LiFePO4 Batteries - Okay Tear Me Apart ;-)

[electric1]

Also, on a large bank, say 400Ah, .01C is 4A, your MiniBMS shunting is rated at only .75A?

I want to clarify this common misconception about shunting. Shunting is never intended to dissipate entire finishing current. Its only meant to slow down the charge on cells which reach HVC a bit sooner than others, to equalize their SOC. Once any cell reaches HVC BMS will force the charger off to prevent overcharge. Shunting is not meant to prevent overcharge, its only meant to equalize the SOC a little bit at a time. Depending on how much imbalance there is between cells equalization might take several charge/discharge cycles.

However, this is mostly applies to traction packs with large number of cells and imbalance is more evident when discharge is regularly exceeding 2C levels. In house bank with 4-8 cells and less than 1C loads you will never see any imbalance unless you are mixing cells from different sources at different initial SOC ( which is easy to address by charging cells individually with 4V bench supply to get them on same SOC level)

[s/v Jedi]

Hi Electric,

I know you are past all this but this also means you are well qualified for this: can you explain the BMS in general and compare your design with that? I'm not sure if I understand the functions of the BMS completely and also if your design addresses all that or not. Don't be afraid to use tech talk where needed. You are the first person I meet that can explain this to me I think.

I am also interested in your view of rules of thumb for sizing the battery bank. How do you look at a short big load (go up to 2C or more?) and how about medium but sustained loads. In which SOC range do you like to work (you mentioned longer life when not fully charging) etc.

thanks,
Nick.

[imcbride]

Hi Charlie,

Originally Posted by CharlieJ

Allow me to clarify/amplify the ABYC Standards (E-11)

Thanks for the details. Please correct me if I'm not interpreting them correctly in this case.

For spade connectors, we're only talking about ~.75A, so as long as the connectors can withstand a physical 6lb pull for a minute they're OK. electric1, are these locking spade connectors or would they come apart from simply pulling on them?

As for soldering, the connection pad/solder pad size of 1.5x the diameter of the connecting wire isn't too restrictive. I've seen other BMS cell boards that probably meet this criteria by forgoing the spade connectors in favour of soldering.

Iain

[jallum]

Originally Posted by electric1

Yes, momentary push to reset BMS and start/stop the charger. Again, I don't see HVC happening in your case since you use chargers made for LA batteries and they don't go as high to reach HVC on a LFP batery.

HVC on the ThunderSky cells I have (300Ah) is listed at 4.25v and LVC is 2.8v. Since I'm shooting for 24v nominal and banks of eight in series, that'd work out to 34v(!) on the high-side, and 22.4v on the low side. That seems to line up nicely with the statements above.

(http://www.thunder-sky.com/pdf/20092131633.pdf)

[imcbride]

Originally Posted by electric1

Shunting is never intended to dissipate entire finishing current. Its only meant to slow down the charge on cells which reach HVC a bit sooner than others, to equalize their SOC. Once any cell reaches HVC BMS will force the charger off to prevent overcharge. Shunting is not meant to prevent overcharge, its only meant to equalize the SOC a little bit at a time. Depending on how much imbalance there is between cells equalization might take several charge/discharge cycles.

That is very good to clarify, thanks. It gets sticky, though, at the point of "BMS will force the charger off". This is what we've identified as a problem.

In house bank with 4-8 cells and less than 1C loads you will never see any imbalance unless you are mixing cells from different sources at different initial SOC

Unfortunately, this is bound to happen to someone, sometime. The BMS is just protecting the battery bank. But our charging source needs protection too.

If the BMS is going to be more or less fool-proof, it needs to protect the charging source (alternator diodes). What if a corroded connection in the battery wiring (granted, a maintenance failure) shorts and causes the BMS to open the solenoid during a full current bulk charge. The field collapses and and uncermoniously fries the alt diodes.

All we need is an alternator regulator with some sort of field control circuit, or as S&S suggests, a fancy way to bring the alternator field down gracefully. Genasun says they have a "alternator interface" available. Anyone asked them what this is/does?

