Stupid question of the day

**Mr.Ed** · 26-12-2014, 02:31 PM

Can you use your car battery to charge the bike's battery?

So basically my bike hasn't been started in yonks and obviously the battery doesn't have enough juice to start it now... God knows where my battery charger is so I was thinking of using the jumper cables and leaving it connected to the car for say 15min (?) without even starting the car.

Google searches seem to think it's fine but I'd prefer to hear from you peeps. Anyone ever done that??

Cheers.

**Marshy** · 26-12-2014, 03:45 PM

I haven't just hooked them up and left them.... it'd be fine, but I don't think it'll actually charge very much. I've certainly hooked up a car to a dead flat battery that won't even allow a jump start, and left them hooked up while revving the car for 15 or 20 mins to get a charge into the bike battery. They don't transfer much at idle, unfortunately, as the alternator isn't doing much.

**senator8** · 26-12-2014, 04:07 PM

Wether or not it will do anything or transfer from one battery to the other depends on factors such as composition, internal resistance etc. As Marshy said, best bet is to start the car. If you've got nothing drawing on the electrical system of the car it will still probably be putting out more at idle than the bike would. None of this is ideal. If you're not going to regularly top up charge your stored bike battery you should disconnect it. Every time you flatten it you're reducing it's capacity/life.

**Mr.Ed** · 26-12-2014, 04:10 PM

Thanks gents! I'll start the car and give it a go then... I reckon I'll probably need another battery anyway but at least I get to learn something in the process!

**Mr.Ed** · 27-12-2014, 01:36 PM

Well, it worked!! Bike started within half a tenth of second... impressive.

But now I need to find a way to ride next to the car 'cause the battery is not holding charge anymore hahaha After 15min connected to the car (on and reving a bit) it would barely try and started before giving up completely.

Oh well, time to get one of those lil' lightweight batteries again.

**senator8** · 27-12-2014, 03:52 PM

Originally Posted by Mr.Ed

Well, it worked!! Bike started within half a tenth of second... impressive.

But now I need to find a way to ride next to the car 'cause the battery is not holding charge anymore hahaha After 15min connected to the car (on and reving a bit) it would barely try and started before giving up completely.

Oh well, time to get one of those lil' lightweight batteries again.

It may still recover. 15 minutes is not long enough. Do you have a multimeter?
Try and find your battery charger and let it charge for at least a few hours depending on the amp rating of the charger, and stop trying it and draining it completely

A lithium battery is unlikely to recover from this type of abuse BTW, and they do not like being flattened completely and left. The upside is if you disconnect it (lithium) they have a very low self discharge (approx 10% a over a year)

**stevem** · 27-12-2014, 04:56 PM

Where do you live Ed ? I have a couple of battery chargers. I'm at Engadine.

**Negrogrande** · 27-12-2014, 05:15 PM

I can loan you mine when I'm back on the 30th (I've got a spare) are you still in Bondi

**Mr.Ed** · 28-12-2014, 11:20 AM

Thanks guys! Yep, I'm still in Bondi Andrew. I won't be home till the 4th of Jan now though... but thanks for the offer!

I usually treat my batteries really well but since I lost my charger this one got neglected quite a bit. Pretty sure I still have a LiPo battery somewhere as well... but again, no charger!