Myth of alternator damage cause by BMS disconnect?

[donradcliffe]

My alternator should be protected because there is no HVC between it and the 100 ahr AGM chassis/starting battery. Am I wrong?

[donradcliffe]

Originally Posted by goboatingnow

GooD summary in that link

Thanks for posting the link. I think I am going to make some changes to the storage mode.

I thought the claim for overcharge damage by pulse width regulation was interesting, but at the high pulse frequencies involved I would question whether it is really true. I think I am going to put a scope on the battery terminals and see whether they really do spike over 4 volts per cell. The link had no references, which concerns me that it may be only one man's theories.

[goboatingnow]

Originally Posted by PaulCrawhorn

Below only applies if LFP longevity is a priority.

13.5V = 3.38Vpc

is higher than what I would call 100% SoC at rest, 3.33 - 3.35V is better

around the result of HVC at 3.45Vpc.

Charging setpoint at 13.8V or lower, not holding CV / Absorb stage long.

It serves no useful purpose going higher, just unnecessary extra wear reducing lifespan.

Only go there when needed, loads going to be shortly pulling SoC down.

Storage mode should otherwise be the default if your use case allows. 13.2V = 3.3Vpc is fine, but really anything in the ballpark so long as parasitic draws are not allowed to pull anywhere close to 3.0V
Isolating the LFP bank is safer, let cheap lead service the parasites, then only need to take self-discharge into account.

Be especially wary of BMS, that often means Battery Murder System.


I do not see how.

The issue with voltage only charge current is boat batteries are almost always under load when charging sometimes heavy load. An LFP battery unloaded full SOC is around 18,8v whereas under load (O.25C) is around 13.4v. Hence a simple charge cutoff can result in unused SOC . Battery charge currebt less tail current needs to be factored in.

[goboatingnow]

Originally Posted by donradcliffe

My alternator should be protected because there is no HVC between it and the 100 ahr AGM chassis/starting battery. Am I wrong?

The Answer is yes and no. If the hvc occurs while the alternator is largely unloaded as is typically the case then yes The AGM will dampen the inductive spike enough , will the same be said in a worse case HVC (ie alternator under full load and the AGM is fully charged ) well it's debatable. Viewing it under a scope the battery will not dampen the spike fully , buts that's not to say the alternator diodes won't survive , especially if they are avalanche types. Wil! Any other electronics on the alternator side survive , maybe , maybe not. Stuff that's automotive qualified will most likely be fine.

Since the BMS is detecting the hvc the best way is to command the alternator to shutdown before the HVC switch opens. Others approaches deploy slow opening solid state switches and or deploying TVS diodes throughout the wiring

[PaulCrawhorn]

Originally Posted by goboatingnow

An LFP battery unloaded full SOC is around 18,8v whereas under load (O.25C) is around 13.4v

Check your math that is insane.

3.6V is way higher than anyone should go, data sheet Max V is a "do not approach" limit not a charge spec.

3.45V is plenty high to get to 100% Full. There is no significant extra capacity above 3.33-3.35V at rest, pushing more in is just faster wear, shorter lifespan.

If you really need to get every last bit mAh stuffed in there, shoulda bought bigger cells.

[goboatingnow]

Originally Posted by PaulCrawhorn

Check your math that is insane.

3.6V is way higher than anyone should go, data sheet Max V is a "do not approach" limit not a charge spec.

3.45V is plenty high to get to 100% Full. There is no significant extra capacity above 3.33-3.35V at rest, pushing more in is just faster wear, shorter lifespan.

If you really need to get every last bit mAh stuffed in there, shoulda bought bigger cells.

Sorry typo 13,8v

[s/v Jedi]

Originally Posted by PaulCrawhorn

Check your math that is insane.

3.6V is way higher than anyone should go, data sheet Max V is a "do not approach" limit not a charge spec.

3.45V is plenty high to get to 100% Full. There is no significant extra capacity above 3.33-3.35V at rest, pushing more in is just faster wear, shorter lifespan.

If you really need to get every last bit mAh stuffed in there, shoulda bought bigger cells.

