# nimh batts



## dimitri (May 16, 2008)

Ni-MH is a good type of battery for EV, obviously several production EVs have used them over decades. The devil is in details. If using these 4.5Ah cells in EV you must have number of cells in parallel to get decent AH and then a number of these parallel packs in series to get decent voltage.

So, for example, if you want 144V 135Ah pack, you will need 30 cells in parallel times 20 in series, total of 600 of these cells.

Can you get 600 cells? How much would they weigh? How much room will they take? Can you assemble them in 30 cell packs with sturdy parallell connections to ensure equal current distribution and avoid heat damage?

You'd need some sort of BMS to keep series connected packs balanced, and you'd need a charger that is compatible with Ni-MH profile.

Its possible to do, just need to think it thru.


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## jackbauer (Jan 12, 2008)

I think i can get 600 cells ok. I know its not easy but its worth investigating. I may be able to get a 6v 4.5ah stick for $1usd! Estimated weight for that pack is about 150kg (330lbs). The same in lead acid would be about twice that weight. Just another option to have on tap!


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## EV59RAG (Oct 6, 2008)

But how are you going to solve the charging side of it. Do you have already something in mind that you can share?


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## jackbauer (Jan 12, 2008)

I'll gladly share any ideas on the forum. One potential idea is to make up an 80v 4.5ah stick of cells for use with the shunt element of my compound wound motor.


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## Sunking (Aug 10, 2009)

NIMH have some positive attributes that are useful in EV production. They have a pretty flat discharge voltage cure from 100% to 0%. With an energy density os 70 wh/kg makes them comparable to LiFe4Po of 100 wh/kg at a fraction of the cost. Their big advantage is internal resistance is low which makes them a good choice for high discharge rates needed in EV applications.

On the down side is they cannot be 100% discharged without possible cell damage (polarity reversal), and unless it is one of the newer models has a significant self discharge rate.

Seeing how GM, Ford, Toyota, and Honda all use them in their hybrid designs speaks well for NIMH. I am certain they choose them for the price per Kwh.


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## Jordysport (Mar 22, 2009)

Wow that sounds amazing, £124 for a 72v 90ah pack, thats INSANE!! only issue is obiously charging and dimensions (what size is the 6v 4.5ah?) 

AND YOUR LOCAL well sort of.


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## Jokerzwild (Jun 11, 2009)

I have been looking at NI-MH as well, what I have seen thats been a pain was the 500 cycle recharge life. On the up side was the discharge rate was much higher.


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## jackbauer (Jan 12, 2008)

Just looking at this setup. If i go for 96v 135ah.

so i need 16 off 6 v packs to get 96v at 4.5ah.

I need 30 off the above 16 stick packs which is 480 6 v packs. Now the way i see it is i can charge aany amount of nimh batts in series so i effectively have 30 batteries that need to be charged and discharged. Discharge is easy via diodes. Shottky diodes only drop about .4v. So my main concern here is to charge 30 batts via a bms that catches the tendency in the cell voltage to drop slightly at the onset of full charge.

Other side of the coin is that these packs are intended for use in emergency lighting and are constantly maintenance charged at 250ma with no ill effect 24/7.

they originate in china and as i said the packs are priced at $1usd each but i don't know as yet what will be available. I'll keep you up to date! The company may be interested in a group buy but i think its the way i'm going.


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## Jordysport (Mar 22, 2009)

jackbauer said:


> Just looking at this setup. If i go for 96v 135ah.
> 
> so i need 16 off 6 v packs to get 96v at 4.5ah.
> 
> ...


Could you keep me posted, as it would be brilliant as they are in the UK sort of, can't find any BMS or chargers for Ni-mh tho.


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## jackbauer (Jan 12, 2008)

6v packs are 245mm long by 25mm diameter.


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## Jokerzwild (Jun 11, 2009)

If you can get batteries for that we will figure out the rest.


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## jackbauer (Jan 12, 2008)

thanks guys. this is the company:
http://www.bstbattery.com/en/product.asp?CId=2

they also do 9ah and 13ah
Sent them an email to get pricing.


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## Jordysport (Mar 22, 2009)

jackbauer said:


> thanks guys. this is the company:
> http://www.bstbattery.com/en/product.asp?CId=2
> 
> they also do 9ah and 13ah
> Sent them an email to get pricing.


Hmm, don't show the sticks tho, (7.2V / 6v) they are all 1.2V.


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## IamIan (Mar 29, 2009)

Jokerzwild said:


> I have been looking at NI-MH as well, what I have seen thats been a pain was the 500 cycle recharge life. On the up side was the discharge rate was much higher.


The ~500 cycle for NiMH is for 100% DoD cycles.

~500 cycles does not mean dead at ~500 cycles ... it means you will have ~80% of the original rated capacity by that point... so after ~500 cycles of 100% DoD an average NiMH cell originally rated at ~10Ah will be down to ~8Ah.

Most of the damage to a NiMH cell is in the charging top 5 to 10% SoC... and in the discharging in the bottom 5 to 10% SoC.

