adding a second battery bank

ldiorio

have a 48v solar system with 8-Fullriver AGM batteries which are about 3 yrs old -i discharge them no more tham 25%

am interested in adding a second string of batteries 8-more fullriver batteries

they say you cant add new to old ---can i do this or can someone explain the details to me

thanks 
Lou

Estragon

The risk is that the new batteries will essentially "catch up" to the old ones in age characteristics. For example, if your old bank is in good shape, it's life expectancy might be another five years or so, and the new string would have about the same in parallel.

With AGMs, it's harder to really tell what shape they're really in as you will have no records of SGs.

If you never discharge more than 25%, it may be cheaper in the long run to discharge more deeply as loads grow rather than adding a second string.

In other words, you can add a second string of (identical) batteries, but it may not be most economical.

Marc Kurth

Lou,

When you ask a room full of battery engineers if you can cost effectively add new to old, it can be like starting a food fight in a cafeteria. Opinions run strong and deep. The optimal approach is to replace all batteries, period. But not everyone does this.

I have had detailed conversations with the engineers at both Concorde and Fullriver, and I can present a case for either argument. The devil is in the details......... Standing voltage, measured storage capacity, average DOD history and why you are adding, are the starting point.

Just today (literally) I approved the addition of a third string of 2v, 900 ah batteries to a 2.5 year old battery bank. The original string is still at 94% capacity and the individual cell voltages are all very, very close to the one year old second string - which are close to new battery spec's.

I do realize that many people here will say that it should never be done, and that I'm an idiot. My field experience has taught me that it is like asking: "How long is a piece of string?"  Well, it depends..... I have been designing/selling/supporting battery banks for 9 years, so I have strong convictions on what I've seen work - and more importantly, the failures!

Do consider that for a given daily load, older batteries with a reduced storage capacity, will effectively be cycled more deeply than new - potentially accelerating wear. It all depends........

Marc