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Cheap LiFePO4 BMS?


jetzi

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There is plenty of evidence that many of the recommendations (even from manufacturers) about LiFePO4 charging voltages are way too high -- see here for example:

 

http://nordkyndesign.com/practical-characteristics-of-lithium-iron-phosphate-battery-cells/

 

Charging to anything above 3.65V/cell (14.6V for 4 cells) is a bad idea even for short periods to get to 100% capacity; even at 3.65V the voltage should be dropped as soon as the current falls to C/30. For long periods (float/absorption) the cell shouldn't be held above 90% capacity which is 3.4V/cell or 13.6V. All this assumes perfectly equalised cells.

 

There's a goldmine of practical information on the Nordkyn site, anyone rolling their own BMS should read it before starting -- or failing that, after they've killed their first set of batteries... ?

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1 minute ago, IanD said:

There is plenty of evidence that many of the recommendations (even from manufacturers) about LiFePO4 charging voltages are way too high -- see here for example:

 

http://nordkyndesign.com/practical-characteristics-of-lithium-iron-phosphate-battery-cells/

 

Charging to anything above 3.65V/cell (14.6V for 4 cells) is a bad idea even for short periods to get to 100% capacity; even at 3.65V the voltage should be dropped as soon as the current falls to C/30. For long periods (float/absorption) the cell shouldn't be held above 90% capacity which is 3.4V/cell or 13.6V. All this assumes perfectly equalised cells.

 

There's a goldmine of practical information on the Nordkyn site, anyone rolling their own BMS should read it before starting -- or failing that, after they've killed their first set of batteries... ?

They arnt LifePo4s Ian they are LifeMgPo4s from memory but I agree with you and it's what I have been doing for the last year and a half or so 

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2 hours ago, Dr Bob said:

Agreed. The point I was trying to make to Richard is that as temp goes down, you have less in the tank to use. We've been in a marina for the past 2 weeks on shore power with the Li's isolated and I can see the voltage (at rest...well nearly, maybe a 500mA draw) decreasing with temp.

Your battery monitoring takes 500mA? Blimey! Base load on NB Melaleuca is 800mA and that's the BMS AND the internet router AND the back boiler pump AND the inverter standby AND the digital alarm clock AND three laptops in sleep mode AND the radio memory protect.

 

MP.

 

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24 minutes ago, MoominPapa said:

Your battery monitoring takes 500mA? Blimey! Base load on NB Melaleuca is 800mA and that's the BMS AND the internet router AND the back boiler pump AND the inverter standby AND the digital alarm clock AND three laptops in sleep mode AND the radio memory protect.

 

MP.

 

If you leave valence batteries long enough the Bms will discharge them how long I have no idea but it does happen 

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1 hour ago, MoominPapa said:

Your battery monitoring takes 500mA? Blimey! Base load on NB Melaleuca is 800mA and that's the BMS AND the internet router AND the back boiler pump AND the inverter standby AND the digital alarm clock AND three laptops in sleep mode AND the radio memory protect.

 

MP.

 

What a cock up! Whata mistaka to makka!

No idea why I put 500mA in.  It uses about 25mA spread across the BMM8, the BG8S and the automatic BEP switch......so only 20 times less. Its still a fair whack and likely far higher than your arduino system......add to that the BMV as well compared to yours.

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2 hours ago, peterboat said:

They arnt LifePo4s Ian they are LifeMgPo4s from memory but I agree with you and it's what I have been doing for the last year and a half or so 

I agree with Ian's post. Plenty of evidence out there. Read through the threads on the Cruiser forum as well. Hundreds of pages of it.

I dont think the Mg in your Valances make any difference to the voltage. My thunderdskys contain Yttrium so are LiFeYPO4s to be more exact but it just does something to the low temperature behaviour rather than overall voltage.

The nordkyndesign guy talks a lot of sense.

