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Do I really need a DC/DC Charger?

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  • #16
    Thanks heaps for this very in depth reply. Very interesting points you have raised, and now more for me to consider. I really appreciate your input, and having been down the road I am now travelling is reassuring. I will take onboard what you have just said and look at purchasing a DC/DC charger to suit my situation. As I have said in the past, its not going to get a heap of use, so dont want to be spending a heap of money.
    Thanks again for all of your input TCT888.
    '06 Grande V6 Pearl White - Factory Alloy Bar
    Fyrlyt Driving Lights - GME UHF
    Electric Brake controller - Anderson plug
    Cooper AT3's
    Towing Tracktrailer Tvan

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    • #17
      Very informative. I'm not familiar with including a DC to DC charger in a dual-battery setup. Do you have any recommendations for sources of more specific instructions on how to install the charger in an existing dual-battery setup, and perhaps opinions on whether a alternator diode booster would achieve the same (and/or why one should or shouldn't use a booster)?
      2008 Prado Grande: ARB + Warn in front, Kaymar rear, Steinbauer, Beaudesert, REDARC, 2" lift, TJM sliders, ARB airlocker, etc.

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      • #18
        First off the Redarcs, Cteks etc only charge at a maximum of around 14.4@25C for the AGM setting.

        You can certainly fully charge an AGM battery at 14.2V, in fact 13.5V or any voltage higher than the fully charged terminal voltage will do it eventually, higher voltages just reduce recharge times.

        Float voltages in car DCDC chargers are generally a waste of time as the battery will most likely never be on charge long enough for charge voltages to become an issue.

        As for charge rates, those in a typical touring setup want to charge their batteries as fast a possible so would be unlikely to use batteries that can only accept low charge rates. If however you have then yes a DCDC charger could be used as an expensive resistor to limit the current.

        As for the use of booster diodes versus DCDC charges, this has been covered to death in the 120 & 150 electrical sections if you care to use the search engine.
        LeighW
        Avid PP Poster!
        Last edited by LeighW; 29-07-2017, 04:06 PM.
        HKB Electronics, manufacturer of the Alternator Voltage Booster, Silver 2008 D4D,Lifted,Underbody protection, Alternator Voltage Booster, Tiger Z winch, Lightforce DL, Air Horns, Tanami Drawers, Drop down fridge slide, Outback cargo barriers, Rotronics dual Battery system, Polaris GPS, HF/UHF/VHF, Radio speaker combiner, Long ranger water tank, Diff breathers, Inverter, Snorkel and others

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        • #19
          ..
          Last edited by TCT888; 31-07-2017, 08:44 PM.

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          • #20
            The voltage specified is max voltage, in a hot engine it wont be that high, most charger manufacturers specify around 14.4V@25C.

            You have left a lot out about how DCDC really work, your DCDC charger does not charge at your 14.7V constantly, depending on the SOC of the battery it will start charging at a low value maybe 13V for example ie its current limiting stage, the voltage will gradually increase as the battery charges to maintain a constant 20A in the case of a 20A till it reaches its maximum charge voltage, ie 14.4V@25C, it has to do this to prevent itself burning out. With a 20A charger 14.4V will not be reached until the battery SOC gets to around the 90% SOC point.

            The charger manufactures used to argue that lower charge rates were better for batteries when they only had 20A charger, then 25A came along, then 30A then 40A amp, now they can produce higher charger rates they have dropped the argument.

            Their are thousands running out there running 100Ah-200Ah of with simple VSR setups with no issues, as for alternators, many are running 400Ah of batteries directly off the alternator, unless you run huge cables then you won't have an issue. I have never heard of anyone having an alternator failure directly attributable to battery load.

            I have worked with batteries for over 40 years, in industry most batteries are float charged, not a smart charger to be seen and where talking batteries worth tens of thousands of dollars!

