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Supplimenting solar with a B2B charger vs Isolator

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  • Supplimenting solar with a B2B charger vs Isolator

    Hello folks,

    I have a bit of a question about the most effective way to combine a solar powered system and charging with the alternator.

    I've seen people use B2B chargers, such as the Sterling one, in their campervan. This in theory provides a more appropriate 3-stage charge. My [I]suspicion[/I] is that it's not worth the money because rarely would you be driving long enough to get past the "bulk" stage in most situations.

    I'm most familiar with a simple voltage regulated relay or an isolator, which are different setups but considerably cheaper. They'd just provide a simple 13.8V charge to the system as long as you're driving from what I understand. This seems like the most effective choice to me.

    And finally, I also see some folks combine the two (solar and alternator charing) with one of the Ctek chargers, but I can't even find one that combines solar with an alternator at more than 20A so it'd only work for relatively small systems and is also expensive compared to a relay and a small MPPT controller.

    Does anyone have a preference here? I'm working off of some of my assumptions so the chances that I'm missing something are pretty high!

  • #2
    [U][B]"because rarely would you be driving long enough to get past the "bulk" stage in most situations."[/B][/U]

    I you are going to drive only short distances then you will need charge other than your alternator, a lead acid battery needs to be at full charge to get best life.

    The VSR ,voltage sensing relay / Isolator / option is the most simple and providing you have the wiring to suit will get the most into your battery in the shortest time.

    Long term the problem is that a VSR system is batteries in parallel and often 2 different types of battery so not good for battery life.

    The DCDC chargers, as they are more commonly known, overcome the batteries in parallel and the differing type of battery problem. I have dealt with redarc brand a lot and they have been very reliable. But not cheap.



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    • #3
      Originally posted by Donkey_in_training View Post
      My [I]suspicion[/I] is that it's not worth the money because rarely would you be driving long enough to get past the "bulk" stage in most situations.
      I can understand how you might think that, but far from reality. A vehicle alternator can generate more energy in an hour than a roof full of panels can generate in a few days.

      A vehicle alternator on say an RV is in the neighborhood of 800 to 1200 watts day or night, rain or shine, cold or hot. Easily generate 100 amps with a flip of a switch all day and night long.Just 30 minutes run time and you have 50 AH @ 12 volts generated. Care to guess how long days it would take a 250 watt panel to generate 50 AH @ 12 volts? Try 1 to 2 days if the RV is parked in blistering sun without any kind of shade from sunrise to sunset.

      When the RV alternator or external charger is running. the Solar Shuts off and collects dust because it is not a Stiff Source providing the higher energy state. It gets reversed biased and turns off. In fact with a good Isolator, drive once a day or every other day, you have no real need for solar.. A $75 isolator will run circles around $2000 of solar panels and controller. 1 hour engine run time equal days of solar even in the middle of winter, at night with it snowing.
      MSEE, PE

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      • #4
        Sunking,

        I'm on the same page as you as far as the alternator being the best resource. What I'm more curious about is whether or not a $450 B2B charger is any more effective than a $100 Isolator. It seems that most people wouldn't be driving enough get a proper 3-stage charge so the B2B wouldn't really have much advantage. Perhaps it's in what Bala is saying in that different batteries in parallel isn't good for balancing reasons.

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        • #5
          Originally posted by Donkey_in_training View Post
          What I'm more curious about is whether or not a $450 B2B charger is any more effective than a $100 Isolator. It seems that most people wouldn't be driving enough get a proper 3-stage charge so the B2B wouldn't really have much advantage.
          No real advantage at all. 3-Stage Charging for the most part is just a Marketing Term. It has one advantage and one advantage only. It is faster and that is all that can be said for it. What is not said, it is harder on batteries. You are trading speed for less cycle life. If you look at a utility, communications companies, military, emergency communications and other critical mission operations who use battery plants DO NOT USE 3-STAGE charging algorithms. They never have and never will use 3-Stage on Pb batteries. They all use Float Chargers and on 12 volt PB batteries Float Voltage is 13.2 to 13.8 volts depending on manufacture and alloy used in the battery grids and plates.

          So what is the difference between a 3, 4, or even 5 Stage Charger 20 amp charger, and a 20 Amp Float Charger? Well I am here to tell you not a lot of difference. In a Nut Shell the only difference is the multistage charge has a variable voltage output under the control of either timers or microprocessor. A Float Charger has a Variable voltage output under manual control you set once and forget it. Otherwise the both are exactly the same 20 amp charger using the exact same parts.

