Charging Lipo's help

[Peter6bee]

I hope this is in the right area. If not then I do appologise.

I'm new to the battery charging lark and have viewed several posts to try and get my head around it but just want to make sure I've got it clear before I blow my cells up. I appologise if I am asking something which has been covered but my searching has not been able to answer my questions fully due to my limited newbie knowledge.

I have a Blade MCP X and the charger which comes with it but I've also been given a Pro-Peak Prodigy charger (Ist gen not II). I have also bought a 6 way parallel charging lead to use with the prodigy charger.

I have been charging my single lipo cells at 0.2a and 3.7v on the included charger but want to use the Prodigy to charge multiple cells.

I have
2 x 200 mAh cells (the ones that come with it)
2 x 250 mAh cells (2nd hand ones)
4 x 300 mAh cells (bought new)

My questions are as follows.
Can I charge different capacity cells together or would it be better to keep them seperate?

I understand that in parallel the voltage remains the same at 3.7v but I am a little confused in relation to the ampage.
If I am charging 1 cell at 0.2 amp would I charge 2 cells at 0.4 amp 3 at 0.6 amp etc etc or would it remain at 0.2 amp regardless of the amount of cells.

If 0.2 amp is best for 200 mAh cells is 0.3 best for the 300mAh cells?

Thanks in advance for any answers / info anyone can give me.

Cheers
Pete

[waveydavey]

You are probably better charging at the same capacity. IE 200 ones together 250 together etc. You are right about the current, if you are doing the 200mah at 1C then you would need 0.4, 2C would be 0.8 and so on. This is based on 2 packs as above

[Peter6bee]

Fantastic. Cheers Dave. Is the 200 mah I've been charging at over cautious then? Is 400 mah better for 200 mah cells?

I'm not overlly fussed with the life span of the cells within reason as £8 for 2 cells is not a major amount to be fair.

[UKHelibob]

It is better for the cells to charge them at low rates, within reason. So 1C is usually recommended to give them a long life, so 200mA (each) for your 200s. Some people regard the cells as consumables and charge at higher rates, sometimes much higher. Your choice.

I regularly charge cells of different capacities and when doing so I base the charge rate on that of the lowest cell being charged as each cell will receive the same charge current regardless of its capacity.

[Peter6bee]

Cheers for your reply Bob. Things are becoming clearer :-)

[heathy]

This will be of help now and as you get bigger packs chargers.... Electric RC info, how-tos and such - TJinTech

[cjcj1949]

Lots of people will tell you that charging lipos in parallel is ok as lots of people do it. To a certain extent that is true. The problem is that lipos are meant to be charged at a given current and not a given voltage. The reason for this is that if you charge at a given current you can predict what is going to happen. We know that if you charge at 1C that it will take 1 hour to charge a lipo. This is called knowing what is going on and is the usual way of doind things. Noone would think of constant voltage charging a lipo because the charge current would be very difficult to predict and so we would not know how long the lipo would take to charge. Everyone agrees on this and there is much discusion and opinion on charging at 1C, 2C or higher. Few people think there is an advantage in charging at higher currents other than to save time.

With parallel charging there is a problem as soon as you connect two cells together at a different voltage. This is constant voltage charging and large currents flow from one cell to the other. The higher cell is discharging and is ok, the lower cell is charging at very high rates. Very difficult to predict as an unknown voltage is being placed directly across a lipo.

If the higher voltage cell has a higher capacity than the lower voltage cell then the current pulse will be higher and last longer and will do more damage. To take it to the logical conclusion, someone may be told that parallel charging is safe, that no restrictions exist on capacity matching and that no voltage matching is required. Anyone doing this could think that connecting a flat 100mah directly across a fully charged 5000mah would be ok. The charging current would be very high and the small cell would either be destroyed or not last very long.

It is possible to parallel charge and to do it reasonably. You do need to know exactly what you are doing. The basic assumption is that everything works out in the end. This is correct, but it is only half of the solution. The other half is the dynamic situation, where some cells will be charging while others are discharging.

Keeping all cells the same type and age, the same voltage, and the same temperature will give reasonably predictable results. It will still be a process that has a lot of safety factors removed. You will have to be careful to connect the cells the right way round or they will probably explode, unless fuses are fitted.

Series charging, in contrast, is completely safe. It is not usable for 6S backs as 36S chargers do not exist. For 1S or 2S packs it is viable.

[Peter6bee]

Thanks health. Interesting read there

Thanks Cjcj for all that info too.

Something I've picked up on that I do not fully understand is that a 3.7v battery are at 100% capacity when at 4.2v. If I am right I should still set my charger to charge at 3.7v and not 4.2 yes?

I also need to get my head around how using the voltage in the battery changes as I thought this was just the "flow" of the power coming out so to speak and was constant. How does that change when the battery is discharged?

