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Battery bank - multiple parallell sets or large single set?


Wetkit
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Hi Guys,

 

I need to start looking at replacing my existing battery bank.

Currently I have 2 banks of 48V at 100AH in each bank, but the battries is over 10 years old now.

 

If I have to size my new battery bank correctly, I would need about 800AH or more. Problem is that it is way to costly right now.

I'm happy to run with 400AH.

 

Now, I can do it with 4 x 100AH parallell strings, 2 x 200AH strings or a single 400AH string.

 

Thing is, with multiple strings, I can buy a new string say every 3 to 4 months.

With a single string, I would need to save for at least a year

 

Now the question is, if all the battries have the same specs, which configuration would be better in the long run?

Multiple smaller strings or single big string?

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MPPT is rated 60Amp and chager on inv is rated 75Amp, so coverd there.

 

The only thing I can pick-up from the web is they say to try and stay with max of two strings.

Reason for this they say is that any fault on a single battery in a string, will cause the other other battries in that string to overcharge.

With multiple parallell strings this can result in the other strings to also discharging into that faulty battery, resulting in increased fire risk.

Some sites also mention that the outside battries of multiple strings might charge more and work harder due to copper losses.

Each string also needs it's own fuse or protection device.

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MPPT is rated 60Amp and chager on inv is rated 75Amp, so coverd there.

 

The only thing I can pick-up from the web is they say to try and stay with max of two strings.

Reason for this they say is that any fault on a single battery in a string, will cause the other other battries in that string to overcharge.

With multiple parallell strings this can result in the other strings to also discharging into that faulty battery, resulting in increased fire risk.

Some sites also mention that the outside battries of multiple strings might charge more and work harder due to copper losses.

Each string also needs it's own fuse or protection device.

 

I had 5 strings of batteries and towards the end of last year one of the cells in one of the batteries went shorted - that caused the faulty battery as well as the other 3 batteries in that string to become very hot because the faulty battery and the other 3 in the string were basically being overcharged by the other strings and the charger.  I then designed the battery disconnector.

 

It disconnects a string if the voltage of any of the batteries in the string become too low or too high.  It is still version 1 where you have to set the upper and lower limits by means of presets, but I'm still improving on the design and the next version I might be using microprocessors to achieve more accurate cut-off limits and hysteresis.

 

PCB bottom

PCB top

In circuit

Boxed

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@Wetkit,

I also read the same thing about multiple strings, but I think it's how you wire everything up.

My setup is going to have 3 strings and each string will connect directly to busbars, and from the busbars to the inverter/chargers.

 

@Superdiy

I want!

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@Wetkit,

I also read the same thing about multiple strings, but I think it's how you wire everything up.

My setup is going to have 3 strings and each string will connect directly to busbars, and from the busbars to the inverter/chargers.

 

@Superdiy

I want!

 

Your 3 strings, each connected to the busbars, are still in parallel and therefor if any cell in any of the batteries fail, the other batteries in the string, containing the faulty cell, will be overcharged by the healthy strings and/or charger.

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Ok, so the more I am reading up, the more I learn :D

 

So, bottom line is that not all batteries is created equal, even if it is in the same batch.

Now when you parallel these, the minute small differences will grow bigger over time.

Some strings might charge less and other more. How long this takes, I cannot tell.

Solution might be to mix the batteries between strings every couple of months?

Does sound like a lotta work :(

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@superdiy, looks like a very professional product there :)

Estimate on cost of one of those units?

 

Difficult to say right now, I don't want to speculate on a price, but I'd rather design and build the microprocessor controlled version first - then I will calculate the cost.  Currently I'm busy with my PV installation and that keeps me occupied all me time.

 

Just keep in mind that a single unit works for a single string in a 48V system - either 4 x 12V batteries or 8 x 6V batteries or 12 x 4V batteries or 24 x 2V cells.  You'll have to have 1 unit per 48V battery string - if you have 4 parallel strings, you'll need 4 battery disconnectors.  The relay I'm currently using to disconnect the string is rated at 30A for 250V AC - it should handle about 30A at 60V DC - that is the current for the single string only.  I can also make a unit for 24V and a unit for 12V, but I'll have to use relays with higher contact current ratings for the lower voltage versions.

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