Mixing chemistries for most economic performance and range

We all know that Ping is one of the cheapest long lifetime LiFePO4 packs out there(Calendar life is still being tested by the community, lol.). However, it lacks the ability to supply great current. A123s are great at supplying the high amps when you need it most(going up hills, accelerating from a start, etc.) but are fairly expensive for any reasonably long-distant range. However, the motor is usually consuming a lot less current when it's reached "top speed", so the a123s are being used for a lot less than their potential for a great majority of the time. So why not get the best of both worlds and use a lot of Ping and a little of the a123s(or other high-C alternatives like 20-30C liPos?)?

As some of you might know, I'm working on an electric scooter which I've developed some electronics for. I think I have the necessary electronics experience and materials to create the necessary "current splicer" circuit, haha. So, what do you think of the idea and does anyone have ideas on how the best of both worlds can be combined at the technical level?

I think the ramifications of this basic idea would be most drastic for motorcycles. Instead of going with a huge ton of a123s to get any decent range, why not use a lot of the low C-rate lifepo4 and use a little of the premium content when the current needs are high? It seems like it would drastically cut down construction costs but yet have nearly the same range of performance(Assuming decent range is desired from the beginning).

Word.

The motor may be using less current, but that doesn't mean less current is being drawn from the battery pack, due to the controller. So ensure this is the case before you go ahead.

This is something I've thought about too. Using 4S A123's in Parallel with each of my 12v AGMs. During acceleration the A123's would help with the 200 amp burst current. Once speed was maintained the A123's would charge thru the agm's. My cut-off is 10.5 volt for my AGMs, well above the 8.4 for the 4S A123. I recently bought 4 A123's to experiment with and a CBA II is on order. Going to cycle them with a 12 10 AH SLA in P an see how they do.

Tiberius (nick) on here and the pedelecs board, has done this using NiMH cells to provide the high current peak supply to "fix" a problem with a LiIon battery...

see:
http://www.pedelecs.co.uk/forum/electric-bicycles/1261-torq-battery-mod-storms-up-hills.html

I'm looking at doing a similar thing to potentially extend the life of my main LiIon

I don't think you can do that.
If you connect the A123s in parallel to Ping LiFePo then what will happen is this:
Since the A123s capacity is only 2300mAh, and Pings pack is 20000mAh, then during the burst the A123s will supply most of the current until they are depleted, and their voltage drops beneath the LVC.
During the A123 discharge the current from the Ping pack split 2 ways: 1) To the load (motor) 2) To the A123 pack to try to replenish them.
The A123 will try to drop the voltage because it it is loosing charge, but the Ping pack still has most of it's charge thus trying to keep the voltage high. The Ping pack cannot supply the current demand required to charge the A123: (Vping - Va123)/(Rint_ping+Rint_a123), so the Ping BMS will engage the cutoff, and not charge the A123 at all, thus you will have an empty A123 pack and a full Ping that is cutoff.
This is why you cannot connect two packs in parallel that have different capacities. The pack with the higher capacity will eventually start charging the lower capacity pack because the lower capacity's voltage will drop before the higher one's. And in addition to charging the lower pack it will keep on supplying the current to the load.

Dave-s said:

I don't think you can do that.
If you connect the A123s in parallel to Ping LiFePo then what will happen is this:
Since the A123s capacity is only 2300mAh, and Pings pack is 20000mAh, then during the burst the A123s will supply most of the current until they are depleted, and their voltage drops beneath the LVC.
During the A123 discharge the current from the Ping pack split 2 ways: 1) To the load (motor) 2) To the A123 pack to try to replenish them.
The A123 will try to drop the voltage because it it is loosing charge, but the Ping pack still has most of it's charge thus trying to keep the voltage high. The Ping pack cannot supply the current demand required to charge the A123: (Vping - Va123)/(Rint_ping+Rint_a123), so the Ping BMS will engage the cutoff, and not charge the A123 at all, thus you will have an empty A123 pack and a full Ping that is cutoff.
This is why you cannot connect two packs in parallel that have different capacities. The pack with the higher capacity will eventually start charging the lower capacity pack because the lower capacity's voltage will drop before the higher one's. And in addition to charging the lower pack it will keep on supplying the current to the load.

Click to expand...