            If you only need to replace 20Ah then you would be considered a very light user, most I know of need to replace 100Ah-200Ah in a four to five hour drive time, your not going to do that with a 20A charger, in my case I generally have to replace around 150Ah in 4- 5 hours, my setup consists of a VSR and booster diode for in car battery charging and 30A DCDC to manage they Lithiums, this system has been running happily now for around 12 months with no issues. Before that I had a 100Ah in car and two 100Ah in the camper all charged directly off the alternator for over six years with no problems, only moved to a DCDC charger as the Lithiums have a much higher terminal voltage versus SOC then AGM's plus I can customise the settings.

            Mine aren't claims by the way, I have been across 1000's of installs and get constant feedback, many have gone from VSR setups to DCDC chargers only to find the performance of the system to be much worse and then having to revert again, DCDC have there place but they are not the be all end all some make them out to be.

            For most as long as the alternator puts out a reasonable charge voltage a simple VSR setup will perform much better though granted battery selection is important.
            LeighW
            Avid PP Poster!
            Last edited by LeighW; 29-07-2017, 08:32 PM.
            HKB Electronics, manufacturer of the Alternator Voltage Booster, Silver 2008 D4D,Lifted,Underbody protection, Alternator Voltage Booster, Tiger Z winch, Lightforce DL, Air Horns, Tanami Drawers, Drop down fridge slide, Outback cargo barriers, Rotronics dual Battery system, Polaris GPS, HF/UHF/VHF, Radio speaker combiner, Long ranger water tank, Diff breathers, Inverter, Snorkel and others

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            • #21
              Originally posted by TCT888 View Post
              Well, if you want to actually charge your new AGM battery correctly, then YES!

              A DCtoDC Charger will work in stages, normally at least 3-stages, Boost, Absorption and Float.

              The problem with simply daisy-chaining from your primary battery, is your alternator/battery will only send the charge through to the auxiliary battery at a maximum output of 14.2volts <<< THIS IS NOT ENOUGH VOLTAGE.

              Not true!
              ps daisy-chaining implies connecting in series so not sure why you have used that term?


              Your auxiliary AGM battery requires a 14.7v absorption charge rate to achieve a full 100% charge, and then hold a correct float charge at 13.8v, and this is exactly where a DCtoDC charger comes into play.

              Maybe your AGM does, but mine does not need 14.7v and can be fully charged at 13.65v. It will also be more than happy with a 13.2v float if I used my BCDC1220. Are you saying you weren't able to fully charge your battery with less than 14.7v? Out of interest what aux battery do you have?

              By daisy-chaining from your primary at a maximum of 14.2v, you will never correctly achieve a full charge on your battery, nor a correct float-charge voltage.
              C'mon, really!!

              THEN we move onto the Amperage side of things. Your new AGM battery will have what is called an 'Initial Charge Rate' for the size of the battery, Again, yours might, mine has no amerpage limit for alternator charging most will not like anywhere above 10-25 amps, dependent on the AH size of your battery, and you cannot control this without the use of a DCtoDC charger.
              Are you not referencing an 'initial charge rate' when using a battery charger here?? How do you control this with your starting battery?

              If you simply hook your battery up as a daisy-chain, it will suck as many amps as it possibly can through the cable size you have laid, like someone dying of thirst it will DRINK as MUCH as it can, as QUICKLY as it can, theoretically you can 'cook' your battery and it will drop cells, giving you a very short overall battery life.
              you are suggesting a force-fed situation like using a large DC charger on a small Ah battery

              You will get plenty of back-yarders that hook batteries up as a daisy-chain and tell you its fine, but they are simply uneducated in the requirements of deep cycle batteries, AGM/GEL/LifePo etc.. which all require very specific charging rates and stages.
              please, enlighten us on your education and qualifications...

              Batteries arent cheap! I've done the cheap and nasty daisy-chain setup myself and ruined batteries after only a year, which should have had upto 10 year lifespan all because I wasnt educated..... so I educated myself with the above and much more regarding car power and auto electrics.

              *Remember to always have a breaker/fuse at EACH END of the setup, as close to each battery/charger as practical, with the correct amperage break setting for your setup.

              DC to DC Chargers can be purchased from as little as $70 for a generic rebadge such as PowerTrain or Matson brand, which will give you a correct 3-stage charge rate for AGM etc..

