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Old 08-09-2015, 19:23   #4666
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Hoopla
Thank you for your input. From my research their are a few ways to get around this issue with the alternator, not everyone is going to have the same system set up as you suggest.
I am just interested in hearing from people who have simular set up to mine and how they did their install if they are happy to share. You will notice that the last post I sent was actually directed to Eric.
Also a lot of the information I have been reading here is a couple of years old, so I am interested in reading the latest information.
Cheers
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Old 08-09-2015, 22:09   #4667
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Hello Nick,

Alternator voltage begins to matter with LFPs when they get close to full. As long as they are still absorbing, they don't give balance issues. Keeping the end-of-charge voltage on the low side is a great way of hiding cell balance issues. It also places less stress on the cells, but charging is not all about voltage: it is also a matter of how long you hold it (absorb) before stopping.

Some boats make short occasional use of the engine only and can get away with somewhat approximate setups on the basis that stopping the engine stops the charge.
Those who motor frequently and for long periods need a way to stop charging. Using a "low voltage" like some have done is not ok, unless it is so low that it essentially fails to charge and then what is the point...

The catamaran I mentioned earlier already had Next Step charge controllers, so I adjusted them to get 14.0V absorption for 30 minutes and 13.35V "float" (which is too low to charge LFPs and just high enough to prevent discharging them, so it terminates the charge). The owner lives aboard a lot and frequently motors short distances, occasionally longer ones. 14.0V gives decent charging efficiency with shorter engine runs and 30 minutes is not enough to get the bank to the end of the current taper, which is good.
That boat cycles batteries quite significantly, a bit like what you describe. If it didn't, I would have reduced the absorption time and voltage.

One issue with alternator controllers is reliability. Balmars are particularly bad on this front - my experience based on all the blown ones I have seen here - and they are essentially unrepairable due the circuit board being cast in silicone.
Others seem to do significantly better while being simpler and you can put new MOSFETs in them if/when they blow. A good alternator controller should have no heatsink fins and run cold. If it creates enough heat to need them, it means its design is poor and/or intended to be as cheap as possible to manufacture.

If you are not intensely focused on fast alternator charging, you can sometimes opt for charging through splitting diodes. Those will commonly drop between 0.9V at high current and 0.4V at low current. As a result, they tend to also limit alternator output: e.g. losing 0.9V @ 120A would mean a charging voltage of 14.3-0.9=13.4V, which is way too low to actually push 120A, so the system settles for a lower current and lesser voltage drop, which keeps the alternator cooler. When the current tapers near the end of the charge, the voltage drop becomes closer to 0.4V and the charging voltage rises to 14.3-0.4=13.9V, which is not bad.
If you motor occasionally and not for hours or more in a row, it can work out. Otherwise you still need a way to terminate the charge and using external controllers is a practical way of achieving that. Another one is interrupting the field circuit (alternator off), or disconnecting the LFP bank from the isolating diodes - provided there is a start battery left to act as a remaining load.
Some alternators (like the Mitsubishi found on all the new Volvo engines) have external voltage sensing. This opens additional avenues. The options are too numerous to be fully discussed here, but many were here in the past if you take the time to go back and read.

You need to start by assessing what you have got. This includes how your alternators actually behave, what is their regulation voltage, where they take their voltage reference from and how you get the current to the batteries.
Multihulls can greatly benefit from isolating diodes anyway, because you can split each alternator 3-way: port start, starboard start and house bank. This provides very simple redundancy and small DC/DC boost chargers can top up the starting batteries from the lithium voltage up.

Junsi cell loggers are toys really. Their voltage reference is inaccurate and they load the cells unevenly, so leaving them connected is out of the question.

Best regards,

Eric
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Old 09-09-2015, 00:23   #4668
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I have my setup so I can manually change from the engine in emergency's, temporarily connecting my LFP cells in parallel with my starter battery. HVC controls remain in effect when I make this connection. So should I ever forget to switch it off, the LFP bank will be disconnected from everything, requiring manual intervention to get everything working again. My normal charge is done through 900W of solar and 400W of wind power, which is normally enough for all our activities.

