batteries and the ah con

[jimmyhackers]

i wont even get started on sla's and there 20hour rating system

i want to ask about lipos and lifepo4 etc

for rc car experience my 3300mah lipos last about 20mins then they are low voltage. my imax b6 charger from this state says it only puts in around 1700ma in to get it fully charged again.

i did think the batteries were faulty initially, then i read up and realised lipos only do about 60% discharge before u start damaging cells etc. so 1700mah out of a 3300 mah battery seemed right again, if not a bit annoying.

4 other friends using cars and helicopters reported the same from there lipo batteries. all different brands, sizes and shapes.

is the same true of larger lipo bike batteries (branded and unbranded)? so 20ah is really closer to 13 ah of usable power?

is the same true of lifepo4? google seems to think you can go to 80% depletion. making them a better option

anyone got any real world experience or figures for me?

 

[Deleted member 4366]

The larger lipos are essentially the same as the smaller ones. I've had several different brands and types and they've all given pretty close to their stated capacity. That's charging to 4.2v and running down to about 3.5v. The difference might come from the lower discharge rate. On my bikes, I always had about 10 Ah to get enough range, so that the discharge rate would be about 1C average, while as R/C models normally go way higher than that.

 

[Kinninvie]

The 16Ah Multistars are actually underated as tests done by an ES member when they first came out found that all 9 of his batteries put out more than 16Ah.
I recently ran a pair down to 3.41V per cell and got almost 17Ah out of them.
Sadly I think this is the exception rather than the rule.

 

[jimmyhackers]

ahhh ok fair point

does that mean lipos are subject to the same flaws as lead acid.

i.e. higher discharge rate makes for less overall ah capacity in the battery?

 

[trex]

yes.

 

[Deleted member 4366]

But when you have ones as high as 60C, the difference between running them at 1C or 2C is insignificant.

 

[flecc]

i.e. higher discharge rate makes for less overall ah capacity in the battery?

This is true of any battery type, it's called the Peukert effect or Peukert's Law.

Explanatory link
.

 

[trex]

not that insignificant.
his cells are similar to the blue plot (UR18650F, 2600mAH).
If he rides slowly, more than hour, he gets 95%. If he flattens his battery in less than 20 minutes, then the drop will be huge.

source: http://batteryuniversity.com/learn/article/calculating_the_battery_runtime

 

[Alan Quay]

not that insignificant.
his cells are similar to the blue plot (UR18650F, 2600mAH).
If he rides slowly, more than hour, he gets 95%. If he flattens his battery in less than 20 minutes, then the drop will be huge.

source: http://batteryuniversity.com/learn/article/calculating_the_battery_runtime

That's true. It's even described in the post just above yours. Now what's that 'effect' or 'law' called again?

 

[anotherkiwi]

So always pedal and set assistance level to the lowest? Switching up for the duration of a steep hill climb then back down again as soon as it is done?

Tony

 

[trex]

this is a problem affecting mostly small lipo packs without BMS.
If you keep within 2C, that's what most BMSes do by default anyway, you'll get 90%-95% of your battery's capacity.

 

[jimmyhackers]

ok cool so if im drawing around 20 amps i need a 20ah pack to get a reliable 1c and the fullish 20ah

next question lipo or lifepo4?

lifepo4 seems a safter more stable longer lasting option

 

[Deleted member 4366]

not that insignificant.
his cells are similar to the blue plot (UR18650F, 2600mAH).
If he rides slowly, more than hour, he gets 95%. If he flattens his battery in less than 20 minutes, then the drop will be huge.

source: http://batteryuniversity.com/learn/article/calculating_the_battery_runtime

He's talking about lipos, where even the cheapest are 25C.

 

[jimmyhackers]

ive just had another thought. this is between sla and lipo life etc.

as voltage = speed and amps = torque. roughly

a lipo 48v battery will be 12cell 12 x 4.2v = 50.4volts maximum
a SLA 48v battery will be 4 cell 4 x 14.7= 58.8v maximum

theoretically could i achieve a higher speed with the higher voltage sla's?

 

[Stalkingcat]

I think that you're thinking too much
The higher speeds that you are talking about, will hardly be noticeable and will be taken up by the extra weight of SLA's.
If you want some cheap fun, then wire up another inexpensive 12v sla to get your 60V.
If you don't mind splashing some cash and want to get somewhat serious, then start leaning about balance charging lipos.

 

[trex]

jimmy could hold the record for building the cheapest 35mph bike. His is cheaper than d8veh's xiongda that he sold recently.

 

[Alan Quay]

ive just had another thought. this is between sla and lipo life etc.

as voltage = speed and amps = torque. roughly

a lipo 48v battery will be 12cell 12 x 4.2v = 50.4volts maximum
a SLA 48v battery will be 4 cell 4 x 14.7= 58.8v maximum

theoretically could i achieve a higher speed with the higher voltage sla's?

Sorry to put a downer on this, but lead acid fully charged is between 12.7 to 13.3v (ish). You may see a 'float' voltage above this, but as soon as you draw any current, that float will be scrubbed off.

If you use 12.7v the result is 50.8v

I know it's not what you want to hear, but SLA really is a poor choice for electric bikes.

 

[averhamdave]

I run my 48v nominal, MXUS at 14S (approx 60v charged) - using RC Lipo batteries. I use these as I use the same for my large RC Helicopters.
The shunt is soldered and I draw 30a at maximum pull.
I used to run with 15a capacity but my journeys are generally less than 15 miles so I have reduced it to a 10a pack. (It did consist of 9 lipo packs it now has 6)
On that basis I am pulling up to 3C which is absolutely nothing. In the RC application they will pull 20C+

 

[averhamdave]

The worse thing about RC Lipos is that you have to be fairly disciplined and balance charge them regularly. I don't Im afraid - I use a DC power supply set at 58.5v to bulk charge as one big pack.check them infrequently and am always alarmed by how far out of balance they have become! (Which really shortens their life, puffs them etc). Only about 1 in 15 charges is on my large balance charger.

 

[Stalkingcat]

I don't I'm afraid - I use a DC power supply set at 58.5v to bulk charge as one big pack.

I'm afraid of bulk charging with a bms, much prefer to monitor each cell each charge.
For some reason i trust the chargers in-built BMS more than a pack's BMS.
A good balance charger can charge pretty quick these days. It's just that the power supply gets pretty expensive, unless using a computer psu supply.