Add a string!

[Red_Fir]

Soo...
I have a 370 AH string of six Trojan L16's,
I bought them used, they are currently 10 years old and provide yeoman service.
Resting voltage on the string (after charging and sitting 24 hrs disconnected) is 50.4 volts.
Currently they carry the required load for around 15 hrs and then my Radian switches to grid power if sun is inadequate.

I found a good deal on new L16's and picked up another 370 AH string, I would like to parallel the original string, .. from what I read the reduced capacity of the old string will drag down the new string...
Is there any scheme where a large diode could be used as a blocking diode to prevent this? or a Arduino controlled contactor to tie the two together when capacities are matched?

 

[chrisski]

Best suggestion I’ve seen is to charge one battery bank off a second PV bank. The second would be connected to the first with a Dc to Dc converter to charge the newer bank from The older bank. This is limiting and wasteful, but may be the only practical way.

The guy who suggested this was looking at a 320watt converter/charger to join the separate battery bank on a 12 volt or 24 volt system. I don't know how many watts your converter would be for this scenario, but I would guess 1000 watts. A 48 volt converter 1000 watts would be hard to come by.

 

[Zwy]

Soo...
I have a 370 AH string of six Trojan L16's,
I bought them used, they are currently 10 years old and provide yeoman service.
Resting voltage on the string (after charging and sitting 24 hrs disconnected) is 50.4 volts.
Currently they carry the required load for around 15 hrs and then my Radian switches to grid power if sun is inadequate.

I found a good deal on new L16's and picked up another 370 AH string, I would like to parallel the original string, .. from what I read the reduced capacity of the old string will drag down the new string...
Is there any scheme where a large diode could be used as a blocking diode to prevent this? or a Arduino controlled contactor to tie the two together when capacities are matched?

You can't parallel them, the new L16's will suffer when paralleled to the older L16's.

 

[Vigo]

So what are the actual targets here? Trying to get more runtime, or support a larger load? The goals might make the best approach more obvious.

I am not trying to discredit anything said about the downsides of paralleling the two strings. But, i think contextualizing the downsides might change how you feel about them.

Considering the old pack is still in a usable state at 10 years old, and that paralleling the new string to the old string is still probably LESS stressful to it than if it were simply carrying the entire load by itself, i think there’s a good chance this new string could be paralleled to the old string and still have a useful lifespan beyond 10 years. The inherent downside of paralleling batteries of different internal resistances is that whichever is lowest will do the majority of the ‘cycling’ and wear quicker than if paralleled to something of near-equal IR. But wear quicker than..what? Wear quicker than if it were installed by itself, where it would do not the majority of the cycling, but ALL of it? I think it probably poses more problems with ‘tracking’ the system than it does with causing an ‘early demise’.


Now, thats based on several assumptions. You bought the current string used, and its unknown whether they would have made it to 10 years in YOUR usage regime. Could be better, could be worse! I dont know how they were treated before, or how you’re treating them now. Another assumption is that you will remove the older string from service at such point that it is essentially degrading the newer string more than it’s helping it. Do you think you know how to parse that?

With ideal answers to those ‘questions’ i think it would be fine to parallel them, keep an eye on it, and have it be better for both the new batteries and the system’s performance than simply replacing the old string with new and not paralleling them at all.

The only reason i personally wouldnt try paralleling them is if i didn’t want to have to commit any additional monitoring or brainpower to the system, or if i had a cool ‘downcycling’ idea for repurposing the older string to something else.

My .02…

 

[Red_Fir]

Sorry for the long delay!, computing time is a low priority for me...

I recognize their will be a penalty to the longevity of the new batteries, via directly coupling them in parallel,

My question was: Does anyone know of , or is anyone able to conceptualize a blocking diode or Arduino controlled relay scheme to allow the inverter to pull from both banks while keeping them from referencing each other?