# Installing a second Lifepo4 pack



## baerfoot (Jan 2, 2011)

I presently have a Hipower 100 AH 24 cell pack. The pack is rated at 1C, so I try to keep the current draw under 100 amps. This makes for slow acceleration and slow hill climbing. I noticed that 3C cells are now available. I was thinking that I could add a 40 AH 24 cell pack in parralel and increase power and range. Does anyone know how much current I would draw from each pack in this type of configuration if I was drawing 200 amps? Would there be any pitfalls to this setup?


----------



## TheSGC (Nov 15, 2007)

baerfoot said:


> I presently have a Hipower 100 AH 24 cell pack. The pack is rated at 1C, so I try to keep the current draw under 100 amps. This makes for slow acceleration and slow hill climbing. I noticed that 3C cells are now available. I was thinking that I could add a 40 AH 24 cell pack in parralel and increase power and range. Does anyone know how much current I would draw from each pack in this type of configuration if I was drawing 200 amps? Would there be any pitfalls to this setup?


You would actually be lowering you pack capacity to 40 AH (well, actually 80AH cause it's in parallel). Anytime you put cells in Parallel they must be identical in AH capacity, or the lower cells will drain first then get destroyed while the higher cells continue to provide power.

If you want more range you can add another 100 AH string of cells in parallel, and if you want more performance you can either increase your voltage, or just upgrade the whole pack to a 3C setup.


----------



## Guest (Jan 2, 2011)

You monitoring battery current or controller current? You want to monitor battery current for that 1c rating continuous. These can peak to 3c. If you add into your pack a 40AH pack I believe your charger will terminate when the 40AH pack reaches full leaving your other 100AH pack rather undercharged. I would put in matched 100AH cells to bring your total to 200AH. I will be putting in two matched packs of 120 Volt 100AH Hi-Power Cells. If I am not mistaken you want the AH matched for a balanced pack in series or parallel.


----------



## baerfoot (Jan 2, 2011)

TheSGC said:


> You would actually be lowering you pack capacity to 40 AH (well, actually 80AH cause it's in parallel). Anytime you put cells in Parallel they must be identical in AH capacity, or the lower cells will drain first then get destroyed while the higher cells continue to provide power.
> 
> If you want more range you can add another 100 AH string of cells in parallel, and if you want more performance you can either increase your voltage, or just upgrade the whole pack to a 3C setup.


Many times people compare the flow of electricity to the flow of water. My theory is that the two packs would be like two tanks of water of equal heights (voltage), but of different diameters (ah capacity). As I drain out (discharge) or add water (charge) to the tanks they would remain at the same level (voltage). But maybe I'm totally wrong


----------



## jockepocke (Nov 24, 2009)

baerfoot said:


> Many times people compare the flow of electricity to the flow of water. My theory is that the two packs would be like two tanks of water of equal heights (voltage), but of different diameters (ah capacity). As I drain out (discharge) or add water (charge) to the tanks they would remain at the same level (voltage). But maybe I'm totally wrong


You should be totally right, I think TheSGC is thinking about series connection! 

You should get 140 AH in total. 

Gottdi: Would not the li charging algoritm charge the whole pack full? First it charges with constant current to eg 3,65 V, when the pack reaches 3,65 V/cell, each cell should be the same voltage as the one it is parallelled to, since they are parallelled- eg if cell a is parallelled to cell b they will inevitably have the same voltage

Same works for two parallelled packs, at least in theory, or I am too wrong?


----------



## TheSGC (Nov 15, 2007)

Nope, I am not getting my Series and Parallel batteries confused. The water analogy has quite a few holes, cause well, it's water and not electrons.

Here is a good read: http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

Yes, theoretically you get 140 AH by paralleling 100 AH with 40 AH, but in the real world, you don't. Instead you get a 100 AH lithium pack with a 40 AH limit before it will over discharge and destroy some cells, if not the entire pack due to excessive heat. 

With batteries, no matter if they are in series or parallel, you want them to be matched in voltage and AH.


