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Upgrade to Lithiums?

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Johny View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2014 at 12:52pm
I have 192 cells pairs and I often "see" 690 VDC when I start out in the morning at 8:30AM when the charger stopped at 6AM. So 2.5 hours later my cell voltage can be at 3.6 V. That would put your 34 cells at 122.4 VDC.

However. you previously mentioned that the Lead Acid charger was charging to 124 VDC ("The charger I am using used to get the old SLA pack of 96v nominal up to 124 volts max.") so the controller obviously survived that!

On the other hand, assuming the BMS you are using does balancing, you do have to get the cell voltage up to the balance voltage per cell or the balancing won't be effective.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2014 at 1:16pm
Thanks Coulomb and Johny - much appreciated! These lithiums, and how they work, are taking some getting used to!
Cheers
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 4Springs Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2014 at 5:23pm
From the slight amount that I know about your BMS, I think it would need to be reprogrammed for a new battery voltage. But if the bloke who had to do the reprogramming was the bloke selling you the new cells then I reckon it might be done for free!
Sounds like you might be past that idea though, if this cell stays good.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2014 at 1:43pm
Thanks Christopher, I might do it anyway!. The space is there and the two new cells would give me a slightly better top speed (I think?)
The cell that I thought may be fried seems to be OK!
I will check with the distributor and ask him what the cut out voltage of the BMS is at the moment.
Thanks
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote T1 Terry Quote  Post ReplyReply Direct Link To This Post Posted: 09 July 2014 at 1:44am
Quote The charger I am using used to get the old SLA pack of 96v nominal up to 124 volts max. I am unsure what the new BMS will allow the charger to charge up to. At present the BMS stops the charger at about 113v but that is, I think, because four cells commence shunting when they reach 3.65vpc and quickly get to 4.0vpc whereupon the BMS shuts the charging process off.

I'm guessing the BMS you are using is trying to cell balance by burning off some of the high cell capacity once it reaches 3.65v. These things are usually limited to around 0.5 amps, so at this point the charging needs to slow to that 0.5 amp rate so the high cells don't push on to 4.0v. If your charger doesn't have this facility, maybe stopping the charge for 10 mins when the high cells reach 3.8v, then restarting to allow the BMS modules to burn off some of the capacity in the high cells. What cell voltage do the BMS modules stop trying to burn off capacity?

T1 Terry
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 09 July 2014 at 12:55pm
Thanks T1 Terry - I wouldn't know but I'll check with the manufacturer.
Regards
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2014 at 12:04pm
Hi people,
I have completed my upgrade to lithiums after having a couple of faulty cells. As I was advised previously in this thread, I installed 34 cells giving me a nominal 108.8v battery bank. I use the EV-power BMS which is currently configured for 32 cells ie. It stops the charging process when the pack voltage reaches 121v or any cell goes above 4.0v.

Performance is far superior to the old SLA’s!! I still however have two minor problems:
a)     The bms cuts off the charging at 121v (what I call “peak pack voltage”) – the BMS is configured for 32 cells - after which the pack drifts back to about 113v (what I call the “settled pack voltage”),
b)     When approaching the 121v level, the charger is still charging at about 3amps which limits the amount of time a “high” cell has available to shunt to a “low” cell,

These problems are due to my continued use of the old 96v SLA charger so I have decided to upgrade the charger to a lithium charger but I don’t know what size?

My limiting factor is my 96v controller. The manufacturer has advised the 96v controller will actually handle up to 120v though of course it won’t be covered by warranty. This means (I think?) I need a charger which results in a “settled pack voltage” of about 117v while dropping the charge current down to about one amp (or less?) somewhat before reaching the 121v “peak pack voltage”?

If my above logic is correct (and that’s a BIG if) I think maybe a 32 cell TC Charger or similar would give me the end result I need?

Could someone advise on the above, particularly if they have a 32 cell system and even if they don’t? Are there flaws in my logic? I feel a 34cell charger might get my peak pack voltage up to around 130 volts whereupon, once charging stops, the pack may drift down to about 125v which would be too high for my controller? A 30 cell charger may not get the voltage high enough for 34 cells?

Thanks in advance and for all the previous advice,
Cheers
Paul
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Johny View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2014 at 12:14pm
Hi Paul
Does your BMS connect to every cell?
Just on the face of it I would pull 2 cells out and run it as a 32 cell system. Especially if the BMS only monitors/balances 32 cells.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote evric Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2014 at 3:23pm
Hi Paul,
Johny is right. Use 32 cells and then you won't strain your controller. Try your current charger and see how it goes. Ensure the BMS is setup to turn off the charger is it exceeds the maximum. Just monitor it at the end of charge.
Eric

Edited by evric - 06 October 2014 at 3:28pm
Prius Plug-in Conversion: http://www.evplus.com.au ...Holden Barina EV: http://www.evric.kestar.com.au
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2014 at 6:08pm
Thanks Johny and evric,

My BMS appears to monitor every cell. The BMS seems to only cut off the charging if the pack reaches a certain voltage (for 30 cells about 115v and for 32 cells at about 121v). The fact that I have 34 cells doesn't change the voltage at which the BMS stops the charging process. I could have 50 cells and the BMS, in its current configuration, would stop the charging process at 121v or when any cell reaches 4.0v.

