can I combine diferent AH batteries?

homerramirez Solar Expert Posts: 102 ✭✭
Hi, guys....I have a question about batteries, currently I have 4/200ah 12v Napa Power Cycler batteries AGM, all new (one month old) but at $263.00 each is getting expensive to increase my bank size, I found a guy that sell the 100ah12v telecomunication batteries AGM for $100.00 dlls. ea., Is it a good deal to save $63.00 dlls. for every 200ah.? or how will affect my existing battery bank by adding the 100ah. telecomunication batteries. I have my array wired @ 12v.

any help on this is very apreciated.:D


  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    Re: can I combine diferent AH batteries?

    Generally, once the batteries are probably more than a few months or a year old--it is usually not a good idea to mix old and new banks...

    The rough thinking is that the new batteries will tend to "take the load" because they have lower internal resistance and more active plates (as they are new) and will tend to cycle more and deeper than the rest of the bank... Until the "wear" down to roughly the same state as the other (older batteries) in the bank... So, the theory is that the new batteries will die about the same time as the old batteries (all things being equal--which they are almost never).

    The bigger issue you have is that you have different sized (and perhaps even different brands/construction) batteries that you want to bring in. Unmatched batteries will not share the load very well at all and, presumably, the "lower resistance" batteries will carry most of the load... But you have so many variables--it is difficult to guess what would happen in your case (different size/age gives you different resistance, slightly different chemistry/metallurgy/construction may give different voltages).

    The above discusses the theory of failure, mostly, for batteries in parallel.

    For batteries in series, another issue with AGM batteries is they cannot be "equalized" or overcharged (at least, very much) without generating internal pressure and venting gas. So, if you have two batteries in series, one may reach full charge, while the other does not (older battery, the two batteries where not fully charged when connected, older battery has more "leakage" current than the newer battery, and/or some other reason)... And you will not be able to fully charge both batteries becasue they are not balanced in capacity/behaviour. So, either you will undercharge one battery (and cause early life failure) or overcharge the other battery (and cause early life failure).

    There are other methods that can overcome some of the above issues--but probably not worth the cost/time/confusion to worry about at this time.

    You might try setting up a second system (panels, charge controller, and/or a 12 volt to 12 volt battery charger) with the other batteries and use a marine switch to combine strings when needed (heavy loads, etc.)...

    To be honest, the above is a lot of speculation--and it would be difficult to predict what your experiences will be... But, remember a small difference of 0.10 volts between two 12 volt batteries can represent a ~10% difference in charge level...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: can I combine diferent AH batteries?

    Mixing batteries from different "batches", is not a good idea. From different Mfg's, you can be assured they will have different chemistry's (1% more of this, 3% less of that) and therefore, different voltages, even .1V difference in design voltage, will mean one will be undercharged, or the other, overcharged. AGM, if overcharged, cannot re-combine gasses fast enough, and so they vent. Once your H & O2 are vented, you cannot add more water, you have just lost capacity. Even at 1% loss per week, in a few weeks, you have reduced your battery capacity. The only way it's feasible, is to build 2, separate battery banks, and once a week, after a full charge, switch to the other bank, and run off it for a week. You may have to reprogram your charge controller to a different voltage for each brand of batteries.

    In your case, research for each prospective battery, what it's recharge, and float voltage is. I'll bet they are not the same from brand to brand.
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  • chilefrans
    chilefrans Registered Users Posts: 2
    I have a 48 V powers system. I hv connected  (4) 12v 100AH batteries to run 48v system  . (12V x 4x 100AH) x 3 
    The 3 sets are connected together in parallel connection. each set is connected is series  to make a 48V 100AH. 
    I would like to add one set of (12V x 4 x 200AH)  eg 
    (12Vx4x100AH) + ( 12Vx4x100AH) + (12Vx4x100AH) + ( 12Vx4x200AH) 
    is it safe to add one set of 12V x 4 x 200AH  battery?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    Is it safe? If the batteries are of the same type (all AGM, all FLA, etc.) / Brand / Model family--They probably will operate with reasonable safety...

    You still have the question of mixing old and new batteries. And I really like to avoid more than 3 parallel strings just from the issue of wiring (lots of parallel connections) and lots of cells to check (flooded cell, you have 4x 24 cells per string to check; and just more chances that a cell will go bad eventually--And you have to keep an eye on everything).

    And when wiring parallel battery strings, take a look at this website on how to paralel batteries so that they share current between strings:

    Keep an eye on all the batteries. A quick check of the voltage across each battery to make sure they are all nearly equal is a start (any battery that is high or low with respect to the others should be investigated).

    If you have a DC current clamp meter, you can also monitor the charging/discharging current to make sure each string is carrying its share of current (roughly, they probably will be identical current sharing).

    At some point, you will probably want to replace all the batteries with larger AH rated cases, and fewer parallel strings.

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