the charger doesn't try to shove too much charging current into it. Some automotive chargers have these smaller battery settings. A charger for these smaller batteries will be significantly less expensive than automotive chargers. Then again, you may want to have an all-in-one charger you can also use on all your vehicles. Your choice, here.

    Next, I've never seen a battery charger which is *powered* by batteries, though it seems they really ought to exist. If not, you will to also need to buy an DC-to-AC voltage "inverter" that runs off of 12V batteries and produces 120 VAC (or 240 VAC, depending where you live). Make sure the inverter's wattage rating is 50% or more of that needed by the charger.

    The best lead-acid battery chargers have 4 stages of charging:

    0) An optional "precharge" stage automatically used only if the battery is seriously discharged. I can easily see this happening in lead-acid batteries for non-automotive uses (like for your kayak or in a UPS). The charger will determine whether or not a precharge is necessary.

    1) A fixed, relatively high current stage that quickly recharges the battery to about 75% capacity. This isn't harmful to the battery and saves a lot of charging time compared to a stupid "voltage, only" charger. This is why many modern chargers are touted as having a (very small and simple) microcontroller running the charging procedure them.

    2) A fixed voltage stage that charges the battery up to 100% of its capacity.

    3) Finally, the charger should go into a "float charging" mode that will keep your battery at 100% charge when the battery isn't being used and the charger is turned on. This mode is used to store the battery for indefinitely long periods of time. L-A batteries, like all other types, slowly self-discharge. L-A batteries will damage themselves if left to self-discharge for several months at a time or longer.

    These stages occur in the listed order and are automatically controlled by the charger. You just connect up the charger, turn it on and then wait for the "Done" indicator to light up. All you have to do is get a charger with a suitable charging current rating for your particular battery size. The charger has no idea what size batteries it is connected to, so you have to buy the right size charger in the first place.

    I think you're looking at an outlay of about USD $100 to $150 for both the charger and the inverter, together, for, say, a motorcycle-sized 12V battery. Your battery is most certainly a "sealed" (gelled electrolyte) battery, but these don't need a special charger.
  10. Dec 2, 2010 #9
    You can pickup a charger from your local hobby shop that will use a 12V DC power source. They are built with DC inputs so the radio control hobbyists can charge batteries out in the field where power isn’t available.

    Just make sure it has a setting for the lead acids you’re charging.

    This is probably safer and better for your batteries than just hooking them directly to the truck.
  11. Dec 2, 2010 #10


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    @ pascor
    As I scanned down this thread I was waiting for someone to point that out!
    There is a simple, safe solution and that is to find the maximum charge rate for your kayak battery. Then put a beefy, high power resistor in series when you are 'parallelling them up'.
    Question - what value of resistor?Taking the worst possible scenario (the little battery is really flat) then the resistor should be 13÷Max permitted current. This would mean that, whatever the state of charge, there would never be too much current. However, this is too pessimistic and would not allow a reasonable charge current if the volts on the small battery are still about 12V. Remember- if you really flatten your small battery, you will totally knacker it, if it's lead acid so you really ought to make sure that its voltage is not lower than 11V.
    Assuming that there is some life in your battery (or you could even check the volts with a cheap meter) then you could use a much smaller resistor and still not blow up your battery. If you are sure it's charged to a safe level then a series resistor of 2÷Max Current will avoid explosions.
    These are not ideal solutions but will only cost pennies compared with buying ' the proper thing'.
    The portable booster chargers that the rescue services all check the 'dead' battery voltage before charging at a high rate. Just a posher version of what I was suggesting.
  12. Sep 14, 2011 #11
    You can't properly charge a 12 volt battery directly by another 12 volt battery ! If you try you will ruin the sealed battery because it won't get charged up properly.. The battery under charge needs, ONLY AT TIMES, a voltage greater than 12 volts, perhaps as high as 14.0 volts. Even your truck battery gets charged at 14 volts. So, the way to go is 1) Get a 120v sealed lead-acid battery charger (NOT an automotive/motorcycle charger) with enough power for: 2) A 120 volt inverter to power the 4-stage charger. The inverter power capability only needs to be able to output, perhaps, 150% the max current needed by the charger.
  13. Sep 14, 2011 #12

    jim hardy

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    are you handy at soldering?

    Simplest would be as somebody suggested a beefy resistor in series with your little battery. I'd suggest a 6volt automotive headlamp because its resistance is nonlinear and a strong function of current through it..
    That'll charge your gel-cell to whatever your truck battery is.

    I do not believe charging three pounds of gelcell will significantly discharge a fifty pound truck battery.

    When current drops to near zero (0v across lamp) start the truck and both batteries will soon be topped off to 14v.

    Those 4 stage chargers are really nice - i once built one for fun. TI makes the IC's, they even have built in temperature sensing to make just the right voltage for lead-acid.
    If you're into elecrtonic projects that'd be an interesting one - a charger to plug in your cigar lighter. All you need is 2volts of boost. Small switcher would do that,.

    Dave Johnson over at DiscoverCircuits forum had something similar for his "Circuit of the Month" a couple years back .

    but - keep it simple.
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