(Potentially) Noobish questions about LiPo/LiIon charging

[blc]

Hello all

First post, new to the forum and all that .  Compared to the experience on tap here I've only got a rudimentary understanding of electronics: I can follow a schematic, lay out a circuit on a stripboard and wield a soldering iron competently enough (though I haven't tackled SMD yet), but it has been a very long time indeed since I've designed my own circuit and I've never done anything as remotely complex as this.  Therefore please forgive me if I've overlooked something obvious or I'm barking up totally the wrong tree.  I've tried to do as much research as possible already, but I think I've hit a bit of a brick wall.

I'm trying to design a mobile power supply for the Raspberry Pi.  My "rough plan" is to have the Pi powered by a 5v boost converter (preferably one with a low-voltage dropout), and have the charging circuit and some sort of "fuel gauge" battery meter on the same PCB.  I want the system to be able to seamlessly switch from battery power to external power when a charging cable is plugged in - in the same way that a mobile phone or laptop would do - and then revert to battery power when charging is complete and the external supply is removed.  The output of the charging circuit feeds the battery, which in turn feeds both the load (the boost converter and the Pi) and the "fuel gauge".

LiPo charging was covered way back in , but I want to be absolutely sure that I have everything correct before I even start designing anything; LiPo/LiIon batteries have the potential to be legitimately dangerous, and I don't want to mess around, even with protected cells...

First of all, if I have a LiPo battery where all the cells are in parallel and properly balanced, it is my understanding that - for the purposes of charging - these can be treated as a single cell.  Is this correct?  Potentially I could be dealing with some pretty hefty capacities - over 2/3Ah - so I think a compact single-cell solution is out of the window.  I can't find anything reasonably small with a capacity greater than 2.2Ah, which won't last long with a device that sucks around 0.6A under full load (although that entirely depends on your definition of "reasonably small"). 

Second, I want this to be an in-system charger, which I understand that charger ICs can handle perfectly well.  I can't quite get my head around this part though.  My understanding is that charging a LiPo is a two-stage process, the first being constant current and the second being constant voltage with a steadily decreasing current.  Let's assume that the load can potentially draw up to 1A, which is, conveniently enough, the charging current of the battery (these are likely to be pretty close to the real values).  Let's further assume that we are in the second phase of charging and the current being supplied to the battery by the charging IC at this point is only 500mA; if the load is drawing the full current - i.e., 1A - how does the charging IC "know" that only 500mA is needed to charge the battery when the circuit overall will be drawing 1.5A?  This is further complicated by the fact that the current consumption of the Pi may well be in constant flux; it's not going to be drawing a full load all the time, and could swing wildly in a short space of time.

Furthermore, if the charging phase is in constant current mode, supplying the specified charging current of 1A, and the Pi is drawing a full load of 1A, does this mean that the charger IC will have to be able to cope with 2A?  The cells I'm going to be using (or at least planning to use) for prototyping will be at least 4.4Ah, possibly up to 6.6Ah, and can sustain a maximum charging current of 0.5C.  For simplicity's sake, I'll probably stick to a charging current of 1A.

Thanks in advance for any help.

[peter.mitchell]

First of all, if I have a LiPo battery where all the cells are in parallel and properly balanced, it is my understanding that - for the purposes of charging - these can be treated as a single cell.  Is this correct?  Potentially I could be dealing with some pretty hefty capacities - over 2/3Ah - so I think a compact single-cell solution is out of the window.  I can't find anything reasonably small with a capacity greater than 2.2Ah, which won't last long with a device that sucks around 0.6A under full load (although that entirely depends on your definition of "reasonably small"). 

You can use cells in parallel as much as you want and treat them as a single cell, provided that each cell has an equal internal resistance and the connections between cells are very very low resistance, keeping them equally charged/discharged. The "lowness" of this resistance is dependent on charge/discharge current, due to voltage drop, more draw you need lower resistance ect.

Second, I want this to be an in-system charger, which I understand that charger ICs can handle perfectly well.  I can't quite get my head around this part though.  My understanding is that charging a LiPo is a two-stage process, the first being constant current and the second being constant voltage with a steadily decreasing current.  Let's assume that the load can potentially draw up to 1A, which is, conveniently enough, the charging current of the battery (these are likely to be pretty close to the real values).  Let's further assume that we are in the second phase of charging and the current being supplied to the battery by the charging IC at this point is only 500mA; if the load is drawing the full current - i.e., 1A - how does the charging IC "know" that only 500mA is needed to charge the battery when the circuit overall will be drawing 1.5A?  This is further complicated by the fact that the current consumption of the Pi may well be in constant flux; it's not going to be drawing a full load all the time, and could swing wildly in a short space of time.

Generally, the cell will be charged with constant current until it reaches the cell gets to a certain voltage (generally between 4.10v and 4.18v for 4.2v lithium), then the charger switches to constant voltage.

*I ASSUME*

I assume, that once the voltage goes below this level again (maybe with a hysteresis) it would return to constant current. So basically if the cell is being charged, not drained the IC will maintain the CV, and the current would be limited by the draw, and if the cell is being discharged the charger wouldn't be able to maintain the higher threshold voltage so the charger would stay in CC mode.