If on a normal basis you will use in the middle ~80% of the SoC range of the cells you can expect closer to ~1,800 cycles before that same ~10Ah cell is reduced to effectively ~8Ah.

This is the kind of thing the Hybrid packs all do... right off the bat they cut out the efective capacity by not charging up past ~90% SoC... and they dont' discharge down past ~10% SoC... then they try to do what else they can to reduce the damaging effects at various temperatures... so they reduce the power when the NiMH cells are very cold or very hot.

This results in the vast majority of HEV NiMH battery packs like the Gen-I and Gen-II Honda Insight's all having ~150,000 mile & ~10 year warranties... and this includes the harsher usage patterns a HEV will put a battery through than a EV will.

The upper end of energy density of current NiMH cells I know of are in the ~90Wh / kg range.... and I know the Insight Gen-II NiMH cells discharge at up to 17C rates.

Often times as much as a EV battery buyer cares about kwh / kg ... other factors matter too ... it is almost never the same cell that offers all three... kwh/kg ... kw/kg ... and $/kwh.

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In the end... determine what you need from your battery pack ... then go out and look to see what you can find to meet those needs... it might end up being PbA ... or NiMH... or LiFePO4... or Li-Ion ... etc... keep your needs in mind and look at the numbers for the whole battery system... including any needed BMS and charging systems.


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## jackbauer (Jan 12, 2008)

Looks like I can get the batteries so just need to look at charging and layout. From what i remember nimh batts drop slightly in voltage as they approach full charge so just need a detection system and some form of low voltage cuttoff to prevent 100% dod.

I won't know the price for a few weeks but it will be much less than lead acid or li-ion. Not sure whether to go for 135AH or bump it up to 180AH. Potential plan is to put some cells in a housing where i remove the fuel tank.


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## dimitri (May 16, 2008)

According to the data on the Web site and dimensions of your 6V stick, the stick contains 5 C size cells in series.

Since Ni-MH individual cells are 1.2V nominal, its better to figure out whole pack using individual cell sizes.

So, if you wanted 120V pack of 135 AH, you'd need 30P100S config, which means 3000 cells of C size.

These cells on Ebay go for approx $2.50 per cell, obviously a small quantity price, could be better if you ordered 3000 cells, I am just using it for rough numbers.

So, 3000 cells even at $2 per cell would be $6000. Comparable pack made from TS LiFePo4 cells from EV Components will cost about $5500.

So, unless you get these Ni-MH cells at much much better price ( like "fell off the truck" kind of price ) then its not worth the trouble, IMHO.


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## IamIan (Mar 29, 2009)

I attached a chart that shows the different NiMH charging terms.

Just keep in mind the limits of all of them.

Every single one of the commonly used end of charge methods for NiMH happen after the cell has pasted 100% SoC... that up swing in the slope of the Voltage and the Temperature begin when the battery is in the top ~95% of the SoC... by the time you reach any of those point includign the PVD , -V, dT/dt , etc... by the time you reach any of them, you have already past 100% SoC.

Thankfully NiMH is reasonably tollerant of over charging... so people use these method to signal the end and limit the amount of over charging the cell is subjected to.

If you have no time restrictions ... use a charging method more like those that Li use which are designed to reduce the risk of over charging ... sense Li will not tollerate it... charge under constant current until you reach a specific voltage for NiMH it is ~1.39V ... than switch over to a constant Voltage charge at that voltage until the current rate drops bellow a set minimum value... this makes the charging time take much much longer... but for the same reason it is used in Li batteries ... it is a low tech method of greatly reducing the risks of over charging a battery.

Of course it might just be easier and definately faster to charge the NiMH the conventional way... after all they are pretty tollerant of overcharging and such... so they should still operate for many hundereds of cycles without significnat issue.

Don't forget to include an occasional at least 1 or 2 times a year battery cell balancing... if you don't the cell will drift away from each other and the whole packs performance will drop.

If you are interested in getting the maximum service life out of them ... you might also want to design a method to do 2 or 3 low current re-conditioning cycles every year or so.


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## Jordysport (Mar 22, 2009)

dimitri said:


> According to the data on the Web site and dimensions of your 6V stick, the stick contains 5 C size cells in series.
> 
> Since Ni-MH individual cells are 1.2V nominal, its better to figure out whole pack using individual cell sizes.
> 
> ...


he says he can get them for $1 per stick roughly (6v 4.5ah) so a 120v 135ah pack would cost $600


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## IamIan (Mar 29, 2009)

dimitri said:


> So, 3000 cells even at $2 per cell would be $6000. Comparable pack made from TS LiFePo4 cells from EV Components will cost about $5500.


$1 a stick = $600 for ~16kwh is still ~$38 / kwh.

I would also say ... don't forget the BMS chosen for the NiMH ... vs what kind of BMS will be used for the LiFePO4... it is the total battery package vs the total battery package... the LiFePO4 can end up a better deal... but they might not... $1 a stick is a killer price.


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## jackbauer (Jan 12, 2008)

iamian thanks for the info its been a while since i designed charging circuits. As you stated cc charge to a particular voltage then switch to cv is my basic plan and its how they are used in the origional application.

