Dan Baker via EV wrote:
Thanks guys for the information. Ken and I are reaching out for help so
don't spank us yet for putting in something dangerous lol.
Hi guys,
I'm not trying to "spank" anyone. Just get them to look before they leap!
Did you ever have a (usually teenage) friend say, "Hold my beer and
watch this..." What followed was one of those disasterous "Darwin award"
moments that you can only laugh about years later.
I've *been* one of those "teenagers" myself. I've done some amazingly
stupid things out of ignorance, or just to "see what happens if..." So
some of the things in this discussion set off alarm bells.
In my case, where I plan to use a BMS but want to reuse a lead charger,
a BMS and voltage cut-off so the charger doesn't just send amps to the
shunts when all cells reach full charge is a bad idea?
It is a bad idea. It only works if
- You KNOW the voltage and current that the charger actually delivers
when the BMS tries to shunt current around a cell.
- You KNOW the BMS can safely shunt the charger's current at that time.
- You KNOW that something will turn the charger off before it sits too
long under these conditions.
You will have to test this for yourself. Do not trust the charger or BMS
manufacturer's marketing claims. They will say anything to "make the
sale", and will not accept *any* responsibility if things go wrong!
Without a CAN bus, how does the charger know when to turn off?
Exactly! A lead-acid charger may *never* turn off. Many simply stay on
"forever", on the assumption that a lead-acid battery won't be harmed
(too much) by being left on a "float" charge forever.
Does it need to turn off?
Yes! This is intolerable for lithiums. You have to be SURE the charger
turns off, or the cells will get charged to death.
Most of the pre-built packs for sale I have seen only employ 2 wires
(sometimes a third for temp) to the battery which say they have an on-board BMS.
The cheapest (and so most common) scheme is no BMS at all. They depend
on the cells being matched well enough that you won't need a BMS until
they age a bit and start to drift apart.
The second-cheapest scheme (which covers alomst everything else) is
something equivalent to a zener diode across each cell. When it reaches
some voltage, it shunts the charging current away from the cell.
This only works if the charger is delivering a current that is *lower*
than the shunt can safely handle. Most shunts are tiny; they can only
bypass maybe 100 milliamps of current.
If the shunts can't handle the charger's current, they burn up or fail.
Then you have no protection. The next thing to fail is the cell (perhaps
spectacularly).
Even if the shunt can handle the charging current, it is converting it
to heat. This heat is dumped into the battery box. Having large numbers
of shunts all producing heat for long periods is a bad thing, as heat
kills lithium cells. That's why you must *know* that the charger will
shut down in some relatively short time period.
I know in lead charging, the amperage draw slowly drops to near 0 as
the pack reaches full charge.
Yes, if all the cells are good. If a cell is bad, or the pack is hot, or
old, the current does not drop off at full charge. Lead-acids can "take
it" for a while (this is how you equalize them).
With Lithium and a BMS, when the cells reach full charge
(determined by the BMS) and the BMS shunts the load, is that load
significant? I have looked at a few BMS boards so far, none seem to have a
relay or signal that controls the charger power so I'm assuming a fully
shunted BMS isnt that much of a load.
If the BMS is any good, it will have an output that either tells a
"smart" charger to shut off, or can control a relay to force a "dumb"
charger to shut off.
Assuming your BMS has such an output, you also want to be sure it is
"fail safe". That is, if the BMS fails, it STOPS the charger. Too many
cheap ones can fail "on".
--
Any intelligent fool can make things bigger, more complex, and more
violent. It takes a touch of genius, and a lot of courage, to move
in the opposite direction. -- Albert Einstein
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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