I do use yunsi for cell monitoring, and have found no problems with them.

But that is my situation, you might find they don't work in your situation.

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Old 13-09-2015, 15:35   #4669
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

It is interesting feedback about the Junsi. Someone had disassembled and partly reverse-engineered one some time ago (other forum however) because its calibration kept changing all the time. It was also powering itself from the lower two cells rather that the whole battery and this was one of the reasons why it was throwing the bank out of balance.

These design issues could have been fixed in later versions, but it would be worth having a close look at individual cell currents. I suppose you are using the 8S version with the alarm output controlling a disconnect?
They are so cheap that it is almost baffling. A single precision voltage reference would already cost a significant fraction of the value of the whole device!

A couple of years ago I tried designing a simple marine BMS board people could assemble as a kit, no microprocessor or complicated components, but safe and robust. Making it safe (so I could sleep at night) was the issue: it got too complicated to control without a CPU and I had to review that idea.

In order to make these things safe, you need redundancy. Then you need to keep checking for any failure and shut down properly if one of the circuits has failed.
Many people don't realise that these little BMS circuits that hold a contactor on while everything is ok are anything but fail-safe. The most likely failure mode is blowing the transistor that is holding the contactor on. When that happens, 99% of the time the transistor turns into a short-circuit and the contactor stays on. The failure is completely invisible (it is on when it should be on) until the board tries to drop the contactor on a HV/LV event. Then nothing happens.

In terms of engineering risk management, it is the worst case scenario: the failure is unpredictable and then completely undetectable until it has had consequences.
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Old 13-09-2015, 16:58   #4670
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by OceanSeaSpray View Post
Many people don't realise that these little BMS circuits that hold a contactor on while everything is ok are anything but fail-safe. The most likely failure mode is blowing the transistor that is holding the contactor on. When that happens, 99% of the time the transistor turns into a short-circuit and the contactor stays on. The failure is completely invisible (it is on when it should be on) until the board tries to drop the contactor on a HV/LV event. Then nothing happens.

In terms of engineering risk management, it is the worst case scenario: the failure is unpredictable and then completely undetectable until it has had consequences.
It would be important to know why this boards fail & that their specifications have not been exceeded. A PM has been sent re this a matter.

Regards Bill
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Old 13-09-2015, 18:08   #4671
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

MOSFET transistors fail in switching applications due to stresses, typically voltage spikes at turn-off and current inrush at turn-on.

How much stress they endure depends on the load they are switching, their rating and how well protected they are by additional components. The issue is that stronger, more resilient devices cost more. If you want a fail-safe design, you also need to double them up, so it is now twice a higher cost compared to a cheap board using a single low voltage transistor for each output.

Solenoids should always have a reverse spike protection diode. The more current they switch, the harder they are on the control circuit. Because the problems arise from the transients at switching, it is hard to define what is or is not within specifications. The answer is to built the control circuit to industrial strength and it is more costly.
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Old 05-10-2015, 11:41   #4672
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I've been running a set of 4 Balqon 700 AH cells as my house bank for the last (almost) 2 years. So far so good. They typically sit at about 50% charged when not at use and charge up before a cruise. Mostly weekend cruising at this point.

Primary charging is by an older Xantrex Freedom SW3012 (815-3000) inverter charger that I closely monitor when charging. The Inverter/charger is shut off when we are at the dock and I'm not monitoring it.

Underweigh charging is via a balmer 100 amp alternator with a MaxCharge MC 612 using Mainesails charging parameters (more or less).

The bank uses a HousePower BMS without a disconnect relay and cellboards without the shunt resistors.

When originally installed the pack was top balanced to 3.8 VPC on the bench then assembled in the boat. With the pack usage monitored with a Victron BMV600s.

I have been noting that one of the cells tends to have a higher voltage under high rate charging than the others and then falls to within a few mV of the others when at rest. to me this indicated a need to top balance again. Why the cell is out from the others I cannot say. It could be that the original top balance what not done as well as it should have been or perhaps an issue with the cell. The cells were from the balqon "clearence" at a great price.