              Happy for you to ask questions if you wish.
              As I have both the referenced BCDC1220 and isolator type set-up in my rig I have
              decided to 'chime in'...
              Cheers
              Micheal.

              2008 GXL D4D Auto. GOING... GOING... GONE
              2015 GXL 1GD Auto. And it begins again...

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              • #22
                With regards to the 14.7V required to charge an AGM, you only need to supply a voltage greater than the resting fully charged terminal voltage of the battery to fully charge it, increasing the voltage as the battery charges is advantageous as it will reduce recharges times as the battery charge state increases and for cell equalisation if required.

                An alternator charges a battery by applying a constant charging voltage, as it has a much larger current generally than a DCDC charger it does need to current limit to protect itself as a DCDC does, it can therefore replace the bulk charge much quicker. A DCDC charger has to current limit initially to prevent itself burning out, so is generally much slower replacing the bulk charge when the battery is low, I'm referring to your typical 20A charger, it may have an advantage of being able to pump more current into a battery that's at a high state of charge though if the car has a low voltage alternator fitted.

                Generally comparing the two, a normal alternator will recharge a battery discharged to 40% SOC much quicker than a 20A DCDC charger can, a 40A DCDC charger would recharge the battery in approximately the same time, I'm referring to in car batteries.

                I covered this above and so did 120D4D but to provide some more info below is the charging requirements for an Optima battery, you'll see they vary quite a bit depending on how you intend to charge the battery, when a manufacture states charge voltage is 14.7V you need to look at the charge regime they are referring to. For example when the manufacture states 14.7V he could be referring to the battery being constant current charged at 20A, when the battery voltage reaches 14.7V the charging should be stopped to prevent overcharging the battery. If the battery is constant current charged at 5A then the charging my need to terminate at 14.4V to prevent overcharging. Same for temperature, dumping large currents into the battery when its in a low SOC and cold should not cause a problem and this is how alternators charge and one of their advantages, they can replace the bulk charge quicker but then the charge current reduces quickly as the battery approaches a fully charged state. DCDC chargers though start out at a much lower charge rate generally simply because they can't supply large currents, they maintain that charge rate to a much higher voltage when the battery is approaching fully charged, this can lead to thermal runaway if the charge current is to high hence why for battery manufacturer might state a maximum charge rate of 20A for example.

                Look at the charging specifications for a typical optima battery below and you'll see what I mean:

                Recommended charging information:
                • Alternator:
                  • 13.65 to 15.0 volts, no amperage limit.


                Battery Charger:
                • 13.8 to 15.0 volts, 10 amps maximum, approximately for six to twelve hours.
                • Cyclic Applications:
                  • 14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below one amp, finish with two-amp constant current for one hour.

                • Rapid Recharge:
                  • Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below one amp.

                • Float Charge:
                  • 13.2 to 13.8 volts, one amp maximum current, time indefinite (at lower voltage).
                  • Strictly adhere to all limits.

                It all depends on specific battery how you intended to charge the battery as to what the allowable charge voltages and currents are.
                LeighW
                Avid PP Poster!
                Last edited by LeighW; 01-08-2017, 10:49 AM.
                HKB Electronics, manufacturer of the Alternator Voltage Booster, Silver 2008 D4D,Lifted,Underbody protection, Alternator Voltage Booster, Tiger Z winch, Lightforce DL, Air Horns, Tanami Drawers, Drop down fridge slide, Outback cargo barriers, Rotronics dual Battery system, Polaris GPS, HF/UHF/VHF, Radio speaker combiner, Long ranger water tank, Diff breathers, Inverter, Snorkel and others

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                • #23
                  Hi Leigh,
                  Slightly off topic, but what are your recommendations regarding the best Lithium Iron Phosphate battery charging process? I have one of these as a second battery, and find it can get be charged up to 16v using the 240v charger supplied, but of course the alternator can't get anywhere as high as that.
                  The battery has a BMS that cuts in at 16.5v and cuts out at about 12.5v
                  Ta
                  Rob
                  Rob.
                  '08 GXL V6 Manual with: O/L Bar, Cibie Oscars, Safari Snorkel, Revalved Ironman 45710 Struts & 45682 Shocks, Dobinsons & King springs, MickeyT STZs, Eaton E-locker on rear, ScanGaugeII, InaWise TPMS, and a Tvan controlled via TowPro.

