We have a small stand alone system 12v that runs a few lights, our composting toilet fan, a 12v TV , and a 12v pressure pump. Last May we upgraded our battery bank to 6 x 2v 500ah Sonnenschein batteries. Then a couple of months later we thought again and decided to add another 6 of the same batteries ( making it a 12v 1000 ah system ) and an inverter to run our 2 Vestfrost freezers. We already had enough panels for this We bought the original batteries from Energy Matters and they said it would be ok to add 6 more as long as we did so fairly soon ie within 6 months, so we ordered then in early Sept. They told us unfortunately they were out of stock and would have to come from Germany. This would take 12 weeks but the supplier was very reliable yadayada .......... Now they say they wont be here until 25th Feb..........they are willing to refund our money but they also say “One more suggestion... had a talk to our most experienced stand alone system installer about adding a new string of batteries to an existing 6 month old string and he said it should be ok. He suggested that you mix + match, old+ new batteries in each string to minimise any differences in internal resistance in the batteries. So in each string of 6 batteries, you would have 1new, 1 old, 1ne w, 1old, 1new, 1old. You could also add extra parallel links to equalise. This is not such a big issue with 12V battery bank but more for 24v, 48V banks where there are more cells in series.” This sounds feasible but also could be a story just to keep the sale ! so I am hoping someone with more knowledge than us can tell the difference please ! frosty
Hi Frosty, The decision is up to you of course but you don't really have a lot of choice. It's a pain when this happens and it's one of the reasons we have three systems at our place after years of adding on more panels when we can affort to. I don't understand why old/new/old configuration would reduce the difference in resistance but I'd love to know the explanation. Can you ask the company and let us know the answer? I have added batteries and considerably reduced the life of the newer ones when I started experimenting with solar so I try to avoid that now. I've also added a new battery to an incorrecrtly sized system after a year and there was no noticable reduction in performance. If you've taken care of your existing batteries and they're less that six months old, then I'd say take the chance. Irene
Hi Frosty The problem with mixing old with new cells is that the charger/regulator will read the resistance from the weakest cell (ie the oldest) and will charge the entire bank accordingly. Thus your newest cells will be charged as if they were slightly degraded, which will rapidly bring them down to the state of the older cells. If your cells aren't too old it's not much of a loss but I would still be wary. A similar problem can occur when linking cells in paralell https://www.rpc.com.au/products/batteries/deepcycle_faq.html You could try separating the battery banks, use one for the inverter or other high consumption appliences like your fridge and the other bank for general purpose. I believe the setup is a bit tricky though and you may need separate regulators/chargers. cheers Steve
Thanks everyone we are still trying to decide what to do ......... this is a big investment using our super and we bought Sonnenscheins because we hope they will last us the rest of our lives ( they say they will give 15 years and we are arround 60 ) its does seem very risky specially when as little fish points out we are going to be running 2 strings in parallel .......... but it just doesnt work to run the 2 separate as 2 freezers are too much to run on half wondering whether to keep the existing for DC only and save more and buy bigger batteries for the inverter set up frosty
That's exactly what we've done too when our savings have allowed us to. It's quite useful having more than one system because you can redesign the new one exactly how you want it and still use some of the power (if you've a controller which dumps some of the power) to help the older one along. You also have at least lights and and internet if either of the systems goes down either because of a failed inverter or batteries of just to do without it for a day or so for maintenance. In times of no sunshine it's useful too to be able to switch over to another system if you need it to keep something running. It's much better having a really good system rather than having money in the bank - but I'm sure you know that already !
Just out of interest, it sounds as if you have a great set up, what made you choose the 240v freezers? - was there other stuff to run too? (you can get regulators that will charge 2 banks seperately, you could get a second regulator and run them both off the same set of panels, you could also get a device from a 4wd shop to do this but why complicate things - i would get 1 old battery and 1 new, wire them in parallel then wire in series to the next lot of 2 and so on - this should remove the resistance problem (if any). Sonneschein batteries should be the "bees knees" and from all accounts 6 months or even a year at reasonable discharge levels should see no real difference from new. I would be a bit annoyed with the supplier but don't stress on the batteries.