I’m sure there are spec sheets showing something different, but often it says:

Max charge voltage: 3.60V
Charge rate: 1C

Let’s say it’s a 100Ah cell then what they mean is that you get a full charge by charging CC at 100A until the voltage hits 3.60V which is the termination voltage. This will work, but who can do that charge rate!?

So when you go at a lower charge rate, like 0.5C or 0.25C, then the cell will probably be fully charged at a lower voltage. Unless the specsheet shows a graph of charge rate vs termination voltage, you’re only gonna learn this using a coulomb counting battery monitor.

[CatNewBee]

Originally Posted by s/v Jedi

I’m sure there are spec sheets showing something different, but often it says:



Max charge voltage: 3.60V

Charge rate: 1C



Let’s say it’s a 100Ah cell then what they mean is that you get a full charge by charging CC at 100A until the voltage hits 3.60V which is the termination voltage. This will work, but who can do that charge rate!?



So when you go at a lower charge rate, like 0.5C or 0.25C, then the cell will probably be fully charged at a lower voltage. Unless the specsheet shows a graph of charge rate vs termination voltage, you’re only gonna learn this using a coulomb counting battery monitor.

Stop charge is 3.65V
Stop discharge 2.9V.

You can go more conservative, but that are the figures for the operation range.

[PaulCrawhorn]

Originally Posted by s/v Jedi

I’m sure there are spec sheets showing something different, but often it says:



Max charge voltage: 3.60V

Charge rate: 1C



Let’s say it’s a 100Ah cell then what they mean is that you get a full charge by charging CC at 100A until the voltage hits 3.60V which is the termination voltage.

No, Max Voltage is just that, a "DO NOT APPROACH" maximum voltage you should ever subject the device to under any conditions, very stressful to the cells.

Stresses in excess of the absolute maximum ratings can cause permanent damage to the device. These are absolute stress ratings only, functional operation of the device is not implied at these or any other conditions in excess of those given in the operations sections of the data sheet. Exposure to absolute maximum ratings for extended periods can adversely affect the device reliability.

If you care about the longevity of your bank, avoid going to "maximum" capacity. Basing your definition of 100% SoC on datasheet maximum as your CV (Absorb stage) setpoint AND holding Absorb for more than a minute or two is going to greatly reduce lifespan.

Letting the bank sit there for hours, even more so. It is best to target RESTING Volts as your benchmark, thousands of different charge profiles get you to the same result.

It is pointless to try "cramming" the tiniest bit (say the 99th percentile) extra capacity into the battery, seeing how many more mAh you get when you chase the last few .0x resting volts, anything above 3.33Vpc is just "surface charge effect" showing you went too high.

And for no constructive purpose, completely insignificant gain in actual measured capacity utilisation.

Same with current rate, anything much over 0.4C is harmful to longevity, that 1C example rate is another "DO NOT APPROACH" maximum, very stressful to the cells.

Now of course your bank your choice! If your use case dictates max capacity utilisation plus the fastest possible charging are very important, and you do not care about getting more than the 2000-3000 cycles from your LFP investment

then just follow those industry norms interpreting datasheet maximums as if they were charge recommendations.

[PaulCrawhorn]

WRT DoD for longevity start cutting unimportant loads, especially high current ones, at say 3.2V. Safety / comms / nav essentials that are low current continue to service at the expense of bank longevity. But refrigeration, heating/cooling, etc should be considered non-essential well before the bank hits 3.1V, giving you essential operations for many more hours, even days if needed.

Of course, the above applies to solar-only, if you have an on-demand charge source, that 3.1-3.2V range would be a signal to crank her up, at a higher voltage means much longer cell lifespan.

[donradcliffe]

Originally Posted by PaulCrawhorn

No, Max Voltage is just that, a "DO NOT APPROACH" maximum voltage you should ever subject the device to under any conditions, very stressful to the cells.




If you care about the longevity of your bank, avoid going to "maximum" capacity. Basing your definition of 100% SoC on datasheet maximum as your CV (Absorb stage) setpoint AND holding Absorb for more than a minute or two is going to greatly reduce lifespan.

