is this the reason why old battery shouldn't be connected in parallel

The old battery is lossy and has high resistance in the circuit. So the new battery ends up carrying 90% of the load, till it is as worn out as the old one, then they  both die.

bill von novak said:

solarking said:

when a old battery is connected in parallel to a new battery the new battery will be not properly charged due to the voltage difference between the two also the new battery will be charging the old battery when the charger is off. When the new battery continues to take care of the old battery it will lose it's life is this correct? or am I missing something?

Not exactly.

When you use them cyclically, the new battery gets most of the charge and discharge current (because it's ESR is lower.)  That means the older battery doesn't work as hard, but also doesn't get a full charge - which reduces its life.  Meanwhile the new battery is getting charged and discharged harder, which also reduces its life.

However, if you let them float for a long time (i.e. UPS service) then eventually they both do get charged. That's about the only application where it's not too bad to use dissimilar batteries, because a long enough float will eventually charge both.

Great post. But I will respectfully add that the "long enough float time" can vary, so it doesn't only apply to UPS service. I know that you did NOT say "UPS only" but some folks will carry that message.

Plus the resistance shift due to aging varies across the lines of battery type, design and specific manufacturer.

In my business I see a lot of systems designed for 10% to 20% average daily DOD. These systems spend a good amount of time on float. So in my experience, the charge equalizing between old/new batteries will indeed tend to happen over time, but even with "shorter" float times than continuous.

All of this assumes that the "old" batteries have been properly capacity tested and are inherently healthy of course!

Marc

bsolar said:

there are problems with any 'multi-cell' battery pack or bank setup with something called balance, it reduces output and causes too much load on other cells in the pack or bank .. no matter how you slice it a multi cell bank or pack will always go imbalanced thats a huge problem when you have alot of money in a bank .. you definitely dont want to introduce an imbalance knowingly ... in fact a wise person would figure out an alternative charging method to make sure they get a balanced charge instead of frying the whole bank at once on the nuke setting now and then .. of course this doesnt matter if your independently wealthy and/or have the cost involved figured in to replace them at set intervals .. i find it comical to watch youtubers explaining their immense system while frying their half-dead bank while talking .. they invested their life savings into that bank and found out the hard way solar isnt about battery power ...

You may be talking about a few different things here.  (If not, sorry about the misunderstanding.)

Lead acid batteries in series are balanced by performing an equalize charge.  (They actually tend to balance during regular cycling as well, but EQ charges do it a lot faster and more effectively.)  This works because once a lead acid battery reaches full charge, it will continue to accept current - but use that current to split water rather than increase charge level.  Thus you want to minimize EQ charges to reduce water usage.

Other batteries in series (like li-ion) do NOT have this mechanism, and can be quickly damaged by overcharging.  Thus the need to either balance during installation and then monitor, or actively balance during usage.

Most of the above discussion is about parallel connection, which is a different issue.  It's a good idea to use similar age batteries in any two parallel strings for some of the issues listed in previous posts.