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, 1new, 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 !

Regards
Anne in WA

Never ever mix different strings of batteries on the same inverter, unless the batteries have not been drawn on/down.

Not an absolute expert on this, but my experience with my own system gave me enough knowledge to know this sounds very shonky.


Please be careful

Thanks DB

do you mean it cause problem with the inverter ? we thought the only risk was with the old batterys dragging the new ones to their level .......

Anne

I have no 'qualifications' in solar matters but I built and have successfully maintained my own system for twenty five years without any outside help. I would go the latter route even tho there's a bit of mucking about connecting newer to older. best way to balance out any differences ...

but this issue is nit pickingly technical I think in light of other problems that could happen during the life of the batteries ... for example: regulator fails bats overheat equals shortened bat life; someone forgets to add water for a while bats o'heat equals shortened bat life; long period of cloudy weather and without a LARGE CHARGER equal to the job of restoring 1000A so bats discharge below 11.8 volts and consequently their life is shortened ... all sorts of things more serious than connecting large bats of a very slightly different age together ... I 'm assuming these bats are supposed to last 25 year?

Anne,

Potential issue for both your invertor and your batteries.

Double checked with brother in law who is solar electrician and he confirmed my thoughts that mixing different age batteries shortens the life of the batteries. It does perform a balance, but it is a 'downward' balance as the newer batteries have there life shortened.
The advice here is to check with the manufacturer before the installer.

Older/newer batteries on same string may also give feedback issue to the inverter (invertors are generally designed to receive charge at an 'equal' rate so to speak, and cope with occasional variances in feed but not constant variances) so depending on the invertor rating/manufacture, may cause damage to the invertor.

My advice is simply be careful, do your research and go to other 'solar' installers and manufacturers. I bet my bottom dollar you get different advice.

There are some shonks in the solar industry.

Thanks DB

I agree about the shonks its getting to be like dealing with politician !

Anne

pls allow me to restate my view with other words.

I am assuming the batteries you speak of are top grade, say twenty five years anticipated life?

where yr power demands are not excessive and WA seems to have plenty of sun at the moment it is probably safe to assume yr existing bats will not have undergone excessive dischrge/charge cycles.

so it could be said yr bats are 'almost as good as new'.

it is intuitive I think to quantify 'almost as good as new' in the following way:
at the stated time of delivery of the second set of bats the first set will have been in service for 11 months or 11/300ths of their expected life.

thus the original question can be put this way: is it practical to connect batteries that have 289 parts of their expected life remaining to batteries that have 300 parts of their expected life remaining?
looking at the numbers it seems to me that it IS practical to do this.

however having 1000A of storage raises another issue. should accident, equipment failure or human error allow yr storage to fall below 11.8v ... and there has been a dearth of sunlight in the eastern states since late winter which could have allowed this to happen and WA's turn must surely come especially if you live in the SW ... have you considered the difficulty of mechanically recharging such a large mass of storage?

wilburandsally wrote:
Thanks Wilbur

the batteries are top quality and we estimate 15 ears life if we discharge by 20% but at present we are only using theortically 5% - yet readings early morning before panels start to charge are 12.7 or 12.8 v .........

I should have explained in my original post that we do actuallly have mains power and a grid feed system. The stand alone system is part of our preparations for WTSHTF and also in the meantime allows us to export more of what our GF system makes......... so for now there is always the option to use the grid in cloudy weather ....... we do also have a battery charger than can run on the grid or on our generator

Anne

If you are going to 12 batteries you would be better off with them in series and a 24v inverter?
series is considered better for batterylife than parralel

But that would not help with your 12v appliances.

Try asking your question on

solarpowerforum.net

or

solarpaneltalk.com

there is a lot of experience on those sites

Thanks Bala

we have certainly considered the 24v option - we even tried it years ago with a 24 to 12v converter and boy was that a disaster ! Within a year both the batteries and the converter died ! lucky they were relatively cheap batteries........

we wish now that we had just bought the bigger 2v batteries but at the time we didnt have the money, then few months later we found we could hence the present stuff up ..........

thanks for the links Ill go and have a read

Anne

Not sure why the 24-12 reducer should be the cause of failure as such.
Only reason i would thinks is if you did not have a low volt cut out in the system and the batteries got to low.
Any inverter or reducer will use power itself though.
I have a small 24-12 reducer on my system, it runs a clock,a couple of lights sometimes, and a 12v relay that runs a 24volt pump, it runs of the load terminal on my solar charge controller and I have the controller set to cut out if the batt volts get low.

you would get more power from the 24v set up than parralell 12v set up and longer battery life.

For longest battery life you really need to consider going 24v series instead of 12 parralel.

Stand alone is expensive as you know. Its not not the great thing that a lot of people think, it can mess with your mind and wallet,

Regards

Max

I have known quite a few with stand alone power as soon as the mains are within cooie they are first to connect

Just remember one thing connecting 2 batteries or more together with a different amp/hour in series and the new AH will be what the lowest one is. Now depending how many deep discharges you have done with your current batteries and depending if you have fully charged/floated/equalized atleast every month, never taken your current batteries over 2.41 volts per cell. Then your 500AH batteries will be lower in capacity.

Personally if you are going to stay on 12 volts then with that new bank have then as a separate array. Then by keeping the spare bank fully charged when the used bank gets to 12.2 volts, switch over use the other bank then fully charge the other bank. Using this method will ensure real deep cycles are avoided and you should see many years out of both arrays.

That dealer that told you to mix the batteries only wants your hard earned cash so if he has offered a full refund THEN GET YOUR MONEY BACK and never deal with them again. In the 8 years we have been off the grid I have learnt a lot and today there are too many shonks around willing to tell you anything to get a sale.

Also the sonnenschein battery is considered on of the best and the price of them is justified if you can afford it. It may be worth considering getting an array of cheaper batteries the same capacity and doing what I said above about the 2 separate arrays.

Regards Bryan

Thanks Bryan
RuddyCrazy wrote:
WE have used them very sparingly ( we use no more than about 7% )and I presume they are fully charged each day as the regulator seems to cut back the charge going in from mid morning most days

but they have never been on a charger just the panels o does that then do the float /equalise etc ?

the voltage in the morning before they start to charge is usually arround 12.6 to 12.8 which i think is fully charged ?

I dont understand the bit about never taken them over 2.41 v per cell ?


the problem with splitting the 2 arrays is that we only want to use about 500wh of direct DC power and the balance through an inverter to run freezers - and cant run the DC plus either fridge on a 500ah battery bank.

we will never discharge the batteries below 20% as we can either switch the appliances to the gird or have the charger come on automatically

that is why I asked !! but I did think Energy Matters wdere one of the better companies ....... I wont deal with thr locals anymore ...... so who else is there ?

we do want Sonnnscheins ...... we have used Trojans with not bad results but now we dont want flooded so we saved up to buy the best

we are considering runing on the new batteries for 6 months much as we have used the existing ones then add them together later. This means putting off running our freezers on our own power and hoping the grid doesnt go down

the other option is not taking the new 500ah batteries and instead buying larger ones - still sonnenschein - but that is more money plus another regulator !!

thanks
Anne

Anne,

I am with Bryan on this one. He explains far better than I do.

Coincidentally, I hav 12x 1700ha Sonneshien (Sp?) batteries and they are fantastic.

'Main grid power' is now only a couple hundred meters away from me and I can honestly say I never want grid power back. I enjoy being electricity self sufficient.

I spoke to my solar installer about the issue of mixing batteries and he strongly advised against it and warned against less reputable people in the solar business. I guess I can only speak from my personal research and what my local expert and brother in law tell me.

Good luck