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Old 20-04-2013, 17:41   #121
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Yeah, I come from the RC world on this, via electric bikes.

Cells in paralell will stay balanced, but if you have one bad cell in one of four large 3,3v packs it will cause trouble. This pack will lose voltage faster than the rest, and will cause constant balancing issues.

With, say 5 13v batteries with balancing leads, you easily find the bad cell. You just disconnect the balancing leads from each other for a while, run the bank down to 35-40%, then check each battery with the cell-log. Bad cell will have lower voltage.

.manitu
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Old 20-04-2013, 18:48   #122
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Originally Posted by manitu View Post
Yeah, I come from the RC world on this, via electric bikes.

Cells in paralell will stay balanced, but if you have one bad cell in one of four large 3,3v packs it will cause trouble. This pack will lose voltage faster than the rest, and will cause constant balancing issues.

With, say 5 13v batteries with balancing leads, you easily find the bad cell. You just disconnect the balancing leads from each other for a while, run the bank down to 35-40%, then check each battery with the cell-log. Bad cell will have lower voltage.

.manitu
This is all correct. IF I have a bad cell, it's hard to find. Especially if it's just weak or has a trivial internal short. For big failures, I have fuses that will lead me in the right direction.

For marine, the large prismatic cells vs the cylindrical in RC are not supposed to fail. Optimistic perhaps, but based on much slower rates of charge and discharge relative to the capacity of the bank in this application. I don't think anyone has posted of a cell failure if the proper voltages were maintained. But our marine community with LFP is still small.

Attached is a picture of the config I presently have (but extended for 12P cells) along with the fusing where a complete cell failure would be evident. I am thinking of changing it to the configuration shown on the bottom, but undecided between this and removing all fuses for other reasons. It's still a work in progress. The fuses may not be needed at all or even desirable.
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Old 21-04-2013, 09:33   #123
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Re: Lithium batteries (for the rest of us)

The way I see it:
Top picture: a cell short circuiting may blow all the relevant parallel fuses. The opening of the fuseís will not prevent the relevant parallel strings discharging their energy into the faulty cell.
Bottom picture: As above but may prevent the complete bank discharging into the faulty cell if the fault current is sufficient to open the relevant fuse. If the rating of the fuses is such that each fuse shares an equal fraction of the load an unbalance may open all the fuses.
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Old 21-04-2013, 11:20   #124
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Re: Lithium batteries (for the rest of us)

Fundamental to this whole discussion is that there have been few, if any, LFP cell failures reported when they are used as a house bank and have not been abused.
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Old 21-04-2013, 11:39   #125
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Re: Lithium batteries (for the rest of us)

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Fundamental to this whole discussion is that there have been few, if any, LFP cell failures reported when they are used as a house bank and have not been abused.
When I talked with a LFP company engineer regarding a house bank setup recommendation, he suggested that I keep it simple with as few cells as possible. He said that cell failures are few and far apart, much less than with LA batteries. He also said their batteries on electric carts, which routinely discharge down to 10% as they want to extend range, very rarely have problems.

I understand that there are no number comparisons in the statement above but it certainly pushed me into keeping things simple. Boats are to be enjoyed...
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Old 21-04-2013, 11:59   #126
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When I talked with a LFP company engineer regarding a house bank setup recommendation, he suggested that I keep it simple with as few cells as possible. He said that cell failures are few and far apart, much less than with LA batteries. He also said their batteries on electric carts, which routinely discharge down to 10% as they want to extend range, very rarely have problems.

I understand that there are no number comparisons in the statement above but it certainly pushed me into keeping things simple. Boats are to be enjoyed...
You and Charlie are probably right. However, there is not a good consensus of experts. The current fuses are only there at the suggestion of the PhD who owns the company that sold me my cells. But it is flawed, as is the alternative I showed before.

I'm still thinking about removing all the fuses, but if I leave them, a better configuration is attached. It's the only one that I think will not only isolate the cell from parallel, but also serial connections. Although middle cells shorting may take the whole bank offline.
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Old 23-04-2013, 10:27   #127
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Re: Lithium batteries (for the rest of us)

your fuses across the batteries at the point labeled 6v, could cause reverse flow from a high v battery through a low v battery. would this trickle cause any problems?
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Old 25-04-2013, 06:40   #128
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Re: Lithium batteries (for the rest of us)

Did anyone mention that the LIFEPO are almost unaffected by temperature - this is an obvious advantage...
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Old 25-04-2013, 06:50   #129
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Re: Lithium batteries (for the rest of us)

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Did anyone mention that the LIFEPO are almost unaffected by temperature - this is an obvious advantage...
Except when it gets really cold..... Not a huge issue other than for high latitudes cruising.. My bank will not be left on-board during the winter season....
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Old 25-04-2013, 06:53   #130
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Re: Lithium batteries (for the rest of us)

I actually meant high temps...the lifespan is not reduced by hot temps on LIFEPO whereas LA's lifespan is greatly reduced by hot temps.
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Old 25-04-2013, 06:59   #131
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Re: Lithium batteries (for the rest of us)

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I actually meant high temps...the lifespan is not reduced by hot temps on LIFEPO whereas LA's lifespan is greatly reduced by hot temps.
Yes this is a huge +.... I see far too many builder slapping LA batts in ENGINE/GENERATOR ROOMS! The owner is then pissed when the batts last two years....
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Old 28-04-2013, 18:02   #132
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Re: Lithium batteries (for the rest of us)

This not entirely true, Li batteries die as a result of heat, but not external, internal heat kills them. If there is no way for the cells to dissipate their internal heat they will fail, I have 2 cells that this happened to, the inner 2 cells in a parallel string of 4 cells, the outer 2 cells are still functioning, even though the charge voltage went up to around 4.6v.
This is not an issue during normal use and the cell voltages staying within the safe parameters, and 3C discharge voltages are the max. but if something goes wrong with the charge controller, the cells will get internally heated due to over voltage, they need some method of dissipating this heat. if the ambient temp is above 60 deg C, there is no hope of the cells cooling themselves, they will gradually heat soak, so environmental temp is still an issue. My recommendation would be a blower fan through the battery box and vented outside the engine room when the battery is charging or very heavily discharging, this will remove the heat and any possible electrolyte vapours so there is no vapour build up. The cells vent electrolyte vapour if overheated, no one ever intends to deliberately over charge their batteries, that doesn't mean that accidents will never happen, better to be safe than sorry.

