New 16-cell Battery Management System (BMS)

[bobmcree]

Bob,

Another question.

Some SLA chargers, for some reason, need to detect a certain minimum amount of voltage at their output before they turn on. This is often discovered when attempting to charge a pack that is completely dead. How will this affect the usage of such a charger with your BMS? Any work-around?

when first connected, the fet in the negative charger lead will be full on, so the charger should start if the pack would start without the bms.
when the bms is powered on, if any cell is below 2.1v the charger will start with the current reduced to the value used during shunt activation. The cell needs to be up above about a volt before it can be detected as low voltage, so the system is not perfect.

the best workaround is to find the weak cells and bring them up with a current limited supply, like the kit i am providing that runs from 1-25v DC out @ 1.5 A cont.

Bob,

Another question.

Some SLA chargers, for some reason, need to detect a certain minimum amount of voltage at their output before they turn on. This is often discovered when attempting to charge a pack that is completely dead. How will this affect the usage of such a charger with your BMS? Any work-around?

 

[fechter]

Sorry to hear about your problems with LifeBatt. Some people are not worth working with. I've had problems in the past working with business types. They are a different mindset than technical types. Different priorities. I usually just want to do what's right. They want to make money. Guess that's why I'm nearly broke all the time.

 

[kyakdiver]

I resemble that remark Fetcher....

 

[bobmcree]

I resemble that remark Fetcher....

me too, unfortunately. i have not made a penny on lifebatt cells, and i did a lot of promotion for them. all i wanted was good batteries for my friends and fellow ebikers, and i would have been content to go on for a few more months with no profits with the promise of lower prices on the horizon, but i will not tolerate being attacked with a bunch of lies on a public forum. i do appreciate the way my friends have stood up in my defense.

the latest rev of the boards for the bms will be here wed. so we should be able to start shipping kits friday or so. these boards will have the adaptive current limiting that cuts the charge current based on heat sink temperature, and only one status led. the 16 leds were cool, but they took a lot of real estate. if enough people want them i might put them back on the surface mount version.

 

[chas_stevenson]

Bob,

The function of the 16 LEDs were to be ON when each cell was in a charge condition and OFF when the cell was fully charged as I understand it. This sounds like a very good way to find bad or questionable cells. I for one like this feature and would like it to remain. I am sure the LEDs can be wired on the board without holes for them or their resistor. I could add them to board myself if I knew where to connect them. Maybe you could add them as an option in the board diagram.

My Vote,
Chas S.

 

[Ypedal]

I 2nd the motion for the cell level LED's.

If this compromises reliability then , hell.. no worries.

But if it's only a cost factor, i'll willingly pay for it personally.

Having the LED's light up as the cells get full, gives you an indication of pack health and ballance over it's life. If cell X is always the last to fill, then you know wich one to target on a repair, and also helpfull when dealing with a customer remotely.

 

[GGoodrum]

I was going to eliminate the LEDs, mainly as a way to reduce the parts count, both to lower the cost, and to reduce assembly/test time. Not having them in the way would also allow the use of bigger heat sinks, which means a higher shunt current could be used. We can still do versions that include the LEDs

After the first couple of times of watching all the LEDs come on within seconds of each other, I decided I didn't need individual LEDs on mine. If there is a problem with a cell, you will know during discharge, because the LVC will start tripping too soon. At that point, I'd check the individual cell voltages to weed out the bad cell, not wait until charging. Anyway, to each his own. If you want LEDs on each channel, it will still be possible to do so.

-- Gary

 

[bobmcree]

Hi Bob,

I think you heart is at the good place, sorry to say this but I never like the attitude, from LifeBatts people. Except for you and Gary who are always's welling to share knowledge and welling to find a solution.

I know you are made a lot of effort, of thinking, and trying to made this BMS solution in our and and I'm really proud of you for all of the good work you have done!

Have nice day Bob and thank's again, to take your time for helping us, for me is really appreciate!
Bikeraider!

