I understand that series wiring of batteries makes sense, since EV's and other applications require a voltage (eg 48v) which is far higher than the voltage of an individual lithium cell (eg 3.7v).

But we have boost circuits. Why not make parallel packs with a boost converter?

Is avoiding losses or cost of a boost circuit the only reason packs are built in series instead of parallel?

 

I think a buck will be more efficient than a boost. Probably not by much.
Higher voltage and lower current is common to reduce the loss in wires.

 

Car EV's have primary batteries of several hundred volts, not 48V.
That's to reduce the battery current when delivering more than a hundred kW of peak power the motors require under fast acceleration.
You don't want to convert voltages with the resulting losses, when dealing with that much current and power.

The only EV voltage conversion is when charging its standard lead-acid 12V lights/accessory/control-electronics battery from the main battery.
(It's interesting that they are still using lead-acid batteries in cars after over a 120 years of such use).

 

have you heard of current war? (Tesla/Edison or AC/DC)
do you know why AC won? because it could be transported using higher voltage, thus minimizing losses.

the same exist on a low end of the spectrum where one operates with just few Volts... you CAN have a lot of power by paralleling bunch of cells. but the WIRING is going to be prohibitively expensive...and heavy. EV economy revolves around minimizing weight - specially when batteries themselves already weigh a lot, making EVs a lot heavier than comparable vehicles with ICE.

so you have a 4.00V battery and want to deliver 4500A. hint... do the match on conductor that will do that and ensure less than 5% voltage drop.

now increase the battery voltage to 400V (100x increase) and for same power current is only 45A (100x decrease).
the first one was thick like your thigh, the later is like your finger. now suppose you want to make an inductor for your DCDC converter. which wire will be easier to wrap around some ferrous core? how many turns? do you begin to see the problem?

 

Part of the 400V and 800V car batteries is what kind of transistors are available.

 

yes, wiring is only one of the problems...

 

I think there's another reason: It may be more difficult to boost a low voltage. For example, this LiFePO4 cell has a Recommended discharge cut-off voltage of 2.0V. Other chemistries charge to 1.5V. But I haven't seen any OTS boost module that accepts less than 3V input.
https://liionbms.com/pdf/a123/charging.pdf

Apparently, low-power devices with a single cell use voltage doublers. The most efficient of which might be the "cross-coupled" design described here.
https://en.wikipedia.org/wiki/Voltage_doubler#Cross-coupled_switched_capacitors

Anyone have a schematic for a high-efficiency voltage doubler?

 

I understand that series wiring of batteries makes sense, since EV's and other applications require a voltage (eg 48v) which is far higher than the voltage of an individual lithium cell (eg 3.7v).

But we have boost circuits. Why not make parallel packs with a boost converter?

Is avoiding losses or cost of a boost circuit the only reason packs are built in series instead of parallel?

Hi,

Well a simple analysis is that if we had 10 cells of 4v each that would be 40v, and with a 40 watt load the current through each cell would be 1 amp.

Now with any kind of converter we have to consider the efficiency. With a boost converter boosting 10 cells of 4v each in parallel which is still just a 4v battery, with a 40 watt load if the efficiency is 80 percent the cells would have to put out 48 watts, which means each cell would have a current of 1.2 amps which means the cells run down faster, and that 8 watts is completely lost to heat which means some parts are going to heat up.
If we went to a 90 percent efficiency, we'd get just 1.1 amps through each cell and 4 watts lost to heat, but it may be very hard to create a boost converter to go from 4v to 40v at 90 percent efficiency. We may even end up with only 75 percent efficiency which means 1.333 amps per cell and about 13 watts lost to heat.

Also, connecting cells in parallel requires thicker wires.

There is the converse effect to consider too though, that is the series resistance. The series resistance increases by the number of cells. In the case of the 10 cells, that would mean the series resistance is 10 times any one cell. That leads to more of a loading effect which reduces the voltage which also decreases efficiency.

The solution is to put some on parallel and some in series.

Still yet another consideration though is the efficiency of the device being powered by the battery pack. A higher voltage motor will draw less current than a lower voltage motor. That could offset the effect of increased series resistance.

Converters are usually only used when they have to be which could include some kind of convenience factor.

 

The solution is to put some on parallel and some in series.

I think the ratio of parallel vs series would depend on losses of each. The trade-off might favor parallel or series, depending on the voltages, current, converter efficiency, and load.

Converters are usually only used when they have to be

My application is a 35W mono class D audio amplifier. It sounds better with a 24V supply than a 4V supply.

My battery is 4 LiFiPO4 cells with a discharge cutoff of 2V, or about 8V in series at lowest charge. So i have to boost, it's a question of boosting 8V to 24V (4S), 4V to 24V (2S2P), or 2V to 24V (4P).

I think 35W is per cell:
4S: 8V @ 4.5A
2S2P: 4V @ 9A
4P: 2V @ 18A

I assume the converter won't work as hard, ie will be more efficient, the smaller the difference between Vin and Vout.

However, my charging system will be simpler if the cells are parallel. So it would be great if i can get better than 90% in a 4P configuration.

I'm looking at this boost IC, which can exceed 90% efficiency under certain conditions. I'm not sure how to interpret these charts. If i'm reading it correctly, i'll get about 95% with 2S2P or 4S, and about 85% with 4P.
https://www.ti.com/lit/ds/symlink/tps61289.pdf


I'm wondering if there's any benefit to putting a boost on each individual cell in a 4P configuration. That's per cell 2V @ 4.5A, which looks like better than 90% with this IC.

 

I understand that series wiring of batteries makes sense, since EV's and other applications require a voltage (eg 48v) which is far higher than the voltage of an individual lithium cell (eg 3.7v).