I'm looking at a price of between $1 to $1.50 per 6v 4.5ah stick. These are not stolen or otherwise illegally obtained. I used to work for a company that uses them by the hundreds of thousands per year hence the price. It may well work out to be twice that price i'm not sure for a few weeks.


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## dimitri (May 16, 2008)

No doubt, if I could get 3000 cells for $600 I would not hesitate 

My comparison cost was based on 37 cells of $150 per cell, which is about right considering there is no cell of equal size, so I went for middle between 100AH and 160AH cells from here

http://www.evcomponents.com/SearchResults.asp?Cat=34

As for apples to apples, building a pack from 3000 little cylinders is going to be a lot of fun labor, so obviously its not that simple to compare the two, even when you take BMS into account.


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## Jordysport (Mar 22, 2009)

dimitri said:


> No doubt, if I could get 3000 cells for $600 I would not hesitate
> 
> My comparison cost was based on 37 cells of $150 per cell, which is about right considering there is no cell of equal size, so I went for middle between 100AH and 160AH cells from here
> 
> ...


they are in sticks i cannot see the issue of the building the pack, my pack would be 480 sticks est.


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## jackbauer (Jan 12, 2008)

dimitri not as bad as that i reckon. A 12v 45ah battery would require 20 sticks and my plan would be to build them in 12v 45ah blocks. Now if i go for 96v at 180ah i get 40 x 96v 4.5ah batts that need to be charged and monitored. Yes its work and its not easy but if it can be done i'll try


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## Jordysport (Mar 22, 2009)

jackbauer said:


> dimitri not as bad as that i reckon. A 12v 45ah battery would require 20 sticks and my plan would be to build them in 12v 45ah blocks. Now if i go for 96v at 180ah i get 40 x 96v 4.5ah batts that need to be charged and monitored. Yes its work and its not easy but if it can be done i'll try


here here, if you could get it up and going jack i would praise you like a god,


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## dimitri (May 16, 2008)

jackbauer said:


> dimitri not as bad as that i reckon. A 12v 45ah battery would require 20 sticks and my plan would be to build them in 12v 45ah blocks. Now if i go for 96v at 180ah i get 40 x 96v 4.5ah batts that need to be charged and monitored. Yes its work and its not easy but if it can be done i'll try


Assuming you will connect 2 sticks in series to get 12v then parallel 10 of these pairs to get 45ah, then parallel some more of these 20 stick blocks to get more AH and put those blocks in series to get more volts. 

The issue I see is trying to keep them balanced. If individual cells in 10 cell strings ( 2 sticks in series ) will begin to drift over time, then you have no way to even know this , let alone do something about it. This will create a danger of overcharge-overdischarge of individual cells, with all the consequences.

Ideally, you want to parallel as many individual cells as possible to get final AH size, then put them in series and put BMS on each block of parallel cells. But in your case this is not possible unless you break those 5 cell sticks apart, which will create a nighmare to reconnect all tabs and wires.

So, you will have to make some compromizes.....

Since ideally you want to keep those tubes and wires to simplify overall wiring, you would be better off paralleling as many sticks as possible to create 6v blocks of max AH, then put those 6V blocks in series with BMS on each block. This way potential imbalance can only exist within 5 cell strings, which is less risky I guess.


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## jackbauer (Jan 12, 2008)

Thanks for the tips. To be honest I'm more concerned with actually getting my hands on the batts right now the tech side of it is easy compared!

Your idea of the parallel 180ah 6v packs is a great idea as it means i only need 16 bms circuits instead of 40.


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## Amberwolf (May 29, 2009)

Could I buy some of those sticks through you? I'm trying to build or find a better battery for my recycled-parts ebike than the 46 pounds of lead I'm carrying right now. 

If a 6v 4.5ah stick is $1, then I could actually afford to build a battery pack out of them that I could use. I'd need a minimum of 24V (4 series sticks) at around 30Ah (8 parallel sets of sticks for 36Ah). That's 32 sticks total, so $32 plus shipping. Since they're small, most likely all of them would fit into at most two USPS flat rate priority boxes, so shipping wouldn't cost very much. If I skimp on groceries and meals for a month, I think I could scrape that up. 

Probably if they *are* available at that price, I'd want to get 36V or 48V worth of them, so I have some spares around when I toast things experimenting.  That would take a lot longer to get the money, though, the way they've been cutting my hours at work.
________
Live Sex Webshows


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## Jordysport (Mar 22, 2009)

Amberwolf said:


> Could I buy some of those sticks through you? I'm trying to build or find a better battery for my recycled-parts ebike than the 46 pounds of lead I'm carrying right now.
> 
> If a 6v 4.5ah stick is $1, then I could actually afford to build a battery pack out of them that I could use. I'd need a minimum of 24V (4 series sticks) at around 30Ah (8 parallel sets of sticks for 36Ah). That's 32 sticks total, so $32 plus shipping. Since they're small, most likely all of them would fit into at most two USPS flat rate priority boxes, so shipping wouldn't cost very much. If I skimp on groceries and meals for a month, I think I could scrape that up.
> 
> Probably if they *are* available at that price, I'd want to get 36V or 48V worth of them, so I have some spares around when I toast things experimenting.  That would take a lot longer to get the money, though, the way they've been cutting my hours at work.



