Individual battery replacement.

alright71
alright71 Registered Users Posts: 15 ✭
in Off Grid Solar & Battery Systems #1
I apologize if i am repeating a previously asked question...i have looked amongst the 10s of thousands of post but did not find this particular question )

I have come across a paper which was written by a Manager at Sprint. It discusses commissioning, testing, and replacement of VRLA batteries. If i understand their application, it is for a backup scenario not necessarily "off grid" but i don't think it makes a difference.

Attachment not found.

The huge item of note which i see is that this paper discusses the replacement of individual batteries within the array as they go bad instead of replacing the whole array. Over the years this results in a mismatch of batteries (ages and/or brands) but this appears not to be a factor

I know this is contrary to our normal practices, and that is why i decided to raise the question. Is it possible, particularly in larger battery arrays (20+ batteries), to replace batteries as they go bad as discussed in this paper?

Thank you in advance for your thoughts on this!

Blake
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Comments

  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: Individual battery replacement.

    It's only contrary to the tech'os and parrots. Been there , done that many times. All the new battery is going to do is drop down to level of the others over time. It's all about economics, everyone would like a perfect matched set of batteries, it reality they are all a little different.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #3
    Re: Individual battery replacement.
    alright71 wrote: »
    If i understand their application, it is for a backup scenario not necessarily "off grid" but i don't think it makes a difference.

    Interesting question... I think it might make a difference that their batteries are for backup and not for off-grid. An off grid battery is cycled frequently... at least every day. A backup battery spends most of its life in float.

    The reason we recommend not to mix old and new batteries is that the batteries will not share the charge-discharge cycle equally. If there is no charge-discharge cycle (as in floating batteries), it probably does not matter that you mix old and new batteries.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Individual battery replacement.

    vtMaps has got it. When you're actually cycling batteries regularly the small differences get bigger faster.

    So the issue is when you're replacing one in a string/bank, how bad are the others? If they are not too far off good it doesn't make much difference at all just as Blackcherry said.

    Trouble is around here when we get questions about people putting "one new one in" it usually is on a bank that's years old and at least deeply cycled if not badly treated. If they are also expensive batteries it's like chucking money out. It will still work but you will still be losing some of the value of the new battery.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #5
    Re: Individual battery replacement.

    I see another angle to this that deals with multi-bank arrangements.

    If you have 2 banks, having different age/condition and 1 CC:

    you will likely have a problem getting BOTH banks to full charge state simultaneously

    &/OR you will overcharge one of the banks.

    My solution was to get 2 CCs (ya I know expensive but a guy has to have his toys) so that each 'identical' bank gets its own specific charge regime from 2 arrays.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Individual battery replacement.

    Clarification to what westbranch said:

    This two controller/two battery system only works if the two banks of batteries are kept electrically separated. If they are connected on positive and negative while charging, then it acts as one bank being charged by two controllers and there is no advantage.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #7
    Re: Individual battery replacement.

    ...and a related question: would it be less-bad to switch out a battery in series rather than in parallel? I.e. if you had 2 x 12V batteries in parallel to give you 12V, and you swapped out an old one for a new one, then the difference in charging currents would be substantial. But what if you had 2 x 12V in series and you swapped out an old one for a new one?
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #8
    Re: Individual battery replacement.
    stephendv wrote: »
    ...and a related question: would it be less-bad to switch out a battery in series rather than in parallel? I.e. if you had 2 x 12V batteries in parallel to give you 12V, and you swapped out an old one for a new one, then the difference in charging currents would be substantial. But what if you had 2 x 12V in series and you swapped out an old one for a new one?

    My thoughts as well, only I have 3 parallel strings, which is one of the reason I'm going with 2 volt batteries for my new bank.
    I believe I've been very lucky to get 11 years out of the present bank, never ever expected that. And I'm still learning, even after all this time. Perhaps operating an off grid system is like life itself - - a never ending learning process, and if we come to the point where we think we know everything, it will be time to hang up the keys.
    Many of my neighbours consider me an "expert" in off grid. Well I may know a lot more than those who know nothing about the subject, but I'll never consider myself an expert.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Individual battery replacement.
    stephendv wrote: »
    ...and a related question: would it be less-bad to switch out a battery in series rather than in parallel? I.e. if you had 2 x 12V batteries in parallel to give you 12V, and you swapped out an old one for a new one, then the difference in charging currents would be substantial. But what if you had 2 x 12V in series and you swapped out an old one for a new one?

    The "old" one will act as a lower resistance in relation to the "new" one. In series it will increase the over-all resistance and reduce current flow (which is variable on a charging system so it has to be compared to a non-existent "ideal"). In the case of a failed cell, greater Voltage will be induced in the new battery: the old one might come up to 12 Volts instead of 14 and the new one will be pushed to 16 (not exact numbers; just an example). You will reach full charging Voltage for the new battery sooner.

    In parallel it will act as a by-pass current path around the new one, directing charging power to heat rather than charging. Again you would still get the full charge in the new battery, but it will take longer. The net effect for either is imbalanced charging of both batteries.

