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[bobmcree]

i modified the original schematic for the lv cutoff to show current limit resistors for the opto leds. i have not received an answer from microchip on the safety of operating without them, but it is true that the opto current could cause faster self-discharge for cells left uncharged for months.

when considering this, consider that the opto should trigger the ebrake line causing the throttle to be cut off, letting the cell voltage immediately rise well above the hysteresis point and turning off the opto. the circuit would then draw only a microamp until the no-load voltage drops to the trigger point, and i think if one lets a dead pack sit around for thousands of hours at that point that it is not a great idea no matter how the bms behaves. in practice i think that if a resistor is chosen to set 2 ma or so at the trigger voltage that it will not affect the self-discharge of a dead pack that much. when the voltage drops below the point where there is enough led voltage to bias it on, the leakage will be very small.

the typical rated output current of the TC45 is 7 ma and that is lots more than is really necessary to turn on the optos i specified, so the resistor will probably be a worthwhile investment, at a couple of cents extra per channel. it would certainly increase reliability, and that is always a good thing. i mentioned in a previous post how to calculate the resistor at different trigger voltages to produce the required led current.

the optos i mentioned can handle 70v on the output, so pulling them up to 15v on a typical controller ebrake line is not a problem but i would not pull the output up to the battery voltage if it is over 48v.

 

[GGoodrum]

Okay, so if I'm sticking with the 2.7V cutoff, I use resistors around 800 ohms, or close to to it, right? What is the value of the pullup resistor that is connected to the +5V signal? Is that +5V available in one of the wires in the ebrake cable?

Thanks -- Gary

 

[bobmcree]

Okay, so if I'm sticking with the 2.7V cutoff, I use resistors around 800 ohms, or close to to it, right? What is the value of the pullup resistor that is connected to the +5V signal? Is that +5V available in one of the wires in the ebrake cable?

Thanks -- Gary

the ebrake line is pulled up to 14.5v or so through a 10k resistor in the crystalytes i have, so if one assumes that a couple of milliamps will do it, and the optos will handle the voltage easily.

the opto i specified has a 500% current transfer ratio, so 1 ma of led current will cause the phototransistor to sink 5 ma. other optos may have a different ratio. you want a little extra margin over different conditions, so say it needs 5 ma output current.

when the tc54 triggers at 2.7v the output will drop to .5v and the led needs 1.4v across it to turn on, so that leaves .8v across the resistor at 1 ma or 800 ohms. that will mean that when the opto triggers the bms draw will only go up to 1 ma, and then when the load is removed it will go back off and the quiescent current is almost nothing.

limiting the led current like this will permit permanent attachment of the module to the cells without any concern that the bms will cause the pack to self-discharge more quickly or more deeply. this does have consequences, however;

once the voltage drops below 1.8v the led will not turn on even though the voltage trigger is asserted. this minimum operation level could be lowered easily if it is really a concern; the voltage sensor output of each cell could be pulled up to the cell above it, guaranteeing operation all the way down to 0.5v for the lower cell assuming the upper one stays above a couple of volts.
for example, you have cells of 3.3v each in series and you want the sensor to work all the way down to .5v you connect the tc54 across each cell, but you connect the current limit resistor to the 6.6v point on the cell above it in the string. of course you are stuck on the highest cell unless you want to add a voltage boost circuit, but all that stuff adds cost and complexity. (in this case you would need the resistor to drop 3-4x as much voltage so you would use 3k or so to get the same milliamp of led current.

of course all these numbers are based on the values from the respective data sheets. i am expecting parts today or monday and when several of us get the system built and have some experience i'm sure there can be refinements possible.

 

[GGoodrum]

First of all, I don't ever want the cells to be used below the cutoff, so I don't care if the circuit won't work if a cell gets down below 1.8V. the whole point of this is to keep that from ever happening.

So, are you saying the pullup resistor in the drawing is already part of the controller? In that case, just two wires go from this widget to the controller, the brake inhibit signal line, and the ground, is that right?

