Parallel or separate for batteries.

[Blunderbuss]

Any advice you have on putting multiple batteries on my bike would be welcome? I am going to build my second ebike, I have learnt loads from building my first and I intend to have something that is more flexible. It will be a geared hub (thanks for all the help with this) and I would like to mount two batteries (identical bottle type) so that I can just use one for the shorter journeys but double up for longer. This will also allow me to have them in the main frame as opposed to a rack to help with handling as I do all my miles off road.

So my question is when I have the two batteries mounted should I run each one down in turn, or connect them in parallel – any advantages of either approach?

 

[Deleted member 4366]

If you want to put two batteries in parallel, they must be at the same voltage when you connect them, and you must disconnect them when yoou charge either.

 

[Blunderbuss]

Yes they would be identical batteries, and i would charge them separately off the bike so they were at the same voltage - i was just wondering if there was any advantages of running them in parallel.

 

[chris_n]

Less voltage sag, fewer deep discharges, potentially longer life.

 

[danielrlee]

May i add "a bit more than double the range of a single battery" to the above.

 

[Blunderbuss]

It is looking like parallel is wining the day - the little bit of extra range is intriguing, why would that be?

 

[Deleted member 4366]

It is looking like parallel is wining the day - the little bit of extra range is intriguing, why would that be?

Less sag and the higher the current you draw from a battery, the less capacity it gives. I have a 6Ah battery, which gives the full 6Ah at 1 amp, but at 5A, its capacity reduces to about 5Ah. At 10A, it's even less.

 

[anotherkiwi]

Here is an independant test of the Samsung 29E, you can see the different Ah depending on discharge:

http://lygte-info.dk/review/batteries2012/Samsung INR18650-29E 2900mAh (Blue) UK.html

 

[Benjahmin]

My Ezee battery is getting a bit long in the tooth and a bit saggy on the welsh hills. I have an insat battery built for another bike. So I made a splitter lead and carry the insat battery in a pannier. Sag, what sag?
The other advantage, as already said, is that the current draw is taken across the two batteries, so stressing each one less, so extending it's useable life.
If you're carrying the weight of a spare battery anyway, it seems a no brainer to go the parallel route.
Not sure what it's done for range, haven't had chance to find out yet.