Naive battery question

[james01]

Recently had to buy a replacement battery for wife's ebike (£400 ).
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?

[Tigerbiten]

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 ...........

[stodd]

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]

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]

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.

[Cyril Haearn]

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

[squeaker]

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

[squeaker]

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

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

[Grandad]

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]

Recently had to buy a replacement battery for wife's ebike (£400 ).
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]

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.

[bikes4two]

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?

Go to eBay or Amazon and search for something like 'car jump starter' to see the massive array of LiPo type starter packs - then youtube the topic to see folks starting massive diesel engines covered in snow with these little packs.

[fullupandslowingdown]

I once dreamed of converting a old motorcycle frame into an electrically powered motorbike. Once I did the figures I realised I'd probably need to tow a small trailer with over a quarter of a tonne of car batteries behind it.
Car lead acid batteries are designed for high but brief discharge rates. The quoted capacity however is fiddled from a small discharge current. If discharged at the equivalent rate to say 150 to 250 watts to power a cycle motor, the realised capacity would typically be about a third. Worse, the battery wouldn't last many charge/discharge cycles before the plates become damaged. Deep cycle batteries cost more but used 5 times a week still won't last many years. Legally, if you use lead acid batteries for traction, then they have to be mounted with regard to protection from damage in any accident. It all adds to the kerbside weight, and everytime you climb a gradient, that extra weight sucks the range.
You would need a quality charger, a cheap one simply wouldn't cope with charging a car battery from flat on a daily basis.