Sorry for all the questions but you are making an old software weenie's brain hurt...

-- Gary

 

[bobmcree]

First of all, I don't ever want the cells to be used below the cutoff, so I don't care if the circuit won't work if a cell gets down below 1.8V. the whole point of this is to keep that from ever happening.

So, are you saying the pullup resistor in the drawing is already part of the controller? In that case, just two wires go from this widget to the controller, the brake inhibit signal line, and the ground, is that right?

Sorry for all the questions but you are making an old software weenie's brain hurt...

-- Gary

like others have said before the only dumb questions are the ones to which you think you already have the answer. i am a much better hardware than software designer, unless it is assembly code where it is so much like hardware i am pretty good. the pic is like that in many ways.

yes, there is a built in pull-up resistor inside the controller on the ebrake line, which is the bottom right hand corner if the power leads are on the right on a crystalyte 35a unit. . the 3 pins are: a current limited 14v supply for systems using a hall sensor ebrake switch, ground, and the signal line which is pulled up to the 14v supply. i think this is common to all the systems.

without a pullup there will be no output signal from the open collector phototransistors in the optos, so for testing you could use a 10k resistor to the battery voltage if it is 48v or less, or pull up the 10k to the middle of the pack at a reasonable voltage.

the led in this opto can handle up to 50 ma, and the sensor can provide 7, so that part should be fail safe, and if more noise immunity is needed the led current and output current can just be increased by using lower value resistors.

 

[bobmcree]

the standby current is 1 MICROAMP! that is 1 million hours to discharge an amp hour.... that is OVER A HUNDRED YEARS!!!

OK, that's pretty low. I guess I shouldn't worry about that.

i was going back over this thread, and owe fechter an apology for "shouting" at him. the standby current of a microamp is so cool that i wanted to make sure everybody noticed it, but that was not really the question he asked. he was expressing the concern that the bms might cause dead packs to commit suicide more easily on the shelf, and it is a valid concern maybe for helicopter rc guys who use it and put it away for weeks or months.

i think for us ebikers that if the pack asserts the brake under full load to signal us, then lets us get a little further at lower throttle while it cuts on and off, AND assuming we have a drainbrain or wattsup or cycleanalyst to do the undervoltage and other monitoring, it is a good choice for a do-it-yourself bms.

a friend said "wow, you should patent that!" but of course there is really nothing patent-worthy here, just the sharing of ideas between friends. the schematics i published are not copyrighted, so feel free to chop them up to fit.

the technique of isolating different systems operating at different voltages with optic isolators is very basic, and the idea of using the signal to pull down the brake line was not mine anyway.

i saw a commercial bragging about all the new patents they had on their razor blades. gimmee a break! if you need 35 patents on a new razor blade design to keep people from stealing your intellectual property there is something wrong. could it be as simple as the fact that there are just too many lawyers?

anyway, i will try to refrain from shouting in caps when it might be taken the wrong way. it was not a dumb question, and as it turns out there is a solution.

 

[fechter]

No sweat Bob, I did not take it as shouting. 1 microamp is an incredibly low drain, much lower than any op-amps I've looked at for this application.

 

[rf]

i forgot to mention, that since the original request was for a cutoff at a lower voltage, the 2.1v version of the tc54 is also available at mouser for the same price. this is as close to the requested 2.2-2.3v target possible with a standard part, but could be adjusted upwards with a resistor pair.

i feel that is a bit low for good cell life, but my experience is limited. using the 2.7v parts it is not possible to go down. but with the 2.1v parts you can always go up. it does require a resistor pair that raises the quiescent current from 1 microamp to 100, but that is still quite small.

Why tip-toe around? A123 Systems *recommends* cutoff at 2-volts. They don't say that's the absolute minimum. Recommended. Below 0-degrees C., they recommend a 0.5-volt cutoff.

The R/C guys have been finding these cells to be very robust. Some are charging at 70amps and having good results.


