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  • LiFePo4 Boost Battery

    I'm thinking of putting together a small LFP boost battery pack. I will be able to use it initially to help extend the life of my expiring lead acid battery that I currently have (one string is on it's way out). That way I could limp through yet another winter on the 2 remaining strings. Eventually, when I replace my storage with an AHI battery pack, it will be useful in extending the usable storage of the battery when heavier loads come on.

    LFP is well suited for handling high power applications efficiently, whereas AHI should give me plenty of long term storage, that will allow me to save on battery expense in the long run. The small LFP pack will improve the efficiency of the AHI pack, help extend it's usable life, and allow for a deeper discharge, which helps to minimize generator run time. It should be able to extend the useful DOD of the AHI bank from about 50%, to as much as 80% if need be.

    I'm thinking of using an mppt charge controller to do double duty. One is to separately charge the LFP battery. The other is to send power to the AHI battery when requested (ie when the main battery voltage drops below a preset level in combination with a load detected). I'm thinking one or two extra cells should be enough to raise the voltage high enough for the charge transferring. And if one of the LFP cells fail for whatever reason, it won't be a disaster in which I lose my whole battery pack. I do plan to operate the LFP within a safe range, so I actually don't expect to have a premature failure.

    The other option would be to use dedicated PWM charge controllers, but not sure if there's units out there where one could limit current flow to preset levels (for the charge transfer)? Thoughts about the best way to go about this technically?

  • #2
    Go ahead and try to close in two batteries with as much as 1/10th volt difference and see what happens. I dare you to try it.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Go ahead and try to close in two batteries with as much as 1/10th volt difference and see what happens. I dare you to try it.
      That's why I plan to take the safe route and run the boost current through the charge controller. I did mention that one option is to have an mppt charger such as a Midnite Classic, do double duty. When the sun is out it would be switched to the LFP battery pack to charge it. Normal operation would have it switched to the main bank, and run current from the LFP pack. That way you can limit current.

      Other option could involve putting in place dedicated PWM chargers, but thinking the mppt option may be preferred?

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      • #4
        Doesn't this approach just cost you more than replacing the batteries over the next 2-3 years?

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        • #5
          Originally posted by donald View Post
          Doesn't this approach just cost you more than replacing the batteries over the next 2-3 years?
          I assume your talking about FLA batteries. No, it should cost less in the long run. AHI are rated for 10,000 cycles at an average daily DOD of 25%. Plus, I really don't like lead acid batteries. I look forward to having a system where partial state of charge is no concern, and no maintenance required! Plus no gassing or acid smell.

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          • #6
            Originally posted by northerner View Post
            AHI are rated for 10,000 cycles at an average daily DOD of 25%. .
            Are you gullible enough to believe one word of that claim? I think you are. It will be 30 years before that is proven from a company with a product of less than 1 year on the market. Did you ever stop to think why a 30 year battery only has a 2 year warranty and if you want a 5 year pro-rated warranty cost extra? Are you clueless that there are FLA batteries with a 10 year warranty and been in biz for 50 years and more?
            MSEE, PE

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            • #7
              Originally posted by Sunking View Post
              Are you gullible enough to believe one word of that claim? I think you are. It will be 30 years before that is proven from a company with a product of less than 1 year on the market. Did you ever stop to think why a 30 year battery only has a 2 year warranty and if you want a 5 year pro-rated warranty cost extra? Are you clueless that there are FLA batteries with a 10 year warranty and been in biz for 50 years and more?
              Yes, I am gullible to believe that. I am a bit of a risk taker. We shall see in about 30 years time.

              They have sent out their products for third party testing, but not sure where there cycle ratings are derived from. I'm not concerned about warranties as if for some reason they don't pan out, then a warranty won't help anyway. The warranty they have in place to cover manufacturing issues which should show up within the first year or two.

              So, looks like it will take 30 years to turn you into a believer!

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              • #8
                You will need to be able to control the discharge current of the LFE. Will you be able to do that with available DC charge controllers?