 

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4 hours ago, BEngo said:

What is a WH and  an AH?  A Watt Henry and an Ampere Henry? ?

Wh and Ah I understand, but many do not, as clearly demonstrated in many, many battery theads.

  

N

I am insensitive.

 

As in case insensitive! Yes you are probably right, h not H. But what is a shift key between friends!

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4 hours ago, peterboat said:

Yes as soon as I stop the voltage rises very quickly,  I would say I have lost two thirds of my range, in the summer it would do 30 miles between resets now its 10 miles! I have actually stopped using it for the moment mainly because its cold and the chances of damaging the batteries is very real for no good reason 

That raises the point that if the batteries are reluctant to give up their charge due to cold, is there actually any danger of damage due to over discharge, if no more than the AH that would have been available at higher temperature, are taken out. In other words the actual voltage under cold discharge is the voltage due to the chemistry, minus the voltage drop due to the “internal resistance”, the latter being the resistive element plus the reaction speed element. It is only the minimum voltage due to the chemistry that one shouldn’t go below.

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1 hour ago, nicknorman said:

That raises the point that if the batteries are reluctant to give up their charge due to cold, is there actually any danger of damage due to over discharge, if no more than the AH that would have been available at higher temperature, are taken out. In other words the actual voltage under cold discharge is the voltage due to the chemistry, minus the voltage drop due to the “internal resistance”, the latter being the resistive element plus the reaction speed element. It is only the minimum voltage due to the chemistry that one shouldn’t go below.

The answer is Nick I dont know what I do know is that the eco light comes on a lot when its cold which is a limiter so I have decided to use it only on warm days to avoid doing damage to the batteries

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16 hours ago, Dr Bob said:

Maybe I am the only one but I think you are bonkers charging an individual cell to 3.9V regardless of what the manual says. That is just asking for problems and risk of something going wrong is much higher. Maybe every now and again take it to full to re sync the BMV but that is charging until you get to the CV stage and tail current drops to 4-5% NOT a cell voltage of 3.9V  or wotever. Be very very careful when you get up to those high cell voltages. 

On your SoC measurements, I would just not bother. They are never going to be accurate. Are the settings on your BMV accurate? Do you know the actual capacity? If not, you are just guessing. What is the temp of you batteries?

Measure at rest voltages as you discharge (or near at rest) and record the bank voltage and amp hrs used. Plot a graph of that data and use that to estimate how far from full you are. A glance at the voltage tells you then the state of your batteries. I do look at the Ahrs used on my BMV but after 2-3 months since the last resycn it is a load of cobblers. 

There is a temperature effect on the SoC as lower temps give lower capacity. This last week mine have been down to 10degC and looking at the literature that means maybe 10% capacity less. Reading just the voltage is better. My system has 3 things drawing power which are not connected via the BMV shunt (the cell monitor, the auto disconnect switch and the BMS) all taking power ...maybe 20Ahr per month. Your batteries will be doing something like that internally so that will confuse the BMV SoC reading. Just looking at voltage is better.

Received and understood.

 

Having said that, the nordkyn, (sp?), site refers to a memory effect which can gradually/rapidly reduce capacity. It would therefore seem that there is a need to balance getting to a proper 100% reasonably often, against avoiding too high a voltage all the time.

 

It seems that I have one cell out of 16, (4 batteries), that is out of balance enough to reach 3.81V when charging at 14.6V. I suppose I could see what reduced voltage would mean it gets no higher than 3.65V, (maybe 14.2V or so), check out the voltage spread, and see how it looks.

 

I think, if they are left charging at the manufacturers float charge of 13.8V, balancing continues so, maybe, I could let them float for a while, and see if the wayward cell gets balanced.

 

The manual says they can be left on float at 13.8V permanently, (although I wouldn’t do that, given I would be charging via genny or solar). In those circumstances, it seems they would self balance over time.

 

Again, I could see how low the charging voltage can be taken before the self balancing stops. It might be 13.6V which seems to be a safe voltage?