Richard

 

[GGoodrum]

I agree, a123 cells are VERY robust, but they do have their limits, nonetheless. I have tortured many cells in high-powered RC helicopters for the last year, or so, and I have packs with hundreds of cycles on them that are still going strong. Most of these setups use anywhere from 10 to 20 a123 cells in series, mostly in a "1p" configuration. On the larger helicopters, that weigh about 12-15 pounds and have rotor discs around 1.5-1.8m in diameter, it is quite common to pull 4000-5000W peaks, whch translates to about 70-80A, which the a123 cells have no problem delivering. It wasn't until I started using a123 cells in ebike setups, however, that I started killing cells. The reason is over discharging.

Bob's handy LVC circuit is just what is needed to protect against over discharging, and I think 2.7V is the right number, not 2.1V. The reason is that in an ebike application, where we are using 4, or more, a123 cells in parallel, it takes quite a bit to get a voltage drop under load down to even 3.0V per cell unless the pack is at the end of the capacity. Even a 50A load is only 12.5A per cell for a 4p setup, and only 10A for a 5p configuration. An a123 cell will supply 10A continuous quite easy for the duration of the capacity at a voltage well above 3V per cell. On my 18s5p setup the voltage under max 40A/2400W peaks stays right at 55-56V for almost the full duration. That is about 3.1V per cell. At the end, however, the voltage drops very quick, and can go under 2.5V per cell in the blink of an eye. Using 2.7V as a cutoff, I can still get almost the complete capacity of the pack (2.2-2.3Ah per cell...), but catch it before the cells are close to being damaged. If the cutout first occurs when under a full load, which is usually the case for me (i.e. -- going up a hill, for instance...), if you back off the throttle, the voltage will bounce back to something above 3.0V per cell and it still gives you a bit of "get home" reserve if you ease off the throttle.

I had a couple of incidents that killed some cells in a 16s2p pack. A couple were completely dead-shorted. A couple more had "resting" voltages down to about 1.8V. The rest were at, or above 2.6V per cell. All these recovered fine and are still working to full capacity. After being off the charger for a half hour, or so, they will all have voltages in the 3.50-3.55 range. The ones that got down below 2.0V have since been recovered, but now the best they will do after a bit off the charger is about 3.39-3.42V, and their capacity is reduced now to about 1.8-1.9Ah, or a 20% reduction.

-- Gary

 

[rf]

...

Bob's handy LVC circuit is just what is needed to protect against over discharging, and I think 2.7V is the right number, not 2.1V. ...


-- Gary

You had a couple incidents where you damaged cells due to less-than-optimal shut-off procedures. (Been there, done that.)

Now that a reliable shut-off is available you're not willing to fully trust it?

When does damage occur? When cell voltage drops well below 1-volt. Or, according to A123 Systems, when a cell is stored below 2-volts for an extended period. (If you're going to store them you're supposed to put a partial charge in them anyway.) Various testers have repeatedly discharged to 0-volts with no loss of capacity. Two volts seems amply far away from reversal.

The A123 cells have a limited operating range. Crowding it more, for the sake of being extra-double-safe doesn't necessarily seem appropriate.

Dewalt set their shut-off at 2.5 volts. But their balancer is poorly designed, so even 2.5 volts doesn't help.

You may be right that 2.7 is a better number. But I don't see the reasoning yet. Reports are that these cells tend to get better with age. My gut tells me that taking care to discharge fully may pay dividends. Perhaps that's my experience with unrelated chemistries talking. Just a nagging feeling ...


Richard

 

[bobmcree]

i received a response from microchip that the tc54/tc111 is not internally current limited on the output, so it is necessary for reliabile operation to use a current limiting resistor on the led in the opto to keep the output current down below 7 ma. i think for the 2.7v device i would use 470 to 820 ohm resistors. the resistor does not need to drop the led voltage when the input is above 2.7v, so you base the calculation on operation from the trigger point down. the current transfer of the opto must be considered if not using the ones i suggested.

 

[GGoodrum]

You had a couple incidents where you damaged cells due to less-than-optimal shut-off procedures. (Been there, done that.)