For once though he is not in the USA, yay. so they would have to be shipped to the usa which would be about $100 and then customs duty on top of that,


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## jackbauer (Jan 12, 2008)

I have no problem selling to others , IF i can get them and IF the price is right and IF people understand i'm not a dealer or a business and offer no warranty or returns of any kind whatsoever. I'm based in Ireland so shipping would need to be calculated and paid in full by any interested parties. Best plan would be to let me see how things work out first. I have a box of samples so will be playing about with them over the next few days and will update the thread.

Sorry if i seem harsh with the above just better safe than sorry.


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## jackbauer (Jan 12, 2008)

ok just some back of the envelope design. I'll preface this by saying i KNOW the limitations and problems involved. In their normal environment these batts are charged at 250ma 24/7 by a simple transformer , rectifier and 2.5w resistor. I've expanded on that a little here.

Now if we had a 2 step cv source that would switch from a high charge rate (say 500ma) to a low maintenance charge of 250ma this would aid cell balancing.

I have enough samples to construct a 12v 22.5ah pack and have ordered some 40a 200v shottky diodes for this purpose. I'll start doing some charge/discharge experiments to see how it works out. In theory each 12v battery cannot "see" any other battery either during charge or discharge and is charged individually.


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## IamIan (Mar 29, 2009)

jackbauer said:


> ok just some back of the envelope design.


Forgive me if I am missing something.... but....

I think you might have some issues with that kind of use of diodes.

Do you have a different intended connection between how they will be discharged and how they will be charged?


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## jackbauer (Jan 12, 2008)

Well i planned to use shottky diodes in the discharge path due to low forward voltage drop compared to normal diodes. Charge diodes can be ordinary silicon type. I don't think i have any paths for one battery to "see" another but could well be the case! Basically one set will allow current out of each battery and one set will allow current in for charging.


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## dimitri (May 16, 2008)

Can't you just parallel these sticks and treat them as a single larger cell without any diodes?


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## jackbauer (Jan 12, 2008)

As far as i'm aware one cannot parallel nimh without major problems hence the diodes.


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## Amberwolf (May 29, 2009)

In lower power applications like laptop battery packs, I have seen MOSFETs used for such flow control, rather than diodes, because the power wastage across them is much less.

It would increase the complexity of the circuit, but you could individually control sticks this way, too, by "turning off" sticks that reached charge state first while charging, and also "turning off" sticks that reach discharged state first while in use. Of course, *that* would *greatly* increase the complexity, requiring basically a microcontroller to sample the voltage across each stick periodically, then command the MOSFET drivers to their appropriate states. 

If I were to design a pack for my ebike with these, I would do it this way, but probably use analog circuitry for the comparator and control work because I don't know programming.  

Either way, the MOSFETs would still add to the complexity, but even if you simply used a switch that turned on either the "charge" MOSFETs or the "discharge" MOSFETs, and was setup to be exclusive so neither could ever be on when the other was, it would still work and would probably save you a lot of power wastage. 

If you were to pull say, 2C out of each stick, at 6V, assuming a (high) MOSFET RDSon of 0.002 ohms, you'd only have a 0.018V drop across it. That's only a bit more than a tenth of a watt power loss (0.162W). 

If you use diodes, you'll lose a lot more: 0.15V to 0.45V is a typical range for Schottky drops, so at 2C that's 1.35W to 4.05W. But the voltage drop may be higher than that, because at higher currents, the voltage drop tends to increase.

At higher currents for discharge or charge rates, of course each loss goes up proportionally. 

Also, Schottky diodes often have higher reverse-leakage currents, meaning that they don't do as good a job blocking the way you want to use them as regular diodes would. If they did, then regular diodes would probably have gone out of fashion a long time ago, for most purposes. 
________
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## jackbauer (Jan 12, 2008)

Thats food for thought I wonder could each 96v "battery" have its own supervisor microcontroller to handle charge and discharge? My gut feeling is this will work just needs some design and experimentation. I may still need diodes to prevent each 12v stick seeing each other as fets would not block this out.


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## IamIan (Mar 29, 2009)

jackbauer said:


> As far as i'm aware one cannot parallel nimh without major problems hence the diodes.


You can... the diodes and other methods try and solve some of the issues that can come up.

SoC is not as well related to terminal voltage in NiMH as it is in other battery types like PbA and Li.

Two different cells rated for the same capacity and at the same SoC... might not be the same voltage... depending on how well matched the cells are and how well the thermal management of the cells is done... these variations can be very tiny or it might be more significant.

These variations grow further and further apart over time / use.

When a dV occurs even if it is just 0.0001V it will result in current flow... out of one cell into another... this can lead to further oack balancing issues over time.

As the cells get more out of balance you have increased risk of individual cells experiencing over charging events and over discharging events.

The low tech option is to re balance the cells SoC ( not the same as Voltage ) during the charging cycle ... or maybe every month or so... then do reconditioning cycles ( every year of so ) of smaller sections of the whole battery pack.... as close to single cell level as you are willing and able.

