5th year of living dangerously with LiFePo4 batteries

On Tue, 05 Jun 2018 11:42:49 -0700, Frank Whiteley wrote:

Sadly, not entirely unrelated to this thread. This was the plane used
in the recent glider tow.

https://electrek.co/2018/06/04/sieme...rototype-fire-
crash-death/



A few years back there were a lot of cheap LiIon batteries on sale as SLA
replacements that turned out not to have a BMS system - just cells in the
case. Obviously, these would be at least as flammable as an SLA of
accidentally shorted.

IIRC these were impossible to distinguish from batteries with a proper BMS
inside the case, so, are they still around and being sold to the unwary?


--
Martin | martin at
Gregorie | gregorie dot org

On 06/05/2018 01:11 PM, Martin Gregorie wrote:
On Tue, 05 Jun 2018 11:42:49 -0700, Frank Whiteley wrote:

Sadly, not entirely unrelated to this thread. This was the plane used
in the recent glider tow.

https://electrek.co/2018/06/04/sieme...rototype-fire-
crash-death/



A few years back there were a lot of cheap LiIon batteries on sale as SLA
replacements that turned out not to have a BMS system - just cells in the
case. Obviously, these would be at least as flammable as an SLA of
accidentally shorted.

Replace "at least" with "far more'. SLA's are way down at the bottom of
the list when it comes to flammability.


IIRC these were impossible to distinguish from batteries with a proper BMS
inside the case, so, are they still around and being sold to the unwary?



Not sure what a "proper" BMS board is. There are a variety of functions
that may be included, the data sheets tend to vague on what's in there.
See Wikipedia for a primer.

If the battery can disconnect itself from the terminals in the event of
excessive discharge current, too high of charging voltage, or too low
voltage on discharge, then it requires some high-current switches to do
the job. It's a dangerous assumption to think that a battery must have
those capabilities included.

On Tue, 05 Jun 2018 14:35:42 -0600, kinsell wrote:

Not sure what a "proper" BMS board is. There are a variety of functions
that may be included, the data sheets tend to vague on what's in there.
See Wikipedia for a primer.

I'd say it must have these functions:

- cell-balancing charging management
- low-voltage shut-down
- some sort of high-current limiting
This need not be expensive: I'd accept built-in replaceable fuses
and holders or solid state current limiters that temporarily
disconnect the battery when the load becomes excessive.
Neither are exactly what you'd call new technology.


If the battery can disconnect itself from the terminals in the event of
excessive discharge current, too high of charging voltage, or too low
voltage on discharge, then it requires some high-current switches to do
the job.

Inexpensive built-in fuses can handle that perfectly well.


--
Martin | martin at
Gregorie | gregorie dot org

On 06/05/2018 03:20 PM, Martin Gregorie wrote:
On Tue, 05 Jun 2018 14:35:42 -0600, kinsell wrote:

Not sure what a "proper" BMS board is. There are a variety of functions
that may be included, the data sheets tend to vague on what's in there.
See Wikipedia for a primer.

I'd say it must have these functions:

- cell-balancing charging management
- low-voltage shut-down
- some sort of high-current limiting
This need not be expensive: I'd accept built-in replaceable fuses
and holders or solid state current limiters that temporarily
disconnect the battery when the load becomes excessive.
Neither are exactly what you'd call new technology.

Not new technology, but the switch needs to handle the maximum current
that the battery can be called on to deliver. Many applications are
going to involve more than an amp or two to power a sailplane panel.




If the battery can disconnect itself from the terminals in the event of
excessive discharge current, too high of charging voltage, or too low
voltage on discharge, then it requires some high-current switches to do
the job.

Inexpensive built-in fuses can handle that perfectly well.


Fuses can handle the overcurrent, but disconnecting on high or low
voltage isn't going to be cheap, if the battery is rated for high current.

Richard had an interesting post a while ago, where he accidentally used
the wrong charger on an FLP, and battery was destroyed due to swelling.
So much for the idea of a BMS protecting the battery.

-Dave

On Wed, 06 Jun 2018 09:38:36 -0600, kinsell wrote:

Richard had an interesting post a while ago, where he accidentally used
the wrong charger on an FLP, and battery was destroyed due to swelling.
So much for the idea of a BMS protecting the battery.


Using the wrong charger, or a multi-chemistry charger on the wrong
setting, can destroy any battery, regardless of its chemistry.

A BMS probably won't help here because the charger may not trip the max-
voltage switch and the charge balancer is only there to bring any cells
that are under- or over-charged into line with the rest.


--
Martin | martin at
Gregorie | gregorie dot org

One example does not prove all BMS cannot protect the battery.
I see no eveidence that Life is any more prone to failiure than SLA gell cells.

On Wednesday, June 6, 2018 at 4:25:03 PM UTC-7, wrote:
One example does not prove all BMS cannot protect the battery.
I see no eveidence that Life is any more prone to failiure than SLA gell cells.

If we are rejecting all batteries that can be damaged by connecting to the wrong charger, then we are left with nothing. You can make an SLA explode rather dramatically doing that.

Buy one with a fuse built in?

On Tue, 05 Jun 2018 21:06:06 +0000, Jim White wrote:

Buy one with a fuse built in?

Yes, but preferably one with a low voltage cut-off and cell-balancer for
charging as well.

But what I was pointing out is that a year or so back there were brands
that were very similar from the outside and (in some cases) had similar
prices. Some of these had BMS fitted and some just had cells wired to the
terminals. Often the descriptions didn't mention whether they had a BMS
or not.

What I want to know is whether this undocumented mess is still the case
or if you can now read published descriptions and know, with a fair
degree of confidence, whether there is or is not a BMS and current
limiter inside without having to chop the battery open to find out.

If the adverts now give reliable information about this, then I'll
investigate further: if not, I'll stick to SLAs for a while yet.


--
Martin | martin at
Gregorie | gregorie dot org

On Tuesday, June 5, 2018 at 2:44:10 PM UTC-7, Martin Gregorie wrote:
On Tue, 05 Jun 2018 21:06:06 +0000, Jim White wrote:

Buy one with a fuse built in?

Yes, but preferably one with a low voltage cut-off and cell-balancer for
charging as well.

But what I was pointing out is that a year or so back there were brands
that were very similar from the outside and (in some cases) had similar
prices. Some of these had BMS fitted and some just had cells wired to the
terminals. Often the descriptions didn't mention whether they had a BMS
or not.

What I want to know is whether this undocumented mess is still the case
or if you can now read published descriptions and know, with a fair
degree of confidence, whether there is or is not a BMS and current
limiter inside without having to chop the battery open to find out.

If the adverts now give reliable information about this, then I'll
investigate further: if not, I'll stick to SLAs for a while yet.


--
Martin | martin at
Gregorie | gregorie dot org

You are right to want to know what's in the box. In any case, you should have an external fuse immediately following the battery, since the BMS (if present) is unlikely to limit the current sufficient for most glider wiring. The BMS will protect against short circuited terminals, your fuse should protect the wiring you've chosen.

Another point that should be raised: in lithium polymer and some other chemistries, the BMS is essential to safely charge and discharge the battery. In an LFP, its function has more to do with protecting the cells from an early death due to undercharge, cell balance, or overcharge, than fire safety.. You can probably start one on fire by severely overcharging it, as you can do with any battery (even a flashlight D cell). Undercharging or cell balancing will result in unsatisfactory performance, rather than a fire. You can find some vendors LFP specs that tell you what the BMS does.