ken via EV wrote:
My charger is set to be the right voltage but the old cells being 22 in
series string they need to equalized out at the top/end of the charge.
this requires some battery baby sitting,
if your cells are staying very well balanced then your voltage cut off
method/gadjet may be good.
your ebay gadget coud also be be good for those wanting to do a lower
state of charge, like turn the voltage down 5 volts for a 10% lower top
charge.
I have 2 ev scooters with 22 and 24 lfp cells.
This is a harder problem than you might think. Mistakes can lead to
expensive failures, and even fires! I have several concerns in this
discussion:
- Lack of knowledge about the cells being charged:
- Don't know their actual state of balance.
- Don't know the right voltage to charge them to.
- Lack of information on exactly what the charger is doing:
- What voltage and current does it actually charge to?
- What exactly makes it decide the battery is full?
- And, does it really shut off, or keep on "float" charging?
- Human nature: People who are inexperienced tend to:
- Guess.
- Ignore the problem.
- Seek bad advice (that tells them what they want to hear).
- Then go with the cheapest solution.
So, my advice is to learn all you can! Get data sheets for the batteries
and charger in question. If you can't, make measurements for yourself
(don't rely on assumptions, or bogus experts on the internet. or
marketing claims from unknown suppliers).
If you go without a BMS, understand that any minor failure is likely to
escalate into a *major* failure before you notice it! It's like deciding
you don't need any expensive fuses or circuit breakers in your house
wiring; just wire everything directly to the incoming power. Cheap!
Easy! And it works fine, until the first time anything anywhere happens
to fail shorted. Then it burns your house down.
Now, on the subject of a voltage-sensing controller: This is a simple
method of shutting off a "dumb" charger for a lead-acid pack. That's
because voltage is a reasonable indication of state of charge for
lead-acid. Also, overcharging an old or damaged string of lead-acid
batteries may cause early cell failures; but they are not likely to be
spectacular disasters.
For charging lithiums, you really need to know the voltage *and* current
*and* time to turn off the charger. Voltage alone is not enough.
The normal approach is to have a charger that is smart enough to shut
off when the voltage and current and time are all "right". It won't
charge to an excessive voltage; and it will turn itself off if the
current stays too high for too long (an indication that something is
wrong). A good charger will also have some form of temperature sensing,
as the correct conditions are temperature-dependent.
But if the charger is only sensing total voltage, it won't know if just
one cell fails in the pack. That one cell could go seriously
over-voltage, or fail shorted. In either case, this can lead to a fire.
That's why you normally have a BMS. It senses each cell, and can stop
the charger if something goes wrong.
If you are a cheapskate, and don't believe in BMS, then at the very
least I would suggest something like my Batt-Bridge
<http://sunrise-ev.com/LeesEVs.htm#battbridge>. It will at least warn
you that something is wrong *before* the disaster. You can also use the
light from the Batt-Bridge LEDs to control a relay to shut down the
charger (if charging) or motor controller (if driving).
--
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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