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>> Home > Forum > Electrical Related > Li-Poly batteries
Li-Poly batteries
(876 views)
Author Message
AlanP
(Fleet Admiral!)





Forum Posts: 385
16th Apr 2017 21:00  
>> Permalink
mdlbt.com/27524

My main interest is scale boats, but I have a few fast electric, just for the adrenalin boost.
The Blackjack and the Rocket reach the upper 40mph range with no problems in smooth water.
The boat in question is a bit bigger and will handle rougher water. At the moment I am running it on two three cell 4000 mah 25-50c batteries, these are well used and get warm and swell. For the money I am prepared to spend for my adrenalin rush, I have the choice from Hobbyking of two 4400 mah 65-130c or two 4500 mah 35-70c batteries.
I have never really quite understood the C value of these batteries.
Any advice would be most welcome.

Alan

Midlife306
(Admiral)





Forum Posts: 89
16th Apr 2017 21:20  
>> Permalink
mdlbt.com/27529

Hi Alan,
I'm relatively new to the hobby but the way I understand it is as follows:
If you view your battery as a fuel tank, the mah value is the size of the tank, the bigger the number the longer the run time will be, the down side is the bigger the number the higher the weight.
The C value is how quickly the battery can discharge/charge, the first of the two number is the rate it will run at all day, the second it the peak the battery can handle for a short surge. If you want faster acceleration I think the higher the number the better.
If it was my money I'd be buying the 4400 mah 65-130C
Cheers
Wayne

pmdevlin
(Fleet Admiral!)





Forum Posts: 307
16th Apr 2017 21:52  
>> Permalink
mdlbt.com/27534

thats prety much it Alan, but be warned, chinese c rating and mah tends to be overstated, the bottom figure being exagerated, and the top one really only there for very short bursts, its more abount the make of the lipo

If your present packs have puffed, one or more of the cells has either over discharged, or couldnt take the draw, once puffed, its pretty much had it. I have used slightly puffed lipos oin a boat, but would never use them in a plane, its just not worth the risk if it fails, and it probably will

AlanP
(Fleet Admiral!)





Forum Posts: 385
16th Apr 2017 22:07  
>> Permalink
mdlbt.com/27537

Thanks for the replies lads, (good explanation Wayne) that's pretty much confirmed what I was thinking.
The 4400mah it is then, the high C rating will help to get it up on the plane without taking too much out of the batteries.

Thanks Alan

John
(Fleet Admiral!)





Forum Posts: 74
16th Apr 2017 23:03  
>> Permalink
mdlbt.com/27540

The simple way to understand the “C” or capacity rating is to think about it as what you can draw from the battery. There are two numbers. The first (lower number) is what you can draw continuously and the second (higher number) can be draw for around 10 seconds. To convert the “C” you multiply the number by the amperage of the battery (not the mah).
So the 4400mah is 4.4 amps multiplied by 65(C) = 286amps which you can draw continuously.
The 4500mah is 4.5 amps multiplied by 35(C) = 157.5amps which you can draw continuously.
As already said the higher the C the better to keep it cool.


Happy sailing.

John.
Trillium
(Commander)





Forum Posts: 63
17th Apr 2017 00:47  
>> Permalink
mdlbt.com/27543

While your comment is quite accurate, the word 'continuously' did make me smile. You would be able to draw 157 amps from a 4500mAh battery for less than 2 seconds - if it was fully charged. 😜
Roy

nasraf
(Commander)





Forum Posts: 70
17th Apr 2017 08:39  
>> Permalink
mdlbt.com/27546

I do not know much about the detail design of Li Po batteries but in the past have spent quite a lot of time and tax payers money looking at the performance of lead acid and nickle cad batteries when starting petrol engines over a large ambient temperature range and have the following observations, my interest in model boats is restricted to modest speed versions and I have enough trouble there with Li Po's.

It is a pity that to impress those who have a limited knowledge that the capacity of these batteries is quoted in ma. hours rather than amp hours, I know it is easy to convert if you have a bit of a mathmatical background but not eveyone does. It took me many years before I understood the difference between energy and power and I think a lot of people still do.

Basically the ma. hr. rating is the amount of "energy" that the battery can store but how much of it that you can get out is very much dependant on the rate of discharge ( i.e. the "c" value with its multiplier ) in general the higher the rate of discharge ( i.e. the "power" ) the less of it you can get out. In addition batteries have an internal resistance so the higher the " amperage " the lower the " voltage " applied to the motor terminals, so as "Watts" ( Power ) equals volts times amps the actual power available to drive the boat is reduced. Also the loss due to the battery internal resistance ends up as heat in the battery which does not improve its life.

It must be almost impossible with the information available to be able to select the best battery available for the high rate discharge uses, I wonder if ayone has done any comparative tests?

pmdevlin
(Fleet Admiral!)





Forum Posts: 307
17th Apr 2017 08:52  
>> Permalink
mdlbt.com/27548

nasraf, have you not got that the wrong way around? The higher the c rating (rate of discharge) is the quicker the lipo can deliver? , you state the opposite, Otherwise why would we spend extra bucks on a higher c rated lipo?

FDor example, I have a certain model plane, if I fly with a 3300mah 25c lipo, I expect and get a shorter flight time, and less power when wanting to do stunts, as the battery is depleting. I know this due to the fact I have a lipo low voltage alarm plugged in, set at a generous 3.7v per cell. So I then fly with a 5000mah 40c pack, the flight time is longer, and the stunts will last longer into the pack due to higher c rating. OK, battery make, and age etc come into play, but this is a generalisation.

Going back to the original post, in simple terms, the higher the mah (amp per hour is a simple move the decimal place) and high the c rating, the faster the boat will run, and the longer it will last. Correct me if I'm wrong?👍

pmdevlin
(Fleet Admiral!)