Right now I am rewiring the house bank wiring so as to move the shunt much closer to the bank and to minimize the alternator to bank wiring length. The alt currently peaks at about 70 amps. It is an older 91-100 and should do better. In the end I will end up with a total charging cable length of less than 3' each way (6' total) through 1/0 with 2/0 to the house panel. The inverter/charger will still have a much longer path which will help compensate for the lack of a LiFePO4 setting.

It looks like setting the inverter/charger to the AGM setting will provide the safest match (bulk - 13.45, Absorb - 14.3, Float - 13.45).

I will likely upgrade to a serpentine belt with a balmar at-160/mc-614 at a future date (driven by a perkins 4.236).

At the moment the pack is paralleled and was brought to at 3.600 v at 20 amp (5 amp per cell or 5/700= 0.7% of capacity). The pack has been sitting overnight without charge to let the cells equalize further. Then Later today I will bring the back voltage up to 3.65 and hold it there till the current drops down to less than 1% of capacity. I'll disconnect the charging voltage and let it rest with the cells in parallel a day or few before reconnecting them in series.

In the re-wiring process I am noting that having an isolated ground alternator would be desirable. In order to minimize the alternator to battery cable length I am taking the ground directly off the alternator to the battery pack (though the shunt of course). This ends up making multiple ground connections that may cause a ground loop.

The system reference ground is the negative buss at the house panel. The house bank is connected there via 2/0 cable. The negative bus also connects to the engine at the starter and from there to a dynaplate.

The start battery negative connects to the engine at the starter and has an isolated charge system (Using a 10 amp charger powered by the inverter after engine start).

Anyway this is something to think through and do some measuring with. Having an isolated ground alternator would break the ground loop from between the house banks and start bank through the engine.

After finishing off the top balance and getting a bit of time in use I will get back to say if that got rid of the cell voltage imbalance on charge.

Comments of an informative kind always welcome.
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Old 05-10-2015, 11:49   #4673
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

One more thought - I notice that Balqon has a manual published for their packaging of their cells. The HIQAP system specifies:

bulk 13.8
Absorb 14.2
float 13.5

interesting that these figures are starting to come inline with the thinking here on CF.

regards
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Old 05-10-2015, 12:06   #4674
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Funny about the float , we don't like to float our Lithiums .

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Old 05-10-2015, 13:24   #4675
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Funny about the float , we don't like to float our Lithiums .

Regards
Yes it is so. 13.5 is higher than the 13.2 that Mainesail recommends (correct me if I'm wrong).
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Old 05-10-2015, 14:01   #4676
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Yes I think you are right . I think Mainsail issues 13.2 float setting on his 614 Balmar regulator to essentially turn off charging the batteries from the alternator . So really what is best is no float at all , just charge and stop .

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Old 05-10-2015, 14:43   #4677
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

EVM, I wonder which cell was going high when fully charged? I have noticed that if left idle for long periods the cels(s) closest to the negative terminal often reach a full charge first, even if left after balancing. If the pack is cycled this phenomenon is reduced.
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Old 05-10-2015, 16:07   #4678
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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When originally installed the pack was top balanced to 3.8 VPC on the bench then assembled in the boat.
This is way too high, but what is done is done. Next time, just charge them to 3.6V max until the current is less than 2-3% of the capacity, like what you are doing now by the sound of it.

Quote:
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I have been noting that one of the cells tends to have a higher voltage under high rate charging than the others and then falls to within a few mV of the others when at rest. to me this indicated a need to top balance again. Why the cell is out from the others I cannot say. It could be that the original top balance what not done as well as it should have been or perhaps an issue with the cell. The cells were from the balqon "clearance" at a great price.
The reason is dead simple: that cell has higher internal resistance than the others. They may have been cheap, but the manufacturing process was not consistent or you got random cells out of different batches etc.
It is also going to keep running just a little warmer than the others and age more quickly and things are only going to get worse.