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                  • #24
                    Hi Rob,

                    Need a crystal ball for that one, there is so much conflicting information about and a lot comes the battery sellers themselves.

                    Personally after reading a lot from early adopters is don't charge higher than 14.2V on a regular basis, don't discharge below 30% and if not cycling store them around them at 30% - 40% SOC . Some sellers say charge to a higher voltage but that is more about allowing their cells balancers to do their thing.

                    I have my charger set to 14.1V and charge stops when charge current falls below 3 amps, some will say that's to high, time will tell, battery monitor indicates Ah going in are a the same as what's coming out so settings look ok to me at the moment. Charging resumes when battery SOC falls below 90% from memory.

                    I was intending to charge directly off the alternator but found with the existing cabling and the higher terminal voltage of the Lithiums 13.1V 30% SOC I could only get around 15A going into the camper batteries, heavier cabling would have solved the issue but not wanting to rewire the car I opted for a DCDC charger, this gives me a higher charge rate and the ability to end charging at the required SOC, ie no float charge.

                    Solar charging can be done by the same charger but I find the original PWM solar controller I had provides more charge current in bright sunshine than the MPPT function in the DCDC does so I tend to use the PWM (set to gel) and switch to the MPPT when the batteries are approaching fully charged.

                    I have wired it up so that either charger is switch selectable for solar, for off car charging either the DCDC can be used or can be bypassed for direct charging off the alternator if required.

                    Without hijacking the thread what's your thoughts?

                    Cheers
                    Leigh
                    HKB Electronics, manufacturer of the Alternator Voltage Booster, Silver 2008 D4D,Lifted,Underbody protection, Alternator Voltage Booster, Tiger Z winch, Lightforce DL, Air Horns, Tanami Drawers, Drop down fridge slide, Outback cargo barriers, Rotronics dual Battery system, Polaris GPS, HF/UHF/VHF, Radio speaker combiner, Long ranger water tank, Diff breathers, Inverter, Snorkel and others

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                    • #25
                      Originally posted by LeighW View Post

                      You have left a lot out about how DCDC really work, your DCDC charger does not charge at your 14.7V constantly, depending on the SOC of the battery it will start charging at a low value maybe 13V for example ie its current limiting stage, the voltage will gradually increase as the battery charges to maintain a constant 20A in the case of a 20A till it reaches its maximum charge voltage, ie 14.4V@25C, it has to do this to prevent itself burning out. With a 20A charger 14.4V will not be reached until the battery SOC gets to around the 90% SOC point.

                      The charger manufactures used to argue that lower charge rates were better for batteries when they only had 20A charger, then 25A came along, then 30A then 40A amp, now they can produce higher charger rates they have dropped the argument.
                      For those playing at home, I hooked-up the BCDC1220 to find out what my Redarc does.

                      The AUX voltage (bottom) is with the DCDC running for about 5 mins. It eventually lifted the voltage up to 14.6v and then dropped off to its 13.3v float setting.

                      Click image for larger version

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                      Obviously with my Traxide Isolator the Cranking and Aux voltages hold at the same level.
                      Cheers
                      Micheal.

                      2008 GXL D4D Auto. GOING... GOING... GONE
                      2015 GXL 1GD Auto. And it begins again...

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                      • #26
                        mate honestly if you want to be with the jones then buy all the fancy gear. I have never ever run one of those dc to dc thingys never needed to for our camping trips. I have a 130ahr in the car as a camping only battery, i do have dual batterys connect by a vsr switch has worked fine for many years now, i have a 60w panel on top for keeping battery charged whilst driving connected to a 30amp solar controller, it runs 60ltr fridge freezer and lights and charging phone, laptop and phones that's all we need when parked up we do have an additional 120w panels we setup. Yes your needs may be different but what do you want outta camping, we choose not to take the house with us and we do fine, my ten cents worth.

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