          Contrary to what you might think batteries are not charged with voltage, they are charged with current. When you set a 3-Stage 20 amp charger to say 14.8 volts, and connect it to a battery in need of a charge, the voltage is not going to be 14.8 volts. Not anywhere close Same for a 20 amp Float charger set to 13.8 volts. Both 20 amp chargers voltage will be exactly the same voltage under the same circumstances. I am not making that up, it is 100% fact. What voltage you ask? Beats the crap out of me, measure it. No one could tell you as it depends on the battery open circuit voltage and its internal resistance both of which are variable depending on conditions. Mathematically I can tell you an battery type under Charge Voltage = OCV + [Charge Current x Internal Resistance.

          Example a typical 12 volt 100 AH deep cycle lead acid battery in good condition at 50% state of charge open circuit voltage is roughly 12.1 volts with an Internal Resistance of roughly .01 Ohms. If you apply a 20 amp charge current the voltage will equal 12.1 volts + [20 amps x .01 Ohms] = 12.3 volts. It does not matter is the charger voltage is set to 13.8, 14.8, or 10,000 volts, the battery voltage will be 12.3 volts with 20 amps of charge current. Could you make the battery go from 12.1 to 14.8 volts as soon as you hook up the charger? Absolutely you could just before it explodes. All you would need is a ([14.8 - 12.1 volts] / .01 Ohm]) 270 amp or higher charger.

          Technically a 20 amp Float Charger charges exactly as fast as a 3 Stage Charger. Simple math tells you that: Amp Hours = Amps X Hours, and 20 amps = 20 amps all day long. So why does a 3 Stage charge faster? Simple the end set point voltage is set higher to 14.8 volts vs 13.8 volts. The 3 Stage Charger pushes 20 amps into the battery 1 volt longer which takes a little more time like 30 minutes to an hour before the Absorb Stage starts where current tapers off. The time difference is Absorb Time. A Float charger takes longer to absorb or saturate to full charge than a 3-stage charger. A Float charger can take up to 24 hours to fully charge a battery where 3-Stage can take up to 16 hours. But there is a cost for that speed. Just like your car engine where you can go 90mph for 16 hours is a lot harder on the engine than running 24 hours at 60 mph. Both took you to the same place. Your engine is going to last a lot longer and make more trips at 60 than 90 mph.

          Sow here is the main takeaway. The longest hardest part of charging any battery is Saturation aka Absorb to full charge. It is a long process with either 3-Stage or 1 Stage (aka Float), just one is slower then the other. Note I did not say one is faster than the other. 3-Stage Absorb time is 4 to 6 hours, and Float is 8 to 12 hours. But a 20 amp charger is a 20 amp charger all day long any day of the week. Neither are worth a dam using solar or an alternator as both suck. At least the alternator can charge 24 hours a day. With solar there are not enough Sun Hours in a day to charge a battery period.

          Now go back to my utility critical mission statement and it makes sense now. They are in no hurry to recharge the batteries. They have ten;s or thousands of dollars tied up in a battery plant and need them to last 10 years. Last thin gin the world they would use a 3-stage charger. But then again they do not need to be recharge in 16 hours, 24 hours is more than fast enough for them be it the site generator or utility power makes no difference to them. It just will not be solar or 3-stage because they have a biz to run and make money with, and no outages can be tolerated.
          Last edited by Sunking; 08-11-2018, 04:00 PM.
          MSEE, PE

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          • #6
            [QUOTE=Sunking;n382183]



            Your engine is going to last a lot longer and make more trips at 90 than 60 mph.


            Just for clarity sake, is this a typo? supposed to be reversed, right?



            2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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            • #7
              [QUOTE=littleharbor;n382193]
              Originally posted by Sunking View Post



              Your engine is going to last a lot longer and make more trips at 90 than 60 mph.


              Just for clarity sake, is this a typo? supposed to be reversed, right?


              You are right my bad typo, corrected. THX

              MSEE, PE

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              • #8
                Sunking, thanks so much. This is [B]awesome[/B]. This answers more questions than I had!


                I have a bit more of a specific question now. I have 150Ah of AGM battery connected by a basic relay to my chassis alternator (it's a factory 89A alternator). I'm also using a Hall Effect Sensor to get a rough measurement of Amps on that connection (this one: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1). After a night of usage the batteries are usually sitting at around 12.2V. In driving conditions, I can see up to 30A on that line (it's a 12ft run of 8Awg, which I know is too small now). It's usually closer to 20A and will continuously drop as I drive. How is the system functioning to set the amps here? I'm assuming that as system resistance goes up and they're charging, then Amperes drop, but I'm not entirely sure where to start and a bit curious why the batteries aren't seeing more charge come in. In other words, is this normal or can you pinpoint over the internets what I'm missing.

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                • #9
                  Originally posted by Donkey_in_training View Post
                  Sunking, thanks so much. This is [B]awesome[/B]. This answers more questions than I had!
                  You are welcome and what I said actually answers your question plus one more thing you are over looking.