Actually, it was worse than "less than optimal..". I forgot to hook up two of the four sub-packs I was using so I ended up with half the capacity that I thought I had.

Now that a reliable shut-off is available you're not willing to fully trust it?

Yes, but after lots of experience using these cells I've noticed a few things. First of all, I've never seen a working cell, just all of a sudden stop working, unless it was over-discharged, so I'm not too worried about a cell, or block of cells being way out of whack with the others. These cells are amazingly robust. You can't kill them by trying to pull too much current out of them, even in 1p configurations, but in our multi-parallel configurations there's just no way that the motors and controllers we're using can pull too much current out of these, even if there was some sort of dead short failure you'll fry the wires before you will kill the cells. All the protction in that case that is needed is a big fuse.

Another unique trait I've seen with a123 cells, both in RC use, and with my ebike setups, is that the power is very consistent, all the way to the end of the capacity. They have very flat discharge "curves". What this means is, if you have a well-balanced pack all the cells will "dump" at pretty close to the same time. What happens is that you get no notice that the pack is done. The power feels just as strong ten seconds before the end of the capacity as it does at the beginning and then the power drops like somebody flipped a switch. With other batteries/chemistries, the discharge curves aren't nearly as flat, so you can almost always detect some drop in performance at the end of the capacity. It is quite disconcerning to literally not be able to tell the difference in power and then have the lights turned off. When I killed the cells by only having half my sub-packs connected, it had just as much power. I couldn't tell that I only had half connected. When the end came, I was halfway up a hill and then it was like a plug came undone. I thought maybe I blew a controller, or something. Had I just stopped right there, I might have saved the cells, but I kept going a bit, trying to figure out what happened.

Anyway, when the capacity is reached, the voltage per cell is going to drop pretty quick, so I'm not sure it matters too much whether the cutoff is 2.7V or 2.5V or whatever. The point is that the LVC function needs to detect end-of-capacity and then remove the load. This should trip the first time when under load at the start of the end-of-capacity dropoff. If you back off the throttle, the voltage will recover above the 2.7V level and it might give you a bit more "get home" capability.

-- Gary

 

[bobmcree]

the 2.1v and 2.7v just happen to be the setpoints of these voltage detector chips which are meant for laptop power reset circuits and just turn out to be a great solution for this need for a simple low voltage cutoff. i did not choose them arbitrarily, but because they are available off the shelf.

as was mentioned, 2.7v is quite low on the discharge curve for a123 cells in multiple parallel setups on an ebike drawing under 50A, but on the other hand 2.1v is the recommended cutoff for the lifebatt cells, and would probably be closer to the optimum choice for small cell count a123 systems.

i mentioned that you can add a resistive voltage divider to the tc54 to shift the trigger point upwards, so certainly for prototyping that might be a good option. you cannot shift it downward except by using a different part.

my own observation so far is that i would probably only cut off 10% of the useful capacity by using the 2.7v cutoff with the cells i have, and i have the parts, so i will put that together first. it would be simple to add channels to provide a warning at the 90% discharge voltage when the first cell hits 2.7v and then cut off at 2.1v. that would give me a couple of miles "reserve tank" function, and wiring the optos to the ebrake line should provide for linear feedback if the gains are adjusted properly, so that letting off on the throttle when the pack kicks out would let one use some remaining capacity, up to the time constant of the throttle control loop and until one of the pack cells drops well below the cutoff at low throttle.

for the prototype stage it is not a big issue that using a resistor pair must by necessity increase the standby current from 1 microamp to 100 to maintain accuracy, but that is still very very small. the parts are laser trimmed by microchip to custom voltages if an intermediate value were required. i would recommend anyone doing a pcb leave room for the resistors then they can be used or left off as the spec changes and custom parts are available.

i don't think you can just put a diode in series with the tc54 to shift the voltage up; as someone asked me. the device uses so little power the leakage current of the diode would probably operate it, and in any case it would not be reliable. you just need a resistive divider that provides enough current to operate the device over temperature and spec, as is shown in the data sheet.