Another thing that happens is that their self-discharge rates can be quite different, and then your bank keeps drifting out of balance forever.
I am really fussy when sourcing cells and price is not the first thing I look at.

Differences in internal resistance is why shunting cell boards don't work. The cell voltage during charging at high current doesn't just reflect the state of charge and then cell boards throw the balance out instead of correcting it. Voltage-based shunting can only be done at very low current and cell boards can't see the charging current.

Quote:
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It looks like setting the inverter/charger to the AGM setting will provide the safest match (bulk - 13.45, Absorb - 14.3, Float - 13.45).
I wouldn't do that either, especially with a shore power charger. The open-circuit resting voltage of a fully charged LFP cell is about 3.35V, so anything above this is going to try and keep shunting lithium towards the cathode and plate it.
With an alternator, at least you get charge termination with the engine is stopped, but solar and shore power chargers are killers with floating banks.
13.3V is a good end of charge holding voltage for systems that run for limited periods (engine typically) and otherwise 13.2V because you need to allow the battery to discharge again.
Also watch out, many shore chargers kick back into absorption again after a set time period regardless and this is totally out of the question. Same with many solar controllers, they kick in again each morning and the battery doesn't have a chance to cycle properly.

14.3V absorption is going to give you HV headaches in fairly short order, it is too high as well (3.575V/cell!). As the cells move up towards 100% SOC, balance gets increasingly touchy. I don't absorb at more than 14.0V and stay lower whenever making the most of the energy available is not a consideration (shore power, solar in the tropics, etc). Even just 13.6V will get you a full battery and overcharge after not that long really.

Inverter/chargers are a pain too with lithium, because you can't properly split the load and charge buses. Considering this and all the issues above, your best bet could be using it as an inverter only.

Quote:
Originally Posted by evm1024 View Post
At the moment the pack is paralleled and was brought to at 3.600 v at 20 amp (5 amp per cell or 5/700= 0.7% of capacity). The pack has been sitting overnight without charge to let the cells equalize further. Then Later today I will bring the back voltage up to 3.65 and hold it there till the current drops down to less than 1% of capacity. I'll disconnect the charging voltage and let it rest with the cells in parallel a day or few before reconnecting them in series.
Letting cells sit in parallel is a waste of time. They just don't equalise, there is no voltage difference to drive any current. I deliberately tested and measured this. You just need to finish charging them in parallel at low current, then put them in series right away and discharge the bank a good 20%, even down 50% if you are not going to use it after balancing. They hate sitting around at 100%.
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Old 05-10-2015, 16:42   #4679
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thanks Ocean....

Yea the 3.8 was an older recommended limit that has been lowered based on experience.

In any production run we can expect differences in parameters. It would be nice to have a whole pile of cells to choose from.... I am hoping that with this top balance the voltage during charge will converge at the top.

I do use the inverter charger as an inverter only except when charging the house bank. When charging I always monitor it, specifically the pack voltage with some spot checks of cell voltages. As the pack voltage rises I dial the charge current limit down so as to not exceed 3.5 vpc (3.45 more typically).

Then the charger is disabled. This inverter/charger will automagically turn the charger on after a power cycle so I isolate (disconnect) the inverter/charger battery cable from the house bank when I'm not using the inverter.

My comment in regards to setting the inverter/charger to AGM battery type was meant to indicate the safer setting for when the Human Screw Up factor was involved. As in forgetting to isolate it.

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Old 05-10-2015, 17:05   #4680
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

No worries. They won't "converge at the top", the issue you are describing has nothing to do with the state of charge. It is the amount of current you are running that is skewing the voltage differently between cells and nothing is going to fix this.

All you can do now is get organised so it doesn't cause your BMS to trip while charging. Keep your charging voltage down a bit so that cell doesn't read HV at high current. Once the current starts tapering down, the difference will also subside.

When I buy cells, I specify I want them straight out of the factory crate, never touched by anyone afterwards, from the same production batch with consecutive numbers, all voltages reading below 3.300V and none differing by more than 1mV.

I am difficult, but I can get that. You are not the first here reporting getting a complete outlier with Balqon...
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