----------



## jockepocke (Nov 24, 2009)

TheSGC said:


> Nope, I am not getting my Series and Parallel batteries confused. The water analogy has quite a few holes, cause well, it's water and not electrons.
> 
> Here is a good read: http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations
> 
> ...


Could you please explain further; I cannot verify what you say at the linked site, it says nothing about paralleling differently sized batteries! 

Why should the 40 AH size battery over discharge before the 100 AH pack? They should discharge/charge the same. If one pack is more discharged it will get a lower voltage, this voltage difference makes current stream from the higher pack to the lower, increasing the voltage is the "low" pack, so they will always stay the same. Having a parallel connection forces the battery poles to have the same voltage! 

In theory, battery A that has 1/2 the size of battery, but exactly the same chemistry has the same voltage as battery B, which should have half the internal resistance of battery A. Say A has 50 AH and B 100 AH, and A has Rinternal=1Ohm, B should have 
Rinternal=0,5Ohm. If applied voltage to these cells is 1V + A's voltage = 1V + B's voltage (Since V for battery A = V for battery B), the current through A is by U=RI, 1=1*I => I=1A for battery A, 1=0,5*I => I=2A for battery B. Same applies for discharge. Accordingly they discharge/charge to equal SOC! 

I have seen examples of significantly difference in paralleled cells with good results! Will try and find some!


----------



## icec0o1 (Sep 3, 2009)

You have two batteries, one has 100 watt hours, the other 40 watt hours. At a certain voltage, depending on resistance in the circuit, the batteries put out a specific amps of power (also depending on their internal resistance). 

Both batteries starting at 3.2 volts will put out let's say 100 amps for 200 amps total. As the batteries get drained, the 40 watt hour one will get drained faster as it has less capacity. It will also drop slightly more in voltage, let's say 3.0V compared to 3.1V for the 100 watt hour battery. Therefore the amp draw will change a bit to something like 120 amps/80 amps for the 100 watt hour and 40 watt hour battery respectively. The voltage difference won't be enough to prevent the 40 watt hour battery from draining to 0 volts first. Further, any time you disconnect the circuit after a drain, the batteries will try to equalize and the 40 watt hour battery will draw current from the 100 watt hour battery, which is wasted energy. It is also a very slow process as the voltage difference would be small. 

In summary, it's not a good idea. You will most likely kill the 40 amp hours batteries. If you got something like 80 amp hour batteries, that'd be better although you're still risking the lower capacity cells of going to 0 volts while your car has power remaining from the higher capacity cells.


----------



## MN Driver (Sep 29, 2009)

TheSGC said:


> Nope, I am not getting my Series and Parallel batteries confused. The water analogy has quite a few holes, cause well, it's water and not electrons.
> 
> Here is a good read: http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations
> 
> ...


This is NOT correct. If there are two strings of cells together or a single string of parallel cells, the total capacity will be the capacity of 40Ah and 100Ah combined and the voltage will be the same.

There will be no 40Ah limit, its not possible because the cells will support eachothers voltage. Neither of the two capacities will destroy eachother because the voltage will not suddenly sag without the parallel cell supporting it.

The only problem you will run into if if you are trying to pull the theoretical 240 amps that you think you might get. If the SOC on the cells is low than when the discharge curve turn south on the combined pack, the cells that drop like a rock when they hit the end of the capacity won't support the cells with capacity left, all of the amperage will draw from the cells with remaining capacity. If the cells with remaining capacity supporting the voltage are the 100Ah cells, you can only really draw 100 amps, if its the 40Ah rated at 3C, that is 120 amps.

Either you would want to have two separate strings and parallel them or if you are using a BMS with an LVC or just an LVC of some sort then you could parallel them at the cell level and you would know when they drop too low. Such as 2.5v or less and know to either cut back on current or stop all together depending on the amp draw rate because they are likely to be done by then. If you are 'bottom balancing' without a BMS, it is probably best to balance them as separate strings.

Pay attention to heat, if you are pulling the cells at higher amp draw than they should be getting pulled which would be toward the end of the discharge the cells that are less capable of putting out amperage will get hotter. There should be a fairly clear voltage sag before this happens though.