Even if the cells are all 100% perfectly matched... Internal resistance varies depending on where in the SoC band you are.... Internal Resistance and capacity vary with changes in temperature.... etc...etc... so even if they start matched ... of time they will still get out of sync with each other and need to be rebalanced ... and occasionally reconditioned.

Allot of the cell to cell issues in NiMH are increased by the lower internal resistance that many NiMH cells have.

Then of course there is the failed cell side of it as well .... a NiMH cell that fails... fails as a near 0 Ohm path... which would immediate try to pull all other NiMH cells in parallel with it over discharged down to 0V.

And of course NiMH voltage depression effects.

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So yeah... there are some good reasons why people tend to avoid direct paralleling of NiMH batteries ... but it can be done ... and it can work... there are just some issues that may come up that should be considered.

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Instead of just SchottkyDiodes... couldn't you use Synchronous Rectification 








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Wouldn't that work better?? or would it just be overly complicated for a minor gain?


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## Amberwolf (May 29, 2009)

jackbauer said:


> Thats food for thought I wonder could each 96v "battery" have its own supervisor microcontroller to handle charge and discharge? My gut feeling is this will work just needs some design and experimentation. I may still need diodes to prevent each 12v stick seeing each other as fets would not block this out.


It might be easier to have each of your "batteries" to have it's own, but you could do it any way that is convenient for you. I would personally do it so that all the *parallel* sticks are wired this way, so that you have a bunch of 6V batteries of whatever high Ah capacity you require. Then simply series connect as many of those as is necessary to get the total pack voltage, just as if you were using golf-cart batteries. 

The MOSFETs would indeed block out anything you want them too, up to their rated D-S voltage. No current will flow thru them unless you turn them "on", by making their Gate pin voltage higher (usually by about 4V-10V) than their Source pin voltage. As long as you clamp the Gate pin voltage to the Source pin voltage, often done with a push-pull transistor arrangement but there are chips to do this, too, then they block just as well or better than a diode. 

Think of them as a solid-state switch, for this application.  

I've attached an image of what I'm thinking of. A very basic version would use only one gate driver for *all* MOSFETs on each side (one to turn on all the Charging MOSFETs, and one to turn on all the Discharging MOSFETs), but the ideal version would have a separate driver for each, so every stick could be individually cut off under problematic conditions. 

The only slightly difficult part is deriving a voltage to use for the gate drivers to reference from, since it is in reference to the Source pin on each MOSFET. Even this is not that hard, especially if already using a microcontroller to monitor each stick, since it is already sampling each MOSFET's source pin during charge, so it'd be able to adjust the gate voltage to compensate as the source pin goes up or down, if that turns out to be necessary.

Since gate voltages can usually be anywhere from 4V to 10V, it might be safe enough to simply use that range, with fully discharged being a difference of maximum 10V from the gate input to source, and fully charged being a difference of minimum 4V from the gate input to source, assuming the sticks aren't higher or lower than that at their outer range levels. 

That's all assuming using the same N-channel MOSFETs for everything. If it is necessary, just use P-channel ones instead, wherever necessary, to move which side the gate is referenced to.

EDIT: Forgot to attach the image--fixed. 
________
GiaNNY


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## jackbauer (Jan 12, 2008)

Amberwolf , looking at your fet idea i'm trying to think if i would have a path for one parallel battery to "see" another? Or would it simply force the fet to turn off? Just thinking out loud.

I like the shotkky diodes for simplicity. Fair point on the voltage drop and resulting power loss but i think i'll do a few experiments and see how it plays out.


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## Amberwolf (May 29, 2009)

I must have turned my brain off when reading what you were trying to do. 

You're right; if the MOSFETS on one path (charge or discharge) are all turned on at the same time, then they won't act as diodes, no.

However, if they're setup the right way, it might still work. See this page (about guitar effects pedals, but the descriptions of what happens are still applicable, and better than I can do right now):
http://www.geofex.com/article_folders/mosswitch/mosswitch.htm


Or you could try out the synchronous rectifier suggestion by IamIan.
________
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## jackbauer (Jan 12, 2008)

What do you make of this:
http://www.saftbatteries.com/Produit_NHP_329_51/Language/en-US/Default.aspx#DL2

Thought our buddies at chevron had this all sown up? Probably only sell to oems.

I'll report on some experiments when the diodes turn up.


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## IamIan (Mar 29, 2009)

jackbauer said:


> What do you make of this:
> http://www.saftbatteries.com/Produit...fault.aspx#DL2
> 
> Thought our buddies at chevron had this all sown up? Probably only sell to oems.


There is allot of myth / mis-information / mis-understanding about the limitations on buying / selling NiMH batteries.

#1 on that list is people mis-understanding how a minimum order production run works... it isn't an evil conspiracy ... minimum order quantities are a very very common thing especially when you are trying to buy directly from a factory / manufacture... it's no conspiracy ... there is a minimum operating cost just to fire up the factory and personel etc... and you can't turn a profit on what you make until what you charge per unit item and together with the quantity of items gets high enough.