Iain

[s/v Jedi]

I am not so sure that the diodes will blow when you interrupt the field current during full output. This does happen when you interrupt the alternator output (every time you try it and I did) but when you interrupt the field current, isn't there enough built up field to automatically provide some "cushioning"? I mean it takes some time to collapse, just as it takes time to build up.

About the high voltage: what Electric1 said is that the battery specs do not define the highest voltage you will see... these specs show the highest voltage allowed. The highest voltage you will get is the maximum voltage of the charger, so say 14.4V for a 12V bank. There is no current "pushed" into a battery after the battery reached that 14.4V level.

About ABYC: they are not talking about circuit boards there. The protection that Electric1 suggests will do fine on a boat and as long as you don't hang 5 pound weights on any wires, the blade connectors will be fine too, probably even when you put those weights on. Remember, ABYC is just guidelines and they target wires that can be caught onto with something with that 5-pounds rule. When you tie it down it's a moot point.

ciao!
Nick.

[electric1]

Originally Posted by imcbride

For spade connectors, we're only talking about ~.75A, so as long as the connectors can withstand a physical 6lb pull for a minute they're OK. electric1, are these locking spade connectors or would they come apart from simply pulling on them?

Spades have holes and connector has a bump that goes into it to prevent it from sliding off. These are very tight and difficult to pull apart. I'd say its definitely more than 6lb force to pull them apart. Also, these connections don't carry 0.75 Amp, these are signaling connections and carry less than 5 mA ( 0.005 Amp ) current. Based on specs posted above I'd say there is no issue with connections at all.

[imcbride]

Originally Posted by s/v Jedi

I am not so sure that the diodes will blow when you interrupt the field current during full output. This does happen when you interrupt the alternator output (every time you try it and I did) but when you interrupt the field current, isn't there enough built up field to automatically provide some "cushioning"? I mean it takes some time to collapse, just as it takes time to build up.

I'm still learning about alternator wiring, up to now I've just done what I'm told, i.e. don't touch while running the engine. A further search of the all-knowing Google turned up Blue Sea's Alternator Field Disconnect resource page: Alternator Field Disconnect - Resources - Blue Sea Systems

Assuming that's correct, and I tend to trust them, all the system would need to do is break the field excitation circuit between the alternator and the external regulator and it would stop the alternator output without frying anything. BMS/battery is happy and alternator is disengaged.

S&S - did you have a problem/concern with this approach?

Electric1 - So we would want to have two capabilities. On HVC, break the alternator field circuit. On LVC, interrupt the actual current flow (solenoid, per your original diagram). Thoughts on how to work the alternator field circuit disconnect into your BMS wiring/design?

The battery specs do not define the highest voltage you will see... these specs show the highest voltage allowed. The highest voltage you will get is the maximum voltage of the charger, so say 14.4V for a 12V bank. There is no current "pushed" into a battery after the battery reached that 14.4V level.

Understood. I've read that repeatedly. Basically, don't try to push them 100% of the spec'd voltage. 3.65-3.7V/cell (Thunder Sky) is more than enough, just leaves you with a few percent short of total capacity.

ABYC: they are not talking about circuit boards there. The protection that Electric1 suggests will do fine on a boat and as long as you don't hang 5 pound weights on any wires, the blade connectors will be fine too, probably even when you put those weights on. Remember, ABYC is just guidelines and they target wires that can be caught onto with something with that 5-pounds rule. When you tie it down it's a moot point.

Thanks, sounds reasonable enough to me. It's the interpretation the regs that is the tricky part.

Iain

[electric1]

Originally Posted by imcbride

That is very good to clarify, thanks. It gets sticky, though, at the point of "BMS will force the charger off". This is what we've identified as a problem.

Unfortunately, this is bound to happen to someone, sometime. The BMS is just protecting the battery bank. But our charging source needs protection too.

