Mixing different age batteries

wxh3wxh3 Posts: 70Solar Expert ✭✭✭✭
I have 3 small 12V 12AH SLA batteries (gel cell I think) that are of different ages...one 3 years, one 2 years, and one almost new. None have been used very heavily..mostly just sitting and having their charge maintained. I know the standard advice is not to mix different age batteries. However, I would rather not go out and buy two new batteries if I can still use these with decent results.

My application has them in 36V series (a small scooter btw.) However, in charging I will probably either charge them individually at 12V or in parallel all together at 12V. I plan to use a small <50W 12V solar panel and charge controller.

I also have a cheap 36V charger (plug into A/C outlet) than came with the scooter but I don't trust it. My previous batteries melted when charging with it, granted after several years of infrequent use.

Question, is the problematic part of having different age batteries mostly in the charging phase or the discharging phase? Note, I will be discharging them in series. The charging will likely be on each battery by itself or in parallel.

I guess the main issue is that it will harm my brand new battery? Shortening life a bit isn't a huge concern for me as the batteries are not that expensive.

Comments

  • mike95490mike95490 Posts: 6,440Solar Expert ✭✭✭✭
    Re: Mixing different age batteries

    Charge individually (each 12V) The problem will be discharging them when you use the scooter.

    You have 18 individual 2V cells, from different lots. One will have lower capacity, one will have higher than all the others. The rest will be scattered in between.

    Cell Reversal is the problem you get when the battery pack reads 15%, and one cell is completely discharged, further use creates a situation where the other batteries in the string reverse charge that 1 cell, which is really a BAD thing. Nicad and NiMh are nearly instantly destroyed, but the lead acid can take a little bit of that abuse, but will go down hill rapidly.
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  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Mixing different age batteries

    seeing that you'd otherwise just have some batteries sitting around aging if you buy new batteries, i say use them. they may not last as long as they could've, but why waste them? when they've died some time down the road then there's no question about getting new batteries
  • wxh3wxh3 Posts: 70Solar Expert ✭✭✭✭
    Re: Mixing different age batteries

    So it sounds like the main risk of using them in series is damaging my older batteries instead of harming my new one. Perhaps I could avoid this by trying to not discharge the batteries too deeply. Granted, that would reduce the usefulness/range of the scooter.

    Since currently the batteries are not being used for anything I think I will try to use them. If I have to buy 3 new batteries before too long, so be it.
  • icarusicarus Posts: 5,070Solar Expert
    Re: Mixing different age batteries
    wxh3 wrote: »
    So it sounds like the main risk of using them in series is damaging my older batteries instead of harming my new one. Perhaps I could avoid this by trying to not discharge the batteries too deeply. Granted, that would reduce the usefulness/range of the scooter.

    Since currently the batteries are not being used for anything I think I will try to use them. If I have to buy 3 new batteries before too long, so be it.

    I would, by all means use them if they are just sitting around. Time alone will kill a battery eventually.

    But I would argue that the opposite of what you suggest is true. That is, the condition of ALL the batteries in a string will soon mimic the age of the OLDEST one.

    Tony
  • BB.BB. Posts: 25,139Super Moderators admin
    Re: Mixing different age batteries

    My guess...

    In series, the weakest cell/battery will set the overall ability of the string...

    Mike is correct, the "damage" will occur when one (or more) cells are 100% discharged and the cell(s) reverses polarity (effectively the current draw from the scooter will reverse charge the cell(s)).

    The problem is that when you have large numbers of cells in series (in this case 18 cells)--it is almost impossible, by monitoring the voltage, to see when one cell has dropped to near 0% state of charge (point at which damage has pretty much begun to happen).

    When all of the cells are "balanced" when one cell begins to drop, all the other cells begin to "fade" at the same time--so it is easier to see by voltage and/or scooter performance that the battery pack is exhausted. Thereby limiting the chance for major battery degradation. This is why "series" battery strings last the longest when all of the "cells" are matched (usually done for high performance Radio Control battery packs)

    To give your pack the best chance, charge each battery to 100% charge individually--sealed cells cannot be "equalized" like a flooded cell battery as sealed batteries would vent/be damaged (equalization fills the need to "overcharge" some cells to allow weaker cells up to 100% charge).

    Once you have all of the batteries brought to 100%, then you can charge them as a single 36 volt string without major issues (each cell has had the exact same amount of Amp*Hours removed, so they should reach 100% charge at the same time when in series).

    The issue of batteries "aging" to the same capacity--In My Humble Opinion (IMHO)--is a result of paralleling old+new batteries together. Newer batteries (lower resistance, possibly higher voltage from new chemistry) cycle more of the load than the older batteries--until they "wear out" to the same state as the weakest battery(ies) in the string.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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