 

Given that Valence have been making these batteries since at least 2006, and still do, it seems surprising that their manual talks about their external BMS regulating at these high voltages. You would think that they would have modified their instructions similar to the prismatic maker referred to in the nordkyn site. 

 

 

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7 hours ago, Richard10002 said:

Received and understood.

 

Having said that, the nordkyn, (sp?), site refers to a memory effect which can gradually/rapidly reduce capacity. It would therefore seem that there is a need to balance getting to a proper 100% reasonably often, against avoiding too high a voltage all the time.

 

It seems that I have one cell out of 16, (4 batteries), that is out of balance enough to reach 3.81V when charging at 14.6V. I suppose I could see what reduced voltage would mean it gets no higher than 3.65V, (maybe 14.2V or so), check out the voltage spread, and see how it looks.

 

I think, if they are left charging at the manufacturers float charge of 13.8V, balancing continues so, maybe, I could let them float for a while, and see if the wayward cell gets balanced.

 

The manual says they can be left on float at 13.8V permanently, (although I wouldn’t do that, given I would be charging via genny or solar). In those circumstances, it seems they would self balance over time.

 

Again, I could see how low the charging voltage can be taken before the self balancing stops. It might be 13.6V which seems to be a safe voltage?

 

Given that Valence have been making these batteries since at least 2006, and still do, it seems surprising that their manual talks about their external BMS regulating at these high voltages. You would think that they would have modified their instructions similar to the prismatic maker referred to in the nordkyn site. 

 

 

Richard the only reason that they can charge at those high voltages is because the master BMS provides cutoff protection, the internal BMS does not it just balances the cells, if you continue like you are at some point you will damage a battery! Plenty of others have done what you are doing and have paid the price! James in London has sold a chap a load of batteries twice because of using to high a voltage in charging [he learnt his lesson and now charges to 80%] I dont want you to be the one that stops insurance companies allowing us to use these batteries on board, by having an accident and your boat catching fire! 13.9 bulk 13.8 absorb 13.6 float = safe and long life of batteries and no loss of capacity in 1.5 years. You cannot buy the external BMSs anymore and have them programed so please stick to what I am saying before you wreck your expensive batteries

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14 hours ago, peterboat said:

Richard the only reason that they can charge at those high voltages is because the master BMS provides cutoff protection, the internal BMS does not it just balances the cells, if you continue like you are at some point you will damage a battery! Plenty of others have done what you are doing and have paid the price! James in London has sold a chap a load of batteries twice because of using to high a voltage in charging [he learnt his lesson and now charges to 80%] I dont want you to be the one that stops insurance companies allowing us to use these batteries on board, by having an accident and your boat catching fire! 13.9 bulk 13.8 absorb 13.6 float = safe and long life of batteries and no loss of capacity in 1.5 years. You cannot buy the external BMSs anymore and have them programed so please stick to what I am saying before you wreck your expensive batteries

Received and understood. Thanks.

 

Any idea how to tell when the batteries are at 20% SOC, or less? I'm thinking a resting voltage of about 12.9V.

 

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8 hours ago, Richard10002 said:

Received and understood. Thanks.

 

Any idea how to tell when the batteries are at 20% SOC, or less? I'm thinking a resting voltage of about 12.9V.

 

I never ever get close to that, mine because of the solar go between 13.1 and 13.9 which as soon as charge terminates drops to 13.4. You are going to be safe down to 12.5 ish although the blurb says that 10 volt is the damage point [I have due to solar panel failure done this!!] remember that 12.5 volts wont inflict damage and if we count 12 volts the bottom 14.6 volts the top, 12.5 -13.9 is a much bigger range than LAs and wont do damage to these very robust batteries

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8 hours ago, Richard10002 said:

Received and understood. Thanks.

 

Any idea how to tell when the batteries are at 20% SOC, or less? I'm thinking a resting voltage of about 12.9V.