Thanks to everyone for their support. Not one person has told me i made the wrong decision. Just to clarify, Gary will be the new distributor for LifeBatt for ebike applications, and i will continue to work with him to provide the bms and high quality battery systems. I will just no longer be associated with LifeBatt USA. I think their cells are probably the best available unless you are willing to harvest a123 cells out of dewalt power packs and build the packs yourself.

 

[stevero2001]

Bob,

I have a general question regarding BMS. I have a Ping style duct tape pack, and I noticed that as the first cells are full that they peak around 3.85v and then bleed down, as the charger cyles on and off. I'm thinking that if I could limit them to 3.7v or 3.6v by putting a zener diode across each cell, that this would speed up the charging by clamping each cell, and also limit overcharge damage. Would this work, and what would the max wattage be needed for the zeners, assuming the charger is 2A? I know 3.65v is the target, but if I was willing to sacrifice the last 5% of charging, in exchange for longer life (something like the Dewalt BMS), would this really help? Also, if it charges to 3.85v vs 3.65v, how much does that really affect lifespan?

Thanks,
Steve

 

[bobmcree]

Bob,

I have a general question regarding BMS. I have a Ping style duct tape pack, and I noticed that as the first cells are full that they peak around 3.85v and then bleed down, as the charger cyles on and off. I'm thinking that if I could limit them to 3.7v or 3.6v by putting a zener diode across each cell, that this would speed up the charging by clamping each cell, and also limit overcharge damage. Would this work, and what would the max wattage be needed for the zeners, assuming the charger is 2A? I know 3.65v is the target, but if I was willing to sacrifice the last 5% of charging, in exchange for longer life (something like the Dewalt BMS), would this really help? Also, if it charges to 3.85v vs 3.65v, how much does that really affect lifespan?

Thanks,
Steve

if you had a perfect zener with a sharp cutoff that could handle your charge current it would work. If the current is 2A your 3.65v zener will need to dissipate 7.3WUnfortunately zener diodes do not have a very sharp knee and the voltage varies somewhat with current.

What you are proposing is exactly what i achieved by using the LM431 and the TIP105. The LM431 works like a perfect zener by using a band gap 2.5v reference as one input of a comparator and the measured voltage as the other input. when the measured voltage exceeds 2.5v the output goes low and it works like a very sharp zener. By putting a resistive divider on the input we can adjust the trigger point to any voltage above 2.5v that we want. The pots on the bms will adjust from about 3.0-4.0v to work with virtually any lithium iron cells.

as for the possible benefits of charging to a slightly lower voltage, i just don't know. a123 has had lots of very smart engineers working on the issue for years, and they seem to think throwing away the top 5% of capacity is worthwhile to achieve more cycles. With our bms you could set the terminal voltage to 3.55 or 3.60 or any voltage you like, then the time at which the cells are held at that voltage after the last shunt trigges is also adjustable. if the charge is cut off as soon as the last cell hits 3.65v i have measured about 3-5% less capacity than when i charge them all the way up to 4.0v. I suspect that utilizing that extra few percent of capacity will shorten the battery life, but how much it does is one of those secrets for which i have yet to learn the secret handshake that would give me access to that information.

 

[PJD]

On the topic of charging voltage, I always assumed the voltage of the cell was tightly set by the redox potentials of the cell's chenistry. For example. a lead acid cell is right at 2.2 volts per cell, and can be fully charged, if one is patient enough , at 2.25 volts per cell. So, assuming the Lifepo4's are nominally 3.2 volts per cell, I had assumed that the safest and least abusive way to charge them to full capacity would be leave them overnight or all day, at at 3.3-3.4 v per cell.

So perhaps a longer timer setting would be a good feature for the BMS?

 

[bobmcree]

On the topic of charging voltage, I always assumed the voltage of the cell was tightly set by the redox potentials of the cell's chenistry. For example. a lead acid cell is right at 2.2 volts per cell, and can be fully charged, if one is patient enough , at 2.25 volts per cell. So, assuming the Lifepo4's are nominally 3.2 volts per cell, I had assumed that the safest and least abusive way to charge them to full capacity would be leave them overnight or all day, at at 3.3-3.4 v per cell.