That's EXACTLY why you can't directly parallel them! But... what about intelligently designed electronics that allow them to be intermixed as needed(And nearly everything being controlled as needed and so on)? That's what I'm pursuing. So any ideas in this direction?

swbluto said:

That's EXACTLY why you can't directly parallel them! But... what about intelligently designed electronics that allow them to be intermixed as needed(And nearly everything being controlled as needed and so on)? That's what I'm pursuing. So any ideas in this direction?

Click to expand...

Yes indeed...
1) Nice meaty Schottky diodes used to isolate the packs from each other. This takes care of cross currents.
2) Make up the high current pack so it has less voltage than the main pack. Then when the voltage sags on the main pack due to a high load the high current pack supplys voltage through its diode to prevent further sag.

torrent99 said:

swbluto said:

That's EXACTLY why you can't directly parallel them! But... what about intelligently designed electronics that allow them to be intermixed as needed(And nearly everything being controlled as needed and so on)? That's what I'm pursuing. So any ideas in this direction?

Click to expand...

2) Make up the high current pack so it has less voltage than the main pack. Then when the voltage sags on the main pack due to a high load the high current pack supplys voltage through its diode to prevent further sag.

Click to expand...

GENIUS!

That'd seem so simple and effective. Now just need to design the charging circuitry between the long-range batteries and performance batteries so the performance batteries can be replenished during those times where it's possible and needed(I.e., you're now cruising at top speed after topping the hill, and the performance batteries are running low. Need to recharge for the next hill!).

Dave-s said:

I don't think you can do that.
If you connect the A123s in parallel to Ping LiFePo then what will happen is this:
Since the A123s capacity is only 2300mAh, and Pings pack is 20000mAh, then during the burst the A123s will supply most of the current until they are depleted, and their voltage drops beneath the LVC.

Click to expand...

One A123-cell will not have a significant lower internal resistance to all those paralleled cells in a ping pack.

Lets say the A123-cell got 10 mohm, and a parallelled row of ping-cells got 20 mohm. If the current cut off for the ping pack is 40 A then the whole pack can supply 120A.

So, the bike is accelerating at 100 Amps for a while. When it reaches a constant speed it is consuming 10A.

If ping is supplying 10A to the motor, 30A can be used to charge the A123 cells. The voltage difference can be 30 * (10+20) /1000 V = 0.9 V. I don't think it's reasonable to get a 0.9 volt difference between two 3.2 V cellgroups even from a very long acceleration or hill. The constant speed phases have to be long enough to balance the cellsgroups between accelerations, though.

I read that Doctorbass put cells with low capacity and low internal resistance in parallel with cells with high capacity and high internal resistance in order to get groups with the same total capacity and internal resistanse. That approach is equivalent to this. Doctorbass doesn't seems to have problems with his pack. I read it didn't need to be balanced very often.

A123 cells have 2300mAh capacity. Discharging the cell @ 60A will deplete the cell completely after a little over 2 min.
Climbing hills can sometimes take over 2 min.
So the voltage difference between the Ping and the A123 will be well over 0.9v per cell.

That is just a matter of putting in enough A123-cells to be able to supply high currents for as long as needed, in my opinion.

Dave-s said:

A123 cells have 2300mAh capacity. Discharging the cell @ 60A will deplete the cell completely after a little over 2 min.
Climbing hills can sometimes take over 2 min.
So the voltage difference between the Ping and the A123 will be well over 0.9v per cell.

Click to expand...

It seems natural you'd have discharging control circuitry from preventing the a123s from over-discharging. In the case you went over two minutes at the whopping 60 Amps + 20 or so amps from your ping battery(Got a large steep hill there? lol), the discharge from the a123s would be cut off and you'd be left with the 20 or so amps from your ping battery(The worst case scenario is the scenario you'd be left with if you didn't add on the a123s, you cheap bahstard. :wink: Jk, Jk). Once the motor starts consuming less current than the ping's ampacity, the a123s are set into recharge mode by the charge control circuitry. If you want a longer "boost", it's as simple as bearing states, use more a123s(And get greater peak discharge currents in the process, yeah yuh!).

16 a123-batteries (to make 48V) contains 420 kJ. If all that energy is used to climb a hill it will be a very high hill.

E=m*g*h -> h=E/(m*g)

Assuming m=105 kg and g=10 N/kg makes a 400 meter high hill.
h = 420000 / (105 * 10) m = 400 m.

