Two Batteries On A Bike - Can't Quite See The Point.

[21QUEST]

Is it just me?

Bike has two batteries on it. Both batteries are being charged at the same time and I'm failing to see the point of that.

I'm not sure how they have been connected to the charging system but it'd have to be in parallel....

Is that even a good idea without an upgrade alternator and/or rectifier?


Cheers
Ben

[fastdruid]

Depends on the bike, I know of old 6v bikes where users used to modify them to run on 12v by various means and ran two batteries in series, or running two smaller batteries in parallel could aid weight distribution.

Druid

[21QUEST]

It's one of THESE

The battery is in one of the panniers, so doubt it's for weight distribution lol. It was apparently put in so "you never have any issues with starting..." . Bike has a kick start too.

Any thoughts regards alternator/rectifier coping with two batteries...asumming they have not been changed/modified?



Cheers
Ben

[fastdruid]

Shouldn't have a problem charging two batteries. Can't see the point in that application but never mind.

Druid

[yorkie_chris]

Good if you use lots of 12v stuff when camping, could build some manner of split-charge relay like you would on a land rover if you went really mental. Then you could have worklights, big winch, fridge etc lol.

[Sid Squid]

Originally Posted by 21QUEST

Is it just me?

Usually.

Originally Posted by 21QUEST

Bike has two batteries on it. Both batteries are being charged at the same time and I'm failing to see the point of that.

It's the same as fitting a larger battery - there's greater battery capacity, the reason for using two is simply that there's probably no easy way to physically fit a single larger battery.

Originally Posted by 21QUEST

I'm not sure how they have been connected to the charging system but it'd have to be in parallel.

Yes, if it were connected in series it would effectively be single a 24V battery. Usually when doing this the batteries are simply directly connected together, sometimes what's referred to as a 'split charge' system is used; there is a relay connected such that second battery is only connected to the charging system, (and thus the first battery), when the engine is running, when the engine is off and the charging system isn't working the second battery is disconnected, this battery is the one used for engine off supply, (the interior lights and radio in a camper for instance), this battery can be used as much as you like - the battery for vehicle use, starting primarily, is unaffected and will start the engine even if the second battery is flat.

Originally Posted by 21QUEST

Is that even a good idea without an upgrade alternator and/or rectifier?

Originally Posted by 21QUEST

Any thoughts regards alternator/rectifier coping with two batteries...asumming they have not been changed/modified

No need, the battey is simply a reservoir for the charge produced by the bike's charging system, the rate of charge won't change - it would simply take twice as long to charge both batteries to the same level. Simply put, if the second battery is identical to the first, the bucket is twice the size, thus the water is twice the depth and will take twice as long to re-fill if it's empty - but importantly if there were a situation in which it were being emptied, (ie engine off consumption), it will be able to supply that consumption for twice as long as long before being discharged.

[embee]

If they are simply connected together in parallel, a common mistake is to use too small cabling. It needs to be of a similar size to the main battery cables since when starting the engine the "remote" battery can be delivering very high current.

As referred to by others, the ideal arrangement is usually to use one as the starting battery, with the second one connected via a relay to isolate it for starting, and then use a split charge controller like this which charges each one according to its needs. This is good practice for caravans/motorhomes etc, it may be a too complicated arrangement for your application, simple sometimes is perfectly adequate in low demand situations.

I once had an ex-police Triumph 650 with 2 big 6V batteries in series.

Originally Posted by fastdruid

Can't see the point in that application but never mind.

The guy was on a round the world trip - seems to me that a spare battery would be a great idea on a little trip like that.

[fastdruid]

Originally Posted by vjones777

The guy was on a round the world trip - seems to me that a spare battery would be a great idea on a little trip like that.

Normally I'd agree but with a carbed bike with a kick start?

It makes far more sense for powering extra stuff but not for just starting.

Druid

It wouldn't make much sense in terms of redundancy unless the battery was protected from say a reg/rec fault. It'd also need some means of isolation to protect from the same things that will discharge the main battery.
For running electronic gadgetry like a laptop, I reckon lead acid performance is fairly ho-hum for the weight. They also don't like the constant cycling.
FWIW, my down hole rig has an 18kg 15V/160Ah Li-Ion pack to drive the winch motor + a 20Ah pack for the sensor package. When I haven't got the down hole rig in tow I have a 2kg 24Ah pack on board for the bike computer which does such tasks as provide routing for my net connection. All batteries are charged from the bike.