temperature compensation for dual battery bank

hillbillyhillbilly Solar Expert Posts: 334 ✭✭
I'm having a bit of a hard time cracking this nut. I'm in the process of wiring up a second battery bank; these will be used and charged completely separately. They are connected to our inverter via a blue seas battery switch. The real kicker is that one battery bank (flooded) is located in an outside battery box, while the new set (sealed) will reside inside the home. I'm not sure how to utilize different temperature probes for each bank while charging.

I have an Outback system, with VFX inverter, MX60 Charger, Hub and Mate... all using one temp sensor on our old bank. So far about the only idea that I've come up with is having a second temp sensor, AND manually unplugging the one sensor and replacing it with the other every time we change the battery switch. Not very elegant, and plenty of opportunity to forget and possibly over/under charge the batteries. Anyone else dealt with this challenge before? Any other ideas?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,812 admin
    Re: temperature compensation for dual battery bank

    Monitoring the AGM is more important (don't want to overcharge/damage your new expensive bank).

    Are you planning on discharging the banks in parallel--or what is your plan for the old flooded cell? Emergency Backup? Operating other loads?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: temperature compensation for dual battery bank

    I have to take issue with this statement: "These will be used and charged completely separately".

    If they are completely separate, there is no problem because each battery bank will have its own array, charge controller, and load.

    This would be a very good idea, because not only will the temperatures of the two banks be different, so will the charge parameters. Especially if one is FLA and the other AGM. You would not only have to swap RTS in and out, but reprogram the Voltage set points on the MX60 every time.

    It might be possible to switch charging & temp sense between banks using the AUX function and some extra circuitry, but frankly the whole thing sounds like problems waiting to arise. I did the "two banks" set-up for a while, manually switching inverter and/or charger between banks using two battery bank switches. It was a pain, and that was with two identical sets of batteries in the same place (sharing RTS).

    A few more system details and maybe we can work out some way of achieving the end results without a disaster looming.
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: temperature compensation for dual battery bank

    Thanks for the responses! Lets see now, about some of the questions and comments:

    1st: we will NOT be using these batteries in parallel, they are not the same age, configuration or type.

    2nd: yes we are a bit more concerned about the newer batteries, so if there had to be some priority those would be "it".

    3rd: the "plan", if you can call it that, is mostly just to have a back up as well as to minimize the amount or level of cycling that the new batteries get in the short term. I hate wasting anything with some good life left in it, and these old batteries still have some pretty good usable life left in them I think. At some point in time I think that the old batteries will just be recycled and we'll be down to just one bank, but then of course at some point the "new batteries" will begin to fail and I like the idea of being able to have a more smooth transition into a new set later on down the road.

    4th: 'Coot, you are of course totally correct that there are other considerations for two battery banks. Somehow changing the charging parameters on the MX doesn't seem as "clunky" and awkward as unplugging one temp sensor and re-plugging in a different one. I don't know, maybe it's just what I'm used to since I change my charging parameters from time to time as it is (seasonal, long term absence, etc).

    If it comes down to it, at some point I could just let go of the idea of having a second battery bank. For the time being however it's a nice bit of insurance "just in case"...
    As for switching batteries; that part should be really simple. I have the switch wired so that all charging and discharging is changed by turning the dial. I'm all ears on some extra circuitry if you have some ideas. What other system parameters would you need, most info is in my signature line...
  • BB.BB. Super Moderators, Administrators Posts: 29,812 admin
    Re: temperature compensation for dual battery bank

    Hmmm....

    Program the Charge Controller(s) for your AGM set points (and manual equalization only), set the temperature sensor on the AGMs.

    Connect the battery switches as suggested. Set all to "both". Monitor the heavy charging/discharging current with a DC Current Clamp to each battery bank. I would hope that you will see the the "lower voltage" bank sharing at least 1/4 of the heavy charging/discharging current. If not (say one is 100 amps and the other is 10 amps), then it is probably not worth the work to "share" the banks.

    Run the banks in parallel and see what happens. Ideally, your AGM's will not cycle as deeply but will see proper charging voltages.

    Monitor the flooded cell bank (specific gravity) and make sure it is happy. Once a week to once every month or so, set the charge controller switch to Flooded Cell and do a manual equalize until the cells are back to 100% charged.

    Repeat.

    And every once in a while, take your DC current clamp meter and under heavy load (and heavy charging) monitor the current flow out of and into the two battery banks. When the Flooded Cell batteries stop supplying at least 1/4 of the load and 1/4 of the charging current--then they probably are not really helping you that much. Keep them off/as spares--or recycle.