People who have no experience with orders like this before items are bought in mass by large companies like Wal-Mart... they just don't understand why Panasonic won't sell them 1 ,2 , or 10 batteries... even Honda from 1999 to 2006 had to buy the NiMH cells they used in Gen-I Insights and Civics i minimum order lots of 10,000 cells... if Honda had only asked for 1,000 cells, Panasonic would have told them no as well.

#2 mis-understanding / mis-information ... there are and have been for a very long time a variety of sources to get NiMH batteries... 12Ah 'D' cells have been on the market to buy for about ~6 years... 14 Ah 'F' Cells have been on the market to buy about ~3 years, the 13Ah 'F' cells have been out to buiy for ~6 years, the ~12 Ah 'F' cells have been out to buy for ~9 years ... 26Ah 'M' cells have been on the market to buy for about 2 years, and ~20 Ah 'M' cells were on the market to buy ~5 years ago.... plus the parasmatic cells shown in the link you found and are used in the Prius for more than 5 years.

#3 mis-information / mis-understanding ... we've seen NiMH AA on shelves to buy for ~10 years becuase that is what people will buy enough of for it to be profitable for every link in the supply chain... if the consumers bought NiMH M cells as often as NiMH AA cells the M cells would be just as easy to find , and you could buy them just like the AA in small packages ... and the price per unit would come down due to economies of scale.

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Also I would say... people chain thousands of tiny cells together to form large EV battery packs... Tesla does it .... several people have done it with the small format A123 cells.... etc... if there is a large enough demand for that larger battery pack ... eventually supply and demand will cause larger and larger formats of those cells to become more and more availible and cheaper and cheaper as economies of scale ramp up.


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## jackbauer (Jan 12, 2008)

Fair points and very informative. I have some experience of production runs etc so I can appreciate that end of the thing. I guess it proves the point in my $1 6v packs. Mass manufacture drives down price and increases supply.

One does wonder why tho that production evs or phevs don't use nimh in greater powers instead seem to ignore them in favor of li ion. Anyway I'm not starting another thread on the evils of auto manufacturers just looking deeper at the subject. Trying to make an informed choice.


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## IamIan (Mar 29, 2009)

jackbauer said:


> One does wonder why tho that production evs or phevs don't use nimh in greater powers instead seem to ignore them in favor of li ion. Anyway I'm not starting another thread on the evils of auto manufacturers just looking deeper at the subject. Trying to make an informed choice.


There are several good reasons. Here are a few from my point of view.

#1> Weight Matters:
Currently NiMH that I can just go out and buy has not improved much in the last ~5 years from the ~90 Wh/kg of energy density... While the mass produced 18650 Li-Ion cell that Telsa went with goes up to ~200 Wh / kg... Even the best NiMH availible still get about 1/2 the Wh / kg that Li-Ion can do... Lighter is better for just about any mobile application... and the wh / kg has been a major struggeling point for EVs for decades.

#2> Media & Public Image:
Currently the media and public have a overly simplified view of battery technology ... they see Li as superior to NiMH , and NiMH superior to PbA... so as a marketing choice Li batteries offer your EV & PHEV better marking value.

It isn't that simple if you want to be accurate... Battries no matter what chemistry they are have rating that make up the actual reality of which one will work best for a given application... with each rating meaning different things.

-----------------
Example of why the specifics matter more than just which type of battery chemistry:

Yes you can keep it as simple as you wish... this is just a look into why the overly simplified public / media view of batteries just by chemistry has flaws.

Yes there are massive variety of batteries out there... and yes they all will have different numbers... but for this example let us pick a few different ones and compare the performance:

Wh/kg ... how much will a battery pack weigh for a given amount of kwh desired?

Wh/L ... how much space will a battery pack take up for a given amount of kwh desired?

W/kg ... how much desired cranking power , or acceleration power will cost you in battery pack weight.

W/L ... how much desired cranking power , or acceleration power will cost you in battery space.

$/kwh ... how much will it cost your budget to get a given amount of kwh desired?

#1> This LiFePO4
#2> That LiFEPO4
#3> This Li-Ion
#4> This NiMH
#5> Used ~10 year old Tech NiMH sticks from a junk yard Gen-I HEV Civic.
#6> Your NiMH you've discussed here:
5 'C' cells per 6V 4.5Ah stick
150kg per 600 sticks??
245mm long by 25mm diameter per stick
$1 per stick

Are you sure about the Weight?
If it is 5 'C' cells per stick... I would expect each 6V 4.5Ah Stick to be closer to ~425 Grams... which would make 600 sticks weigh closer to 255 kg... if each of the 6V 4.5Ah sticks actually does only weigh ~250grams, that is increadibly great news... that would mean you have a NiMH battery at ~108 wh/kg... which would be just wonderful news even if they were ~$50 per stick.

Bellow comparison assums ~420g per 6V 4.5Ah stick... I am Also Asuming your sticks are not rated for more than 4C Discharge rates... if either asumption is incorrect adjustments would need to be made to the bellow.