Reducing the cells from 34 to 32 would, apart from reducing the vehicle's performance slightly, mean the cells were trying to balance while the charger is still puting 10amps into the pack. The charger appears to be programmed to charge at 10amps until pack voltage gets to 116v then reduce gradually to 3 amps up to 124v. I am reasonably sure of this because that is what happened when the charger charged the old SLAs I used to have.

I think(?) reducing to 32 cells would reduce vehicle performance and make cell balancing much more difficult than it already is?

I am probably missing something somewhere?
Thanks
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote evric Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2014 at 9:17pm
Paul,
The question is: what is the maximum voltage your controller can take? You need to charge the cells to at least 3.65V per cell. If your controller can accept 34 x 3.65 volts then all you need is a new charger like the TC Charger. These have to be ordered for the precise number of cells and cell type eg. LiFePO4. These chargers are usually set to 3.65V per cell. I haven't had any experience with the TC Charger, but soon will with my Prius Plug-in conversion. I use a ZIVAN NG3 charger which are ideally matched to the lithium cells ans taper the current off as the charge nears completion to match the low current required for the BMS bypassing. I think the Zivan NG3 chargers are about twice the price of the TC Charger.
Eric
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Post Options Post Options   Thanks (0) Thanks(0)   Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2014 at 10:48pm
Originally posted by Paul9 Paul9 wrote:

I installed 34 cells giving me a nominal 108.8v battery bank. I use the EV-power BMS which is currently configured for 32 cells ie. It stops the charging process when the pack voltage reaches 121v or any cell goes above 4.0v.


Hi Paul,

This must be a very recent change to the way the EV Power BMS operates, but, are you sure that at 121v no cell has reached 4.0v?

The EV Power BMS doesn't normally use pack voltage to determine end of charge, just high cell voltage, unless your's is a special

in any event, if at 3A you are getting 3.55v pack average, then the battery is full (assuming of course the pack is fully top balanced - you will need to check this manually with a volt meter)

also, 117v is 3.45v average, which is exactly where it should be.
anything above 3.45v is fully charged

don't rely upon the BMS to get the battery balanced the first time, that should be done manually to be sure it happens (individually at the cell level)
Matt
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 07 October 2014 at 10:03am
Thanks evric and antiscab,

Thanks evric, to answer your question, the controller, according to the manufacturer, will handle up to 120v which is why I think(?) I need to aim at a "settled" pack voltage of about 117v. This is totally different from the voltage the charger will get the pack up to before the BMS stops the charging process ie. 121v.

Thanks antiscab, as I think (?) I understand your comments, you are saying that the 121v achieved by the charger equates to a cell voltage of 121/34= 3.55v which I agree would be full if the cell voltage stayed at 3.55v per cell which it doesn't. Within an hour of the charging process stopping, the pack voltage will drop to 113.5v which equates to 113.5/34= 3.34v per cell. I don't think 3.34v per cell is full?

The manual for the ev-power BMS says there are two warning messages for overvoltage situations:
a) pack voltage exceeds a preset limit, in my case 121v or
b) any cell exceeds 4.0v

If either of these warning messages lasts 30 seconds the BMS shuts down the charging process. What the manual says appears to happen in practice as I have seen both messages and the charging process is indeed shut down by the BMS.

What I think (?) I need is a charger which will:
a) charge the pack to say 124v
b) during the process at a) above, drop the charge curent to 1 amp or so when pack voltage reaches, say, 121v
c) BMS or charger (or both?) stops the charging process at 124v(?)
d) pack settles back to a pack voltage of about 117v equating to 3.44 per cell which is almost full.

What I think is confusing me about both your comments is that the charger achieving a pack voltage of say 121v (3.55v per cell) is not full if the pack immediately drifts down to 113.5v (3.34v per cell).

My apologies for being a bit slow but lithium batteries are really new to me!
Thanks again,
Paul



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Post Options Post Options   Thanks (0) Thanks(0)   Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 09 October 2014 at 8:25pm
Originally posted by Paul9 Paul9 wrote:



Thanks antiscab, as I think (?) I understand your comments, you are saying that the 121v achieved by the charger equates to a cell voltage of 121/34= 3.55v which I agree would be full if the cell voltage stayed at 3.55v per cell which it doesn't. Within an hour of the charging process stopping, the pack voltage will drop to 113.5v which equates to 113.5/34= 3.34v per cell. I don't think 3.34v per cell is full?



Generally if a cell hasn't been floated before, it won't hold a voltage above 3.45v, full or not.