If the BMS is going to be more or less fool-proof, it needs to protect the charging source (alternator diodes). Iain

I absolutely agree that alternator must be protected from failures and if current diagram has an issue then it must be addressed. The diagram was meant to show how BMS protects the battery. I did not take into consideration possible issues with alternator. I need to do some research on this and would appreciate if someone here has ideas how to address it. This must have come up even when using Lead Acid batteries, for example when someone switches from bank 1 to bank 2 while generator is running, won't that blow the diodes during the switch?
I imagine the issue really lies with alternator being sensitive and unprotected rather than systems at the other end of the wire causing the issue.

[imcbride]

Originally Posted by electric1

This must have come up even when using Lead Acid batteries, for example when someone switches from bank 1 to bank 2 while generator is running, won't that blow the diodes during the switch?

Yeah, we just don't do it while the generator is running. See my link above, though, to the Alternator Field Disconnect switch from Blue Sea. This only works if you have an external regulator, but these are relatively common on upgraded electrical systems. Balmar and Ample Power/Next Step are two common makes.

Per Blue Sea:

The AFD is constructed in such a way that the AFD switch does not close until slightly after the main switch contacts have closed and it opens slightly before the main switch contacts open. This insures that there will always be a path for the alternator's output current when the alternator is producing power. The field disconnect will only work on alternators with an external regulator.

So I guess the BMS could first break the field sense wire, then open a solenoid on the current wire from the alternator to be "sure" that no more current gets to the battery.

Iain

[electric1]

I think the alternator issue can be addressed by RC filter across contactor's terminals. For example a low value resistor, say 10 Ohm, and high value capacitor, say 1000 uF, in series across the contactor. In this case, when contactor opens the current flows thru resistor and charges capacitor, which provides graceful discharge of the alternator's field energy and saves diodes. Once cap is full its like an open circuit and battery is protected. When contactor is closed again, the cap discharges thru resistor gracefully.

Any flaws in this idea?

I read the linked page about field control, but it only applies to alternator models which have external access to field coils, so any solution which depends on it won't work with other alternators. I don't know how many different alternators are out there....

I'll try to think about other solutions too....

[S&S]

Originally Posted by jallum

I was thinking about this... maybe this is me being a dumbass, but what about putting the alternator on a magnetic clutch? Instead of cutting the field wire, you could drop out the clutch and let the field decay as the rotor spins down?

Possible. I don't see why that wouldn't work. Clever.

Now to spec a mag clutch that will handle 15 hp@ 8000 rpm rotor speed.

[senormechanico]

This is a great discussion, but it reminds me of the USA mommy government back in the '70's wanting to pass laws requiring cars to be able to be survivable in a 70 mph crash into a concrete wall. Remember Ralph Nader?

Wiring shorts in your boat, especially in the vicinity of batteries, starter motors, primary wiring and alternators ought to not be the "airbag to save your butt" responsibility of charger designers. It should be your responsibility as a cruiser.

What's next? Auto SSB or cell dialers to call the eco police when you are using too much solar power?

IMHO, of course !

[S&S]

Originally Posted by imcbride

I'm still learning about alternator wiring, up to now I've just done what I'm told, i.e. don't touch while running the engine. A further search of the all-knowing Google turned up Blue Sea's Alternator Field Disconnect resource page: Alternator Field Disconnect - Resources - Blue Sea Systems

Assuming that's correct, and I tend to trust them, all the system would need to do is break the field excitation circuit between the alternator and the external regulator and it would stop the alternator output without frying anything. BMS/battery is happy and alternator is disengaged.

S&S - did you have a problem/concern with this approach?

More from the battery side. I'm not sure that these will tolerate a voltage spike. Also, I'm running a brushless alternator so I have diodes rectifying the field current that I have to be concerned with.

[S&S]

Originally Posted by senormechanico

This is a great discussion, but it reminds me of the USA mommy government back in the '70's wanting to pass laws requiring cars to be able to be survivable in a 70 mph crash into a concrete wall. Remember Ralph Nader?

Wiring shorts in your boat, especially in the vicinity of batteries, starter motors, primary wiring and alternators ought to not be the "airbag to save your butt" responsibility of charger designers. It should be your responsibility as a cruiser.

What's next? Auto SSB or cell dialers to call the eco police when you are using too much solar power?

IMHO, of course !

Just trying not to fry some expensive kit, sir.