But we have boost circuits. Why not make parallel packs with a boost converter?

Is avoiding losses or cost of a boost circuit the only reason packs are built in series instead of parallel?

Boost converters step up the voltage. However, the power is always constant. So, according to the formula P=IV, the current decreases. Which will likely not be sufficient.

 

the current decreases. Which will likely not be sufficient.

You're right! Boosting voltage will reduce the current (amps). However, i think it doesn't matter. My load requires 35 watts. It doesn't care whether it gets 35 watts with high current and low voltage, or high voltage and low current. 35 watts is 35 watts.

Details
To determine if the current is sufficient, I believe we have to look at the application and the battery capacity. The load cares about volts and Watts, not amps.

Load
My application is a 35 watt audio amplifier. My understanding, and i could be wrong, is that it requires 35 watts of power supply, as long as the supply voltage is within the specified acceptable range.
Iirc, my amp can accept a power supply from 10 volts to 35 volts. For 35 Watts, it can be:
- 10 volts @ 3.5 amps, or
- 35 volts at 1 amp, or
- anything in-between.

Battery
Iirc, my LifePo4 cell can deliver up to 70 amps continuous at any state of charge from 2v to 3.7 v. That's more than enough power to deliver 35W at any voltage. I assume a four cell pack can deliver the same power for four times as long. If I boost it from 2v to anything, it will still be enough power, because when you boost volts, Watts doesn't change.

 

I think the ratio of parallel vs series would depend on losses of each. The trade-off might favor parallel or series, depending on the voltages, current, converter efficiency, and load.

My application is a 35W mono class D audio amplifier. It sounds better with a 24V supply than a 4V supply.

My battery is 4 LiFiPO4 cells with a discharge cutoff of 2V, or about 8V in series at lowest charge. So i have to boost, it's a question of boosting 8V to 24V (4S), 4V to 24V (2S2P), or 2V to 24V (4P).

I think 35W is per cell:
4S: 8V @ 4.5A
2S2P: 4V @ 9A
4P: 2V @ 18A

I assume the converter won't work as hard, ie will be more efficient, the smaller the difference between Vin and Vout.

However, my charging system will be simpler if the cells are parallel. So it would be great if i can get better than 90% in a 4P configuration.

I'm looking at this boost IC, which can exceed 90% efficiency under certain conditions. I'm not sure how to interpret these charts. If i'm reading it correctly, i'll get about 95% with 2S2P or 4S, and about 85% with 4P.
https://www.ti.com/lit/ds/symlink/tps61289.pdf
View attachment 358474

I'm wondering if there's any benefit to putting a boost on each individual cell in a 4P configuration. That's per cell 2V @ 4.5A, which looks like better than 90% with this IC.

Hi again,

Yes the method of charging is another tradeoff point. Charging one way may be simpler for your setup than another way.
Charging in series requires a special charger, but with charging in parallel there has to be some care in how much current each cell gets.

 

Putting cells in series to get a higher voltage for whatever they are powering is more efficient because no converter is 100%efficient. In addition, it makes charging much simpler because every cell receives the same current.
Besides that, the series connection reduces the current voltage drop loss in the wiring.

 

the series connection reduces the current voltage drop loss in the wiring.

Unclear. Series and parallel both have wiring.

 

Putting cells in series to get a higher voltage for whatever they are powering is more efficient because no converter is 100%efficient. In addition, it makes charging much simpler because every cell receives the same current.
Besides that, the series connection reduces the current voltage drop loss in the wiring.

Hi there,

I mentioned the efficiency too, which I figure must be lower with a lower voltage converted to higher voltage.
There is also the series resistance of each cell to consider.
The charging could be more complicated though because the circuit has to monitor each cell, or depend on the internal protection of each cell which I don't like to do myself, and some cells do not have internal protection so a special charging circuit would be mandatory.
I also don't like to charge in parallel, but many people do this and they do not monitor each cell individually.

For what it is worth, if we put two 4v cells in series we get 8v and the load draws 1 amp (8 watts), then if we put two in parallel we only get 4v so we need 2 amps (100 percent efficient converter) so 1 amp from each cell, the same current. With less efficiency though (converter for 4v to 8v) the cells would have to provide more current like 1.1 amps, 1.2 amps, or even 1.3 amps, so it's a little bit more.

 

I also don't like to charge in parallel,

Why?

 

Why?

Hi,

Because we have to make sure the current in each cell does not go over the max charge current for a single cell. Ideally, not even that much.

The problem starts when we consider charging two cells in parallel, but gets worse when we go to more cells like 4 for example.

If we have just two cells rated for 1 amp max each and we want to charge fast, then we have to supply 2 amps to that pack. With the two in parallel, ideally one draws 1 amp and the other draws 1 amp; no problem. But what happens when one cell charges up faster due to slightly different aging characteristics. That means one cell starts to draw only say 0.8 amps and the other 1.2 amps, and remember the voltages of each one is the same so it's up to the cell now as to how much current it draws. Then one cell maybe draws 0.5 amps and the other 1.5 amps. Then 0.1 amps and the other 1.9 amps, which means one cell is drawing twice the max current.
To fix this, we can supply just 1 amp to the pack. That means that if they start out at 1/2 amp each and one charges up sooner than the other, the other will draw a max of 1 amp which is within its rating. Unfortunately, this means we can't charge as fast as the individual cells can take, so it takes longer to charge the pack.
It gets worse with 4 in parallel because then we are tempted to charge at 4 amps and that means one cell could end up getting 4 amps.

To help with this problem we can perhaps use protected cells, which have a cutout mechanism built in. The only thing is, then we have to rely on the internal cutout circuit to function properly all the time.