Naive battery question

Electrically assisted bikes, trikes, etc.
james01
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Joined: 6 Aug 2007, 4:48am

Naive battery question

Postby james01 » 15 Aug 2019, 2:36pm

Recently had to buy a replacement battery for wife's ebike (£400 :shock: ).
I calculate that when fully charged it contains around 400 watts, and weighs around 5kg. I just weighed an old car lead-acid battery at 13kg. When fully charged it holds around 480 watts. The battery costs around £40 new. The extra 7kg is of dwindling significance on an already-heavy electric bike, especially if it's used heavily laden. The battery requires only a cheap, readily available 12volt charger. My motoring experience suggests that car battery life can easily exceed 7 years. Does anyone know why there isn't a market for 12 volt heavy utility ebikes running on lead-acid car batteries?

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Tigerbiten
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Joined: 29 Jun 2009, 6:49am

Re: Naive battery question

Postby Tigerbiten » 15 Aug 2019, 4:48pm

Most modern bicycle motors are run at 36v or 48v.
If the voltage drops to 12v then the current draw increases.
Heat in wires goes up at the square of the current so there is now more parasitic heat in the wires.
With a lot more heat in the wires then they need to be thicker to stop them burning out.
Thicker wires means bigger/heavier motors which need better cooling due to more parasitic heat.

Plus car batteries are designed to put out a short sharp blast while it starts a car.
Draw to much out of them and run them flat then you risk damaging them.
To get this to work you'll need a deep draw battery which tend to be more expensive.

So it can be done, but it's not that simple.

Luck ........... :D

stodd
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Joined: 6 Jun 2018, 10:24am

Re: Naive battery question

Postby stodd » 15 Aug 2019, 5:30pm

I'd expect a 400w bike battery to be around 2.5 to 3 kg.
Some old ebikes did use lead acid, but as pointed out, but just any lead acid battery will do (or will quickly die under the load)

DaveReading
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Joined: 24 Feb 2019, 5:37pm

Re: Naive battery question

Postby DaveReading » 15 Aug 2019, 5:46pm

james01 wrote:I calculate that when fully charged it contains around 400 watts, and weighs around 5kg.

Forgive my naivety, but what happened to the days when battery capacity was measured in watt-hours ?

hemo
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Location: West Sussex

Re: Naive battery question

Postby hemo » 16 Aug 2019, 12:09am

E bike battery capacity is either listed as Ah or Wh.
The mid drive market use Wh but fo some reason most hub bikes us Ah.
Ah x voltage = Wh.
Many new to ebikes always quote wrongly that xxx battery is 300,400,500 watts etc, when they should be saying 300,400,500wh.
Watts is the power out put of a battery whilst Wh is it's capacity which can be used to get an idea of battery range using different wh/m usage. A typical fit not bike rider should be able to use 6 -12wh/m, whilst a less fit person will likely be 12 - 20whm.

Reason for SLA not being used is many, Weight to heavy/bulky size, energy density is poor, power output under continual load is poor and reduced range. Lithium is far superior under constant load.


To show how good lithium is, when my car battery failed due to lack of charge (as often it is little Iused)I;mable to use a small 3s li -polymer pack to start my car. The li-po pack is tiny approx 13cm x 6.5cm x 3cm, output 65/130a and 4ah/48wh capacity.

brynpoeth
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Joined: 30 Nov 2013, 11:26am

Re: Naive battery question

Postby brynpoeth » 16 Aug 2019, 5:43am

The explosive power supplied by a starter battery is different to the trickle needed by an e-bike, but how much does a car battery cost?

Another disadvantage of e-bikes, a replacement battery alone costs as much as a decent used car :?
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squeaker
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Re: Naive battery question

Postby squeaker » 16 Aug 2019, 10:29am

DaveReading wrote:Forgive my naivety, but what happened to the days when battery capacity was measured in watt-hours ?

It's a [rude word removed] energy descriptor - bring back Joules :lol:
"42"

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squeaker
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Location: Sussex

Re: Naive battery question

Postby squeaker » 16 Aug 2019, 10:31am

squeaker wrote:
DaveReading wrote:Forgive my naivety, but what happened to the days when battery capacity was measured in watt-hours ?

It's a <i>[rude word removed]</i> energy descriptor - bring back Joules :lol:

Er, when was 'b@st@rd' considered rude? Plain english IMHO :?
"42"

Grandad
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Location: Kent

Re: Naive battery question

Postby Grandad » 16 Aug 2019, 11:48am

o show how good lithium is, when my car battery failed due to lack of charge (as often it is little Iused)I;mable to use a small 3s li -polymer pack to start my car. The li-po pack is tiny approx 13cm x 6.5cm x 3cm, output 65/130a and 4ah/48wh capacity.


That sounds a useful answer to unexpectedly finding a flat battery -open door indicator light not seen in my case. Is it something available on the market or a self assembly set up?

kwackers
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Location: Warrington

Re: Naive battery question

Postby kwackers » 16 Aug 2019, 12:47pm

james01 wrote:Recently had to buy a replacement battery for wife's ebike (£400 :shock: ).
I calculate that when fully charged it contains around 400 watts, and weighs around 5kg.

Seems heavy for the power.
Mine's 540w and weighs 3.5Kg.

What form factor case is yours in?

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

Re: Naive battery question

Postby hemo » 17 Aug 2019, 12:00am

Grandad wrote:
o show how good lithium is, when my car battery failed due to lack of charge (as often it is little Iused)I;mable to use a small 3s li -polymer pack to start my car. The li-po pack is tiny approx 13cm x 6.5cm x 3cm, output 65/130a and 4ah/48wh capacity.


That sounds a useful answer to unexpectedly finding a flat battery -open door indicator light not seen in my case. Is it something available on the market or a self assembly set up?


Li -po is only worth having lying about if it also has other uses during the year, commonly they are used for RC cars or choppers etc.
Apart from the battery cost a charger is needed as well for about £10, Lipo 3s commonly available from Ebay, Hobbyking or RC outlets.

My one was about £40 - £50 as it has high C rate, I use mine to power my spot welder for buiding lion batteries and also use it to power a vaporiser for Oxalic acid clensing.