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                • #9
                  Originally posted by northerner View Post
                  Yes, I am gullible
                  OK good when you get in your car next time look up at the roof liner. I took my wife's lipstick and wrote "[I][U][B][COLOR=#ff0000]GULLABLE[/COLOR][/B][/U][/I]" in bright cherry red. You can send me the bill.
                  MSEE, PE

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                  • #10
                    Originally posted by Sunking View Post
                    Are you gullible enough to believe one word of that claim? I think you are. It will be 30 years before that is proven from a company with a product of less than 1 year on the market. Did you ever stop to think why a 30 year battery only has a 2 year warranty and if you want a 5 year pro-rated warranty cost extra? Are you clueless that there are FLA batteries with a 10 year warranty and been in biz for 50 years and more?
                    I can't use a battery only to 25% DOD. Definitely not one the cost of AHI. I am jealous of those who can.

                    Am I supposed to switch over to LFP when I go below 75% SOC? I can't envision an expensive bank of batteries, that only handles part of the load, and then having another expensive "boost" bank for when the meaty part of the load comes along. Heck, I'll go to PbA first.

                    I have to charge my batteries. To charge AHI in a reasonable amount of time, I need a higher current. As soon as I get to 4A, the RT efficiency goes below 80%. Ruh-oh George. At the highest recommended current, 8A, the RT efficiency is below 70%. Won't work for me, regardless of warranty.

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                    • #11
                      Originally posted by Living Large View Post
                      I can't use a battery only to 25% DOD. Definitely not one the cost of AHI. I am jealous of those who can.
                      I can afford them but would not use them even if it were your money I was spending. They would have to be given too me to operate my ham radio gear. That would be ridiculous as that would require me to buy 2 of the S20-008F that cost $2300, would fill a closet with 500 pounds of battery to replace a pair of Trojan T-105's costing $260 weighing in at 120 pounds that fit nicely under the corner of a small desk, with a warranty twice as long.

                      LL I finally found the final coffin nail in the batteries. Look in the [B]specs[/B] at the discharge curves. All the info one needs to know is in those curves. They would not dare print them in a Table Format. I will give you a hint. Peukert is hiding in there. If you only pull 2 little amps off a 51 AH battery C/25 it is a 50 AH battery. If you pull a realistic 10 amps or C/5 it is a 16 AH battery. So you tel me what you would buy For $1150 you get 600 usable watt hours, or for $260 you get 1400 usable watt hours?

                      Forgot one thing. You will need a a much larger wattage solar panel to go along with a monster sized battery due to th every poor charge efficiency. About 50 to 60% more panel wattage.

                      Yankee that is what high internal resistance cost you. How deep are your pockets. I can fix ignorance, but I cannot fix STUPID.

                      Here is the take away. To compare Aquion to FLA we have to find a real usable equal discharge rate to 42 volts. That is C/8 rating. 100 AH at C/8 on a FLA battery is 125 AH FLA battery or 12.5 amps for 5 hours. It will require an Aquion to be 312 AH to match it.

                      Do the real math of cost. It would take 6 stacks of the S20 stacks costing $1155 each or $6930 wigging 3000 pounds, to equal 4 Trojan 30H 12 volt 135 AH batteries (car battery size) costing $200 each or $800 total weighing 240 pounds. That should wake your butt up. If not you are hopeless
                      MSEE, PE

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                      • #12
                        Originally posted by Sunking View Post

                        Do the real math of cost. It would take 6 stacks of the S20 stacks costing $1155 each or $6930 wigging 3000 pounds, to equal 4 Trojan 30H 12 volt 135 AH batteries (car battery size) costing $200 each or $800 total weighing 240 pounds. That should wake your butt up. If not you are hopeless
                        This thread was a question about technically boosting with LFP, and not the merits of using a certain battery chemistry.

                        In any case, I have done the math. Your math fails to account for cycle life and dealing with batteries that need replacing every few years. Not to mention the maintenance and keeping on top of charging all the while. It's pointless comparing without factoring in cycle life.

                        And by the way, average current draw with my system, even in winter, will be about 1/2 amp per stack, if I go with 12 stacks. So efficiency is not a concern. Battery replacement cost is!

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