This has to be idiotic...so please explain why it is!

[openplanet]

OK, I'm still trying to figure out a way to upgrade my existing, healthy, 2 year old, series string of 4 Rolls S530 6 volt batteries from 24 to 48 volts by adding 4 new ones to the string...WITHOUT violating the apparently sacred rule that one Shalt Not Mix Old And New Batteries.

Here is the idea, which I presume is completely idiotic...but I'd really like someone who knows what they're talking about to try and explain to me precisely WHY!

*Keep my existing array, controller, and 4 battery, 24 volt string.
*Set up a second array with its own controller, charging a second series string of 4 NEW batteries.
*Put those two strings in series...and there's your 48 volts, with old and new batteries being charged independently.

Yes, there will be times when one controller is in bulk while the other might be in absorb, or one in absorb while the other is in float. Why does that matter? End to end it's a 48 volt string, and a 48 volt inverter wouldn't know or care about the details.

I CAN see that there would be challenges getting each (Midnite Classic) controller to have its own WhizBangJr. for determining endamps. But beyond that...???

So again, I know this must be lame-brained. I'm just interested in hearing any clear explanations of WHY.

Thanks.

[sensij]

How does ground fault protection work in the midnight classic? If it relies on a fuse between DC- and ground, your idea is DOA.

Edit... Yep.

http://www.midnitesolar.com/pdfs/DC%20Ground%20Fault%20Protectors%20explained.pdf

[bcroe]

How does ground fault protection work in the midnight classic? If it relies on a fuse between DC- and ground, your idea is DOA.

Edit... Yep.

http://www.midnitesolar.com/pdfs/DC%...0explained.pdf

Could the new system be configured with a reverse polarity ground? With a +24 system and a -24 system, the
inverter could run between the + and -? Bruce Roe

[solarix]

They may be getting charged independently, but they are getting discharged all together. The old and new batteries will have significantly different discharge characteristics and some will discharge faster and create an imbalance. On days when you don't have enough sun to fully charge them, the imbalance will not be removed and you are just going to be fighting to maintain your battery capacity. Also, charge controllers are twice as effective as you double the battery voltage. You are just buying an extra controller (that is uneeded at higher battery voltage) hoping to extract value out of expensive batteries.
This is why I counsel off-griders (especially newbies) to just use cheap golf-cart batteries and affordably replace them when (not if) problems arise and not be faced with the situation of how to deal with expensive batteries that have a problem.

[bcroe]

Could the new system be configured with a reverse polarity ground? With a +24 system and a -24 system, the
inverter could run between the + and -? Bruce Roe

I don't see the problem with discharging the 2 strings together. Sure each half would need to be monitored to be
sure it wasn't excessively discharged; doing that is the price of being able to salvage the earlier battery life.
Bruce Roe

[sensij]

I don't see the problem with discharging the 2 strings together. Sure each half would need to be monitored to be
sure it wasn't excessively discharged; doing that is the price of being able to salvage the earlier battery life.
Bruce Roe

Yeah, I think your approach on this could work. There are positive ground charge controllers that make the +24/-24v scheme possible. Just need to verify the inverter can accept that input, and add overcurrent protection and disconnects on all legs.

Some solar panels may be more susceptible to photo induced degredation when positive grounded, but that is probably not a reason to reject this idea.