Letting the bank sit there for hours, even more so. It is best to target RESTING Volts as your benchmark, thousands of different charge profiles get you to the same result.

It is pointless to try "cramming" the tiniest bit (say the 99th percentile) extra capacity into the battery, seeing how many more mAh you get when you chase the last few .0x resting volts, anything above 3.33Vpc is just "surface charge effect" showing you went too high.

And for no constructive purpose, completely insignificant gain in actual measured capacity utilisation.

Same with current rate, anything much over 0.4C is harmful to longevity, that 1C example rate is another "DO NOT APPROACH" maximum, very stressful to the cells.

Now of course your bank your choice! If your use case dictates max capacity utilisation plus the fastest possible charging are very important, and you do not care about getting more than the 2000-3000 cycles from your LFP investment

then just follow those industry norms interpreting datasheet maximums as if they were charge recommendations.

My choices on longevity are influenced by the knowledge that in a few years I will replace the current lithium bank with something that is cheaper and better.

[s/v Jedi]

Originally Posted by PaulCrawhorn

No, Max Voltage is just that, a "DO NOT APPROACH" maximum voltage you should ever subject the device to under any conditions, very stressful to the cells.




If you care about the longevity of your bank, avoid going to "maximum" capacity. Basing your definition of 100% SoC on datasheet maximum as your CV (Absorb stage) setpoint AND holding Absorb for more than a minute or two is going to greatly reduce lifespan.

Letting the bank sit there for hours, even more so. It is best to target RESTING Volts as your benchmark, thousands of different charge profiles get you to the same result.

It is pointless to try "cramming" the tiniest bit (say the 99th percentile) extra capacity into the battery, seeing how many more mAh you get when you chase the last few .0x resting volts, anything above 3.33Vpc is just "surface charge effect" showing you went too high.

And for no constructive purpose, completely insignificant gain in actual measured capacity utilisation.

Same with current rate, anything much over 0.4C is harmful to longevity, that 1C example rate is another "DO NOT APPROACH" maximum, very stressful to the cells.

Now of course your bank your choice! If your use case dictates max capacity utilisation plus the fastest possible charging are very important, and you do not care about getting more than the 2000-3000 cycles from your LFP investment

then just follow those industry norms interpreting datasheet maximums as if they were charge recommendations.

Big words against all that is known about this require citations to back it up. “Maximum” means “not to be exceeded” and not your interpretation “do not approach” and it will remain so until you show proof.

You show no sign of understanding the relationship with charge rate. Do you understand what I wrote above?

[PaulCrawhorn]

Originally Posted by s/v Jedi

.You show no sign of understanding the relationship with charge rate.

Please quote text I have written here that leads you to think that.

> Do you understand what I wrote above?

If you mean the idea of using trailing current spec to guide charge termination (Absorb stage Hold Time) then yes. However I do not advocate ever using CV stage at all. Bulk / Cc-only "charge TO Vsetpoint and STOP" is what I reco for normal cycling.

Forcing LFP batteries to sit at high SoC for any length of time is harmful to longevity, and there is absolutely no advantage to doing so while charging, unlike the lead imperative to avoid PSOC abuse.

True, some poorly designed balancing gear forces the user to do that, sometimes for many hours or even days! Which to me simply is a red flag, a sign to avoid such kit.

[PaulCrawhorn]

Originally Posted by donradcliffe

My choices on longevity are influenced by the knowledge that in a few years I will replace the current lithium bank with something that is cheaper and better.

Then definitely avoid any of my recommendations qualified by "if you care about longevity" just follow mainstream industry practices, e.g. post #203

[PaulCrawhorn]

Note this topic diversion only started in answer to

Originally Posted by donradcliffe

Can you translate high SOC to some real life pack voltages?

I have no axe to grind here, as I said do what you like, I plan to avoid being unnecessarily argumentative.

But if you go back and read the monster LFP thread, you'll see my points consistently reflect advice and evidence about optimising longevity based on long experience and deep research from, not only T1 Terry and Maine Sail, but many others whose knowledge I respect a lot.

Just one bit from Rod

https://www.cruisersforum.com/forums...ml#post2483118