 

[rf]

Anyway, when the capacity is reached, the voltage per cell is going to drop pretty quick, so I'm not sure it matters too much whether the cutoff is 2.7V or 2.5V or whatever. The point is that the LVC function needs to detect end-of-capacity and then remove the load. This should trip the first time when under load at the start of the end-of-capacity dropoff. If you back off the throttle, the voltage will recover above the 2.7V level and it might give you a bit more "get home" capability.

-- Gary

Thanks for the response, Gary.

I see your point. It is nice to have reserve power. And setting your cutoff a bit high like that might provide it. I've been thinking a flashing display on the CycleAnalyst would be nice to alert when you've hit some preset percentage of discharge.

If your intent is to provide a reserve mechanism then it's probably better to state that and review the design to see if it will do that well enough.

--

Since this thread is supposed to be about schematics I'd like to ask if anyone has a good balancer or charger circuit suitable for A123 packs?

Several folks have suggest that SLA chargers come very close to A123 requirements (besides being cheap and widely available.) So a balancer circuit for use with SLA chargers sounds like a winner.


Thanks guys. Great forum!

Richard

 

[bobmcree]

Since this thread is supposed to be about schematics I'd like to ask if anyone has a good balancer or charger circuit suitable for A123 packs?

Several folks have suggest that SLA chargers come very close to A123 requirements (besides being cheap and widely available.) So a balancer circuit for use with SLA chargers sounds like a winner.
Richard

i think the idea that a cheap sla charger can work with a123 cells is a commentary on how tolerant the cells are of abuse, but does not tell us how the cell life will be affected by using a specific charger. it is true that the cheap charger will run out of voltage before it severely overcharges the a123 pack if it is close to the required power/voltage rating.

most balancing systems simply cut the charge current when the first cell hits the maximum permissible voltage, much more sophisticated systems bypass charged cells and route a constant current through the rest until all are fully charged.

as it becomes realistic to expect thousands of cycles out of the cells, the battery management system deserves even closer attention. if the life cycle could be extended a year or two by using 5% less of the capacity, that might be a value that could be added with improvements to the bms. of course in a $1000 battery it seems like a no-brainer to many of us that a micro built into the bms would store the operation parameters for warranty and diagnostic purposes. a security system could even be built into the battery to a degree against theft, but of course thieves will hack out the cells or find a way to beat any security system we come up with that is practical.

the tc54 i suggested for the lvc could also be used for the hvc and switch the charger into balance mode. at that point it can either be used to individually charge each cell to the cutoff or just charge the whole string at a lower current until all cells hit the cutoff, a much more common approach.

with 3 fets per cell you use a constant current charge through the whole string then switch each cell in or out of the string as it hits the target voltage. this requires an expensive high current low rds-on fet in between each pair of cells in the string during discharge, and the fets to bypass the charge current. it also requires a constant current charger that can operate over the voltage range required to charge the string. this is the best way to do it but it is seldom seen due to the cost and complexity it adds.

there are probably modules for the rc market with 3 fets per cell, but maybe somebody who knows about them can let us know how well they work. i know there are some bms modules for thunder sky 120Ah cells that might be modified to work but i bet using the tc54/opto solution will be cheaper and more reliable than any op amp circuits they are using.

if you put a voltage sensor on each cell and cut back the overall charge current to balance level until they all hit the cutoff it would cost about a buck per cell for parts in small quantity, plus the power supply and any mods it requires. i am charging my packs with some $1000 telcom supplies i bought new for $50 each on the surplus market. lots of these telcom supplies have margining inputs that can be used for feedback and they can be controlled with digital or analog signals.
what i have done for a couple of people in the past is to build controllers for telcom 48v supplies to use them as battery chargers. these have sophisticated control inputs and are sometimes available very cheap. i think you really need good voltage regulation to avoid overcharging our expensive batteries. we would hate to learn 2 years from now that by using 5% less of their capacity they would have lasted 10 years instead of 5.

saving a few bucks on the charger is great if it is done wisely. with lifepo4 it seems like a dc regulated supply is all that is really necessary, but a balancing charger could certainly not hurt, and of course i think a thermal sensor is important just as a fail safe if everythng else fails in a way it never did before. if somebody wraps up the pack and charger in a blanket on their feet because they are cold and then keeps switching it on and off to keep their tootsies warm...

