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Post by tonyqj on May 6, 2017 6:39:33 GMT
If you have an old battery who's capacity has dropped say from 110Ah to 70Ah and you put a new battery of 115Ah in parallel with it then you now have a bank of 185Ah.
As long as the battery chemistries are the same (don't mix AGM with Wet for instance) then there is not a problem.
Remember these new batteries are in parallel, not in series like a torch.
Internal resistance of a battery is nothing more than a function of its state of charge and is altered by the strength of the electrolyte.
I note that no-one who has said "I wouldn't do it" has given a single rational explanation as to why not. It's fine to do. Just keep an eye on the old batteries as they will probably die first by virtue of being older.
Obviously, as I said before, if the old batteries are dead then you wouldn't consider adding new ones to them but adding new batteries to tired ones is perfectly fine.
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Post by kris on May 6, 2017 6:55:16 GMT
So if we take your example of the old battery that's droppep to 70ah, what would it's voltage at rest be? What would the voltage at rest of the new battery be? So what would the voltage at rest be of your 'new' 185ah bank be? Internal resistance has a lot to do with the physical structure of the batteries. Number of plates, thickness of plates etc.
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Post by tonyqj on May 6, 2017 7:15:02 GMT
So if we take your example of the old battery that's droppep to 70ah, what would it's voltage at rest be? What would the voltage at rest of the new battery be? So what would the voltage at rest be of your 'new' 185ah bank be? They would all be identical Kris. They would be in parallel. Stood alone they would still be identical. A 75% charged partially sulphated battery will have exactly the same voltage as a 75% charged brand new battery (assuming the same chemistry as previously stated). What made you think that the voltages of aged batteries are any different to those of new batteries?
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Post by kris on May 6, 2017 7:21:33 GMT
At rest tony.
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Post by tonyqj on May 6, 2017 7:22:18 GMT
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Post by kris on May 6, 2017 7:23:20 GMT
So what would it be in your example?
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Post by kris on May 6, 2017 7:24:05 GMT
And how about the differing construction of the batteries?
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Post by tonyqj on May 6, 2017 7:32:52 GMT
So what would it be in your example? Identical. I'm sure you've seen plenty of charts showing the aporoximate SoC versus resting battery voltage. Have you ever seen one that states "but not if the battery is a few years old"?
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Post by tonyqj on May 6, 2017 7:36:50 GMT
And how about the differing construction of the batteries? How about it? Unless a battery is physically damaged such as with an internal soft short then two batteries of differing capacities will happily 'share' the load between them with the battery with the greatest capacity supplying a greater 'share' of the load. The same thing happens when charging. When batteries age, do you suppose they all age at exactly the same rate? Of course they don't. But the bank continues to function for many months or years with some of the batteries being better than others.
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Post by kris on May 6, 2017 7:41:01 GMT
as I've said you do what you want. Have you ever mixed old and new batteries in the same bank or is this just theory?
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Post by tonyqj on May 6, 2017 7:50:24 GMT
as I've said you do what you want. Have you ever mixed old and new batteries in the same bank or is this just theory? Sure I have. What's your vast experience to the negative then?
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Post by kris on May 6, 2017 7:56:30 GMT
I've lived off grid using batteries for about 30years and I've found it disadvantageous to mix old and new batteries. Much better to have a smaller bank of new batteries in my experience. But as I say you do what you like, I'll carry on doing what I do.
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Post by Ssscrudddy on May 6, 2017 9:29:14 GMT
Well I wont be mixing old & new batteries. My current ones are according to the manufacture only good for 85 cycles at 50% DoD. & according to my log book I've been away from the marina for 90 nights (over 3 years), not including odd weekends that didnt get written down, or forgetting to turn the electric back on. My existing ones they no longer make the same capacity (ie their replacement is 115AH). So I think I'll get the original ones I chose in the 1st post that are supposed to be good for 500 cycles.
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Post by tonyqj on May 6, 2017 10:24:09 GMT
My current ones are according to the manufacture only good for 85 cycles at 50% DoD. & according to my log book I've been away from the marina for 90 nights (over 3 years), not including odd weekends... They're not 'old', they're knackered 😂
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Deleted
Deleted Member
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Post by Deleted on May 6, 2017 10:32:05 GMT
I have never come across batteries with just 85 cycles lifetime. Not that I have looked around much.
Last batteries I bought were 100ah unbranded £55 each on ebay. They have m10 threaded posts which I prefer to ordinary batt terminals.
El cheapos. For my low consumption off grid boat. Don't know how long I'll get but would be happy with a year or two.
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