 

Charge them up to full...ish(!!!). Discharge and measure bank voltage vs Ahrs out.....as you should be doing to create your voltage vs battery state graph but also record cell voltage. You will clearly see when one cell starts going down quickly as it gets in the bottom knee. This bottom knee is then your bottom limit rather than a % SoC. You are top balancing so likely one of the cells will get to the bottom knee well before the others but not a lot you can do about this. Look at the total bank voltage when you are in the bottom knee and that is then the voltage I would have as my bottom point, but difficult to set that in stone as voltage will drop under heavy loads so alarms would ring if say you put the nesspresso machine on when near the bottom end but not in the knee. 

On my bank, the cells seem to be going into the knee under 3.10V but of course yours maybe different.

Edited by Dr Bob
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Just to add to that previous post. Once down into the knee, voltage will drop very fast so even though they say 10.0V is ok, it won't take long to get from 12.0V down to 10V. Best to just stop discharge once in the bottom knee. My system gets one or two cells to the knee but then I am down to 12.6V and the LAs in my circuit take over so I never get down lower. 

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Getting back on topic (I know, sorry...) eight squids for an active equalising BMS that can handle up to 1.2A equalizing current per cell and performs throughout charging, not just at the top knee...

https://www.ebay.co.uk/itm/Lithium-Battery-Active-Equalizer-Protection-Board-4S-BMS-Balance-Li-ion-Lifepo4/322457974994

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10 minutes ago, WotEver said:

Getting back on topic (I know, sorry...) eight squids for an active equalising BMS that can handle up to 1.2A equalizing current per cell and performs throughout charging, not just at the top knee...

https://www.ebay.co.uk/itm/Lithium-Battery-Active-Equalizer-Protection-Board-4S-BMS-Balance-Li-ion-Lifepo4/322457974994

Is it this one? I think he raved about it in a previous video, but not so much in this one?

 

 

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20 hours ago, WotEver said:

Getting back on topic (I know, sorry...) eight squids for an active equalising BMS that can handle up to 1.2A equalizing current per cell and performs throughout charging, not just at the top knee...

https://www.ebay.co.uk/itm/Lithium-Battery-Active-Equalizer-Protection-Board-4S-BMS-Balance-Li-ion-Lifepo4/322457974994

I think most of these battery balancers work throughout charging and NOT just at the top knee. This one is not a lot different but is cheap!

This one needs 100mV to start the balance which will only happen when the charge gets one of the cells over 3.35 -3.40V ie near the top knee and likely over 90% SoC. At voltages or SoCs below that you will have all 4 cells in the plateau voltage and you just wont see a delta of 100mV - unless the cells are so widely out of balance.

I cant speak about the Valance cells and how they balance but having had my Thunderskys for 8 months now and done 2 balances.....first after 3 months and 2nd after 6 months....I am not really sure how good these boards are and are they worth the effort.

My cells were reasonably balanced when I installed them (nominal 480Ahrs capacity). On going to 100% SoC ....ie tail current of 15A at termination voltage.....I had a cell voltage delta of 50mV.

After 3 months, I had a delta of 200mV and wasnt quite down to the tail current limit so maybe 99% full. To balance I had to take 80Ahr out of one of the cells. On these balance boards that would have taken 60-70 hrs! How do you keep in the knee without damaging the cells? I did it manually by using as resistive circuit taking 10A so done in 8hrs. On the 2nd balance excercise I had about a 60mV delta which I sorted in 1 hr putting 6Ahrs into the lowest cell. I really neednt have bothered and likely wont need to balance again for 12 months but will go up to 100% every 3 months to resycn the BMV.

I bought one of the balance units that Tom had posted somewhere earlier for £40 and tried that but it also just balanced at 1A or so (at the 3.5V range) so gave up with that. Tom leaves his in circuit all the time and it works (it would if on all the time) but I cant do that because my 'high' cell once we get over 95% charge is my 2nd to lowest in the plateau range (about 10mV lower than the highest) so likely this would recieve current under normal <80% SoC range which would unbalance it as it would get to the knee even quicker. Tom must be lucky that his high cells in the knee are mid range in the plateau.

