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Which, if any, battery motor topology is more efficient?

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Sep 14, 2015
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I’m pretty new when it comes to electronics and efficiency. I apologise in advance if this is a stupid question.
I would like to know which of the following two scenarios would give me a longer time to run the motor attached to the batteries, if there is a difference.
Senario 1:
6 X 12V 40Ah batteries connected in series to provide 72V 40Ah to a 3000W 72V DC motor.
Batt in series.jpg

Senario 2:
3 clusters of 2 X 12v 40Ah batteries connected in parallel and the clusters connected in series to create a 36V 80Ah source. Then adding a Dc step up circuit to boost the volts to 72V and run the 3000W 72V motor from that.
Atep Up Curcuit.jpg

Would I get the same result in these two scenarios?
Any help would be greatly appreciated.
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series parallel arrays are indeed used in electric cars, but it all depends on the particular battery as to how it performs, and battery manufacs keep stuff like that well secret.


By operating the batteries in parallel, you will shorten their life, also charging them equally will be difficult.

I always thought connectIng batteries in series is more critical with lifetime and charge balance.

If the DC/DC step up circuit has 100% efficiency - wich is impossible - the lifetime should be equal.

But as the voltage of a battery varies from empty to fully charged, this will cause to run the motor with varying RPM.
This may be problematic. The use of SMPS will bring constant RPM.
With an SMPS it is easy to adjust RPM to the mechanical load. This could safe a lot of power.

For the batteries you will need an under-discharge protection. This might be easier with an SMPS.

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Scenario 1 is more efficient, batteries in series no problem, you need to monitor for under / over volts on each.
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Scenario 1 is more efficient, batteries in series no problem, you need to monitor for under / over volts on each.

so, balancing circuits will need? i mean how to charge series group of batteries when it becomes disbalanced?

have never seen old lead-acid batteries charged in series but new Lithium batteries in series are charged with a balanced charger that make sure that no cell is over charged.

The most effective battery technology is the one with the lowest ESR per dollar , per volume, per Ah, per Wh with a matched charger.

The present day choice is a family of Lithium chemistry technology.

For a matched charger, I would choose a forward buck that is capable of being switched with a lower RdsOn, transformed by turns ratio n². This can range from microOhms ot milliohms.

In series, you have a "weakest link" situation where the
cell with the lowest -real- A-h capacity will limit the
usable capacity of the entire stack. This is how sealed
lead acid batteries always seem to fail - 5 good cells
plus one that's got low capacity and high series resistance
once depleted.

My preference would be a high efficiency step-up converter
per battery, parallel at the outputs with current control /
limiting to enforce some reasonable current sharing but
tolerant of cells dropping out at any point. Such a scheme
would also permit adding capacity "chunkwise" without a
whole lot of rip-up.

But the original question only wanted to explore battery
wiring choices. There, you have concerns about wiring
gauge and weight / volume - are you better off sending
the power as voltage or current? At low voltage like
this, insulation is trivial but copper can be hefty, and
long runs definitely prefer voltage.

But the realities of battery capacity (and especially
aging) unevenness may pull you away from the plain
series stack on the discharge side of things, while
the charge / charge retention side might not like the
parallelling. So what's your deal there? Max run time
fresh off the charger, or max run time after a week
of sitting in the airport parking lot?

As the voltage output of a lead acid cell is 2V, 12V lead acid batteries are 6 cells in series, sharing a common case and electrolyte, 24 volt lead acid cells the same. Industrial high voltage batteries loads of 2V cells in series. In fact the only time I seen them in parallel is when I did it!!, but I used matched long wiring lengths to try and equalise the charging current.

Actually the cells in a battery (12V for e.g.) do not share the electrolyte, each cell has its own electrolyte, else there would be current flow between cells....

New 12V batteries are matched <1% of 12.5V = 0.125V
The useful capacity range is only 1V.
A 1% mismatch becomes 8% of the useful range.

The mismatch is effectively only the variation in ESR and state of charge per cell and maintained by monitoring specific gravity per cell.

A parallel battery operation mismatch failure can result in short circuit failures, can be deadly with H2 O2 gassing and a spark.

A series battery operation mismatch failure can result in open circuit failures from the weakest cell by cell voltage reversal during discharge and over voltage during charge.

lead acid batteries are commonly paralleled (and series - paralleled), if they are all the same rating and maker then after an initial evening out they will share quite well, an occasional equalise charge being bring up any under performing cells...

Both series and parallel and matrix Ser/Par arrays are commonly used.

As long as maintenance monitor is included to check battery individual cells for s.g. on a routine basis or batteries by pulse ESR test for out of tolerance ESR.

In Telco operations with 24x 2V cells each weighing 500lbs or so, if one cell is mismatched > x% it is taken out of service. In large CO's arrays are used to make a entire room of batteries to 48V.

Similarly, the same matrix topology is used for high power LEDs, preferring to go for even arrays, like 10x10 for 100W LEDs on one chip @30V, while others are 16x6 for 64V etc. depending on mfg. Higher series arrays allow for slightly greater ESR tolerance and thermal ill effects.

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