    Note--You may find that batteries share charging/discharging currents differently based on state of charge (AGM may use/supply more current at first; then an hour or so later, the flooded cell start to participate more).

    And frankly, without you gathering some data--I am not sure anyone can actually predict how it will all work out for you.

    But it would be an interesting set of tests.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: temperature compensation for dual battery bank

    Bill, I'm NOT looking to use these banks in parallel. That wasn't my intention or question. I would not expect them to play nice together, so I was planing on switching between them from time to time.
  • BB.BB. Super Moderators, Administrators Posts: 29,812 admin
    Re: temperature compensation for dual battery bank

    If I set the chargers up for AGM and monitored the current sharing between the banks... I would give running them in parallel a try.

    For short term use (while monitorting), I don't think you will hurt anything.

    Of course, if you get a shorted cell in the (old) flooded cell bank, it could discharge the entire bank (say you are visiting family over the weekend)... That would be a shame.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: temperature compensation for dual battery bank

    Bill, I'd let you try that with your batteries ;-) but I think I'll pass on doing that with mine. I can't really think of why I would want to connect them in parallel. With a single 3500W inverter it's not like I could draw such heavy currents as to warrant that big of a bank anyways. And I can think of at least a few reasons why I wouldn't want to connect two different battery banks of different type, age, and capacity (and different temperatures). At any rate, that wasn't really the point of the thread.

    Maybe I wasn't clear, but the idea is just to have the ability to use one battery OR the other (with a switch that will change back and forth). I have all that figured out (and wired), but haven't figured a solid way to tell the charge controllers the battery temperature of one battery vs the other. Seems like it shouldn't be so terribly complicated, but it's gone over my modest knowledge base for sure...
    Thanks anyways
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: temperature compensation for dual battery bank

    I was thinking maybe you could use the MX60's AUX output to switch some of the PV to another controller once the main bank was in Float, thus boosting up the FLA's to a proper charge level and keeping them ready for emergency use (switched between the AGM's with a Blue Sea battery switch).
    Not sure of the fine details for your system, but it wouldn't have to be another MPPT controller. With the AGM's being on the MX60's RTS, the outside FLA's would need temp comp with the second controller, but that should handle all the charging differences.

    Issue include total array size and configuration vs. battery bank capacity and how much PV could/needs to be switched and what PV Voltage is available vs. battery bank Voltage (so that an inexpensive PWM controller could be used).

    I hope that gives you a clue what I'm suggesting here. :blush:
  • SlappySlappy Solar Expert Posts: 251 ✭✭✭✭✭✭
    Re: temperature compensation for dual battery bank

    as B.B. says here......Program the Charge Controller(s) for your AGM set points (and manual equalization only), set the temperature sensor on the AGMs.

    have two temperature sensors one for each batt bank, but hook them to a dpdt switch. have it mounted next to where you have both banks switched at.?????? so when ya do switch batt banks the temp switch will be their also. Just a thought?
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: temperature compensation for dual battery bank

    I see what you mean there 'Coot. That does sound a bit complex, but possible. I already have the second controller (morningstar 45amp MPPT, needed for odd array configuration). Total PV array will be about 2.2kw (shortly), with about 1380W run through the MX, and about 850W run through the TS45. Agm bank will be 900ah, while the older bank is probably close to 450-500ah now (originally 700ah). So charging wise I would be happy to divert some extra power over to the old bank during float, but I could also just use and recharge the old bank up a few times a month and then let them sit (full) in between uses.

    Slappy, the idea of a dpdt switch right by the battery switch sounds like what I'm looking for. Not familiar with them, but that sounds pretty simple. I like it!
    thanks
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: temperature compensation for dual battery bank

    A DPDT switch is Double Pole Double Throw. It can switch the two input wires for the RTS between one temp probe and another. You'll need a couple of the jacks and a plug and some patience to solder it all up right.

    In case anyone else is reading this, between the battery type difference, the location difference, and the temp compensation difference the Voltage set points could be off up to 6 Volts depending on the system Voltage. That's why he's trying to do this. ;)
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: temperature compensation for dual battery bank
    In case anyone else is reading this, between the battery type difference, the location difference, and the temp compensation difference the Voltage set points could be off up to 6 Volts depending on the system Voltage. That's why he's trying to do this. ;)

    Precisely! Thanks for the clarification on the switch terminology too.
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