Any charger has internal sensing of its output voltage. This is nearly always in the form of a resistor divider network.
If you place an additional resistor in parallel with one of the sensing resistors, you can influence the voltage that the charger 'sees'. Raise the apparent voltage high enough and the charger will reduce its current output to practically zero. If the additional resistor is in the form of an opto-isolator, or opto-isolator+resistor then you now have externaal control that can connect to a BMS. This is exacty what Cedric Lynch did on his ultra efficient enclosed feet forward motorcycle. The Zivan NG3 was hacked and interfaced to his cell protection boards. Each board could shunt a small amount of current, but if the voltage across the shunt resistor was high enough, it would turn on an opto-isolator and limit the output of the charger. On Fri, 22 Mar 2019 at 06:41, Lee Hart via EV <ev@lists.evdl.org> 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). > > -- > 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 > _______________________________________________ > 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) > -- Paul Compton www.morini-mania.co.uk www.paulcompton.co.uk (YouTube channel) _______________________________________________ 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)
