Axpert 5KVA and Batteries

[andrew9484]

Hello,

I have installed a system comprising 6 x 320W panels (may change to 9); 12 x 100AH 12V AGM batteries configured in 3 strings of 4 batteries.

1. The one string of batteries is a different brand to the rest - they are 2 years old; the rest are new. I know this is not ideal situation; but is it acceptable? I don't want to have to throw the 4 away and incur cost for new batteries unless there is a problem.
2. I am planning to add another string of 4 batteries. Is there any limit of number of batteries the Axpert can handle? i think not as long as charging current limited?

 

[Don]

1 hour ago, andrew9484 said:

The one string of batteries is a different brand to the rest - they are 2 years old; the rest are new.

Different brands of batteries have different characteristics like charge current, discharge rates, bulk charging voltage and float voltages, resistance, etc.

It is not a good idea to mix new batteries with old batteries, even of the same brand. The new batteries would work a lot harder and you will probably end up overcharging the new batteries and undercharging the old batteries and therefore reduce the life of the new batteries.

1 hour ago, andrew9484 said:

I don't want to have to throw the 4 away and incur cost for new batteries unless there is a problem.

Don't throw them away, find something else to do with them. If you use the old batteries with the new ones you will probably find that in 2 - 3 year's time you probably would have to throw away all 12 batteries. You will greatly reduce the life of the new batteries and not get the specified cycles out of them that you are supposed to.

1 hour ago, andrew9484 said:

Is there any limit of number of batteries the Axpert can handle?

As long as you can set the charge current high enough to be able to charge them properly, it is not a problem. The 100 Ah battery capacity is relatively low, so adding another 4 batteries should not be a problem.  

Add the new string of batteries and remove the old string of batteries.

[SilverNodashi]

I concur with Don. Don't mix the new and old batteries. Chances are you'll have to replace the whole bank in less than 400 cycles if you do. Some have suggested using load balancers to charge the batteries more equally (why does that sound so bad in my head?) but there's no guarantee that it will work flawlessly. 

[andrew9484]

6 hours ago, SilverNodashi said:

I concur with Don. Don't mix the new and old batteries. Chances are you'll have to replace the whole bank in less than 400 cycles if you do. Some have suggested using load balancers to charge the batteries more equally (why does that sound so bad in my head?) but there's no guarantee that it will work flawlessly. 

thank you!

[___]

On 3/21/2017 at 9:04 AM, SilverNodashi said:

Some have suggested using load balancers to charge the batteries more equally (why does that sound so bad in my head?) but there's no guarantee that it will work flawlessly. 

It sounds bad because it is bad. There is no way to proportion charge current across parallel strings with balancers. Both the balancers discussed here before proportion the voltage equally across batteries in series, but they cannot ensure that each parallel string gets the right amount of charge current.

The issue isn't on discharge. When discharging, parallel strings naturally contribute proportional to their own capacity, so if you put an 80Ah battery (or an older 100Ah battery that has 80% capacity left) in parallel with a 100Ah battery (for example), and you take the combined bank down to 50%, one battery will contribute 50Ah and the other one will contribute 40Ah, exactly as you would expect. When you recharge them though, the weaker battery might accept only 35Ah while the stronger one with the lower internal resistance will get 55Ah shoved through there, thereby overcharging the one and undercharging the other.

Unless you have individual shunts on each string (so you can measure Ah distribution) and a way to distribute charge current across strings, it remains a bad idea.

[Jako]

An easy way to balance your parallel connected batteries are with an H bridge as shown with the attached picture.

Any of the experts can comment before you do this 

[___]

It's recommended to fuse the red squigly lines. So that a shorted cell in a battery or something bad in that string doesn't pull energy from the other strings, adding fuel to the fire :-)