As far as the single high Ah capacity cell v the multiple small capacity cells in parallel to build the same ah capacity, I'd choose the multiple smaller cells every time. There are a number of reasons for this, averaging manufacturing tolerances and access to connections.

The first point, averaging manufacturing tolerances.
No 2 cell will ever be identical, they are close, but not identical. For example, one cell may have 101Ah capacity, another 99Ah capacity, one cell may have slightly higher resistance than the other, these differences will mean one cell will reach a higher cell voltage while charging than the other, and one cell will reach the end of capacity before the other. If these were single cells in series, balancing would be more of an issue, but these 2 cells in parallel would balance each other out, hope that made sense.

Access to connections.
High capacity cells just have a lot more soft packs linked together to the negative and positive terminals, like plates in a lead acid battery, but inside the plastic case, they are simply bolted to the terminal block, no fancy soldering or welding, just a pinch bolt. If these connections are on the outside, when we suspect a poor connection is causing irregular reading, much higher when charging and suddenly much lower on discharging, a quick check with an IR thermometer while the connections are under a high current load will reveal the crook connection. Clean the two faces and reapply so Alminox, and the problem is solved and the pack resumes normal operation, remember, we are talking about a battery pack that will last a very long time. You can't do that if it bad connection is inside the plastic case, that cell maybe still in good condition, just a poor connection means you have to replace it.

Sorry if this is too deep for the intended scope of this thread, if so, maybe the admin team could transfer it to the lithium discussion thread and place a link here instead.

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Old 29-04-2013, 15:16   #133
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Re: Lithium batteries (for the rest of us)

Has anybody ordered from Balqon recently?
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Old 30-04-2013, 02:27   #134
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Re: Lithium batteries (for the rest of us)

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I just realized that the concept of balancing leads has not been mentioned in this thread. All RC battery packs has a standarized balancing lead, with one wire between each single cell in a battery pack. This allows several packs to be paralleled together, cell by cell.

.manitu
I guess you could have parallel batteries with balance leads, but they would need to be big buggers. If you were talking about a 500ah battery pack made up from 5 paralleled 100ah batteries with each 3.2v battery paralleled as well, if the battery at the end of the string requires the most charging, it has to flow through all the parallel links, if a cell goes high it must flow back to the other cells in the parallel string, you couldn't afford to have voltage drop through the links or you would create cell imbalance as some of the charge current would be wasted as heat, but only in that cell group, so the cables would need to be quite substantial....I guess, why make an easy job difficult is my question.

My 720Ah pack is past 700 cycles now, still in balance, been down to zero state of charge quite a few times now, you just can't trust the sun to come out when it's needed can you. I have 8 x 90ah cells in parallel, 4 sets in series, a 720ah battery, it powers most of the heavy energy users in my house, around 200 ah overnight, has done for the last 2 yrs.
If a cell is going to fail, it does it gradually, not a sudden failure, lead acid is the same generally unless it's a link that fails, then itís just open circuit. The difference is, with Li prismatic cells you can monitor each cell groups voltage very simply and cheaply with a Junsi cell logger. You know when a cell is going down, it pulls all the cells in that parallel group down, even an ah per day will show up fairly quickly when you reach the top or bottom of the charge cycle. It's not a problem requiring an instant fix, but you need to know there is a problem and the overall battery capacity is dropping, the weakest parallel cell group governs that, but you can keep an eye on it, even charge that one parallel group to make up for the loss to buy you a bit more time if needed.
When you have a safe time when you can attend to battery maintenance and finding the errant cell, disconnect half the cells in the pack, say in my case, split it into 2 x 360ah packs, run off one pack and charge the other, charge up the low group of cell in each pack to 3.6v with virtually zero current flow at the end of charge, now watch which group start to lag behind the others and drop their voltage, the problem cell is in that group. Charge that group back to 3.6v, drop all the links off between the cells in that group, leave them for a while, measure the terminal voltages, the crook cell will stand out, bypass it with a length of battery cable if you aren't carrying a space cell to replace it, join the pack back together with one cell from each of the other 3 parallel groups disconnected. You still have a 12v house battery that will stay in balance, the capacity is less the capacity of one cell, in my case that would be 90Ah, till you can organise a replacement. Not the end of the world, Iíd still have a 650Ah house battery till I replaced the bad cell.
The fact you can replace a single cell in a big battery pack at your convenience must make it a much better method of maintaining the power you need to survive. You can't tell when a single cell is struggling in a 12v AGM battery, you don't have access to the individual cell voltages, you don't know until that bad cell has pulled the whole battery pack down.

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Old 30-04-2013, 05:21   #135
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Re: Lithium batteries (for the rest of us)

Again its worth stating , in my experience, franctional C environments, both in charge and discharge as is typical in boats, there is no need to balance cells, after the initial once off balance at the start.

with EV, and the RC crowd, these are in high C environments

dave
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