LiPo efficiency

[MAD Dave]

Can someone please advise me of LiPo efficiency regarding charge out versus charge in. I recall in the days of DEAC NiCd cells a figure of a little over 70% was used when the batteries were charged at the 10hr rate. I am aware that there are many variables to consider but if there is a "Rule of thumb"out there, I would be delighted to hear of it.

[Chris Bott - Moderator]

Dave that is a great question.

I think most people assume that it's 1:1 or 100% but I bet it isn't.
Maybe a few of us should do some discharge tests followed by charges just to see what we find.

[MAD Dave]

I have to admit that I am unaware of any process that is 100% efficient!

[John Olsen 1]

Mine get warm both on charging and on discharging, so I deduce that the efficiency is not 100%. You would need to monitor voltage and current throughout a cycle to get the overall energy in and out. Some of the chargers do some logging, I am not sure how much that would help. I suspect it will be better than the DEACs.

An electric heater can be 100% efficient, all the energy that enters it ends up as heat.

John

[Greybeard]

It does depend where you start your measurement from, the gas/oil well or your electricity meter. If the latter I suggest you assume a figure of 10% and be prepared to reduce it.

[Chris Bott - Moderator]

Why not go back to how much vegetation had to rot down and lie under the sea for thousands of years to produce the oil/gas Greybeard.. c'mon be serious!

Back to the original question. There are a lot of variables that will affect the efficiency, not least the rate of discharge and rate of charge. For a start, LiPos warm up more if you discharge them faster, and the same goes for discharge.

I was just wondering how to test this, should I discharge at 1A on my charger, ending up with a measurement, or should I try at the current that a model uses, using the wattmeter? Would it be different if I discharge at a steady current, or should it be in bursts? Or maybe at a steady current that's equivalent to the average used in flight?

On the other hand, many people use the charge figure from the charger as a measure of the capacity taken out of the battery during the previous flight. While this may not be quite correct, it is often used to check against flight time and help decide how long to fly for to look after the battery. As the charger will have to put a bit more in than was taken out, then at least this difference comes out on the safe side. So maybe it just doesn't really matter.

[Frank Skilbeck]

In Mah terms it may not be that simple, when you discharge a Lipo the volts supplied drops, when you charge it up the volts rise, so yes you do put in more energy than you take out but in mah the charger mah reading maybe the same as the discharged mah. But in watts terms more will be put in due to the voltage drop on discharge.

If i remember I'll do a check next time use my telemetry system, I'll see if the charger puts back the same mah as I've taken out.

[Simon Chaddock]

I am not quite sure why the charge to discharge efficiency is that inportant unless you are concerned at the cost of the electricity you are using.

Surely from a model plane performance point of view all that matters is the total energy available in the battery and the rate at which it can be used.

[Frank Skilbeck]

While I agree with Simon, this is all academic, I did some quick readings on a 2200 mah 3s battery this morning as follows

Battery checker before flight - 97%

Mah remaining on telemetry signal -451 mah as battery was set at 2200 mah this equates to 1749 mah used.

Battery recharge - 1691 mah, battery checker now says 98%.

So based on that it looks like I've had to put in less than I took out, but all so close that it's all down to the equipment accuracy

[Ben B]

As said, academic but a good discussion for a cold and snowy day... the key parameter is how much of the capacity you can actually use and of course that depends on how far you discharge your batteries and therefore what voltage you're happy to discharge down to. IE if you have a low C battery and are discharging at moderately high amps you'll draw a very small amount of capacity before the cell voltage drops "too low". Meanwhile if you're using a 90C battery and are prepared to use a cut-off of 3v you'll get a lot more out. C-rating will also effect "efficiency" in terms of heat production.

So if you want a "high efficiency" battery IE able to use more of the available capacity and producing less heat use a higher C-rated battery. And make sure any connectors are low resistance and tickety boo.

[MAD Dave]

Thanks for the responses guys. My question may be academic and perhaps LiPo efficiency is something one doesn't actually need to worry about with the advent of "intelligent" chargers.

That said, what brought my question to life, was the desire to understand how much charge might remain in a LiPo post-flight. I have only recently started to dabble in electric flight courtesy of a Flyingwings Raven. I have been advised that to maximise the useful life of a LiPo, a battery should not be discharged below 20% capacity; anyone like to comment? I have three 3S 1300mAh batteries and have been slowly extending my flying time whilst monitoring post-flight charge magnitude to try and get an idea of "capacity used". As one might expect, I have observed a measure of variation between the performance of the packs but this may be down as much to the flight profile for each "trip" as regard any other variable. Without terrorising the skies, I have found 16 minutes powered airborne time may about right for 80%-ish indicated discharge; but I'm not sure.

