Domestic Batteries in Parallel

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Dave100456

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Assume you want to achieve 400 Ah at 12v

Now, please ignore size, cost, installation factors and weight.

I have it in my head that from a charge and discharge perspective its best to have for example; 2 x 12v 200Ah rather than 5x 12v 80Ah batteries. Am I correct and why?

Apart from having fewer total cells in the 200 set up are there any other reasons for it being better to have larger batteries.

Thanks

Dave
 
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Yes, I agree with you about possible sources of resistance.

With 5 batteries in parallel, if the positive load is taken from one end of the string and the negative from other end the string there should be equal resistance on all cells, provided as you say there were no poor connections.
 
Interesting question to which I should know the answers having been in the battery business a long time, but I was IT Director and not an engineer and it is getting on for 10 years since I retired.

However: Yes it is better to go for fewer batteries, more particularly for fewer cells which is much the same thing. For best deep discharge performance in VRLA and most other types you go for large cells – Submarines have lots of big (c 4ft x 1ft x 1ft) single, nominally 1.5V, cells. Motive power batteries (e.g. for for lift trucks) are designed much the same way but on a smaller scale and it certainly gives better deep discharge performance.

You can’t ignore weight as the amount of lead in a battery is critical to its performance. Thin Plate Pure Lead designs by Enersys (Full disclosure: I used to work for them) may be considered an exception in that they outperform conventional designs in terms of power density and power to weight ratio by >= 30%, but even there power is dependent on weight within the range.
 
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With 5 batteries in parallel, if the positive load is taken from one end of the string and the negative from other end the string there should be equal resistance on all cells, provided as you say there were no poor connections
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Not according to here . You might have to be a bit more inventive to completely balance a bank of 5 batteries
 
Dave100456 said:
Yes, I agree with you about possible sources of resistance.

With 5 batteries in parallel, if the positive load is taken from one end of the string and the negative from other end the string there should be equal resistance on all cells, provided as you say there were no poor connections.
Click to expand...

Assuming the batteries are identical, which they won't be.

Edit: as may be implied from the previous post by Hoolie.
 
Hoolie said:
Not according to here . You might have to be a bit more inventive to completely balance a bank of 5 batteries
Click to expand...

The simplest way with 5 batteries will probably be using method 3 in the link, using bus bars rather than single terminal studs.
 
Dave100456 said:
With 5 batteries in parallel, if the positive load is taken from one end of the string and the negative from other end the string there should be equal resistance on all cells, provided as you say there were no poor connections.
Click to expand...

Regardless of the theoretical Smartgauge calculations, just taking the positive and negative connections from opposite ends of the bank is simpler and works fine. On my last boat, I had 6 x 110Ah batteries in parallel wired in this way, with no problems. You're never going to run the batteries completely flat, so the theoretical niceties are largely irrelevant.
 
pvb said:
Regardless of the theoretical Smartgauge calculations, just taking the positive and negative connections from opposite ends of the bank is simpler and works fine. On my last boat, I had 6 x 110Ah batteries in parallel wired in this way, with no problems. You're never going to run the batteries completely flat, so the theoretical niceties are largely irrelevant.
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Thank you for relating your personal experience which is often better than theoretical calculations.

That said, on the Smartgauge site the author does show a 3 amp difference in drain for two batteries and states this imbalance is detrimental. How much it matters I'd like to know.

It is the author's phrase:
"The answer is that some of the links have to pass more total current and this therefore increases the voltage drop along their length."
that I can't yet get my head round.

Will look at it in the morning
 
Dave100456 said:
On the site:
http://www.smartgauge.co.uk/batt_con.html

I know its late, but to me (and I'd love someone to explain why) it looks like in method 2 each battery has 5 connections (positive and negative) to pass through before it supplies the load. Since this is the same total resistance for all the batteries, why are the current values different for each battery?
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Don't believe everything you read on t'interweb! Perhaps Mr Smartgauge is vying with Mr Sterling for the prize for the most verbose technical articles.
 
Dave100456 said:
That said, on the Smartgauge site the author does show a 3 amp difference in drain for two batteries and states this imbalance is detrimental. How much it matters I'd like to know.
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Why would it be detrimental? You're not intending, I hope, to discharge the batteries below about 50%. When they're recharged, they'll all take whatever current they can accept until they're full again.
 
pvb said:
Why would it be detrimental? You're not intending, I hope, to discharge the batteries below about 50%. When they're recharged, they'll all take whatever current they can accept until they're full again.
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Yes 50% DOD occasionally but planning on 30% discharge to be the norm.
If batteries are unbalanced, does it only have a detrimental affect if you do deep discharges?

On my old bank, a couple of batteries used to use a little more water and be hotter than the others but they were 7 yrs old.

In this case, would the ones supplying the greater currents run hotter when being discharged and charged?
 
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Hoolie said:
Not according to here . You might have to be a bit more inventive to completely balance a bank of 5 batteries
Click to expand...

Interesting, not read it all, but did not come across battery voltage to load. As the load on the battery increases voltage for a given capacity falls, same in charging, as current increases cell voltage increases. So as the load on the first battery increases the cell voltage falls, if this battery carries on supplying a higher current, it's capacity will also fall. This will result in a lower battery voltage, thus battery two being at a higher voltage due to lower current and higher capacity, will compensate for cable/connector loss and supply full current. The same will then occur for battery three and four, load will stabilize for a given load, in this case by slight variation in battery capacity.

Same for charging, the first battery may initially take a higher charge, but once the voltage increase due to battery capacity increase exceeds the volt drop due to terminal resistance losses, current will balance. As the volt drop due to connector losses is small, This is how a VSR split charge works, charging the engine battery increases capacity and cell voltage, on connecting service battery it's capacity and resistance is lower. So current takes least line of resistance, service bank, voltage will fall, to low to charge engine battery and to high to discharge engine battery. Thus charge goes to service battery, once voltage comes up to engine battery level, it will start taking a charge. Bothe service and engine will balance out to equalize voltage and charge level.

Consider normal loads are 10 amp or less, unless you run an inverter for backup power you will not see 100 amp, so actual volt drop in the real world will be a lot lower, not saying if you supply a bow thruster or inverter some thought in battery layout could be a good thing.

Brian
 
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TQA said:
If you can get them at a reasonable 6 volt golf cart batteries are the way to go 4 of them will give you a 450 ah bank. They cope with deep discharges better than thin plate cells.

I have 6 and get away with leaving them all connected as I charge daily using solar.
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There's the ideal answer - 4 x T105's and LOTS of solar so they really do get fully charged quite often if you never plug in. :encouragement:

If you're using the batteries and rarely plugging into a mains charger then actually getting back to 100% would benefit from some thought as well otherwise you'll just trash them.
 
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