These balance boards are ok if you have small capacity cells ie <100Ahrs but if you have 480Ahrs then a 100mV delta could require days of balancing at 1.2A and my view is that its a bad idea to do that with cell voltages in the knee.

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2 minutes ago, Dr Bob said:

These balance boards are ok if you have small capacity cells ie <100Ahrs...

Indeed. One approach that I’ve seen has been to have a single 100Ah (or thereabouts) LifePo4 battery which can be charged quickly with the engine and then put it in parallel with a bank of Trojans via a boost converter to allow them to have their nice long absorb for 6 hours or whatever, their initial charge having also been achieved with the alternator. So all the heavy lifting is being done by easily maintained Trojans, using the lithium purely to cover the lack of sun through the winter. 

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3 minutes ago, WotEver said:

Indeed. One approach that I’ve seen has been to have a single 100Ah (or thereabouts) LifePo4 battery which can be charged quickly with the engine and then put it in parallel with a bank of Trojans via a boost converter to allow them to have their nice long absorb for 6 hours or whatever, their initial charge having also been achieved with the alternator. So all the heavy lifting is being done by easily maintained Trojans, using the lithium purely to cover the lack of sun through the winter. 

Unless you already have the Trojans and can't sell them, that seems like an odd thing thing to do. Trojans are not cheap, so the cost per cycle of LiFePO4 is unlikely to be less than for Trojans, when you tale into account lifetime and usable capacity. Add the costs of the DC-DC converter and the complexity of either manual or automatic control and it doesn't seem very attractive. If the motivation for this is to keep the LiFePO4 pack below 100Ah for balancing purposes, then that's spurious, balancing large packs is a solved problem, even if the solution is not the one stated above.

 

MP.

 

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22 minutes ago, WotEver said:

Indeed. One approach that I’ve seen has been to have a single 100Ah (or thereabouts) LifePo4 battery which can be charged quickly with the engine and then put it in parallel with a bank of Trojans via a boost converter to allow them to have their nice long absorb for 6 hours or whatever, their initial charge having also been achieved with the alternator. So all the heavy lifting is being done by easily maintained Trojans, using the lithium purely to cover the lack of sun through the winter. 

Yes, that was my idea earlier this year.

Not sure anyone has taken it up. I think Nick was a bit concerned that the LA's would suffer as they would not be fully charged with the low voltage. It shouldnt be a problem as long as the LA's dont get too discharged.

 

 

eta....but my thread was not aimed at Trojans, more the cheapo end.

Edited by Dr Bob
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15 minutes ago, Dr Bob said:

I think most of these battery balancers work throughout charging and NOT just at the top knee. This one is not a lot different but is cheap!

This one needs 100mV to start the balance which will only happen when the charge gets one of the cells over 3.35 -3.40V ie near the top knee and likely over 90% SoC. At voltages or SoCs below that you will have all 4 cells in the plateau voltage and you just wont see a delta of 100mV - unless the cells are so widely out of balance.

I cant speak about the Valance cells and how they balance but having had my Thunderskys for 8 months now and done 2 balances.....first after 3 months and 2nd after 6 months....I am not really sure how good these boards are and are they worth the effort.

My cells were reasonably balanced when I installed them (nominal 480Ahrs capacity). On going to 100% SoC ....ie tail current of 15A at termination voltage.....I had a cell voltage delta of 50mV.

After 3 months, I had a delta of 200mV and wasnt quite down to the tail current limit so maybe 99% full. To balance I had to take 80Ahr out of one of the cells. On these balance boards that would have taken 60-70 hrs! How do you keep in the knee without damaging the cells? I did it manually by using as resistive circuit taking 10A so done in 8hrs. On the 2nd balance excercise I had about a 60mV delta which I sorted in 1 hr putting 6Ahrs into the lowest cell. I really neednt have bothered and likely wont need to balance again for 12 months but will go up to 100% every 3 months to resycn the BMV.