Is it the case that in say a 300mah cell when at full capacity it can output at a full 3.7v but when down to say 50mah it can only manage to put out 3.5v for example?

[UKHelibob]

Lots of people will tell you that charging lipos in parallel is ok as lots of people do it. To a certain extent that is true. The problem is that lipos are meant to be charged at a given current and not a given voltage. The reason for this is that if you charge at a given current you can predict what is going to happen. We know that if you charge at 1C that it will take 1 hour to charge a lipo. This is called knowing what is going on and is the usual way of doind things. Noone would think of constant voltage charging a lipo because the charge current would be very difficult to predict and so we would not know how long the lipo would take to charge. Everyone agrees on this and there is much discusion and opinion on charging at 1C, 2C or higher. Few people think there is an advantage in charging at higher currents other than to save time.

With parallel charging there is a problem as soon as you connect two cells together at a different voltage. This is constant voltage charging and large currents flow from one cell to the other. The higher cell is discharging and is ok, the lower cell is charging at very high rates. Very difficult to predict as an unknown voltage is being placed directly across a lipo.

If the higher voltage cell has a higher capacity than the lower voltage cell then the current pulse will be higher and last longer and will do more damage. To take it to the logical conclusion, someone may be told that parallel charging is safe, that no restrictions exist on capacity matching and that no voltage matching is required. Anyone doing this could think that connecting a flat 100mah directly across a fully charged 5000mah would be ok. The charging current would be very high and the small cell would either be destroyed or not last very long.

It is possible to parallel charge and to do it reasonably. You do need to know exactly what you are doing. The basic assumption is that everything works out in the end. This is correct, but it is only half of the solution. The other half is the dynamic situation, where some cells will be charging while others are discharging.

Keeping all cells the same type and age, the same voltage, and the same temperature will give reasonably predictable results. It will still be a process that has a lot of safety factors removed. You will have to be careful to connect the cells the right way round or they will probably explode, unless fuses are fitted.

Series charging, in contrast, is completely safe. It is not usable for 6S backs as 36S chargers do not exist. For 1S or 2S packs it is viable.

I think that you may have overdone the worries. Of course, if people do silly things when parallel charging then there will be problems, but you can do silly things when charging a single cell or series charging too such as applying too much current or allowing cells to short out by plugging them into charge leads before plugging the lead into the charger.

I would suggest that the main safety factor in parallel charging, as you have highlighted, is to connect cells together when they are at reasonably the same voltage, but not to be too anal about it. I usually fly for 4 minutes and my mixed set of mCPX lipos of varying capacity all end up at about 3.8 volts so I am happy to connect them together because very little current flows between them when connected. It is true that any cells at a higher voltage do discharge into those with a lower voltage, but as providing current is their purpose in life it is not a problem. The current flowing into the cells with a lower starting voltage could be a problem but the matching of initial voltage, as well as the internal resistance of the cells, limits this current and the time that it flows. In this situation the lower voltage cell is not being charged at constant voltage, far from it. The voltage of the cell(s) providing the current drops as current is drawn and the cells soon reach equilibrium.

None of my cells has ever shown any signs of puffing or getting hot when connected together and charged in this way and I am happy to continue to use this method. As I said earlier in this thread I base my charge current on the lowest capacity of the cells being charged and charge at very conservative rates. Considering the abuse of lipos that is often considered normal, such as flying to LVC, charging at high C rates and not storing lipos at the correct voltage, I think that any problems caused by sensible parallel charging pale into insignificance.

I am not clear what you mean when you say that series charging is not usable for 6S packs. Can you please clarify this point ?

[cjcj1949]

I am simply stating what is happening. This is electronic engineering not worry. My point is simply that parallel charging is only as safe as the way it is done. People make inaccurate statements on what happens when cells are connected together, and also how the current divides between cells connected in parallel. The original post was about connecting cells of different capacity together and I've shown that when the high cell is much larger than the lower cell that this is constant voltage charging.

It is possible, as you and lots of others do, to charge batteries in parallel without damaging them. It is not trivial and has lots of pitfalls. Devices that have an output that is very sensitive to input voltage can not be connected in parallel and still have a predictable output.

When two cells are connected in parallel the internal voltage does not change as soon as the cells are connected. Current flows from one cell to the other determined by the difference in internal voltage divided by the sum of the interconnection resistances and the internal cell resistances. This current can be very large.

I am not worried about the risks in parallel charging. There is no problem so long as people do not make mistakes and follow a sensible plan. The only exception is that of a lipo failure. When a cell develops a leakage current as they all do eventually, then there is a clear difference between charging one cell and many. With one cell the fault is detected and the charging stopped. With many cells in parallel the good cells will discharge through the leaky cell. This may well be a problem.

As for charging batteries in series, my point about 6S batteries is just that high voltage chargers do not exist.

[cjcj1949]

Thanks health. Interesting read there

Thanks Cjcj for all that info too.