So perhaps a longer timer setting would be a good feature for the BMS?

the timer is a simple RC so 10 megs and 1000 uf will give you about 2.7 hours. I do not believe you can charge the lithium iron cells adeqauately without raising the voltage up to 3.65v and i have demonstrated that charging them up to 4.00v will provide an additional 3-5%.

 

[GGoodrum]

For those who have asked, I finally have done a new LVC board. This one supports up to 16 channels, and comes with active cutoff, which is implemented using two 4110 high-power FETs. Here's what one looks like:

The basic unit comes with a board-mounted multi-pin connector and a matching plug/harness with foot-long wires that can be connected to the cell junctions on the pack. This way, the cell connections can be check first, before plugging in the LVC board. I've smoked a few TC54 chips, in the past, inadvertantly connecting a channel to the wrong cell.

The picture above shows a second receptacle-type connector, with wires connected, which can optionally be used to plug in an external balancer, or a set of individual cell chargers. An option is also provided for a matching plug/harness, which can then go on the balancer/charger end.

While Bob and I finish up the BMS checkout, I've decided to install these boards on my LiFeBatt packs, so that I can get back to riding. The weather has been too good lately (mid-80s, clear blue skies...), to not be riding. What I will probably do is make up a board without the LVC functionality, and then use it as an external "CMS" unit, between the charger and these LVC-equipped packs.

Here's the link to the LVC page on my site: http://www.tppacks.com/proddetail.asp?prod=LVC-16-Cell

I think I have 90% of the options covered, but if there is some other requirement needed, I'm sure it can be accomodated.

-- Gary

 

[rf]

Thanks again, Bob and Gary for all the great work. And for sharing your ideas so freely.

I saw a post on another thread lauding Exxon or somebody for buying a new-tech battery company and getting involved in alternative energy. Wow! When an oil company buys a battery startup is isn't to further the technology, it's to kill it. No question.

Devising solutions and publishing them freely is a wonderful blow against the empire. Thanks guys!

Now, to sort of take this back on topic ... about that active cutoff.

Is there any reason why the new active cutoff shouldn't also become main power-on? Power-on surge with most controllers causes problems, from wearing out connectors and switches to requiring half-on arrangements with resistors, etc.

Any reason not to connect a switch to the `active shutoff' on the BMS as master power?

Thanks

Richard

 

[oatnet]

As of 3/22/08 I am ending my association with LifeBatt. They have a good product but there is no room for me to make an honest profit and i no longer wish to be associated in any way with them. I made some completely reasonable and factual remarks about the lack of a LifeBatt bms for a system i sold 5 months ago, and don decided to turn it into a personal attack against me. Gary will be their distributor from now on. I cannot work with Don.

Bob, I am sorry you got burned by Don Hardon - lacking significant technical depth himself, Don appears to have a pattern of conning brilliant folks and leeching off them. Whenever the swaying tower of lies his con is based on is threatened, he responds with cutting, ad-hominum attacks. You are better off being clear of the carnage, and not having your good name associated with it.

It looks like your escape from Hardon set off a chorus of cheers on the TidalForce forum too! Funny how Don leaves behind a trail of haters behind wherever he goes - I wonder if he realizes the problem isn't everyone else...

It is obvious that Hardon's personality defects are going to drive away engineers, customers, and suppliers, so I think it is unlikely that LifeButt will become a long-term functional entity. The cells seem capable, so it is a shame that most of us won't be able to use them until Hardon stops constipating the product's delivery with his fantasy pricing structure, and a real entreprenuer steps in and makes a market with them.

-JD

 

[albie]

I think we can simply say (and i am putting it out there) Don Hardon is an asshole who just wants to make a shitload of money on a product which we know has stifled the growth of the Lifebatt product.

I guess we can leave it at that, but if you do want to discuss Don Hardon further, start anotherthread.

