# Chargers on Series for Lithium Pack



## rwaudio (May 22, 2008)

joamanya89 said:


> I'll use a 288V and 160a/h Lithium battery pack, and I would like to know if I can conect more than one charger to it, so I can charge it faster?
> 
> I would like to get 1 hour of charging time or 1 hour and a half...


That's a pretty lofty goal. Do you have a small power station to plug into? That kind of power in 1 hour is over 200A at 240v ac with an efficient charger. That is the typical power feed you would find for an entire house (an older house might only be 100A). Why the need to charge so fast?


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## joamanya89 (Feb 13, 2012)

Just the idea to create a car wich can be able again ( after being out of battery) to drive with a few minutes charging...

But, I'm seeing that is impossible the goal that I'm trying to get...

If that time is impossible, at least is it possible to use various chargers to make it a bit faster than usual...?


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## frodus (Apr 12, 2008)

The problem is you need batteries that can charge that fast. Many batteries have a slow charge rate of 1-2C. 

What batteries are you using?


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## rwaudio (May 22, 2008)

joamanya89 said:


> Just the idea to create a car wich can be able again ( after being out of battery) to drive with a few minutes charging...
> 
> But, I'm seeing that is impossible the goal that I'm trying to get...
> 
> If that time is impossible, at least is it possible to use various chargers to make it a bit faster than usual...?


Yes it is possible, but probably not ideal. If you are looking for a high power lithium charger check out EMW, they have kits or complete chargers.
http://www.emotorwerks.com/emw3/category/component/


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## brainzel (Jun 15, 2009)

I don't know if I get something wrong ...

you would like to get two chargers to the pack to double your Amps going into the pack, right?

In my opinion you should parallel them, not connect them in series.

btw: has anyone did that before? Could I parallel f.ex. two Zivan NG3 with the same voltage settings to work together on the same pack without disturb eachother?


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## Coulomb (Apr 22, 2009)

brainzel said:


> In my opinion you should parallel them, not connect them in series.


Certainly you would connect the chargers in parallel.



> btw: has anyone did that before? Could I parallel f.ex. two Zivan NG3 with the same voltage settings to work together on the same pack without disturb eachother?


I think it would be like trying to connect two DC/DC converters in parallel (outputs in parallel), to charge the 12 V auxiliary battery. It can be done, but you need to adjust the voltage outputs from the chargers / DC/DCs very closely, and/or add resistance to the outputs so that they share the current better. (Otherwise, one charger will output a smidge less voltage, and will deliver almost no current.) [ Edit: I was wrong about this; it's not like the DC/DC case at all. See my post on page 4 of this thread for details. Sorry for the misinformation. ]

The problem with the resistance is that for main pack charging, the losses would be large.

I'm sure it's doable with a little co-operation between the two chargers. Or you could somehow arrange for the voltage-current characteristic to have a gentle cut-off, rather than the usual abrupt characteristic.

It's certainly not commonly done.

Search these forums for a 10 kVA DIY charger, and an even larger 3-phase version.


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## joamanya89 (Feb 13, 2012)

I apologise about saying conect it in series, What I wanted to say was to conect it in parallel...

On the other hand, I have been thinking on this Idea that I would like to know if I'm wrong...

Supose that every time that I want to load my car, I just swich of the pack in the middle of it, so I'll have two packs, and then I conect two chargers (the same one) to each pack, then when I finish the charging, I just swich it on back and done, I have got to battery pack full back and in the half of time...

Excuse me my english...


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## Ziggythewiz (May 16, 2010)

If you charge in series, your packs will become severely unbalanced because the chargers are different.


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## joamanya89 (Feb 13, 2012)

Why? did you aderstund what I have sayed? why would it have diferent charge if they will be both at full charge...


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## Coulomb (Apr 22, 2009)

joamanya89 said:


> Suppose that every time that I want to load my car, I just swich of the pack in the middle of it, so I'll have two packs, and then I conect two chargers (the same one) to each pack, then when I finish the charging, I just swich it on back and done, I have got to battery pack full back and in the half of time...


Yes, that should work. We need to do exactly that with our MX-5 (Miata) conversion; in the 750 V configuration, it's difficult to find a charger that will output the required 820 V. But we can find two 410 V chargers to charge half the pack independently. There should be no problems as long as most of the time the charges finish, so the cells are all at the same SOC (full).

But note that there is no magic here, with getting the charge over in half the time. Suppose you have a 288 V pack, which can be split into two 144 V half packs. So you could use two 2 kW chargers (4 kW total); each of them would charge at about 14 A (14 A x 144 V ~= 2 kW).