I have been monitoring remaining charge post flight using a proprietary battery checker. I leave the battery to settle for about 30 minutes before checking. Whether this is the best procedure to follow, I don't know.

I also record the charge rate (I have standardised on 0.5A) and the elapsed time at which the charger turns off when it believes the battery to be "full". From this, it is simple to calculate the mAh put back in expressed as a percentage of capacity.

The slight issue I have is that the mAh charge added to the declared residual percentage rarely sums to 100%; surprise, surprise maybe! As noted above I have three 1.3mAh batteries but they are of differing manufacturers (badges?).

I would like to gain a better understanding of the variables and errors hence my original question!

I have monitored the battery set over three outings and found residual charge summed with subsequent replaced charge (as a capacity percentage) to be as follows for each battery.....

Turnigy - 102, 100, 109. (Purchased three months ago)

RFI - 95, 91, 95. (~ one year old)

Overlander - 98, 95, 100. (Purchased two months ago)

The first two figures for each relate to 15 minutes flight times on my Tx timer and the latter figures, 16 minutes. My sop has been to land pdq post the timer bleeps.

I would agree I don't seem to have much to worry about but I just got to thinking about LiPo efficiency and wondered whether there was a working figure out there......

[Gordon Bushell]

One thing to bear in mind is that the charger may be a half amp charger, but you can only put a half amp into the battery until the voltage reaches fully charged. At this point I believe most chargers cut the charge rate to keep the voltage constant.

I know my charger only says the battery is full after a period of oscillating between 0.1A and 0.0A charge rate, although the bulk of the charging may be at 3A or more. Things can take even longer if you are balance charging.

My charger, and I think most except the most basic chargers have a counter that measures the mAh put into the battery.

Personally I tend to keep my battery packs above 30% full immediately after landing, but this may be over cautious. I have started to notice with the older packs and low temperature at present that the performance has dropped off, despite plenty of remaining capacity.

[Gordon Bushell]

Depending on how deeply you want to get into it, you might like to look up the peukert effect for light bedtime reading.

Basically, battery capacity is not constant, but decreases as the discharge rate increases.

Or just fly to the timer, and don't worry too much!

[Gordon Bushell]

Looking at my log, the lowest I have taken the remaining capacity on landing is 14% by my handy battery checker. Obviously, by the time the batteries have got home, they have recovered slightly, but I'm not sure by how much.

With 14% remaining, on three occasions I have recharged only 60% to 65% of the alleged capacity.

It is interesting to note that the difference between 30% remaining and 14% remaining is about 150-200mAh from a 4500mAh pack. I reckon peak loads are around 20-22C discharge (static), and the average for the flight around 5C.

Hope this helps.

[Frank Skilbeck]

Dave, you can't use the time taken to charge a Lipo as a measure of charge it takes, the way a Lipo charger works is that it maintains a constant current until the cell voltage is 4.2v and then reduces the current until the current drops to zero at which point it switches off. Different batteries have slightly different internal resistance so some might start reducing the current with less mah returned than others and some of the time will be used balancing the cells if it's a charger which charges through the balance lead ot has a built in balancer. Also as batteries age they lose some capacity and umph and this is determined by how hard a life they've had.

[Simon Chaddock]

In my experience it is not only the depth of the discharge that has an impact on the battery life but also the discharge rate. At a high discharge say 20C the actual cell voltage will drop more than at a 1C rate so the potential for long term damage goes up.

To achieve absolutely maximum duration in my 'endurance' flying have repeatedly taken one 25C rated battery right down to LVC (and opened the throttle a couple of times to make sure!) without apparent ill effect but then it was only being discharged at 1C.

A similar battery on a 20C discharge taken to LVC got distinctly warm and started to puff up.

As other have said the only safe way is to be conservative and limit the flying time to keep at least 20% capacity remaining.

Of course if you have a plane that has potential a duration exceeding one hour even a 20 min flight is not likely to work the battery very hard!

[Ben B]

Those battery checkers give you a ballpark figure but it's quite a big ballpark! They're not accurate- the resting voltage slowly returns to a stable voltage after discharge and how much it comes up by depends on how long the current draw has pulled it beyond the resting voltage. So again if you're pulling amps near the maximum rating of the battery expect the battery checker to under-estimate remaining capacity. I've drawn over max-C rated current on batteries and they've come down on LVC at <3v but after half an hour to cool down and relax a bit they're up again near 3.6-3.7v.