                  Originally posted by Donkey_in_training View Post
                  I have a bit more of a specific question now..
                  OK not going to quote the whole thing, I got it and you know the question. You are are on the right track, just missing some info, and making an incorrect assumption. Resistance stays fairly steady but in reality for Pb batteries Internal Resistance goes down as the SOC rises, Some reduction due to heat as the temp rises, resistance goes down in almost all materials. As SOC rises has an affect and resistance goes down a bit as the battery charges. But forget that, because change in resistance is not the major contributor here and if it were, charge current would go up, not down as the battery charges up. So nip that in the bud right now and forget about it. That dog does not hunt and suited for your wifey powder puff.

                  Remember the formula for a charging battery? [B]Charging Battery Voltage Battery OCV + (Charge Current x Internal Resistance)[/B]. What that is telling you comes from Ohms Law which is a collection of 12 equations that shows how Power, Voltage, Current, and Resistance are related. One that applies here is Voltage = Current x Resistance. Look in the Charging Battery Formula and you will see it applied, [B]Charge Current x Resistance[/B]. Well that equals a voltage so all the formula really says is Voltage1 + Voltage2 = Voltage3. 20 amps x .01 Ohms is 0.2 volts. Understand?

                  If you use my previous example starting at 12.2 volts, 20 amps of charge current, and .01 Ohms of resistance. Well we do not know what the open circuit battery voltage is exactly other than at start and end voltages, but with 20 amps of charge current I can tell you with 100% accuracy the battery ocv will be exactly exactly 0.2 volts lower than I measure with a meter because I know I have 20 amps flowing through .01 Ohms of Resistance.

                  So how does the current taper off and knows when to do so. Ohms Law again and is determined at what Set Point Voltage I set the charger to like 14.8 volts. The Charger will keep pumping 20 amps until the battery OCV = 14.6 volts or .2 volts lower than set point. Where id .2 volts come from? 20 amps flowing through .01 Ohm,s right. Well there is another equation in Ohm's Law that says Amps = Voltage / Resistance. OK .2 volts / 20 amps = .01 Ohms.

                  So the instant the battery voltage goes above 14.6 volts, curent is going to taper off toward 0 amps. Example if the battery OCV = 14.7 volts, and charger Set Point = 14.8 volts leaves a difference of .1 volts. Charge Current now equals .1 volts . .01 Ohms = 10 Amps. When the battery OCV = 14.8 volts and Charger Set Point = 14.8 volts what happens to current?

                  0 volts / any resistance = 0 AMPS

                  No voltage difference, no current flows. So yeah you were on the right track of resistance, but did not consider wire resistance.

                  OK that does not answer why you only see 20 amps on your AGM. Has nothing to do with the above other than Resistance. Not resistance of the battery, but resistance of the 8 AWG wire and the length of the wire. The resistanc ein the wire like the resistance in the battery limits current. Current flowing through the resistance of the wire lowers the voltage. Want more charge current, lower the resistance of the wire by either making it shorter, larger wire, or both.

                  Be careful using a mechanical relay as a Battery Isolator. Sooner or later it will bite you. Eventually when you are starting or turning off the motor the relay contacts will disconnect under load and weld the contacts together and you will not likely notice. A lot of people discover it the hard way. They have been camping for a few days, ready to pull out, go to start the engine and hear Clickity Click of a dead battery. They did not notice the relay contacts welded together putting the RV SLI and House Battery in parallel. Had a good time camping and ran both SLI and House battery down to th epoint they cannot crank the engine and now you are stuck with dead batteries. That is why they use Electronic Isolators so that cannot happen. Some even have emergency bypass os if your SLI battery dies, you can bridge the House Battery to get the engine started and get the heck out of Dodge and be a happy happy camper.
                  Last edited by Sunking; 08-11-2018, 08:30 PM.
                  MSEE, PE

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                  • #10
                    Sunking, I suspected the resistance in the 8AWG wire was the main reason (I didn't really articulate my suspicion in my question). It's not something worth ripping out right now, but it's good to know that it's all functioning according to the math. With the alternator and 200W of solar we're managing pretty well and only have to curb our electrical consumption every few weeks or so to catch up from a particularly cloudy, immobile day.

                    And interesting point on the relay. I didn't think of the case that it would fail in a way that leaves the system connected!

                    Again, you've been incredibly helpful and patient. I think I've followed all of your explanations here. The proof of the pudding is in the eating, though, so I won't know for sure until I have to diagnose a new issue!
                    Last edited by Donkey_in_training; 08-12-2018, 11:15 AM.