But you could just as well have bought one 288 V 4 kW charger (288 V at 14 A). That would charge the pack in the same time.

Or if you charge the pack halves in parallel, you could use a 144 V 4 kW charger (144 V at 28 A). Now each half pack will see about half the current, or 14 A. So all the 4 kW solutions (2 x 144 V 2 kW, 288 V 4 kW, or 144 V 4 kW) take the same time to charge.

How fast the batteries take to charge depends solely on the power of the charger. (Of course, the charger has to have the right voltage for the pack.)


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## joamanya89 (Feb 13, 2012)

Sure, that what I'm talking about, but to make it work as you say, I'll need to find out what's my battery pack limit of power received (I'm not sure if it is about de amperage or the voltage, if you could explain it to me, that would be great), so knowing that I'll be able to separate my pack in two and then get a improved charging time, if I don't do it I'll be just wasting time as you say, because it would be the same as use a higher power charger...

So, could you explain me, how do I know the limit that my battery pack is able to receive in some period... or how to calculate it, or what it depends about ?


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## Coulomb (Apr 22, 2009)

joamanya89 said:


> So, could you explain me, how do I know the limit that my battery pack is able to receive in some period... or how to calculate it, or what it depends about ?


The maximum charge rate is a specification, provided by the manufacturer.

For example, in the attached, the maximum charge rate is stated as 3C. That means that you can charge at up to three times the Amp-Hour capacity of the cell (in this case, they are 40 Ah cells, so 3 x 40 = 120 A). The bulk part of the charge could happen in as little as 1/3 of an hour (20 minutes), in this case.

The attachment is for Thunder Sky cells, the predecessor of CALB cells. It may well have changed, and it could well be different for other manufacturers.


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## joamanya89 (Feb 13, 2012)

So, I should find the higher power charger that I can buy, (buy two of it), to make it works... Thank you verry much for your help!


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## joamanya89 (Feb 13, 2012)

and what should I look for? 288 output voltage charger? more? or less?


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## joamanya89 (Feb 13, 2012)

Sorry about my last question, I wanted to say 144V...

But what changes for example for a 144V/14A or a 48V/41A?

I mean the both are 2KW but, would the 48V charge my battery pack? does the volts change anything? or it depends just on the amperages?


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## sourcefinder (Dec 17, 2010)

You have 288V 160 Ah.

So first of all you should figure out what amount of energy you can get in your house/garage/where you want to charge it.
Usually you have a fuse in your house/garage`s electric network.

Now you take the current a little bit less than this fuse.
We in Austria have at 400V 16, 32 or 64 A.

So I choose 16 in this case and the 400V three phase.
Of course you can also use the small two phase plug.

16 x 400 = 6400 Watt

So in this case I can pull about 6 kW from this plug.
So I would choose one charger with 6 kW for 288V.

If you have a 32A fuse, you can use two of the 6 kW chargers parallel.

This gives you the advantage, that you can use one or both of them.
So when you drive to your friend and he has just 16A you use one and charge two times as long as with two of them....

I think at 288V there are chargers that can charge the pack in one.
But if you have something above 400 V there is no way to use one charger because they cannot handle the voltage. In this case you would need 2 or more in serial. Same here with the energy you can pull out of the plug.

So why buy a 144V Charger?
Buy a 288V charger that has a current your electric network can provide!


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## Coulomb (Apr 22, 2009)

sourcefinder said:


> We in Austria have at 400V 16, 32 or 64 A.
> 
> So I choose 16 in this case and the 400V three phase.
> Of course you can also use the small two phase plug.
> ...


Actually, assuming you have a 3-phase charger that uses all 3 phases, it's about 1.7 times that, or 11 kW.


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## Bowser330 (Jun 15, 2008)

joamanya89 said:


> I apologise about saying conect it in series, What I wanted to say was to conect it in parallel...
> 
> On the other hand, I have been thinking on this Idea that I would like to know if I'm wrong...
> 
> ...



What I am pretty sure you are saying is, having a contactor to split the pack into two separate packs, which can be charged by separate chargers...I do not know much but on the surface I do not see why this could not be possible...


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## ga2500ev (Apr 20, 2008)

The possibility of the pack being out of balance is of crucial importance here. I'll try to keep it brief.

Here is a sample of the LiFePo4 discharge curve:










The key point is that the discharge is nearly flat until the cell is almost empty and then the voltage drops off quickly. Fully discharged cells will be damaged.

Now if one cell in a pack empties, and others still have energy the empty cell will reverse, which will destroy it. 