..........#1..... #2......... #3 ....... #4 .... #5 ...... #6
wh/kg.. 67..... 106 ...... 201 ...... 87 .... 43 ....... 65
wh/L... 147.... 162 ...... 548 ..... 289 ... 139 ...... 56
$/kwh.. ? ...... 306 ..... 1,533 ... 521 ... 285 ...... 37
W/kg... 202 ... 2,133 ... 402 ..... 174 ... 666 ..... 257
W/L.... 441 ... 3,249 ... 1,097 ... 289 ... 2,142 ... 225

Add to the above the BMS needed for each which will alter some of those numbers.

You can also compare the service life given the conditions you will have and the BMS you will have.

Add to the above the Battery Box Requirements which will alter some of those numbers.

-------------
So the specific requirements of your application will make the biggest effects on which battery properties matter the most to you... 

If it is the acceleration & Torque, you want a high power cell that will give you high W/kg & W/L ... 

If Range is more important you will look to Wh/kg & Wh/L.... 

Everybody has a budget so $/kwh always comes in somewhere... but if you are Tesla you have very different priorities when you know you can sell the car for $100,000 each.


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## jackbauer (Jan 12, 2008)

I'll get an exact weight on the sticks this week. I fully accept the points you have made and many thanks for taking time doing so. The other part of this equation is simple cost and availability. If money were no object (within reason) then I would simply buy the best Li Ion pack available , stick it in the car , put the hammer down and drive the width of Ireland on one charge.

Reality sadly bites as this is a budget conversion and my two choices are simply lead acid (either flooded or agm) or the nimh sticks. By a quirk I can get the nimh as cheap if not cheaper than lead acid hence my interest. Even if the pure energy density of the nimh is not much better than lead acid the reduction in weight and better discharge characteristics have me interested.

Looking at your other comments re the manufacturing issues then if I could develop a system for using parallel and series connection of off the shelf c or d cells that are readilly available then this would be a bonus not only to myself but to other converters.

Re the discharge I don't think i'd be hammering the individual sticks too badly. If i'm running 96v at 180ah and I want to pull 500amps for acceleration each 96v stick must source 500/40 amps = 12.5amps which is not even 3C.


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## Jordysport (Mar 22, 2009)

jackbauer said:


> I'll get an exact weight on the sticks this week. I fully accept the points you have made and many thanks for taking time doing so. The other part of this equation is simple cost and availability. If money were no object (within reason) then I would simply buy the best Li Ion pack available , stick it in the car , put the hammer down and drive the width of Ireland on one charge.
> 
> Reality sadly bites as this is a budget conversion and my two choices are simply lead acid (either flooded or agm) or the nimh sticks. By a quirk I can get the nimh as cheap if not cheaper than lead acid hence my interest. Even if the pure energy density of the nimh is not much better than lead acid the reduction in weight and better discharge characteristics have me interested.
> 
> ...


what i think is that these NiMh cells should not be compared with Lithium but should be compared to Lead acid, and when you compare it to lead acid the price is significantly lower and most importantly the weight is also lower. Colin Chapman is my god


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## IamIan (Mar 29, 2009)

jackbauer said:


> Reality sadly bites as this is a budget conversion and my two choices are simply lead acid (either flooded or agm) or the nimh sticks. By a quirk I can get the nimh as cheap if not cheaper than lead acid hence my interest.


I 100% agree... if you can get these 6V 4.5Ah sticks for $1 a stick ... that is actually cheaper than Lead Acid... which by itself is a crazy thing.

For a budget conversion like yours and many other DIY EV conversions ... the lowered price is one of the highest priorities.... your proposed pack of 96V and 180Ah or 640 sticks to give ~17kWh and only cost you ~$640 ... that is crazy cheap... and will be hard for any other battery to compare with for the cost?



jackbauer said:


> Looking at your other comments re the manufacturing issues then if I could develop a system for using parallel and series connection of off the shelf c or d cells that are readilly available then this would be a bonus not only to myself but to other converters.


Yes... but don't get your hopes up to high ... a good BMS for hundreds or thousands of sticks or cells is no small thing... and marketing it out to people is no small thing either... even after you had it all finished ... marketing and selling to people is no small thing either.

years ago http://www.airlabcorp.com/ did something like that... but Haven't heard anythign new from them in a long time.


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## Jordysport (Mar 22, 2009)

How is it coming along?


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## jackbauer (Jan 12, 2008)

Got the diodes on friday so i'm in the process of getting a test rig put together with a simple charge circuit. I'm going to setup a charge / discharge test to monitor capacity and drift etc. Should have some more info by weekend.


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## Jokerzwild (Jun 11, 2009)

how you doin?


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## jackbauer (Jan 12, 2008)

Getting there lol. Busy making the adaptor plate and other stuff. Will be running a discharge on the motor at the weekend.


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## Jokerzwild (Jun 11, 2009)

**

no PRESURE!!!!!!!!!!!!!!!!!!!