Efficiency is of course not 100%. 50% efficiency makes a 200m high hill - still a high one.

swbluto said:

As some of you might know, I'm working on an electric scooter which I've developed some electronics for. I think I have the necessary electronics experience and materials to create the necessary "current splicer" circuit, haha. So, what do you think of the idea and does anyone have ideas on how the best of both worlds can be combined at the technical level?

I think the ramifications of this basic idea would be most drastic for motorcycles. Instead of going with a huge ton of a123s to get any decent range, why not use a lot of the low C-rate lifepo4 and use a little of the premium content when the current needs are high? It seems like it would drastically cut down construction costs but yet have nearly the same range of performance(Assuming decent range is desired from the beginning).

Click to expand...

If you haven't already, you might want to take a look at this thread. It gets most informative at the bottom of the first page.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=6285

Aerowhatt

swbluto said:

Dave-s said:

A123 cells have 2300mAh capacity. Discharging the cell @ 60A will deplete the cell completely after a little over 2 min.
Climbing hills can sometimes take over 2 min.
So the voltage difference between the Ping and the A123 will be well over 0.9v per cell.

Click to expand...

It seems natural you'd have discharging control circuitry from preventing the a123s from over-discharging. In the case you went over two minutes at the whopping 60 Amps + 20 or so amps from your ping battery(Got a large steep hill there? lol), the discharge from the a123s would be cut off and you'd be left with the 20 or so amps from your ping battery(The worst case scenario is the scenario you'd be left with if you didn't add on the a123s, you cheap bahstard. :wink: Jk, Jk). Once the motor starts consuming less current than the ping's ampacity, the a123s are set into recharge mode by the charge control circuitry. If you want a longer "boost", it's as simple as bearing states, use more a123s(And get greater peak discharge currents in the process, yeah yuh!).

Click to expand...

Errm, you're not thinking of using the Pings to charge the A123s on the go are you? Won't you lose a good percentage of energy doing that? (These things can't be 100% efficient!)

If you want to do the recharging thing, have you thought of using some HUGE capacitors as your peak current stores? They'll charge much faster than any battery and I suspect will be much more efficient at charging too. (I'm sure this has been covered in a previous thread though....)

You don't need a charging circuitry between the packs, just connect the cells in parallel and they will charge whoever is lower automagicly.

The post was "Mixing chems" which isn't the same as micing pings & A123's.

I did a test this afternoon with a 12v 9ah agm and 4S A123. Charged them from the same sla smart charger then put them in parallel. Had a separate current meter on the A123's an a watts up into the inverter.

I used an inverter and 2 lamps (75 watt & 100 watt). Couple observations....

1). With the 6 amp load (75w) the sla supply 5 amps, a123's 1

2). When switching in both loads the slas supplyed 12 amps, A123's 2.5

3). Turning off the inverter the A123's recharged the SLA's

4). I then purposely used just the A123's to lower their voltage, re-paralled at 100 w load. For about 30 secs the sla was recharging the A123's at 2.5 amps and then droped to zero.

Looks like it is working as expected, they are balancing each others current based on which chem has the higher voltage. I don't think you need to worry about overdischarging the A123's as long as your SLA's are cut off at 10v per cell (by controller). This would leave your A123's at a very safe 2.5 per cell.

I then charged them in parallel with the sla charger, the A123's started off wanting more current but after about 20 minutes they balanced out.... 1 hour later th sla was using more until float was reached.

So at a first look, I think mixing chems could really help with the high current periods. Going to run a few more tests.

That is interesting, pgt400. At which current do you think the A123-batteries will start to supply the most current? Will it ever? What's the AGMs internal resistance?

One pgt400 is worth a thousand us's.

Looks like the 4 A123s have 5 times the internal resistance as 1 AGM.
Unless you somehow limit the current from the AGM, or connect some A123 cells in parallel it looks like the AGM will always supply 5 times the current the A123 chain supplies.

pgt400 said:

Looks like it is working as expected, they are balancing each others current based on which chem has the higher voltage. I don't think you need to worry about overdischarging the A123's as long as your SLA's are cut off at 10v per cell (by controller). This would leave your A123's at a very safe 2.5 per cell.

I then charged them in parallel with the sla charger, the A123's started off wanting more current but after about 20 minutes they balanced out.... 1 hour later th sla was using more until float was reached.