 

[recumbent]

... I think it is unlikely that LifeButt will become a long-term functional entity. The cells seem capable, so it is a shame that most of us won't be able to use them until Hardon stops constipating the product's delivery with his fantasy pricing structure, and a real entreprenuer steps in and makes a market with them.

Do you honestly think that "Don" controls the price structure of "Lifebatt" batteries?

 

[Dalecv]

I would be interested in seeing a schematic that shows how the cells should be connected to the BMS.

Thank you,

Dale

 

[PJD]

Gary,

Is the cutoff point of your LVC cutoff board easily adjustable? What is the default cutoff voltage?

Thanks,

Paul D.

 

[GGoodrum]

Gary,

Is the cutoff point of your LVC cutoff board easily adjustable? What is the default cutoff voltage?

Thanks,

Paul D.

Yes, it is adjustable, via different versions of the TC54 voltage detector chip. There are versions 2.1V to 6.0V, in .1V increments. I think for most LiFePO4 cells, 2.1V is the right number. I've tested LiFeBatt packs with LVC's using the 2.1V chips, and run the packs down to the cutoff. At that point, the Ah used is right around 9.8 - 9.9 Ah. For a123-based packs, I use the 2.7V version, mainly because a123 cells hold the voltage up higher than any other cell I've seen, all the way to the very end. Then, the cells dump quickly. Using 2.7V as the cutoff ensures that this quick voltage drop is detected as soon as possible.

-- Gary

 

[rf]

Any reason the new 'active cutoff' shouldn't also become main power-on?

Power-on surge with most controllers causes problems, from wearing out connectors and switches to requiring half-on arrangements with resistors, etc.

Any reason not to connect a switch to the `active shutoff' on the BMS as master power?

Thanks

Richard

 

[fechter]

Any reason the new 'active cutoff' shouldn't also become main power-on?

Power-on surge with most controllers causes problems, from wearing out connectors and switches to requiring half-on arrangements with resistors, etc.

Any reason not to connect a switch to the `active shutoff' on the BMS as master power?

Thanks

Richard

I don't see why not. Apparently the 4110's handle the on/off surge.

 

[GGoodrum]

Any reason the new 'active cutoff' shouldn't also become main power-on?

Power-on surge with most controllers causes problems, from wearing out connectors and switches to requiring half-on arrangements with resistors, etc.

Any reason not to connect a switch to the `active shutoff' on the BMS as master power?

Thanks

Richard

Sorry, Richard, I missed this earlier...

I agree, I think this is totaly doable. I believe all you would need to do is ground the gate signal on the FETs, to prevent any current flow, and then release the ground to let in the "surge".

Right now, for my 72V setups, I'm using using a couple of big power resistors and a momentary pushbutton in order to "pre-charge" the controller caps, before switching on main power. This has completely eliminated the big "zaps" I was getting before, which were especially bad after the bike/packs had not been used for a couple weeks. This, however, seems even simper. I might give it a try on my Mariner folder, which is slated to get two new 12-cell LiFeBatt packs, with these LVC boards.

-- Gary

 

[rf]

[...]
Right now, for my 72V setups, I'm using using a couple of big power resistors and a momentary pushbutton in order to "pre-charge" the controller caps, before switching on main power. This has completely eliminated the big "zaps" I was getting before, which were especially bad after the bike/packs had not been used for a couple weeks. This, however, seems even simper. I might give it a try on my Mariner folder, which is slated to get two new 12-cell LiFeBatt packs, with these LVC boards.

-- Gary

Damn, now I'll have to get a couple of these boards. Just to remove the last major bit of funkyness from my bike. (And because I just blew a couple A123s from flying without an LVC.) My 72 volt system has destroyed too many connectors and switches already.

It's interesting that Crystalyte and others seem to ignore the problem.

 

[chas_stevenson]

Bob or Gary,

Can you please explain the difference between the LVC boards and the BMS Boards.

Thanks,
Grandpa Chas S.