# Optimal terminal utilisation of batteries in parallel

Registered Users Posts: 24 ✭✭
When connecting batteries in parallel, is it necessarily optimal to utilise the positive and negative terminal that are furthest apart for output?

• Registered Users Posts: 3,422 ✭✭✭✭✭
A good primer on parallel connection of batteries:
http://www.smartgauge.co.uk/batt_con.html
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Registered Users Posts: 24 ✭✭
Estragon said:
A good primer on parallel connection of batteries:
http://www.smartgauge.co.uk/batt_con.html
Interesting article. So let me get this right:

For connecting 4 batteries in parallel, method 4 is the most efficient and implements positive to positive connections between battery numbers:

1<->2
2<->4
3<->4

and negative to negative connections between battery numbers:

1<->2
1<->3
3<->4

There is no connectivity between batteries 2 and 3.

The positive terminal on battery 2 and the negative terminal on battery 3 are used for input and output.

I use an 80A fuse which would attach to the positive terminal on battery 2.

Have I got this right?

When I connect 2 batteries in parallel, I use 20cm long heavy duty copper cable. It is only just long enough to reach between the 2 terminals. I figured the shorter the better. If I had to wire 2 batteries with another battery in between then I would need longer cable. Does this mean that I could still use the shorter cable between adjacent batteries and longer cables to connect non adjacent batteries or should all cables connecting terminals be of the same (longer) length?

Now I am likely to be connecting batteries in parallel. How should these be wired for optimum efficiency?

With 3 batteries in parallel, can connecting 1<->2 and  2<->3 with the positive terminal on battery 1 and the negative terminal on battery 3 for input/output be improved upon? Is there a better configuration involving 1<->3 connectivity?
• Registered Users Posts: 83 ✭✭✭
edited February 20 #4

As a general rule of thumb, paralleling batteries is never a good thing. Also, adding a new string of batteries to an older string is even worse. A superior option is to trade in smaller capacity batteries of higher voltage to higher capacity batteries of lower voltage.

For example, in order of capacity, you could have one 12V battery of about 120AH of capacity (low), or a string of two 6V batteries like T-105s equaling 225AH (medium), or two 6V batteries like L-16s for 370AH (medium high), or three 4V L-16s for 550AH (high capacity). There are even 2V L-16s. Six of those in series would give you 1500AH of capacity.

Each example has exactly the same string voltage, but the lower voltage batteries add up to much higher capacity.

15 Renogy 300w panels,  Midnight 200 CC, 8 Trojan L16 batteries, Schneider XW6848 NA inverter, AC-Delco 6000w gen.
• Registered Users Posts: 24 ✭✭

When wiring batteries in parallel, I always use identical batteries of the same age and use. I like to have a 260-390Ah battery bank which I can achieve by paralleling 2 or 3 x 12V 130Ah batteries. The downside of putting lower voltage batteries in series as an alternative is that I carry the batteries around and would want to minimise the number of batteries and also limit the mass of each battery to about 32kg.

• Solar Expert Posts: 1,261 ✭✭✭✭
edited February 21 #6

> When connecting batteries in parallel, is it necessarily optimal to utilize the positive and negative terminal that are furthest apart for output?

No, it's almost certainly not optimal. If you use proper gauge cables, variations in internal resistance of the batteries can be greater than the cables. Unequal cabling can offset this - ie better balanced! So the right way to do it is to measure amps (clamp on ammeter) or volts (5 digit meter) on each cell while under heavy load. A measurement checks actual balance (vs assuming).

Method 3 on the Smartguage site is the easiest to understand, fuse and adjust for balance.

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