LiPo batteries basics

[Andreas]

I am not an expert but as far as I know most DIY fans are building their own Lithium Polymer battery packs (aka “LiPo” batteries) in order to be flexible with the performance and form factor.

Compared to Nickel-Cadmium and Nickel-Metal Hydride batteries NiMH Batteries they have much lighter weight, much higher capacity and much higher discharger rates (more punch)

On the downside LiPos have a much shorter lifespan and need special care in terms of charging discharging and storage. In the worst case they can lead to fire.

When you research for batteries you will find terms like “6S2P 10,000 mAh LiPo”

Voltage & cell count:

6S means 6 batteries in “series”. If you connect cells in series the voltage ads up (e.g. 6 times 3,7V = 22,2 V)

BTW, If your battery claims 3.7 V this is the average voltage (fully loaded it would be 4,2 V and discharged 3 V)

2P means 2 batteries in “parallel”. If you connect cells in parallel the capacity ads up.

Capacity:

10,000 mAh (milli Ampere hours) = 10 Ampere hours (10 Ah) indicates the capacity of the battery. The higher the number, the longer the run time.

Discharge rate (“C” Rating)

The C Rating is simply a measure of how fast the battery can be discharged safely and without harming the battery. The “C” in C Rating actually stands for Capacity.

50C would mean you can get 50 times the capacity, so for the 10 AH example above this would be a maximum of 500 Ampere which you can get out without harming your LiPo.

I found this guide which goes into more detail:

https://rogershobbycenter.com/lipoguide/

For beginners it might be much easier AND SAFER to buy a complete LiPo pack such like this one:

Multistar High Capacity 20000mAh 6S 10C Multi-Rotor Lipo Pack

[Sammy]

So using the Waterwolf battery as an example regarding typical capacity & voltage.

And 8 EUR Lipos like following:

https://www.d-edition.de/RC-Modellbau-Zubehoer/Lipo-Akkus/Lipo-3-7V/LRP-Lipo-Flugakku-170mAh-30C-3-7V-fuer-z-B-Blade-mCX---mCX2---mSR.html

Would this mean?

50,4 V divided by 3,7 V = 14 Lipos in series

(Wh / V) * 1000 = mAh = 1510 / 50,4 V * 1000 = 29.950 mAh

29.950 mAh / 170 mAh = 176 Limos in parallel

176 (parallel) * 14 (series) = 2.464 Lipos ?

Even for 4 EUR a piece that would be 10.000 EUR for a battery pack?

This seems wrong. Where is the mistake?

[Andreas]

Hi @Sammy,

The LiPos you are referring to are not sufficient for this use case. You have to take 18650 cells.

50,4 V divided by 3,7 V = 14 Lipos in series

This is correct and brings us to “14S”

If you take good Sony VTC6 cells they offer 3000 mAh which leads to 10 cells parallel to reach the 30 Ah.

Now we are at “14S10P” = 140 cells.

You can get 150 cells at Ebay for around 747 EUR. See the picture:

Cheers
A.

[Marsen]

LiPo Cells are not really very good for eSurfing boards. The 30C or more ratings of these batteries are designed for high power RC such as helicopters where weight is really important. The powerful motors require high current so a high C rating is needed, LiPo’s are commonly 30C plus. Li-ion are at most 7C continuous. As mentioned earlier the biggest draw back of LiPo batteries is very poor cycle life ie less than 300 sometimes not even 150 by which time they will have deteriorated to about 70% of their original capacity. Using a battery at 30C would drain the battery in 2 mins, no good for surfing! A much better option would be Li-ion cells. These use similar chemistry but have different electrolyte. The C rating is about 5 - 7C for high drain cells. However if you want a least 30 mins of surfing at high speed then you need to calculate power used and then work out the size of your battery pack. To do that you need the motor current and voltage.
As an example.
Motor, Turnigy Rotomax 50cc 5300w motor 120A,37V (actual power 4440w)
So to get the 37V we would need 10S (10 in series as explained earlier)
We need a 60Ah battery to give us 120A for 30 mins.
Using Samsung INR1865030Q cells which are high current cells with a capacity of 3Ah that would be a 10S20P battery pack. Or 2.22kWh.
The Wolfwater one above has a 1.51kWh battery pack in a different configuration of 14S. The number of parallel circuits is unknown as there are many different size capacity batteries.
It is unlikely to run at full power for the whole 30 mins so a smaller battery can be used and still get a good 30 mins of use.
Li-ion cells will give you about 500- 800 cycles depending on manufacturer, type and how much you drain the battery by. Using 100% capacity every time will give you the minimum cycles.

[Doubletrouble]

I would just like to add some more safety notes regarding hobby lipo batteries. Regarding safe discharge rates the battery will have two numbers for example 30c 40 c the first is the safe continuous discharge and the second is the burst discharge. Please disregard the second number. Most jet boards are run flat out so you will be using whatever power the battery can put out this number should be well below the constant discharge c rating. Example 5800 mah with a 30c discharge rate should be able to safely discharge at about 175 amps if you run two of these batteries in parallel you double that number so safe discharge is now 350 amps. My drives are not pulling more than 200 amps continuous so this is a safe battery configuration I thought . The problem of safe discharge is complicated by several things. The quality of the battery the resistance due to wiring length size of connectors etc and the amount of heat dissipation you are getting while the batteries are under heavy load.
Lipo batteries are chemistry and just because it says 30c or 60c don’t take that as fact assume the manufacturers are estimating high . The cheaper the battery the more I would distrust the c rating and I have dozens of puffed up hobby batteries with dead cells to back that up.
You should use water cooling for your batteries as well@# your motor and esc. I resisted this because it added more complexity to the build and I did not think it was necessary … it is if you want the batteries to last at all and keeping them cooler can stop a over heating battery from failing completely and catching fire … it’s called thermal runaway and it’s not fun. The other overlooked but important part of battery safety and performance is the connectors and wire size and length. I use 8 mm gold bullet connectors and 8 gauge wire. These work well as long as you take the time to solder the connectors on to the wire carefully and keep the wires the same length on each battery.
In my experience the on,y batteries that displayed anything even close to a discharge rate that they had printed on them were the high capacity low discharge batteries like multistar. Graphene and tattu every other battery particularly those claiming high discharge rates of 40 c or better failed very quickly and even the ones I mentioned failed just less often than other brands . Be cautious with Lipo batteries. Calculate your amp draw then Test it It’s almost always higher than it appears mathematically if your rig pulls 150 amps assume it’s 200 amps and if your battery can discharge 200 amps safely per the label assume it’s 100 to 150 amps and get another battery to run in parallel or get bigger batteries . I learned all of the above the hard way hopefully you can learn from my mistakes and save yourself a baked battery or a fire.