Also be aware that the BMS draws current from the battery continuously, so will take away the surface charge

interesting that it falls to 3.34v though
did you measure individual cell voltages? or watch for red lights just before the BMS disconects the charger?

that may tell more of a story

personally I would use single cell chargers to make sure all the cells are full.
once you do that, it won't matter that the BMS disconnects the charger at 121v, as your battery will already be top balanced

probably cheaper and easier, and having single cell chargers around can be useful

Matt
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 7:46am
Thanks antiscab,

I think my problem goes back to my installation of the lithiums in the first place. Having worked with SLA's before, I had no idea of how important it is to start with balanced lithiums. Upon unpacking the cells they all read between 3.23 and 3.25 volts and as the difference was only two hundredths of a volt I thought they were balanced! Little did I know!

When I charge the pack now I get 5 or 6 cells shunting. As soon as they commence shunting I put a resistor over the shunting cell to slow down the charge on that cell.

I have a single cell charger so I sometimes put that over the lowest voltage cell when charging has finished.

I suppose I am just trying to avoid removing all the cells and doing a charge with all the cells in parallel and then re-installing them, though this is what I will probably have to do. I am just a bit of a slacko!

Thanks
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 8:12am
Paul if you know the shunt bypass current then a trick you can do is to place an appropriately sized resistor in series with the charger that only charges at slightly under the bypass current.

To get the pack correctly balanced you would need to know the exact bypass voltage per cell then apply this voltage * cellCount (or higher is OK).

That way every cell goes into bypass and all is top balanced. It could take a day or so though.

I am still not sure if your last 2 cells (only a 32 cell BMS) are monitored???
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 8:49am
Thanks for that Johny.

I will check with the distributor about the BMS only monitoring 32 cells. Having no idea how a BMS works, I just assumed the BMS would look for a cell to go over 4.0v and then stop the charging. Shows how little I know!

I will also find out the cell bypass voltage. The manual says that the cells bypass at 3.65v and stop the charging when any cell reaches 4.1v. I assume I can believe the manual?

Thanks again Johny,
Cheers
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 8:56am
Originally posted by Paul9 Paul9 wrote:

I will check with the distributor about the BMS only monitoring 32 cells.
I'm assuming that there are small wires from the tops of each cell or there are cell-top modules. The last 2 (of 34) won't have those. Can you eyeball the cells?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 9:15am
Thanks mate,

When I bought the extra two cells I also bought cell modules for them. When I installed the two extra ones I also put the new cell modules on each new cell and wired the cell modules to the adjacent cell modules.

I assume this is what I was supposed to do?

Thanks
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 10:19am
Originally posted by Paul9 Paul9 wrote:

When I bought the extra two cells I also bought cell modules for them. When I installed the two extra ones I also put the new cell modules on each new cell and wired the cell modules to the adjacent cell modules.

I assume this is what I was supposed to do?
Yep - perfect. So you really have a 34 cell BMS system - not a 32 cell. I'm still curious as to why you have always referred to it as a 32 cell unit. Is there a master monitor unit?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 10:44am
There is a master monitor unit which gives me readouts on amps voltage etc.

I refer to it as a 32 cell unit because I have not yet sent it back to the distributor to have it reconfigured for 34 cells. For instance the pack voltage cuts charging off at 121v (3.78v per cell for a 32 cell pack) rather than about 128v for a 34 cell pack. Also, for instance, I can travel 10kms without the State of Charge going under 100%. I assume the BMS reads a preset voltage for 100% full and if the pack voltage is higher than that preset figure then it continually resets the SOC to 100%.

Hence I think the BMS has to go back for recalibration.
Thanks
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2014 at 9:13pm
Originally posted by Johny Johny wrote:

Paul if you know the shunt bypass current then a trick you can do is to place an appropriately sized resistor in series with the charger that only charges at slightly under the bypass current.


I once used a 0.35A LED constant current power supply to balance out a set of 16 90Ah cells, all with BMS modules on them that shunted at 3.65v
I left it on over a weekend

after the weekend, all cells were 3.65v, all bulged, all lost capacity, all had higher internal resistance.
please don't float them

you already have a single cell charger - just leave it attached to the lowest cell until it is full, then move to the next one

what current is the single cell charger?
Matt
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paul9 Quote  Post ReplyReply Direct Link To This Post Posted: 11 October 2014 at 1:09pm
Thanks Matt

My single cell charger is 1.7amps.

That would be ok one cell at a time?

Thanks
Paul
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Adverse Effects Quote  Post ReplyReply Direct Link To This Post Posted: 11 October 2014 at 2:34pm
hehe better than a 200mah one
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Post Options Post Options   Thanks (0) Thanks(0)   Quote antiscab Quote  Post ReplyReply Direct Link To This Post Posted: 11 October 2014 at 10:00pm
Originally posted by Paul9 Paul9 wrote:


My single cell charger is 1.7amps.

That would be ok one cell at a time?


1.7A is fine - just time consuming

the upside, is it doesn't stop you from using the car whilst you are slowly top balancing the battery.

what does the charger do once the cell is full? does it hold to voltage at 3.65v (or some higher number)? or does it just shut down?

if the former, it may be worthwhile using it with a timer, to minimise the amount of time it's holding the cell at 3.65v

for obvious reasons, charge the whole pack until the BMS cuts out before using the single cell charger
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