    Having seen and used batteries in both types of configurations I can tell you that you get fewer problems with the single series string as opposed to parallel connections. Argue the case all you like, but that's what happens in the real world. It isn't just what can go wrong it is what is most likely to go wrong.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #10
    Re: Individual battery replacement.
    Cariboocoot wrote: »
    The "old" one will act as a lower resistance in relation to the "new" one. In series it will increase the over-all resistance and reduce current flow (which is variable on a charging system so it has to be compared to a non-existent "ideal"). In the case of a failed cell, greater Voltage will be induced in the new battery: the old one might come up to 12 Volts instead of 14 and the new one will be pushed to 16 (not exact numbers; just an example).

    Isn't it the other way around? An older battery offers more resistance, so there will be a higher voltage difference across the old battery. A new battery offers less resistance, so there will be a lower voltage across it. So you will end up overcharging the old batt and undercharging the new one, no?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Individual battery replacement.
    stephendv wrote: »
    Isn't it the other way around? An older battery offers more resistance, so there will be a higher voltage difference across the old battery. A new battery offers less resistance, so there will be a lower voltage across it. So you will end up overcharging the old batt and undercharging the new one, no?

    Nope. For "old" battery think "discharged" battery: draws more current for the same applied Voltage, therefor lower resistance. Also produces less Voltage and (most important) has less capacity.

    And yes you will overcharge the "old" battery if the "new" one is being properly charged or undercharge the "new" one if you control charging by the "old" one. In reality you tend to get a bit of both: too much power to the old and not enough to the new.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: Individual battery replacement.

    Past a certain point--Either you have to get the system working and replace one failed cell... Or, you choose to replace them all.

    I believe the "matching" issue with adding a series cell is that your older cells slowly have higher and higher self discharge. While the new cell has less.

    So, your charging/equalization will be "harder" on the new series cell because you are forcing more current (i.e., over charging) to bring the other cells back to "full charge" (as needed).

    And--The AH capacity of the entire string is limited by the maximum capacity of the "smallest AH" cell. With a new cell, they can be as much as 20% less capacity then cycled cells (take 10-100 cycles to get a battery "aged in" -- very roughly).

    And if you have an old bank with 75% capacity cells, adding a new cell (to replace a failed cell) will still only give you an overall useable bank capacity of 75%--The new cell cannot add its "extra available" capacity in a series string.

    I would suggest to look at it from a financial point of view. If the replacement cell (or battery) will give you enough life to the balance of the bank to make it financially worth it to you (i.e., 12 batteries, replace one at ~8% cost--and bank will last more than 8% longer--i.e., a year or more), then why worry (too much) about the one new cell--It should be replaced when the rest of the bank is replaced (if new cell is more than a year or so old at bank failure). You got your money out of the replacement cell.

    There is always the issue that you don't know for sure if the bank will last Y Years longer or not--A bit of a gamble going on here.

    I still remember Tony's (Icarus) and his now semi-retired bank... He replaced (and/or removed) a couple batteries well into their service life (more than 5 years???) and the balance of the batteries have something like 10 years on them now--and still working.

    So, even from the same lot, you can see 2:1 life ratios between a "Good" and "Bad" battery in the same application with the same maintenance--Feeling lucky? :p

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Individual battery replacement.
    BB. wrote: »
    So, even from the same lot, you can see 2:1 life ratios between a "Good" and "Bad" battery in the same application with the same maintenance--Feeling lucky? :p

    -Bill

    Yup: that's the worst of it. No matter how much you study the theories, do the research, work out the plan ... still a crap shoot.

    Not a very comforting thought when you're laying out hundreds or even thousands of dollars for something. :cry:
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #14
    Re: Individual battery replacement.
    Cariboocoot wrote: »
    Yup: that's the worst of it. No matter how much you study the theories, do the research, work out the plan ... still a crap shoot.

    Not a very comforting thought when you're laying out hundreds or even thousands of dollars for something. :cry:

    that's the case for me every time i go to buy a car too. :roll:
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  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
    #15
    Re: Individual battery replacement.

    I have been using VRLA batteries 'Diverted' from the smelter after they come out of service of the telcos. I just picked up 54 Panasonic 2 volt single cell 1040 AH batteries. There is enough for 2 strings of 48 volts and also a 12 volt string which will become spares. In the short term I will have 2080 AH of batteries at 48 volts and as the cells fail, I can replace the bad cells with the 6 spares, then in the big picture I'll have a single 1040 AH 48 volt string with 30 spare cells. After that, they will get broken down to 12 volt strings and still run something. That is the theory that I'm working on at least.

    This works out to 112 kwh of power (gross at the 8 hour rate) for about $1100 not counting the lead core charges, which tends to keep up with inflation, so it tends to work out for me. Each 48 volt string will have a seperate 200 amp breaker, so they could be charged together or individually. While an EQ charge is not reccomended, you can do a longer absorb charge to kind of do a similar thing.

    The new grade B panels came in so in the next few weeks they should go up to help charge these batteries. My goal is to use cheaper batteries, which tend to not last as long, and more panels, which tend to last a long time. A different approach, but I'll let you know how it goes.

    Skip
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
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