You will get the capacity, but might not quite get it at the full amp rate throughout. For the most part the discharge of the cells will remain on the flat part of the curve at least until the first cell(in parallel, otherwise string and itreally depends on the comparison of the curves at a specific amperage which is a function of internal resistance and would require testing) drops off and then all of the discharge is essentially being supported by the cell with capacity left.

So in the end you will get at least 40Ah of 220 amp capability, beyond that it *could* be less. If you are lucky the 40Ah cells will have a similar discharge curve to where things work out better, hard to tell. ...but under heavy discharge, both will sag and will basically be forced to share the load, it's just hard to tell which will take the brunt. The lower internal resistance cell(if they were the same capacity) would generally take that current, but with the cells of a different capacity it really depends more on the voltage sag at a specific amperage and the discharge curve associated with that draw.

Will it work? For capacity yes. Is it ideal? No Is there a compromise to this setup? Yes, heavy amperage draw at a lower SOC could be an issue and might take some testing to figure out if it will work as you wish it to.


----------



## TheSGC (Nov 15, 2007)

Under the section of a "weak cell" is the explanation. A lower capacity battery in parallel with other batteries is a "weak cell". 

This is how I explain it to the soon-to-be graduating EE/CE students in my lab-

In Theory Land, when you add batteries in parallel, you are increasing the AH capacity of the pack. This give you 100 AH + 40 AH = 140 AH total. Now, when you start discharging from that pack, you are removing AH from both packs, and the 40 AH will be empty first, then the 100 AH will continue discharging until empty.

In Real Life, when you add batteries in parallel, you are spreading the discharge current between all cells in parallel. This gives the effect of adding some more AH, for example a 10 AMP load on the 100 AH pack is now a 5 AMP load on the 100AH and a 5 AMP load on the 40 AH pack. That same 10 AMP load would last for 10 Hours on the 100AH pack, but now the split 5 AMP load will last for only 8 hours on the 40 AH pack. Once that 40 AH pack is depleted, the 100AH will continuing to want to provide power, but at the same time the 40 AH pack is drain to beyond dead, which damages the chemistry and causes excessive heat, which leads to severe damage and possible explosions. 

So adding a 40 AH pack to a 100 AH pack just brought down the useful power to 80AH, (in this example a 10 AMP load spread across both packs in a parallel before damage occurs.)


----------



## jockepocke (Nov 24, 2009)

TheSGC said:


> Under the section of a "weak cell" is the explanation. A lower capacity battery in parallel with other batteries is a "weak cell".
> 
> This is how I explain it to the soon-to-be graduating EE/CE students in my lab-
> 
> ...


No, you are misinterpreting. The cell is called weak, because it is broken, and that is why is has the lower capacity in the example! 

You have several faults in argumentation, firstly, cell current is not equal. Second, if one cell were to discharge before the other, it would hit the voltage drop a Li cell sees at end of discharge (http://www.thunder-sky.com/pdf/201072311158.pdf) and thus a voltage difference between the discharged pack and the other, causing the pack with a higher SOC to charge the lower SOC one. Please think a bit about this!


----------



## MN Driver (Sep 29, 2009)

TheSGC said:


> Under the section of a "weak cell" is the explanation. A lower capacity battery in parallel with other batteries is a "weak cell".
> 
> This is how I explain it to the soon-to-be graduating EE/CE students in my lab-
> 
> ...


If you put a 40Ah cell in series with a 100Ah cell, your 5 amp load works that way because the current is shared because of their series connection and after 40Ah you would kill the 40Ah cell if they were fully charged before you began, if they were middle SOC you would kill the lower capacity cell on either charge or discharge and when charging if they were balanced at empty.

This is NOT the same when they are in parallel, you can't drain the 40Ah cell beyond dead unless you also have drained the 100Ah cell. The voltage of the opposing cell will prevent that, as there is less capacity in one cell, the internal resistance of that cell and the fact that it is empty would normally cause that cell to drop in voltage but in parallel the load gets pulled from the cell trying to prop the voltage up, so basically the cell with remaining capacity starts to become the burden of holding that load up.

If you kill a cell it is because the cell with the remaining capacity is pushing more amp than it is rated while the empty cell sits idle. Either that or both cells die when they both drop in voltage.