 

[GGoodrum]

I have had very good luck using a number of the better SLA chargers with a123-based packs. The trick is to have packs in multiples of 4s. Most SLA chargers have the same sort of constant current/constant voltage charging profiles that all of the RC LiPo/a123 chargers use. The difference is that the SLA chargers have a fixed cutoff voltage and the RC chargers have a setting that controls the cuttoff based on the number of cells in series. With a 48V SLA charger, for example, the battery is charged at the max current rating until the voltage reaches about 58V. Then it switches to the CV mode were the 58V voltage is held while the current gradually reduces down to a point that is usually about 10% of the max rate. The optimum CC/CV cutoff voltage for a123 cells is 3.65V, so a 16s pack needs 58.4V. A 36V SLA CC/CV charger has a cutoff around 44V and a 12s a123 pack needs 43.8V. You get the idea.

Almost all RC balancers are very simple. They monitor the cell voltage and if each cell is above about 3.0V, it will start balancing the pack by putting a 150ma load on all the cells that are higher than the lowest one. When all the cells are within about .002V of each other, the process stops. This is exactly how the Thunder Power TP-210V 10-cell balancer works, as does the also popular 6-cell AstroFlight "Blinky". When connected to the TP-1010C 10-cell charger via an included data cable, the TP-210V will allow the TP-1010C access to each cell so that the charger can monitor and display the voltages for each individual cell during the charge process. The balancing continues, independent of what the charger is doing. If the charger detects that the cells are out of balance by more than .2V, it will automatically go into a balance charge mode where the charge current is reduced to 300mA.

The trend these days is to have integrated chargers and balancers. Some, like the Hyperion units, link chargers and balancers together, like the TP units. Some now are doing independent charging of each cell, which is kinda what Bob is describing above, I think. The Balance Pro HD, fro FMA Direct, is one of those, and will charge up to a 6s pack at a 10A rate.

For my own use, what I'm doing is configuring 10s4p a123 packs that have 10-cell balancing plugs. I'm doing a 10-cell version of the LVC circuit (using the 2.7V chips...). I will run two of these packs in a 20s4p configuration, and I will use two of these 10-cell LVC boards, one for each pack, which will plug into the balancing connectors on the packs. After a ride I will unplug the LVC boards and use two TP-1010C/TP-210V combos to balance and charge the packs.

-- Gary

 

[bobmcree]

Almost all RC balancers are very simple. They monitor the cell voltage and if each cell is above about 3.0V, it will start balancing the pack by putting a 150ma load on all the cells that are higher than the lowest one. When all the cells are within about .002V of each other, the process stops. This is exactly how the Thunder Power TP-210V 10-cell balancer works, as does the also popular 6-cell AstroFlight "Blinky". When connected to the TP-1010C 10-cell charger via an included data cable, the TP-210V will allow the TP-1010C access to each cell so that the charger can monitor and display the voltages for each individual cell during the charge process. The balancing continues, independent of what the charger is doing. If the charger detects that the cells are out of balance by more than .2V, it will automatically go into a balance charge mode where the charge current is reduced to 300mA.

thanks gary, i have not kept up with rc charger design. i can see how the balancing system you describe is fairly easy to implement, and would not be too expensive, but it does not scale up very well to charge a system at 20A or more, where the balance current needs to be pretty high to avoid stretching out the charge cycle. such a system would need to provide for quite a bit of power dissipation in the bms and would add weight and cost.

