battery wattage question

Electrically assisted bikes, trikes, etc.
tigersid1974
Posts: 1
Joined: 7 Feb 2019, 11:59am

battery wattage question

Postby tigersid1974 » 7 Feb 2019, 12:28pm

Hi all i recently bought a secondhand e bike of ebay . Upon receiving the bike i checked the battery rating and it is 36V 10AH 360 watts but the bike has a 250 watt controller and 250 rear hub motor. Is this battery too powerful for this bike? as i do not want to burn out the motor or controller.I contacted the seller and he said that the bike came supplied with that battery surely this cant be right? Any help would be appreciated.

rjb
Posts: 3058
Joined: 11 Jan 2007, 10:25am
Location: Somerset (originally 60/70's Plymouth)

Re: battery wattage question

Postby rjb » 7 Feb 2019, 1:09pm

The battery you have is fine. 36v X 10 amps hours = 360 watthours. This is just an identification of the battery capacity. The battery voltage is fixed at 36 volts and the motor will be rated for this voltage and will only consume a sufficient current to allow a max power of 250 watts. A bigger capacity battery providing it is rated at 36 volts will allow longer running times. :wink:
At the last count:- Focus Variado, Peugeot 531 pro, Dawes Discovery Tandem, 2 Dawes Kingpins, Raleigh 20, Falcon K2 MTB dropped bar tourer, On One Pompino, Longstaff trike conversion on a Falcon corsa. :D

thelawnet
Posts: 1639
Joined: 27 Aug 2010, 12:56am

Re: battery wattage question

Postby thelawnet » 7 Feb 2019, 3:00pm

tigersid1974 wrote:Hi all i recently bought a secondhand e bike of ebay . Upon receiving the bike i checked the battery rating and it is 36V 10AH 360 watts but the bike has a 250 watt controller and 250 rear hub motor. Is this battery too powerful for this bike? as i do not want to burn out the motor or controller.I contacted the seller and he said that the bike came supplied with that battery surely this cant be right? Any help would be appreciated.


The battery is not 360W, it is 360Wh. That means that you can run it continuously at 240W for around 1.5 hours, say, or at 720W for half an hour.

The battery consists of a number of cells which can be wired in series, parallel or both. Series increases the voltage, parallel increases the capacity as well as reducing the current taken from each battery.

The battery will consists of a number of probably 3.6V cells, each of which might be around 3.3Ah in capacity. This would be mean that if you had 3 such cells in parallel you'd have 3.6V, 10Ah capacity. And for 36V you have therefore perhaps 30 cells, in 3 parallel runs of 10 giving 36V 10Ah

Batteries have a maximum discharge rate that they are rated for. This varies and could be from as low as 4A for a lithium cell up to 30A.

Your battery motor is I think NOT 250W peak, but rather higher than that. 250W is a continuous rating.

Quite likely the motor can put out more than that.

Your controller numbers is actually what determines that. You say it is a 250W controller, but it probably isn't. It should have a maximum current, and that can be used to calculate the true drain.

For example, if the controller is rated at 15A, then the 36V voltage means there is 36 * 15 = 480W maximum power.

If we are drawing 15A from the battery, then because the battery is using at least 3 parallel series, this means the actual current required is just 5A per cell.

Overall there is a balancing act in terms of cell capacity vs. power.

For example, a very bright bike light would pull typically 2A at full brightness from a cell. In this case, a battery like this one

https://batterybro.com/products/panason ... button-top

which has 3.2 Ah capacity, and max 4.875A continuous current, would provide 1.6 hours of use, and would be a good choice

In a bike application, it's perhaps not so great in that the max current is quite low, though that is a continuous current, whereas your bike probably only uses high power for brief periods.

A battery like this one https://batterybro.com/products/sony-18650-vtc5a has a significantly lower capacity, of just 2.6 Ah, but it has a max current of 35A. Because of the sheer number of cells in a bike battery this is overkill. For certain applications it will be more useful.

In the abstract there is no particular reason to believe your bike battery is outside of spec, because the 360W 'capacity' is not the same kind of measurement as the 250W max 'power', and because in any case the thing that needs to be matched is 'voltage', which for all parts of the system should be 36V. The controller and motor determine how much 'current' is being drawn from the battery - if the controller only pulls out 36W, say, then that isn't an issue for the battery - there's not really such a thing as 'too much capacity'.

It is quite possible your battery is no good, but that's perhaps because it uses fake cells or whatever, but from the specs there's no indication of a problem.

hemo
Posts: 293
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

Re: battery wattage question

Postby hemo » 7 Feb 2019, 11:46pm

Which e bike ?
Most likely the battery uses 2500mah cells arranged as 10s4p, though some batteries may also use 10 x 10ah li-polymer pouch cells.