Furthermore, if the charging phase is in constant current mode, supplying the specified charging current of 1A, and the Pi is drawing a full load of 1A, does this mean that the charger IC will have to be able to cope with 2A?  The cells I'm going to be using (or at least planning to use) for prototyping will be at least 4.4Ah, possibly up to 6.6Ah, and can sustain a maximum charging current of 0.5C.  For simplicity's sake, I'll probably stick to a charging current of 1A.

Thanks in advance for any help.

The charger IC will only supply what it is set to supply and will appropriately current limit itself. So the IC will only have to cope with it's set current, the issue associated with this is even though you have the IC charging, the cell will still go flat because you are drawing out more than you're putting in.

Of course things vary from IC to IC and some may have features which allow more efficient charging of a battery whilst supplying a load.

[blc]

Many thanks for the reply.  I had thought I was on the right track with running cells in parallel, but it's good to have a second opinion.

I've been doing a little more research this evening about this - as mentioned above, feel free to correct/slap me if I've got any of this wrong. 

What I wasn't clear on before is how the current is gradually reduced when it comes to the constant voltage phase: does the charging IC apply a gradually reducing current limit, or does the battery gradually draw less current and the charging IC just supplies as much current as is needed. 

From the reading I've done this evening, I think I understand this a little better.  During the constant current stage, the charging IC just lets the battery slurp as much current as it needs - up to the programmed charging current - whilst gradually increasing the voltage.  Once the constant voltage phase kicks in the battery will gradually draw less current and the charging IC just supplies as much as is needed (which will gradually reduce over time as the resting voltage of the battery approaches 4.2v).

Therefore, if that is the case, then there should not be a problem - provided I can find a charging IC that can supply a current which is greater than the maximum current draw of the load.  The more that the load draws, the less will be available to charge the battery - the net result being that the charging time is simply elongated. 

My principle challenge at this point is to source a charging IC that can supply up to 1.5 to 2A and is reasonably priced.

My sources of information are the following links, in decreasing order of how useful/informative I found them:

https://sites.google.com/site/tjinguytech/charging-how-tos/how-lipo-chargers-work
https://sites.google.com/site/tjinguytech/charging-how-tos/lipo-terminology
http://electronics.stackexchange.com/questions/35864/lithium-ion-charging-schematic?rq=1
http://electronics.stackexchange.com/questions/15722/algorithm-for-charging-li-po-batteries
http://code-42.blogspot.co.uk/2011/03/li-po-batteries-explained-part-4.html

[digsys]

Many thanks for the reply.  I had thought I was on the right track with running cells in parallel, but it's good to have a second opinion.

Make it a 3rd opinion. We make up 5-15KWHr Li packs for Solar car racing (Aurora solar), and usually put 5-7 cells in parallel. Our next generation
pack will have ~12 in parallel. The benefits are that neighbours can "condition" weaker cells, and IF a cell goes high(er) impedance, you have
a LOT of redundancy !! IOW, you don't SUDDENLY lose all power and are pretty much screwed :-)

What I wasn't clear on before is how the current is gradually reduced when it comes to the constant voltage phase: does the charging IC apply a gradually reducing current limit, or does the battery gradually draw less current and the charging IC just supplies as much current as is needed.

This is where it gets VERY MESSY, unless you design your own Li Charger system. Manufacturer specs usually state "if you want the longest life / best
performance", you MUST limit the CV current to C10, or C20 (ie1/20th C). You can definitely go higher, but the cell life will diminish.
In racing we don't give a rats, and if we only get 10-20 cycles life, FINE ! But back to you -
Once the IC has reached CV changeover, and as LONG AS your P/Supply can deliver FULL Load AND "trickle" currents, the ACTUAL current going in to
the cells will be "around" the correct level. In fact it will likely be a bit higher. You need to examine the manufacturers charge curves carefully, find
the CC/CV changeover point (lets assume it's 4.10V), then calculate how much over their recommended CV current (and how long) you end up.
Basically, it's NOT ideal, but unless you want 100% cycle life, it won't be too bad.
I have JUST finished a design (3rd gen), where I split up the DC Input into 2 paths - one SOLELY for the battery charger, so it's mostly ONLY on standby,
and the 2nd path is to the DC Output. Either a PNP FET/BJT switches the battery instantly in event of Power fail. (Some people even use a tiny relay).
If you go that way, you can also include a battery load test function, that puts a say 1/10 load for 5 mins, once a day.
A little more work, but it's the "ideal" method.

During the constant current stage, the charging IC just lets the battery slurp as much current as it needs - up to the programmed charging current - whilst gradually increasing the voltage.  Once the constant voltage phase kicks in the battery will gradually draw less current and the charging IC just supplies as much as is needed (which will gradually reduce over time as the resting voltage of the battery approaches 4.2v).
Therefore, if that is the case, then there should not be a problem - provided I can find a charging IC that can supply a current which is greater than the maximum current draw of the load.  The more that the load draws, the less will be available to charge the battery - the net result being that the charging time is simply elongated. 

Yup, pretty close. The charger IC "should" ALSO limit the current to C10/C20 in CV mode, IF you want full cycle life.
That's where you need to make a decision !!

[blc]

Excellent, many thanks for the reply.

Thankfully I don't need the battery to be cope with a massive current draw, so I'll definitely be using protected cells. I'm not overly concerned about battery longevity at this point, as this isn't going to be a commercial product; it's mostly a project to get me off my lazy backside and start working with SMD. Of course it has practical applications, so I do want something that works

I think I've got a good idea of what I'm doing now, so it's on to design and part selection, methinks.