Why is mixing batteries such a bad idea?

2twisty2twisty Solar Expert Posts: 199 ✭✭✭
I've read through the FAQs that I could find and several threads. I'm sure the info is here, but I can't seem to find it.

Why is mixing battery capacities and or ages such a bad thing?

If I have a 24v system, why can't I have 1 string that is 200Ah and another that is 100Ah?

What is so bad about expanding an existing system by adding a new string of batteries that are of different age from the existing ones?

Why can't I make a string composed of different AH/Ages? For example, 4 6v batteries of differing capacities and or ages for 24v...

I'm understand that doing this is not "best practice," and I understand (from what I've read) that it impacts efficiency, but does it cause DAMAGE? Will doing this shorten battery life appreciably?

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Why is mixing batteries such a bad idea?
    2twisty wrote: »
    I'm understand that doing this is not "best practice," and I understand (from what I've read) that it impacts efficiency, but does it cause DAMAGE? Will doing this shorten battery life appreciably?

    Yes, it will cause damage. Actually, it is a bad idea to hook up parallel strings of identical batteries.
    To understand why, read: http://forum.solar-electric.com/showthread.php?14674

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • 2twisty2twisty Solar Expert Posts: 199 ✭✭✭
    Re: Why is mixing batteries such a bad idea?

    I read that thread, and it makes a lot of sense.

    There was another part to my question, however, concerning batteries of differing ages. Of course, if you only ever have one string, that's about impossible, but still, the question stands.

    I like the idea of redundancy that 2 strings can provide. If I want to have 2 strings for redundancy, why do both strings have to be the same age?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Why is mixing batteries such a bad idea?
    2twisty wrote: »
    I like the idea of redundancy that 2 strings can provide. If I want to have 2 strings for redundancy, why do both strings have to be the same age?

    They don't have to be. But the new batteries will quickly be pulled down to match the age of the old batteries. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,639 admin
    Re: Why is mixing batteries such a bad idea?

    Batteries are very sensitive to minor changes in "resistance"... We are talking about 0.001 to 0.010 ohms here (very roughly). All it takes is a differences in age, differences in cable lengths, battery cell temperatures, etc... to "steer" the current to the lower resistance path. For parallel connected batteries, the "better battery/electrical path" will carry much of the energy.

    For series connected batteries--All cells get the same charging/discharging current, but may have different AH capacity due to age and/or mixed battery models, etc... The battery/cell with the "lowest AH capacity" will run out of power first (while the others still have capacity to supply current). Once a cell goes "dead" it can actually begin to reverse charge--Which for most rechargeable battery chemistries is the kiss of death.

    Also, batteries as they age begin to have higher self discharge. In the case of series cells, the only way to "bring the cell with higher self discharge" back to full capacity (equalization) is by "over charging" the rest of the cells in the string. Equalization is hard on a lead acid cell--gassing can erode plates and the Oxygen the forms on the positive plate (electrolysis releasing hydrogen and oxygen gases) will cause positive grid corrosion. So--too much equalization will age faster too.

    Lead Acid batteries can vary in capacity by upwards of 20% -- So adding smaller batteries in parallel to increase capacity can almost be swallod up by the main string's own variation in actual capacity (and capacity changes over time).

    In the end, mixing old and new batteries can leave you will a bank where you are constantly chasing battery problems. A few months ago a cell/battery failed, a few months from now another one may fail.

    Yes, you can keep a bank limping along by replacing batteries that fail--but it may simply not be worth the effort to you.

    Note, you can get some batteries that fail sooner than others in a bank--If you have 8 batteries and one fails--It may be worth replacing the one failure (especially if you can get a used battery at a good price) if you expect years more life from the others. If, however, the bank is old, you may be better off replacing them all at the same time.

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