I bought one of the balance units that Tom had posted somewhere earlier for £40 and tried that but it also just balanced at 1A or so (at the 3.5V range) so gave up with that. Tom leaves his in circuit all the time and it works (it would if on all the time) but I cant do that because my 'high' cell once we get over 95% charge is my 2nd to lowest in the plateau range (about 10mV lower than the highest) so likely this would recieve current under normal <80% SoC range which would unbalance it as it would get to the knee even quicker. Tom must be lucky that his high cells in the knee are mid range in the plateau.

These balance boards are ok if you have small capacity cells ie <100Ahrs but if you have 480Ahrs then a 100mV delta could require days of balancing at 1.2A and my view is that its a bad idea to do that with cell voltages in the knee.

The good thing to say about this balancing device is that it does active balancing, ie the excess charge isn’t dumped as heat, it is transferred to other less charged cells. Consequently it is better suited to balancing whilst not being charged (no overall loss of charge, apart from efficiency factor).

As to this sentence “so likely this would recieve current under normal <80% SoC range which would unbalance it as it would get to the knee even quicker.” I’m not quite sure what you are saying, don’t forget that cells in series all receive the same amount of charge current (and hence charge) regardless of their voltage.

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1 minute ago, MoominPapa said:

Unless you already have the Trojans and can't sell them, that seems like an odd thing thing to do. Trojans are not cheap, so the cost per cycle of LiFePO4 is unlikely to be less than for Trojans, when you tale into account lifetime and usable capacity. Add the costs of the DC-DC converter and the complexity of either manual or automatic control and it doesn't seem very attractive. If the motivation for this is to keep the LiFePO4 pack below 100Ah for balancing purposes, then that's spurious, balancing large packs is a solved problem, even if the solution is not the one stated above.

 

MP.

 

The approach has been posited by someone who already has a Trojan bank and lots of solar. In the winter his solar can’t keep up (obviously). So rather than run the engine for many hours every day he can run it for just a few hours to get the Trojans to 80% and the lithium to 100%. A boost converter costs less than a tenner and it then trickle-charges the Trojans to 100%.

 

3 minutes ago, Dr Bob said:

 I think Nick was a bit concerned that the LA's would suffer as they would not be fully charged with the low voltage.

That’s what the boost converter is for :)

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10 minutes ago, nicknorman said:

The good thing to say about this balancing device is that it does active balancing, ie the excess charge isn’t dumped as heat, it is transferred to other less charged cells. Consequently it is better suited to balancing whilst not being charged (no overall loss of charge, apart from efficiency factor).

As to this sentence “so likely this would recieve current under normal <80% SoC range which would unbalance it as it would get to the knee even quicker.” I’m not quite sure what you are saying, don’t forget that cells in series all receive the same amount of charge current (and hence charge) regardless of their voltage.

No, what I am saying is that when not charging but under normal discharge or rest, these boards will transfer charge from a high cell to a low cell - (except the board in question will only start when the delta is 100mV which will only be when charging or in the bottom knee). Lets assume then the charge transfer is happening when in the plateau. If I have say my cell 2 which is 15mV lower than the high cell ..call it cell 4 (at little load) then the balance board will transfer charge from cell 4 to cell 2 until the delta is <10mV. My problem is that cell 2 is the first cell to reach the knee when we charge so it doesnt need the charge from cell 4 when in the plateau range. It will only make it get to the knee quicker......and yes I have used a multimeter to check the voltages are real!

The key here is that the voltages of all 4 cells when in mid range of SoC are not identical and can be up to 20mV different but that difference MAY NOT BE the same when approaching the knees.

Edited by Dr Bob
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