Mixing old and new batteries

[Aussiemike]

Hi Guys, Just joined today and have a simple question.
My system.

2x 200w panels in series.
2x150 amp batteries in series
80 amp charge controller
24v 3kva inverter.

I started out with a 12v system and switched over to 24v.
The battery I initially had for three years has now got a new friend next to him.
I read something a while back which I cannot find again about old and new battery mix.
How important is it?
Will I do much damage in the short term?
Cheers
Aussiemike

[SunEagle]

Hello Aussiemike and welcome to Solar Panel Talk

Usually adding new batteries to a system with old ones hurts the new ones. The reason is the old ones make the new ones do most of the work and soon the new ones have hurt themselves and reduced their life down the old ones. I use the "one bad apple ruins the whole bunch" story.

But basically the old batteries have built up sulfur on the plates and internal resistance. That makes them not the best path for the current to take during charging or discharging. The easy path is through the new batteries which then do most of the work and soon suffer.

Short term the system should work depending on how bad the old batteries are. But if the new batteries cost a lot of money then you have wasted it because you will soon have to replace the entire set.

[Sunking]

Buy 2 new batteries.

[Mike90250]

Buy 2 new batteries.

Or, run the ones you have till they fail (1-3 months) and then buy the new batteries. Either way, you are getting new batteries soon.

Don't forget to think about 6V 200ah golf cart batteries (4 in series for 24V gives you 24V @ 200ah) for pretty reasonable prices.

[Aussiemike]

Thanks you all for your imput.
Much appreciated.
I will take your advice.
Just for the record does this apply the same for parallel and series connections?
Cheers
Mike

[SunEagle]

Thanks you all for your imput.
Much appreciated.
I will take your advice.
Just for the record does this apply the same for parallel and series connections?
Cheers
Mike

Pretty much for any type of battery system. You also have to have all of the same type of battery make and model.

Any type of mixing and matching leads to different charging and discharging paths and aging of each battery. It is best to have the entire system age at the same time.

[inetdog]

Thanks you all for your imput.
Much appreciated.
I will take your advice.
Just for the record does this apply the same for parallel and series connections?
Cheers
Mike

Many would say that putting old and new batteries in series is worse, since the two batteries will have different capacity, different self-discharge current, and generally will make it hard for a charger which sees only the sum of the voltages to properly regulate the current that both batteries share.
Possible very bad outcomes are overcharge of one battery, reverse charge of one battery, etc.
When you put them in parallel, both batteries see the same charging voltage, so will probably not be damaged, but they will not share the charging and load current equally, making the new battery work harder.

[Sunking]

Many would say that putting old and new batteries in series is worse,

Yes Sir. +1.

New batteries will have much lower Internal Resistance. In series circuit I1 = I2. That means the old batteries will be over charged and the new batteries under charged.

Overcharged battery + Undercharged battery = 2 dead batteries.