Parallel Li-ions

[allotmenteer]

I've been offered £50 off a spare li-ion battery pack from the place I bought my hub motor kit. The price is £150 for a 6.8Ah li-ion pack. The shop had phoned me to check how things were going with my replacement pack (many of you will be aware of what happened to my first battery pack).

One battery gets me about 14 miles of undulating terrain. I reckon this is about 8-9 miles of actual use up hills and against headwinds with the remaining 5-6 miles coasting downhill.

Ideally I want to use my bike for a 16-18 mile each way commute (probably 10 miles where I really need the assistance). Now I know I could just run off one pack and carry one pack as a spare, swapping one over half way to work. This would have the advantage of only running each pack down about 50-60% which would prolong the life of the batteries.

What about hooking the two packs up in parallel and running that way? This would have the benefit of halving the current drawn from each pack which would greatly reduce the wear on them. I imagine the voltage would sag less as well with the reduced strain on them. Is this an advisable thing to do? Does anyone have any experience of doing this? I have read on other forums that people do this sort of thing (and blow-up cells etc) but most of the examples are from American forums where they are running packs at 72V and pulling 40 Amps + from the packs. Back in the real world I only want to run at 24V and have a 15Amp controller so I don't think I'll have those sort of issues.

I'd be glad of any advice on this idea particularly from anyone running parallel packs.

Paul

Oh - it has crossed my mind that the shop needs to offload their ageing (and deteriorating) li-ion battery packs. Maybe I should ask for a bigger discount?

 

[coops]

Hi Paul,

I'm sorry I have no experience of this issue, and Li is possibly different but parallel NiMH setups have come up on this forum before - Baboonking for one has used them on a Currie bike though I think he now uses two 24V in series for 48V!

I see where you're coming from though with the reduced voltage sag & strain, and it'd save swapping over too, plus you may get a bit more speed, but again as I said I've no experience so I'll leave it to the electronics whizzes to advise properly .

About the Li ion packs needing "offloading" - you may be right, they do deteriorate with age so I've heard and should be used new rather than bought in advance, so you could "haggle" if you wanted .

P.S. I think the batteries to be used in parallel need to be charged separately? i.e. not charged in parallel? I could be wrong and you probably know better, but I'd wait till you have strong advice anyway before trying owt!

Hope thats useful

Stuart.

 

[flecc]

There wouldn't be an issue with the controller or any other bike part, but I'm not sure about the batteries themselves. I'm not thinking of the cells but the fact that it's not the cells that issue the outputs but the integrated control electronics on Li-ion. Basically you'd be connecting two electronic circuit outputs to each other, and that raises questions. For example, if two audio amplifier outputs are directly coupled other than in a correctly designed bridge circuit, they tend to go unstable and one can blow. Even if you find someone who has done it successfully, it would only apply if it were with the identical battery type, since the circuits vary.

As said above, with NiMh it's ok, as long as they are charged separately, but there's a chance of an unstable result with Li-ion. Under load with current being withdrawn it would probably be ok, but the initial connection would have to be on an own risk basis.
.

 

[allotmenteer]

Hi Paul,

P.S. I think the batteries to be used in parallel need to be charged separately? i.e. not charged in parallel? I could be wrong and you probably know better, but I'd wait till you have strong advice anyway before trying owt!

Hope thats useful

Stuart.

Thanks Stuart. I would of course be charging them separately since I only have one charger! Though from my basic grasp of the subject (a tenuous grasp at that ) I would have thought charging in parallel would be ok. The charger is 29.4V 2 Amp so charging in parallel would be at 1 Amp for the double pack. (I'm sure I'll be corrected on this one ).

Paul

 

[allotmenteer]

There wouldn't be an issue with the controller or any other bike part, but I'm not sure about the batteries themselves. I'm not thinking of the cells but the fact that it's not the cells that issue the outputs but the integrated control electronics on Li-ion. Basically you'd be connecting two electronic circuit outputs to each other, and that raises questions. For example, if two audio amplifier outputs are directly coupled other than in a correctly designed bridge circuit, they tend to go unstable and one can blow. Even if you find someone who has done it successfully, it would only apply if it were with the identical battery type, since the circuits vary.

As said above, with NiMh it's ok, as long as they are charged separately, but there's a chance of an unstable result with Li-ion. Under load with current being withdrawn it would probably be ok, but the initial connection would have to be on an own risk basis.
.

The batteries would be identical so I'm not too worried about that aspect.

I imagine if one was fully charged and the other wasn't then the fully-charged one would try to discharge into the other one but I was thinking that the BMS (battery management system) on the li-ions would prevent one of them discharging into the other if they were slightly (or quite a bit) out of balance. Wouldn't the internal fuse blow (I think they have one that trips under adverse conditions and can be reset).

Is there some way of preventing current flowing into the other battery? Perhaps a sort of one-way valve for electricity. I've heard diodes mentioned on other forums but these seem to have problems with loss of energy or suchlike. As you may guess I don't know much about electrickery other than reading these sorts of forums.

It appears all the lithium packs sold with electric bikes are all wired up in series, 7 cells for 24V and 10 cells for 36V. This may give me a clue as to whether I should try rigging two packs in parallel without an extra BMS to govern the 2 packs

Paul

 

[coops]

As you may guess I don't know much about electrickery other than reading these sorts of forums.

Your grasp of the subject seems better tha mine however, given what happened to your last Li-ion & other possible technical problems, I'd tread very carefully Li can give unwanted side-effects, if you know what I mean

Take care & safe cycling Paul,

Stuart.

 

[flecc]

The batteries would be identical so I'm not too worried about that aspect.

I imagine if one was fully charged and the other wasn't then the fully-charged one would try to discharge into the other one but I was thinking that the BMS (battery management system) on the li-ions would prevent one of them discharging into the other if they were slightly (or quite a bit) out of balance. Wouldn't the internal fuse blow (I think they have one that trips under adverse conditions and can be reset).

Is there some way of preventing current flowing into the other battery? Perhaps a sort of one-way valve for electricity. I've heard diodes mentioned on other forums but these seem to have problems with loss of energy or suchlike. As you may guess I don't know much about electrickery other than reading these sorts of forums.

It appears all the lithium packs sold with electric bikes are all wired up in series, 7 cells for 24V and 10 cells for 36V. This may give me a clue as to whether I should try rigging two packs in parallel without an extra BMS to govern the 2 packs

Paul

I don't think there'd be an instant unsafe disaster if there's fuse protection as you say, but a fuse is likely to have enough current capacity to damage the electronics of the management systems. That's if there is an output fuse of course, on my Li-ions the only fuses are in the charge path, the electronics being the sole protections for the the output current. The recent posts by Kraeuterbutter (wow!, just checked and I'd spelt that right!) mention some coupling boxes for lithium batteries in parallel, used in conjunction with chargers etc.

Charging in parallel is an absolute no-no with NiMh, but may be ok with lithium. The charge path is not the same as the output path on lithium of course, different circuit routes. One thing's for certain, lithium batteries do have quite a potential for danger if misused.