Hybrid lipo/18650 pack, different capacity ?

[cycleops612]

OK, so manifestly too messy for consumers.

But it has intriguing mass appeal. batteries are evS weak spot. various chemistries have strengths and weaknesses. Logically, big improvements to the weakness can be made by picking the best chemistry (or combined) for the job.

What is needed (& may exist), is a consumer level, cleverly programmed controller with both batteries attached (2 or more inputs).

It can intelligently and rapidly use either (mosfet switches?) alternately, or combine both batteries in parallel when sufficiently equalised (an optimum state the controller strives toward to maximise reserve power on tap).

small lipo and large lifepo4 pouch paks seem the best contrasting chemistries to combine as a sample hybrid example.

When ever power usage drops below a comfortable output for the lifepo4, then it uses spare amps to top up the burst reserve lipo.

Regen, if u must on an ebike, naturally suits fast charge lipo.

Its not a dissimilar paradigm to an hybrid car or phev - choosing from or combining multiple power sources for the wheels.

Vanilla chinese controllers cost ~$20, so using multiple slave/sub, controllers on each battery to simplify or for robustness isnt a prohibitive option.

am not a tech nerd as u can see, but off the top of my head, maybe the forums for "cycle analyst" are populated with suitable tragics . the "wiki controller" thread here on es may help too.

 

[flat tire]

Interesting concept, I guess you would have to make the 18650 pack a lot bigger than the lipo to avoid constantly charging it which is what would happen with proportionate energy capacities as the lipo discharge curve is way different than any 18650 chemistry with much higher voltage under discharge and static voltage after depletion.

 

[cycleops612]

craigS
"I've convinced myself that you'd like the max charge rate of the lipo to be close to the max discharge rate of the bulk pack. That way you hit both limits at the same time."

Perhaps, as I suggested, this is an example of how each (or one) of the batteries having a dedicated cheap controller, could simplify matters?

The bulk pak can be limited to whatever discharge u like. It neednt limit bulk pak size.

 

[EDS]

You can parallel different capacity packs. I don't think the relative IR is important either. They always stay perfectly balanced to each other. They have to. Their terminals are on the same nodes in the circuit. RC plane guys used to do it (probably don't bother any more since lipos are cheap & high performance now). Parallel charging was & may still be common. You can parallel charge different sized packs together. HK sells 6-way parallel charge cables.

It would seem to be important to have them very closely matched when you make the initial connection which puts them in parallel, since the higher voltage battery will quickly charge the lower one. With only their IR to slow the current, the charge rate could be very high. However, some people were fairly casual about it and didn't seem to have problems. I'd still match them before connecting just to be safe, but as long as you leave them connected, they will stay perfectly matched.

That's at the power cable voltage. Individual cells could still go out of balance unless you also parallel the balance cables. You can do that (I have), if you want to balance charge the whole thing, but it will mask bad cells. It may be better to only parallel the power leads, and periodically check on the individual cells.

 

[bnoooogers]

Just want to say thanks to craigsj for the best examination of hybrid batteries that I've seen on these forums. It's an interesting problem and I appreciate your thorough approach!

 

[EDS]

I take back my statement about the relative IR not being important. Sorry if this is redundant to something above - I haven't reviewed all the other posts closely, but I skimmed Craig's last post enough to notice that I was being chastised, so I thought about it a little more.

Paralleling packs together generally works out if the packs are similar except for capacity, even though packs of different capacity have different IR. The IR tends to be inversely proportional to the capacity, so the small packs put out less current & the large packs put out more current. Those currents are proportional to their capacity, so they tend to discharge at the same rate. Because of the parallel connection, even if there are differences, they stay perfectly balanced (or automatically balance themselves if you prefer).

If the packs are dissimilar in more ways than capacity, then it may not work out as well. If you parallel a very high-capacity, very high IR pack, with a very low capacity, very low IR pack, (which is backwards from more common combinations) then the little pack may be worked pretty hard when you come off the throttle. With those four properties at extremes, the big pack would return to the presag voltage, since it would not have lost significant capacity, but the little pack would stay at the sag voltage, since it never really sagged, but rather it discharged down to the sag voltage. So the big pack would recharge the little pack at a current of the difference between the sag voltage & the resting voltage, divided by the total IR of the relevant circuit. With a sag on the order of volts, and an IR much less than an Ohm, the current could be excessive.

You still have a few things working in your favor. 1. If the whole point is to reduce sag, & you manage to make something with very little sag, then the balancing currents may be acceptable. 2. A high-discharge rated lipo, as would be used for the purpose of the OP, also tends to be able to take a high rate of charge. 3. Self-balancing would not occur for very long. The hybrid pack may be rebalanced before anything has a chance to heat to a problematic level.

If you're going to try something extreme, it would be good to know what you are doing - or better yet, just do it anyway, wear safety glasses, & post pictures.

 

[cycleops612]

Proof of the "fundamentalness" of this thread, is the grid itself.

It requires just such a hybrid of power sources to work properly - from base load to degreesof on tap variable power like hydro & gas turbine.

 

[zackclark70]

seen someone have good success with a bike that had a 10ah lipo pack with Bluetooth bms built in ( can set the max cell voltage ) and a dc-dc converter set so the lipo took the peaks and short trips and there sla in the trailer would charge the lipo at a slow rate