Finally if a cell gets overcharged, that will destroy it too.

So it's real important that cells do not become completely empty, or overfull. The best way to manage this is to make sure that all the cells have either the same amount of energy, or the same state of charge. This process is known as balancing.

In any pack, one cell will have less capacity than the others. This is the cell that will finish charging first, and the one that will empty first. It's the smallest bucket, and defines the capacity of your pack. There are two types of balancing methods, and each manages that weakest cell.

The popular balancing method is call bottom balancing. This method attempts to make sure that all the cells have exactly the same amount of energy, and so will all empty at exactly the same time. It is done by emptying the energy out of all the cells by discharging each cell to a virtually empty voltage (2.5V is popular). Then the entire pack is charged until the first cell (the weakest one) registers full. While none of the other cells are completely full, since they all started empty, and they all got exactly the same amount of energy, on discharge, they will all empty together.

Top balancing makes sure that each and every cell is completely charged to full charge. This is done by monitoring each cell and diverting energy when the cell indicates that it is full (reaching a voltage of 3.65-3.7V). Charging is done until all the cells indicate that they are full. On discharge, the pack is monitored, either at the cell level, or by monitoring the difference in voltage in half packs. When one cell starts to empty, it's voltage will change the monitoring voltage, at which point you stop driving.

In both instances, the pack is charged as a whole. If you split the pack, then bottom balancing is impossible because there's no way to guarantee that each half pack gets exactly the same amount of energy. Specifically if you stop both chargers when the first cell indicates full, there's no guarantee that the cells charged by the other charger have exactly the same amount of energy. They could be more full than the half that caused the stop, which means that half the pack may drop off while the other half still has energy. Top balancing is doable, but you will have to monitor the pack and stop driving when the weakest cell starts to drop off.

Be aware of the issues and you'll save your pack.

ga2500ev


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## GizmoEV (Nov 28, 2009)

brainzel said:


> btw: has anyone did that before? Could I parallel f.ex. two Zivan NG3 with the same voltage settings to work together on the same pack without disturb eachother?


I do this with a Zivan NG1 and an NG3 when I need a quick charge. I usually use my NG1 but if I need a little faster charge I use my NG3 and both for the fastest charge. I get either 14A, 41A, or 55A charge into my pack. Both chargers have the same charge algorithm. Both chargers cut back current at nearly the same time. The NG3 is a little earlier due to the fact that I have a 20 foot long cord between the charger and the pack so there is some voltage drop making the charger believe the pack is at a higher voltage than it really is. If I shut off one charger before the other after they have started to cut back the current then the one left running increases the current due to the slight drop in sensed voltage.

Since the chargers are isolated this works just fine and they do not seem to fight each other.


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## joamanya89 (Feb 13, 2012)

GizmoEV said:


> I do this with a Zivan NG1 and an NG3 when I need a quick charge. I usually use my NG1 but if I need a little faster charge I use my NG3 and both for the fastest charge. I get either 14A, 41A, or 55A charge into my pack. Both chargers have the same charge algorithm. Both chargers cut back current at nearly the same time. The NG3 is a little earlier due to the fact that I have a 20 foot long cord between the charger and the pack so there is some voltage drop making the charger believe the pack is at a higher voltage than it really is. If I shut off one charger before the other after they have started to cut back the current then the one left running increases the current due to the slight drop in sensed voltage.
> 
> Since the chargers are isolated this works just fine and they do not seem to fight each other.


Could you please explain me a bit mor about ir? cos I was thinkking to put two chargers, one in haf of the pack and one in the other half, and have a siwtck to cut of the pack on the middle, so I can run the wo chargers for a master charging, is it what you have done?


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## Ziggythewiz (May 16, 2010)

No, he is running them in parallel, so he can charge at slow, med, or high rate depending on the charger(s) being used.


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## GizmoEV (Nov 28, 2009)

joamanya89 said:


> Could you please explain me a bit mor about ir? cos I was thinkking to put two chargers, one in haf of the pack and one in the other half, and have a siwtck to cut of the pack on the middle, so I can run the wo chargers for a master charging, is it what you have done?





Ziggythewiz said:


> No, he is running them in parallel, so he can charge at slow, med, or high rate depending on the charger(s) being used.


Correct, I'm charging with them in parallel. I personally wouldn't charge half of my pack with one charger and the other with another charger if I could avoid it. I do not want to introduce any pack imbalance. Charging each half separately has the potential of imbalance. The reality is that, for example, two 3kW chargers each charging one half of the pack will charge the pack just as fast as one 6kW charger charging the whole pack.


