DO NOT DO THIS!

You can't safely charge any battery using jumper leads from another battery. If the other battery matches the flat one, but is fully charged, the current will be so high that you will probably start a fire. You will definitely damage the flat battery, and probably both of them. Jumper leads work on cars by having enough resistance that the flat battery survives, but you should never leave them hooked up between two batteries in an attempt to charge one from the other. Car batteries are also designed to supply very high currents, so can accept high currents without too much damage. Bike batteries are not like that.

The main problem is that electric bikes, even really old ones, normally run at 36V, some older ones at 24V or 48V, but a single car battery is 12V. At best a car battery charger will do nothing for you. If your electric bike is old enough to have a series of lead-acid batteries (that work in a similar way to car batteries), a car battery charger might be able to charge those one at a time without damaging anything.

If your bike is a more modern one it will use lithium batteries and you won't be able to charge it at all with a car battery or battery charger. Unfortunately, if you open the battery case up and attempt to charge it, you could easily set the whole thing on fire. Look up some of the videos of hoverboard fires if you want to see what happens.

You really need to find the correct charger and use that. You can probably buy one from Walmart, but you may have to find the spare parts section on their website to do it.

I will edit this answer to be more useful if you can supply more details.

DON'T CHARGE YOUR EBIKE BATTERY WITH CAR CHARGER

Most probably you have 36V Lithium-ion battery. Car charger won't match this battery and in worst case you can get an explosion in your garage:

(this image is clickable and leads to Youtube)

Search "lithium battery explosion" on YouTube, you'll figure out that it can be very dangerous.

EBIKE CHARGER DC PLUG TYPES

To find a charger for your battery you need to take a look at charger plug on it. Most common plugs are:

After this, you can go to AliExpress or any shop you prefer and find charger for 36V battery with plug that matches your battery.

FIND CHARGER ON ALIEXPRESS

Aliexpress search results: http://www.aliexpress.com/af/lithium-ion-battery-charger-36V.html?ltype=wholesale&SearchText=lithium+ion+charger+36V

Average price for charger you need is $30.

36V CHARGER IS ACTUALLY 42V CHARGER

This is actually the most confusing part for beginners. When you open charger page you'll see this:

It's fine that voltage of "36V charger" is 42V. This is because the voltage of Li-ion and Li-Po cells is:

• 3.7V for epmty cell
• 4.2V for full cell

36V battery contains 10 cells, therefore battery voltage goes from 37V when it's empty to 42V when it's full. You'll ask - why it's called "36V charger" and not "42V charger" and I don't have a better answer than: for historical reasons.

OUTPUT CURRENT

Finally, some words about output current which is 2A on screenshot above. 2A means that if your battery has 10Ah capacity, it will be charged from empty to full in 5 hours (10Ah / 2A = 5h). 2A is most common output current for average ebike batteries like yours.

Definitely not do this you need a lithium battery charger as these are Smart chargers and regulate the trickle charge into the battery through a BMS board. Find out what voltage your battery is ie:-36v and if so you need a battery charger higher normally 42v but it's gotta be over the 36v but not exeptionally higher so usually it's 42v and that will be suitable to charge properly