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                    • #11
                      Originally posted by Donkey_in_training View Post
                      It's not something worth ripping out right now, but it's good to know that it's all functioning according to the math. With the alternator and 200W of solar we're managing pretty well and only have to curb our electrical consumption every few weeks or so to catch up from a particularly cloudy, immobile day.
                      If it is working and meeting your needs, leave it alone. If you need more, you know where and how to get it. Just one word of caution if you have not already addressed it. A Fuse or Breaker for that charging battery and its location. Make sure you place a fuse or breaker directly on the battery Term Post. In fact you need two. One one the 8 AWG going back to the Relay, and another one going to Load Distribution. Fuses and Breakers have one purpose and one purpose only and that is to protect the wire. For fuses to be effective must be placed at the SOURCE, and the Source in this case is the Battery at the Term Post. If a battery wire were pinched and shorted out, you have a fire on your hands. A fus eon the battery term post stops that from happening.

                      Originally posted by Donkey_in_training View Post
                      And interesting point on the relay. I didn't think of the case that it would fail in a way that leaves the system connected!
                      Well that is a characteristic of DC vs AC. In AC power the voltage and current reverses itself 120 times a second which means 120 times a second the voltage is ZERO VOLTS and that is called Zero Cross-Over. It also means there is ZERO CURRENT 120 times a second. Here is why that is important. When you open a circuit like open a Contact Closure, or Turn Off a Switch under load ([SIZE=8px]current flowing[/SIZE]) there is an Arc. Current wants to keep flowing. Ever hear of Arc Welding? Millions of people make a living Arc Welding. AC is not as prone to arcing when a switch is opened in Low Voltage applications ([SIZE=8px]under 1000 volts[/SIZE]). DC is another animal and arcs easily. The higher the current and/or voltage, the more intense and hotter the arc is. Damage is accumulating and results in one of 2 things. Pitted contacts which mean high resistance and poor contact. Or welded contacts.

                      Good luck to you.
                      MSEE, PE

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                      • #12
                        Since the 90's, most cars have a power distribution center, with one well placed cable feeding the center, and the center has the fuses for all the other loads in it.
                        And generally, the starter solenoid and starter have no fuse.
                        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                        solar: http://tinyurl.com/LMR-Solar
                        gen: http://tinyurl.com/LMR-Lister

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                        • #13
                          Originally posted by Sunking View Post

                          For fuses to be effective must be placed at the SOURCE, and the Source in this case is the Battery at the Term Post. If a battery wire were pinched and shorted out, you have a fire on your hands. A fuse on the battery term post stops that from happening.

                          Good luck to you.
                          Yep, I've got two fuses on that line; 80A on both ends; the terminal of the chassis battery as well as leisure battery because both are chassis grounded.

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                          • #14
                            Sorry to dig an "old" post, but I find this discussion about multistage charging (B2B charger in my case) VS a simple ACR or Battery Isolator pretty spot on about my questioning right now regarding how to charge my AGM battery while driving...

                            I'm not knowledgeable enough to understand the details of everything mentioned here, but what I think I understood from what Sunking says is basically "you dont NEED multistage charging, it probably has its advantages but a simple relay will allow an alternator to charge an AGM battery to 100% too, it will just take longer but will do the same work, without decreasing the battery life as B2B charger manufacturers want you to believe"... Am I correct ?

                            If that statement is correct I'm on the market for a reliable ACR or other "smart"(or not) battery isolator.
                            One thing I'm concerned about is I want the alternator to charge my "house" battery while driving, Ideally the ACR "connects" when the engine is running (alternator providing electricity) AND the car battery is full. I've seen a few ACR with "dual sensing" or things like that, that can connect if any battery is getting chaged.. but idealy I don't want the ACR to connect the 2 batteries when it senses the solar is charging the house battery (when I'm not driving)... solar is precious so idealy I'd like to focus on charging the house battery with solar, not the car battery...

                            Thanks for all the data available and all the knowledgeable people that participate on this forum, my brain is exploding !

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                            • #15
                              Originally posted by ben_mtl View Post
                              I've seen a few ACR with "dual sensing" or things like that, that can connect if any battery is getting chaged.. but idealy I don't want the ACR to connect the 2 batteries when it senses the solar is charging the house battery (when I'm not driving)...!
                              Following along. But just taking a stab here, as I'm hoping someone with much experience can swoop in. One simple solution would be a hard on/off switch for whatever means you plan to charge via starter batt/alternator. If you really want to isolate for solar, I would think that's the only sure fire way (poor choice of words).

                              Side note. From what I understand, the Sterling B2B unit will only charge the second/aux battery when the starter batt is at a certain voltage, i.e. if the starter batt is low it will prioritize accordingly and wait until the alternator brings it back to nominal voltage. That's what attracted to me. Not the 3-stage charging, which I didn't know enough about anyway (until reading Sunkings post).
                              I'll probably still add an on/off to to that line just because I like the magic of solar.

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