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## jackbauer (Jan 12, 2008)

Ok little bit of an update. Spoke with my battery guy last week who told me the bst batteries are not the best. They work ok for 10 to 20 cycles then capacity falls off. He recommended another type made by gp batteries. They would be more expensive but not too. I've attached a data sheet for anyone interested. They do seem to have very high discharge and charge rates. Paralleling with the schottky diodes works well. I've run a few cycles now on a 12v 22ah pack.


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## Jokerzwild (Jun 11, 2009)

Do you know the price and do you happen to have a picture with what you have done?


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## jackbauer (Jan 12, 2008)

Don't have a price as yet and will post up some photos soon as poss.


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## Bowser330 (Jun 15, 2008)

jackbauer said:


> Don't have a price as yet and will post up some photos soon as poss.


yes please do! thank you.


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## Jordysport (Mar 22, 2009)

no pressure but any news in terms of the Nimh cells??


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## jackbauer (Jan 12, 2008)

Nothing yet i'll hopefully be talking to the guy later in the week to firm up price on the gp cells. I did come across these:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=390083964028&_trksid=p2759.l1259
which at 10ah would reduce the cell count and paralleling. Right now i'm dancing between nimh and lifepo4 options. I'm still leaning toward nimh due to cost mainly and also their tolerance for abuse.


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## Jordysport (Mar 22, 2009)

jackbauer said:


> Nothing yet i'll hopefully be talking to the guy later in the week to firm up price on the gp cells. I did come across these:
> http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=390083964028&_trksid=p2759.l1259
> which at 10ah would reduce the cell count and paralleling. Right now i'm dancing between nimh and lifepo4 options. I'm still leaning toward nimh due to cost mainly and also their tolerance for abuse.


those Ni-Mh cells are the same price if not more than Lifep04


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## jackbauer (Jan 12, 2008)

Right now i'm cycle testing the bst packs. If they are a runner thats great. The one thing i am certain of is i'm not making a lead sled!


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## jackbauer (Jan 12, 2008)

Just got an email from my battery guy. He had his "finder" in china do a bit of digging on my behalf and came up with these:
http://www.pkcell.net/pro-ni-nh6.html
I'll have price and availability tomorrow.


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## Jordysport (Mar 22, 2009)

ooohh ahhh, they look very swish, easier to fit into the car also, very similar shape and style to the Thundersky ones, what sort of price range are you expecting?also those 80ah ones would be interesting. $100 per pack of 80ah or $300 or $800 lol, hope its around $300.

Good work btw.


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## Jokerzwild (Jun 11, 2009)

I got price from those guys allready they are not the best price out there for NIMH. Your looking at $3.50 a D Cell.


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## jackbauer (Jan 12, 2008)

Just in case anyone thought I was joking.


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## Jordysport (Mar 22, 2009)

did you get a price for those other cells, the big packs?  
thanks
and $1.00 for those other sticks, is very cheap, never doubted you lol/, 

keep up the good work.


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## Bowser330 (Jun 15, 2008)

jackbauer said:


> Just in case anyone thought I was joking.


7.2V for 2$, please tell me more....

Even if they were 2AH....100 in parallel would be 200AH for 200$....

7.2V 200AH for 200$ nice.....

300V 200AH for 8,400$

I wonder what the C rating is....


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## Jordysport (Mar 22, 2009)

Bowser330 said:


> 7.2V for 2$, please tell me more....
> 
> Even if they were 2AH....100 in parallel would be 200AH for 200$....
> 
> ...


if you read the thread they, are only good for 10-20 cycles.


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## jackbauer (Jan 12, 2008)

They are 4.5ah. I've been told they have a poor cycle life. I have a string of 5 on test for the past few days so a few weeks will tell the tale.


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## IamIan (Mar 29, 2009)

jackbauer said:


> They are 4.5ah. I've been told they have a poor cycle life. I have a string of 5 on test for the past few days so a few weeks will tell the tale.


What cycle testing method are you using?


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## jackbauer (Jan 12, 2008)

Its a bit crude just setup on the bench. I have 5 sticks in parallel with shottky diodes giving a 22.5ah 12v pack. Discharge load is 4 12v 75w headlamp bulbs running about 25 amps load for approx 1c load. A simple zener and relay circuit cuts of at 9.5v. Charge is just under 500ma per stick over night. Each stick has its own charge resistor and diode. I'm just keeping a manual note of discharge time over the past few days but i might setup a timer triggered by an ldr for more accuracy. So far its at least proven that i can parallel nimh sticks and expect them to discharge evenly.


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## Jokerzwild (Jun 11, 2009)

I know you are still checking on purchesing a large amount I just want to keep this thread in everyones mind because if we can pull this off its the best deal out there.


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## Jordysport (Mar 22, 2009)

Any news or is the cycle life still the main problem??


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## jackbauer (Jan 12, 2008)

Cycle life didnt seem bad on the samples i tested. The issue is no one is talking to me anymore. Not the manufacturer in china or the guys here in Ireland. Absolute frustration at every step


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## Jokerzwild (Jun 11, 2009)

I have been trying to use my lighting company based out of China to buy those items but they will not sell them 2 me. First they told me they have no such item. I had our factory manager visit with there's they said that they would not be intrested to sell that item too us. Sorry guys I tried on this end.


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