5th year of living dangerously with LiFePo4 batteries

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.

On 06/05/2018 03:44 PM, 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.



So has anybody gotten specs out of K2? They don't publish them openly,
but instead require that you register and request them. I did that and
they never replied. Maybe they blacklist anybody who can actually read
spec sheets.