Something I've picked up on that I do not fully understand is that a 3.7v battery are at 100% capacity when at 4.2v. If I am right I should still set my charger to charge at 3.7v and not 4.2 yes?

I also need to get my head around how using the voltage in the battery changes as I thought this was just the "flow" of the power coming out so to speak and was constant. How does that change when the battery is discharged?

Is it the case that in say a 300mah cell when at full capacity it can output at a full 3.7v but when down to say 50mah it can only manage to put out 3.5v for example?

The 3.7V figure is confusing. It refers to the nominal voltage of the cell and is something that a designer needs to consider. It is best forgotten about except to remember that if someone mentions a 3.7V cell that they mean a lipo.

A lipo is considered flat at 3.0V and fully charged at 4.2V. They do not last very long if discharged to 3.0V. Discharging to 3.8V opencircuit (off load) or 3.6 to 3.7 onload keeps the lipos going for a long time but gives shorter lives.

Voltage is also called emf or electromotive force. Electric Current is the flow of electricity through a wire from the positive of a battery to the negative.
Ignore this bit if you want.
(It is actually the flow of subatomic particles called electrons. Unfortunately electrons are negative in charge and flow from the negative to the positive. The flow of negative charge in one direction is the same as positive charge in the other and we pretend that positive charge flows in the wire. It does not if the wire is metallic. Inside a battery there are both positive and negative charge carriers)

The output from a full cell at no load is 4.2V. As soon as you start to take current out the voltage will drop. The voltage drop can be calculated by multiplying the current in amps by the internal resistance of the battery in ohms. A 2200mah 25C battery has an internal resistance of around 5 milliohms at roo temperature. A 200 mah battery would be 50 milliohms. As you discharge a battery its open circuit voltage will drop as well. A lipo has about half its charge left at 3.85V. The internal resistance rises as the battery gets colder and this is the reason they do not work so well in cold weather. Batteries with a high C rating have lower internal resistances than batteries with a low C rating, although some manufacturers are more optimistic than others.

[UKHelibob]

I am simply stating what is happening. This is electronic engineering not worry. My point is simply that parallel charging is only as safe as the way it is done. People make inaccurate statements on what happens when cells are connected together, and also how the current divides between cells connected in parallel. The original post was about connecting cells of different capacity together and I've shown that when the high cell is much larger than the lower cell that this is constant voltage charging.

It is possible, as you and lots of others do, to charge batteries in parallel without damaging them. It is not trivial and has lots of pitfalls. Devices that have an output that is very sensitive to input voltage can not be connected in parallel and still have a predictable output.

When two cells are connected in parallel the internal voltage does not change as soon as the cells are connected. Current flows from one cell to the other determined by the difference in internal voltage divided by the sum of the interconnection resistances and the internal cell resistances. This current can be very large.

I am not worried about the risks in parallel charging. There is no problem so long as people do not make mistakes and follow a sensible plan. The only exception is that of a lipo failure. When a cell develops a leakage current as they all do eventually, then there is a clear difference between charging one cell and many. With one cell the fault is detected and the charging stopped. With many cells in parallel the good cells will discharge through the leaky cell. This may well be a problem.

As for charging batteries in series, my point about 6S batteries is just that high voltage chargers do not exist.

I think that we are in agreement on most points. mCPX lipos can be safely charged in parallel as long as suitable precautions are taken. Your point about faulty cells is a good one and could cause problems. In practice a faulty cell is not likely to produce enough current to fly the model and such a fault will soon become apparent.

Sorry, but I still don't get your point about 6S charging. I have a 6S serial lead with mCPX battery connectors, main power leads and a balance lead that I made myself and can balance charge 6 mCPX cells in series. Although I usually charge in parallel I occasionally charge them in series so that I can monitor individual cell voltages. Such leads are available commercially I believe (3S ones certainly are) and my charger is easily able to supply the required voltage and current and it is nothing special, just one of the many '4 button' clone chargers available everywhere.

[cjcj1949]

Sorry we are at cross purposes. I was refering to the difficulty in series charging 6S batteries. Charging two 6S batteries would need a 12S charger and 3 6S batteries an 18S charger. Charging any number of batteries in series is ok, just practically different above 6. I know higher chargers are available.

Done sensibly parallel charging is probably capable of treating cells better than higher rate charging. Charging 1S cells in parallel is a special case I think as each cell is individually tested before each test by being used. In multicell batteries it is not so easy to spot a faulty cell. Also with small batteries it is easier to get them out of the house if something goes wrong.

Lipos are unpredicatable, but NiCds used to go up in smoke, often on the way back from the field in the back of the car. Most incidents do not get reported.

Things go wrong with models from time to time which is why we fly in private field most of the time.

[UKHelibob]

Thanks - I understand what you mean about 6S now

The message is - be safe