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## Bowser330 (Jun 15, 2008)

GizmoEV said:


> I do this with a Zivan NG1 and an NG3 when I need a quick charge. I usually use my NG1 but if I need a little faster charge I use my NG3 and both for the fastest charge. I get either 14A, 41A, or 55A charge into my pack. Both chargers have the same charge algorithm. Both chargers cut back current at nearly the same time. The NG3 is a little earlier due to the fact that I have a 20 foot long cord between the charger and the pack so there is some voltage drop making the charger believe the pack is at a higher voltage than it really is. If I shut off one charger before the other after they have started to cut back the current then the one left running increases the current due to the slight drop in sensed voltage.
> 
> Since the chargers are isolated this works just fine and they do not seem to fight each other.


wow ive been on these boards awhile and am always learning something new. Thank you for sharing your experience Gizmo!


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## GizmoEV (Nov 28, 2009)

Bowser330 said:


> wow ive been on these boards awhile and am always learning something new. Thank you for sharing your experience Gizmo!


You are welcome. Just remember that if you use two or more chargers in parallel at most one can be non-isolated, the rest have to be isolated. I understand that you can get a fireworks show otherwise.

I didn't mention that my NG1 is a 115Vac charger and the NG3 is 230Vac. That doesn't really matter except that each charger should be plugged into its own circuit.


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## joamanya89 (Feb 13, 2012)

But how is it in parallel? you have 3 chargers charging at the same time? I mean, If you want the faster charging, is there 3 different chargers conected to a 3 diferent points on the battery pack charging at the same time?


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## GizmoEV (Nov 28, 2009)

joamanya89 said:


> But how is it in parallel? you have 3 chargers charging at the same time? I mean, If you want the faster charging, is there 3 different chargers conected to a 3 diferent points on the battery pack charging at the same time?


No, only two chargers. The Zivan NG1 is able to put about 14A into my pack by it self. It is hooked across my 20 cell LiFePO4 pack. The other charger, a Zivan NG3, is also hooked across the pack and by it self it puts out about 41A into the pack. When I hook both up at the same time and plug both in the current from each adds to a total of 14A+41A=55A. Slow charge is just the NG1, medium charge is just the NG3 and fast charge is both at the same time. Because the chargers are *isolated chargers* they work just fine this way. Isolated chargers have no conductive path between the wall AC and the battery pack. Non-isolated chargers do and that is why you can't use two non-isolated chargers at the same time.


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## joamanya89 (Feb 13, 2012)

Alrgiht so it doesn't metter if you conect two chargers at the same time to the same pack of batteryes? as you say they would get of at almost the same time...?


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## GizmoEV (Nov 28, 2009)

joamanya89 said:


> Alrgiht so it doesn't metter if you conect two chargers at the same time to the same pack of batteryes? as you say they would get of at almost the same time...?


That is correct as long as the chargers are _*isolated*_ chargers. That is critical!


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## Coulomb (Apr 22, 2009)

joamanya89 said:


> Alright so it doesn't matter if you connect two chargers at the same time to the same pack of batteries? as you say they would get of at almost the same time...?


Ah! I have misled you earlier... sorry!

While the chargers are in their constant current region (until the battery gets to 80% or so state of charge), they will adjust their outputs so that they run "flat out", or at whatever current limit they are set to. After that, one will very likely do a disproportionate share of the work, but that doesn't matter terribly much. [ Edit: in GizmoEV's case, he has two chargers from the same company, presumably with the exact same algorithm, so they will probably share pretty well even in the last 20% of the charge. With two different branded chargers, they may not share so well, but it really doesn't matter. There will be no harm to the chargers or the battery pack. ]

I was comparing this to the case of two identical DC/DC converters each connected to half the pack, both charging the 12 V auxiliary battery. Sounds similar, right? But the thing is, with the DC/DCs, you want them to share the load very closely; any imbalance in their load means a slight imbalance of half of the pack compared to the other half, and this imbalance will only grow with time. (If you top balance when charging, that will largely go away when you next charge, but some effect will still probably be present.)

That's a different situation to what we're discussing here; here, if one charger takes 10% more load than the other, you really don't care. In GizmoEV's example, one charger is three times the current of the other one, and for most purposes, it doesn't matter. (It might matter if you had two sources of AC supply, each with a low current breaker on them, but let's not go there.)

Obviously, it's a whole lot more convenient to have two chargers the same voltage, and not have to break up the pack with another contactor. One charger could break down, for example, and you could still charge the whole pack without having to swap leads half way through.

My apologies for the misinformation earlier. I'll go and add a comment to the errant post so as not to lead others astray.


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