WetEV
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 1:10 pm

It is always interesting to see new variations on a theme. Any reasonably hilly or stop and go might easily have enough energy input during regeneration to handle dynamic rebalancing. But on a flat and few stops trip, seems unlikely to be enough regeneration.

Other schemes I've seen did energy sharing between adjacent cells, or between a range of cells, or powered the 12V or other subsystem(s) off the strongest cells.

But do note that installing such a system on a current Leaf would be a very major engineering project, at best. Even using the Linear parts mentioned above shown above will require a lot of attention to analog details. If I was building my first production EV, I'd probably not risk doing something like this. Second or third EV, I'd want to consider this.
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Levenkay
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 1:43 pm

Giving each cell its own isolated "augmented charging" crutch that's capable of delivering enough power to compensate for capacity variations among the cells sounds horrifically complex (unreliable), expensive, and bulky. Much better to just use bigger cells, strive for tightly-matched characteristics during manufacture, and then just live with however the "weakest link" cell turns out. I'd far rather see spare volume in the packs (if any) devoted to cooling them.

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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 2:36 pm

Levenkay wrote:Giving each cell its own isolated "augmented charging" crutch that's capable of delivering enough power to compensate for capacity variations among the cells sounds horrifically complex (unreliable), expensive, and bulky. Much better to just use bigger cells, strive for tightly-matched characteristics during manufacture, and then just live with however the "weakest link" cell turns out. I'd far rather see spare volume in the packs (if any) devoted to cooling them.


You're might be right. But still, until someone actually makes one and then crunches the numbers will we actually know. After all, we do have balancing systems already on our vehicles that aren't that much less complex than what we're talking about, the difference being that power would have to be passed through a transformer instead of a shunt. Many people thought and still think that our current shunt-type battery balancing systems are horrifically complex (unreliable), expensive, and bulky. But so far I haven't heard of any problems with any of the balancing systems on Leafs, Bolts, Volts, Teslas or any other EV's. This new type of system could replace the curent battery ballancing system and would also increase overall efficiency (no wasting power through shunts.)
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Levenkay
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 5:25 pm

IssacZachary wrote: After all, we do have balancing systems already on our vehicles that aren't that much less complex than what we're talking about, the difference being that power would have to be passed through a transformer instead of a shunt

As I understand it, the balancing network is implemented with resistors that can be (via a semiconductor switch) connected across each cell of the battery if/when the BMS wants, which seems about as simple as it can get to me. Since transformers don't work at DC, "passing power through" one means converting that power to high-frequency AC with active electronics, with yet more electronics to synchronously rectify that power back to DC again for delivery (otherwise the diode losses would be frightful) Measured any way you want, it IS "much more complex" to try and move the energy around than to just let some of it dribble away.

lorenfb
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 8:58 pm

Levenkay wrote:As I understand it, the balancing network is implemented with resistors that can be (via a semiconductor switch) connected across each cell of the battery if/when the BMS wants, which seems about as simple as it can get to me.


Yes, it's as simple as that, i.e. a resistor and a MOSFET, or just a MOSFET with the desired R(on) resistance.

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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 11:08 pm

You guys are exaggerating. Diode losses wouldn't be frightful, and the BMS in our Leafs is a whole lot more complicated than just a mosfet and a resister. The Leaf's BMS has 96 isolated driver circuits (you can't drive mosfets in a series off a common node) that likely also form part of the voltage sensing circuits (better than running 96 shunts down to the bottom cell) that communicate (optically?) with the main BMS board. Off the shelf BMS kits that are comparable to the Leaf's BMS cost well over a grand for obvious reasons.

For an example:
http://www.orionbms.com/products/orion-bms-standard

I'm not saying a dynamic balancer would be practical (unless you're a perfectionist) but it definitely is possible without huge efficiency losses. The technology to make such a device and to make it small, compact and efficient exists now. We left the stone age quite some time ago.
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lorenfb
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Re: New device boosts road time for Leaf by 50%

Tue Nov 28, 2017 11:54 pm

IssacZachary wrote:You guys are exaggerating. Diode losses wouldn't be frightful, and the BMS in our Leafs is a whole lot more complicated than just a mosfet and a resister.


No one's implied such as the total implementation!

Levenkay
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Re: New device boosts road time for Leaf by 50%

Wed Nov 29, 2017 2:17 pm

IssacZachary wrote:You guys are exaggerating. Diode losses wouldn't be frightful
No? I was imagining delivering into a single cell at about 3.8V though a full-wave bridge with two forward drops. I expect Schottky diodes would be unacceptbly leaky (it's considered a design challenge for the BMS to just *measure* the cell voltages without draining the pack unduly), so that would mean delivering only 3.8/(3.8 + 1.6), or about 70% of the input transformer power. I call that frightful. I may have picked a bad topology to imagine, though; if the transformer is operated in a flyback configuration, only one diode would be needed, so that would cut the 30% loss in half. But that's still pretty bad.
IssacZachary wrote:and the BMS in our Leafs is a whole lot more complicated than just a mosfet and a resister.
Yes, but I'd expect it to be heavily integrated. From occasional curious glances at trade-magazine articles/adverts for such ICs, I gather that they are capable of handling a small number (like eight or so) of directly-connected cells, with appropriate level shifters for measuring the cell voltages. I imagine they can also generate the gate drives for directly-connected balance-resistor FETs as well, so from the component count standpoint, the LEAF's per-cell charge-balancing arrangements DO consist simply of a transistor and resistor (or just the mosfet as lorenfb suggested), plus 1/8th of an IC and its attendant powersupply and communication infrastructure.

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IssacZachary
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Re: New device boosts road time for Leaf by 50%

Sat Dec 02, 2017 9:09 am

I don't see why Schottky diodes couldn't be used. Since you'd have to connect them in parallel to a different cell at a time in order "power balanced" them there's no reason not to disconnect the whole circuit when not in use. This is different than the transistor-resistor setup that's permanently connected in parallel to each cell.

And even if dynamic balancing this way is only 50% efficient, you'd still theoretically get more range than without it.

But would it be worth it? I think we can all agree that it doesn't seem likely that the benefits would outweigh the costs at this time and age. Actually I'd prefer to try to add some 18650 cells in parallel to each degraded cell to bring them back up to 60Ah. Still, I'd love for someone to try. You never know what the actual outcome will be until someone tries. If we nay-said everything like everyone else does we wouldn't be driving EV's. ;)
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