This is with cells in parallel, which would be the same as to two full strings in parallel, just switch the words around for the situation. Not sure what the OP was planning, two series strings of the 40Ah and 100Ah or to parallel at the cell level.


----------



## icec0o1 (Sep 3, 2009)

jockepocke said:


> No, you are misinterpreting. The cell is called weak, because it is broken, and that is why is has the lower capacity in the example!
> 
> You have several faults in argumentation, firstly, cell current is not equal. Second, if one cell were to discharge before the other, it would hit the voltage drop a Li cell sees at end of discharge (http://www.thunder-sky.com/pdf/201072311158.pdf) and thus a voltage difference between the discharged pack and the other, causing the pack with a higher SOC to charge the lower SOC one. Please think a bit about this!


Correct, the lower SOC cell will hit the knee early in the drive, staying right above 0 volts getting high charge and discharge currents as you drive. Those cells won't last more than 500 cycles.


----------



## jockepocke (Nov 24, 2009)

icec0o1 said:


> Correct, the lower SOC cell will hit the knee early in the drive, staying right above 0 volts getting high charge and discharge currents as you drive. Those cells won't last more than 500 cycles.


It does not work in that way, I am sorry for over-simplifying. As the packs are in parallell, no voltage difference can occur at all. Please read MNdrivers text, he just explained it much better than I did.


----------



## icec0o1 (Sep 3, 2009)

jockepocke said:


> It does not work in that way, I am sorry for over-simplifying. As the packs are in parallell, no voltage difference can occur at all. Please read MNdrivers text, he just explained it much better than I did.


I retract my statement after a few minutes of thought. You're mostly right.


----------



## TheSGC (Nov 15, 2007)

At this point all I have to say is browse every RC forum for paralleling batteries, they will say the same thing I do: When putting batteries in Series or Parallel, never mix voltages or AH. 

I know I will never mix AH or voltages in a pack from all of my experiences.


----------



## icec0o1 (Sep 3, 2009)

TheSGC said:


> At this point all I have to say is browse every RC forum for paralleling batteries, they will say the same thing I do: When putting batteries in Series or Parallel, never mix voltages or AH.
> 
> I know I will never mix AH or voltages in a pack from all of my experiences.


it'll only become a problem at high C charge/discharges like 20-40C. It shouldn't be as much of a problem at 3C.


----------



## baerfoot (Jan 2, 2011)

Interesting to see such a spirited debate. I appreciate everyone's input. I wish there was someone out there on this forum that actually had the experience of different sized packs. I don't like the idea of being the guinea pig. Could be an expensive mistake! 
I do wonder though in SGCs line of thought he does not talk about the current flowing back and forth between the packs. Granted with LIFEPO4 chemistry there is very little voltage difference during most of the discharge cycle. So I'm still undecided.....


TheSGC said:


> In Real Life, when you add batteries in parallel, you are spreading the discharge current between all cells in parallel. This gives the effect of adding some more AH, for example a 10 AMP load on the 100 AH pack is now a 5 AMP load on the 100AH and a 5 AMP load on the 40 AH pack. That same 10 AMP load would last for 10 Hours on the 100AH pack, but now the split 5 AMP load will last for only 8 hours on the 40 AH pack. Once that 40 AH pack is depleted, the 100AH will continuing to want to provide power, but at the same time the 40 AH pack is drain to beyond dead, which damages the chemistry and causes excessive heat, which leads to severe damage and possible explosions.


----------



## baerfoot (Jan 2, 2011)

I found a similar thread on an RC forum. Also many different opinions:
http://www.rcuniverse.com/forum/m_9592140/anchors_9595640/mpage_1/key_/anchor/tm.htm#9595640


----------



## spdas (Nov 28, 2009)

Wouldn't the amps on both drop equally? Given the amps would drop out of the 40AH battery say 2x faster and thereby the voltage also would be dropping. Would not the higher voltage of the 100AH battery tend to charge up the 40ah battery? And not the 40ah drawing down to -0volts as discussed in the above chats? thanks
Francis


----------