The trend these days is to have integrated chargers and balancers. Some, like the Hyperion units, link chargers and balancers together, like the TP units. Some now are doing independent charging of each cell, which is kinda what Bob is describing above, I think. The Balance Pro HD, fro FMA Direct, is one of those, and will charge up to a 6s pack at a 10A rate.

i agree that the integrated approach makes the most sense now that simple switching power supplies and low cost controllers have made it cost effective and the cost and lifetime of the cells justifies significant investment in a bms that protects the investment. packs have a realistic life expectancy of several years under heavy use, so the bms must be up to the task.

i am also planning on using the 2.7v devices on the a123 packs i will have on my bike which will be used at <15A per string peak.

i will use the 2.1v devices on the bms for the cruzbike, but we will also have a CycleAnalyst with the programmable low voltage cutoff.

the factory for LifeBatt tells me there will be a 4.0v cutoff in the bms on the pack we will be getting for the cruzbike next week. i'll let you all know how well the system works out.

 

[rf]

Lots of good information and ideas.

I bought a cheap microcontroller development kit recently. Been wondering if I should use it to build a discharge cutoff or a charging balancer. It's beginning to sound like a good idea to use it for both -- a full-on BMS. All kinds of possibilities, and relatively cheap.

The discharge cutoff is simple. The charge balancer needs a good all-around algorithm and cheap way of controlling the bypass, or whatever, to each cell. Need a controller with lots of I/O lines.

Envisioning a 72-volt SLA charger -- or perhaps two 36s would be cheaper and closer to the right current level. A pack with a multiple of 24-cells for an SLA version of 72-volts. All kinds of possibilities for the other parts. Leaning towards multi-channel A/Ds, since they're cheap, and let you adjust levels to your whim. Could even add an LCD display and show cell voltages, but that's getting carried away.

Lots to think about. Guess I'll start with a 24-cell monitor and turn it into an LVC. Need to figure out where to put the flashing lights ...

Richard

 

[rf]

It just occured to me, if I use some sort of `switch matrix' chip I might be able to get away with one A/D and fewer I/O lines on the microcontroller. Assuming it's all reasonably fast.

Richard

 

[fechter]

An analog switch MUX chip would be one way to do that. I don't know if you can find one with a high enough voltage input range. One way around that is to put a divider on all the inputs to bring it down.

 

[GGoodrum]

One of the unique characteristics of a123 cells, over other LiFe and Li-Ion chemistries is that they can be charged at very high rates. I have personally charged single-p packs at 20A rates and they barely get warm. What we really need is an intelligent charger/balancer design that can individually charge a block of cells at a high rate, say at least 20A. You'd be able to do a 4p pack in about a half an hour.

-- Gary

 

[rf]

One of the unique characteristics of a123 cells, over other LiFe and Li-Ion chemistries is that they can be charged at very high rates. I have personally charged single-p packs at 20A rates and they barely get warm. What we really need is an intelligent charger/balancer design that can individually charge a block of cells at a high rate, say at least 20A. You'd be able to do a 4p pack in about a half an hour.

-- Gary

Half hour or less charge sounds great! Probably want a `slow mode' switch for using a `borrowed' outlet without blowing a fuse ...


Richard

 

[rf]

Here's an interesting item ... and LVC for R/C, 2s to 7s A123s, daisy-chainable.

http://www.slkelectronics.com/MM7/index.htm

Wonder what those chips are. (More photos on the `instructions' link near the bottom of the page.)

They also sell the LipoDapter, a simple voltage cutoff for charging with a dumb power supply.

 

[GGoodrum]

What this does is to cut the throttle input to the RC receiver. The way RC radio signals work is through a PPM pulse train, from 1ms to 2ms long. These pulses repeat every 20 ms, I think. A 0 throttle condition is represented by a pulse 1ms long, 50% throttle is 1.5ms long and full throttle is 2ms long. You would need some more logic in order to use it in an EV application.

-- Gary

 

[fechter]

That BMS looks pretty good. It should be easy enough to find the optical isolator and rewire the output so it can activate the the brake switch input of a controller.