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. 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? Without a CAN bus, how does the charger know when to turn off? Does it need to turn off? 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. I know in lead charging, the amperage draw slowly drops to near 0 as the pack reaches full charge. 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.
Thanks again Dan On Fri, Mar 22, 2019 at 8:45 AM Steve Heath via EV <[email protected]> wrote: > Wise words indeed. The price of a BMS is so cheap these days that it is > almost inconceivable not to fit one. > > Steve > > On 22/03/2019 06:42, Lee Hart via EV wrote: > > 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). > > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > Please discuss EV drag racing at NEDRA ( > http://groups.yahoo.com/group/NEDRA) > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20190322/abca33cf/attachment.html> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
