Combining new and old batteries

[RoyceC]

I have 6 Rolls-Surrette 2V 1050ah FLA batteries connected in series in our off-grid system. We bought them a year and a half ago. We want to increase the size of our system and will need to double the number of batteries. I read that combining new batteries with old batteries will cause the new batteries to deteriorate faster. They are only a year and a half old so they are kind of new, but not brand new. How fast will the new batteries deteriorate given the age of the old batteries?

Or, is there a way to combine the outputs of two separate sets of batteries before it goes to the inverter. Is there some kind of high wattage diode?

[paulcheung]

You want to add another set of 1050 AH battery on your 12 volt system? how do you plan to charge them properly? Just out of curiosity how do you design a 12 volt system with such large AH capacity? normally people want so much power usually use 24 or 48 volt system.

I have two set of Rolls S460 and S530 batteries wired in 48 volt system, I use two disconnect breakers to separate and combine them to my inverter, because they are two different capacity and two different age batteries, so I can charge them differently when necessary.

[mapmaker]

How fast will the new batteries deteriorate given the age of the old batteries?

Ultimately the bank will last as long as your older batteries will last... that means if, for example, your old batteries are 1.5 years into an expected life of 8 years, they would now have an expected life of 6.5 years. The new batteries will also have an expected life of 6.5 years.

EXCEPT:

1) If the addition of the new batteries means you are not discharging as deeply as before, that will increase the life of the bank.

2) If you put those new batteries in parallel with the old set, you will shorten the life of both sets relative to the life they would have in a series configuration.

As Paul mentioned, you really should consider reconfiguring your system to 24 volts. It means that your inverter must be replaced with a 24 volt inverter

Many/most of the other components in your system may still be useful in a 24 volt system.

Also, you may need to increase your solar array... what do you have now for an array and controller?

--mapmaker

[RoyceC]

I will be increasing to 24 volts when I get the new batteries and replacing my old 12-volt Trace inverter with a new 24-volt Outback inverter. I will be increasing the solar panels to about 2800 watts from the 700 watts I am running now. I am also going to get another charge controller so I will have two 60-amp MPPT controllers on the new upgraded system. I am using Morningstar charge controllers. Thanks for your responses.

[Sunking]

I will be increasing the solar panels to about 2800 watts from the 700 watts I am running now. I am also going to get another charge controller so I will have two 60-amp MPPT controllers on the new upgraded system. I am using Morningstar charge controllers. Thanks for your responses.

At 2800 watts demands 48 volt battery.

[RoyceC]

I don't understand why it would require a 48volt battery array. I will have two 1400-watt panel arrays, each connected to a separate 60-amp charge controller. The output from the two charge controllers will be connected in parallel at the battery array. Is there something I am missing?

[inetdog]

I don't understand why it would require a 48volt battery array. I will have two 1400-watt panel arrays, each connected to a separate 60-amp charge controller. The output from the two charge controllers will be connected in parallel at the battery array. Is there something I am missing?

The word require is perhaps a little strong.
More precise would be that good design principles, including being able to use only one CC, avoiding paralleling batteries, and allowing 1/4 the cross sectional area wires for a comparable percentage voltage drop all argue for 48 over 24.
If you are starting with an inverter that only accepts 24, you might want to stay there, but since you are moving up from 12v anyway you would be best off going all the way to 48.

[mapmaker]

More precise would be that good design principles, including being able to use only one CC, avoiding paralleling batteries, and allowing 1/4 the cross sectional area wires for a comparable percentage voltage drop all argue for 48 over 24.
If you are starting with an inverter that only accepts 24, you might want to stay there, but since you are moving up from 12v anyway you would be best off going all the way to 48.

He now has a single string of six 1050 ah 2 volt cells (1.5 years old). He wants to double his battery bank capacity. He would have to quadruple it to go 48 volts.

--mapmaker

[inetdog]

He now has a single string of six 1050 ah 2 volt cells (1.5 years old). He wants to double his battery bank capacity. He would have to quadruple it to go 48 volts.

--mapmaker

Or start over from scratch with different (e.g 4 volt) batteries.

[mapmaker]

Or start over from scratch with different (e.g 4 volt) batteries.

Scrapping a 1.5 year old battery bank AND buying a new inverter? Financially, that's gotta hurt.

We don't know what his peak loads are... a 24 volt system might be appropriate.

Of course, you are correct about just needing one controller at 48 volts... maybe. If his expanded array has different orientations or panel specs, he may be better off with two controllers.

--mapmaker

[RoyceC]

Money is always an issue here so I want to utilize as much of the equipment I already have.

So far the word on this forum is that the new batteries will only lose a year and half of their lifespan if I combine them with the old ones. That is acceptable. These batteries have a 10 year warranty so that's a long time. If there is a diode or isolator that can keep the new batteries electrically separate from the old batteries, would that be better?

I wish we had more money for this. When we got here three years ago we inherited a 200-watt system that was cobbled together with borrowed used solar panels, cheap batteries, and old equipment. I have improved it considerably, little by little. I got us more solar panels(used ones) increasing our power from 200 to 700 watts. I got a good MPPT charge controller, I got them to spend on a set of premium batteries. Now, with this new grant, I can upgrade further, but not all the way to a solid 48-volt system. Meanwhile, time goes by and everything gets older.

Thanks for all your comments, BTW.

[inetdog]

If there is a diode or isolator that can keep the new batteries electrically separate from the old batteries, would that be better?

You cannot isolate them both for charging and for discharging without using several very high current diodes per bank and dealing with the resulting voltage offset problems or using separate charge controllers and still losing power in diodes during discharge. I do not know of anyone who has done this.

[RoyceC]

Thanks for all the input on this. I also have not heard of using diodes to separate batteries but just thought I'd ask in case someone has successfully done this.