So at a first look, I think mixing chems could really help with the high current periods. Going to run a few more tests.

Click to expand...

I've got thousands of miles of experience (on 4 bikes, three riders) running AGM's and LiFePO4 in parallel with each other. It saved me about $2,200.00 over an all LiFePO4 pack that had enough power density to run the motorcycle on it's own. No fancy interface though. Just powercheq balancers on the AGM's and stock Ping BMS boards on the LiFePO4. All charge and discharge together without issues. Another nice outcome is that eventhough the AGM's do the heavy lifting for acceleration and hill climbing, they generally are still above 50% DOD when 3/4th's of the max range is used up! They should last a long time with that usage and already have more miles on them than previous, all lead packs, have lasted on the same platform.

Aerowhatt

bearing said:

You don't need a charging circuitry between the packs, just connect the cells in parallel and they will charge whoever is lower automagicly.

Click to expand...

You want to limit the charging current to prevent over-charging current of the a123s and also the excess current supplied by the pings. This is partially what the circuitry is for.

And, the ping's and a123s are definitely of different chemistries. Same foundations, but they definitely have different chemical make-ups. Once you mastered mixing those, then mixing other chemistries would be a cake-walk like extension.

torrent99 said:

swbluto said:

Dave-s said:

A123 cells have 2300mAh capacity. Discharging the cell @ 60A will deplete the cell completely after a little over 2 min.
Climbing hills can sometimes take over 2 min.
So the voltage difference between the Ping and the A123 will be well over 0.9v per cell.

Click to expand...

It seems natural you'd have discharging control circuitry from preventing the a123s from over-discharging. In the case you went over two minutes at the whopping 60 Amps + 20 or so amps from your ping battery(Got a large steep hill there? lol), the discharge from the a123s would be cut off and you'd be left with the 20 or so amps from your ping battery(The worst case scenario is the scenario you'd be left with if you didn't add on the a123s, you cheap bahstard. :wink: Jk, Jk). Once the motor starts consuming less current than the ping's ampacity, the a123s are set into recharge mode by the charge control circuitry. If you want a longer "boost", it's as simple as bearing states, use more a123s(And get greater peak discharge currents in the process, yeah yuh!).

Click to expand...

Errm, you're not thinking of using the Pings to charge the A123s on the go are you? Won't you lose a good percentage of energy doing that? (These things can't be 100% efficient!)

If you want to do the recharging thing, have you thought of using some HUGE capacitors as your peak current stores? They'll charge much faster than any battery and I suspect will be much more efficient at charging too. (I'm sure this has been covered in a previous thread though....)

Click to expand...

Have you checked the cost of capacitors lately? Sure, you may be able to economically purchase stop-and-go capacitors(Although, I have my doubts about the economics of that in comparison to $150 of a123s), but anything with a high enough capacitance for a noticeable hill? hahahahaha.... it gets expensive. Really expensive. The point of this endeavor is for the "most economic" combination for getting satisfactory range and satisfactory acceleration and hill-climbing speed, not necessarily maximizing all the parts' efficiency. Some of the current and upcoming RC motors will allow relatively high hill-climbing speeds(More than 30 mph on a 7% up-hill, SRSLY.) given the current is great enough and REALLY great acceleration(I put a small RC motor on my scooter and trust me, it accelerates very well in comparison to standard hub motors.), and some of the conventional means of obtaining that isn't economically optimal if you also want any serious range.

swbluto said:

And, the ping's and a123s are definitely of different chemistries. Same foundations, but they definitely have different chemical make-ups. Once you mastered mixing those, then mixing others chemistries would be a cake-walk like extension.

Click to expand...

Hm, I'm not sure about that. Both are LiFePO4. What's your source?
Their discharge curves are almost identical.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=5362&p=102163&hilit=Ping+2.0+Reports+discharge+curve#p102163
http://www.zeva.com.au/tech/LiFePO4/images/A123-discharge-1.0ohm.jpg

swbluto said:

You want to limit the charging current to prevent over-charging current of the a123s and also the excess current supplied by the pings. This is partially what the circuitry is for.

Click to expand...

Since they both have a voltage of 3 - 3.2 V over 90% of their capacity range I don't think a charge current limiter is necessary. 0.2V / 0.030 ohm = 7A which is okay with the A123:s.