Forum Posts: 307
17th Apr 2017 09:04  
>> Permalink
mdlbt.com/27553

internet is littered with info, but this link is good, easy explanations!

http://www.rchelicopterfun.com/rc-lipo-batteries.html

nasraf
(Commander)





Forum Posts: 70
17th Apr 2017 11:12  
>> Permalink
mdlbt.com/27559

pmdevlin

Sorry to confuse and you are right in what you say.

What I was trying to point out was the relationship between energy storage and power availability. A big battery will usually have a greater energy storage capacity and depending on its design a greater power output but this is not always the case.

As I would imagine the construction and manufacture of LiPo's is a bit of a " Black Art " as was the manufacture of many lead acid batteries and the likes of Boeing and Samsung have found out at great cost to themselves, if anyone has done any controlled tests on batteries used by the likes of our members it would be useful.

nasraf
(Commander)





Forum Posts: 70
18th Apr 2017 09:17  
>> Permalink
mdlbt.com/27590

The link to rchelicopters.com that pmdevlin has indicated I found very informative and I think should be read by all those using lipo's. In addition if they follow its recommendations is likely to save them a lot of money as to the life they will get out of their batteries.

One thing I discovered was the meaning of the two "c" values shown on the batteries which from what previous contributors have said looks a bit confused.

From the web site article the lower of the two numbers is the maximum charge rate that can be applied to the battery without causing its immediate destruction i.e. assuming a capacity of 1000 ma.hrs and a "c" of 5, 5 amps would be the absolute maximum charging current. However if this rate is used the number of charging cycles that can be done before the battery is seriously damaged would be greatly reduced. The author of the article recommends that the rate should never exceed 1 c if you want to get a good life.

The higher "c" rating is in general better understood and is an indication of the maximum short term discharge rate that can be drawn. Going back to an example of a 1000 ma.hr battery a "c" of 25 would give a discharge current of of 25 amps but not for long and the internal battery heating would not do much for the battery life.

pmdevlin
(Fleet Admiral!)





Forum Posts: 307
19th Apr 2017 22:33  
>> Permalink
mdlbt.com/27620

to add to that, its also regarded as good practise to balance charge the lipo, using the balance lead, and and appropriate lipo charger, I believe all lipo chargers have this facility. This means the cells are all equal when fully charged, and you can view this on the charger display. It also means that none of the cells will be overcharged, and risk fire etc. The down side is that as the charger enters the final stage of balancing and achieving fully charged, it takes a while, as the current reduces, so patience is a virtue, and never leave unattended whilst charging, that applies to all chemistry, I have seem a nihm explode due to over charging, its not just this lipo myth that is risky!😉

RNinMunich
(Fleet Admiral!)





Forum Posts: 599
21st Apr 2017 09:37  
>> Permalink
mdlbt.com/27633

Hi from freezing Munich 🤔
Balancing is not just "good practise" it is a safety essential!!
Otherwise the "good" cells can be overcharged and overheat, with the consequences we have seen with Boeing and Samsung 😲
That is why you need a charger specifically designed for LiPos, and why one should pay more for a decent LiPo which has better matched cells from the start 😉
Cheers Doug 😎


"Retirement is when you stop living at work -
and start working at living/boat modeling!" 舰队的海军上将 😉 Doug
rolfman2000
(Commander)





Forum Posts: 65
22nd Apr 2017 12:32  
>> Permalink
mdlbt.com/27655

My use of lipos involves drones (racing and camera), fpv fixed wing long range aircraft, and boats (mostly reasonably fast electrics). And my charging regime is to number all my similar cells, so all can be rotated, evenly used and charged. I normal charge twice, then balance charge the third. This has kept all my cells with even capacitance and in good condition. I say that, so far (touch wood) that I haven't managed to puff or blow any packs.

RNinMunich
(Fleet Admiral!)





Forum Posts: 599
22nd Apr 2017 12:40  
>> Permalink
mdlbt.com/27656

Hi Rolfman
sounds like you are using / charging single cells.
Then mounting some as a 'pack' and then balancing. Correct?
If so a good scheme to prevent excitement 😡 during hi amp discharge in a fast electric.
I used to number & use my NiMH cells in sets in a similar manner. Most of them lasted several years
Cheers Doug 😎


"Retirement is when you stop living at work -
and start working at living/boat modeling!" 舰队的海军上将 😉 Doug
AlanP
(Fleet Admiral!)





Forum Posts: 385
22nd Apr 2017 15:07  
>> Permalink
mdlbt.com/27661

By gum, my post woke a lot of people up 😁😁😁

Alan

RNinMunich
(Fleet Admiral!)





Forum Posts: 599
23rd Apr 2017 14:18  
>> Permalink
mdlbt.com/27687

and Good So!
I think a lot of people (without the benefit of a long career in electronics) needed a 'Wake Up Call' on the dangers of and correct handling of LiPos. When possible try to find the manufacturers specs. in the net. I only buy from dealers who also post the specs. as PDF. No Spec. No Buy 🤔 The best manufacturers also print the max charge and discharge rates on the LiPo / LiFe etc pack itself. Rule of thumb: Max charge 2C, Max discharge; depends whether you want to run for ca 1sec or an hour or two 😉 Remember: 1000mAH (or 1AH) means the accu (strictly speaking LiPos etc are accumulators and not batteries 😉) can theoretically deliver 1Amp for 1 Hour IF hi quality and in perfect condition 'In this the most perfect of all possible worlds', or 2Amps for 1/2 hour and so on. But if you 'drain the tank' completely it will probably be ruined. Cheers Doug 😎


"Retirement is when you stop living at work -
and start working at living/boat modeling!" 舰队的海军上将 😉 Doug