oldswamm wrote:I just found this thread yesterday, and after some thought (and reading) decided it’s a viable idea if carefully implemented. First off some comments.
If you are using ‘start’ caps, you are looking for trouble (IMHO). They are NOT continuous rated. They are only connected during the motors startup in their intended application. You should be looking for ‘run’ caps, which are almost never over 100uF, and for a given capacity are physically 4 to 10 times larger than a ‘start’ cap.
The hair dryer in a link in an earlier post doesn’t have a GFCI in it. It would HAVE to have a ground connection to be one. <edit> Alan B pointed out to me that the last statement is untrue. GFCI can and do work with only 2 wires. <endedit> I would consider GFI to be essential for this circuit, so beware. They are commonly sold for heat tape (for water pipe), as here:
Here’s the circuit I’ve come up with to control one of these chargers. The LCD display interface isn’t included in the schematic and isn’t labeled on the board. RC4-7 are the data pins, and RB6&7 are the R/S and CS.
In an effort to make it ‘fool proof’ I intend to glue a pin header, hardwired for the digital representation of the number of cells, to the battery’s charge connector. If I eliminate a cell from a pack I would reprogram (rewire) the header (removing bad cells tend to cause packs to ‘change’ from time to time which could lead we fools to connect them at the wrong voltage). It would allow the processor to determine the charge voltage needed and could also function as the start charge signal. Obviously the charger’s battery connector would have to have a matching connector. I would connect switches to the 2 least significant bits so the human could communicate with the processor (with the battery disconnected).
To use programmable current I would probably add another couple pins to the header on the charge connector to program the current. 6 bits for 'S', and 2 bits for 'P'.
My choice of processor is based on the fact that I have some, and the software to program it. I have 18F parts that would give more usable pins, but my newer programming SW is corrupted and I don’t feel like fooling with it just now.
Also note that the PC board is designed to be hand built. If I had boards built I would redesign it (vias are really handy). I might even remember to include mounting holes. For the prototype I'll just etch it on an oversize board and drill holes wherever.
I’ll build the first prototype in the next few days and try to figure a way to test it (I haven't found suitable caps), so if you see an obvious error, please let me know. I just drew this this morning, so it’s POSSIBLE there’s a mistake.
Skippic wrote:Guys this is somewhat off the subject. This thread is about a charger, not a BMS.
I'm not saying that a BMS isn't needed for this type of charger, but maybe we should make a different thread for it.
oldswamm wrote:I couldn't sleep last night for thinking about this, so I got up and drew it.
See original for diagram
This circuit should work from less than 10s to 24s, and would draw about 60uA from the 24s (obviously the resistor could be changed to optimize it).
Would wake up from sleep mode, transmit a 24(?)bit ID # (plus a sync string, start and stop characters, and CRC) at 100khz or so, then go back to sleep for 1/2 sec or so.
The processor is old and obsolete, but I have a bunch of them, and since the bulk of the power used goes into the transmitted signal, a more efficient one wouldn't reduce consumption much.
I haven't gotten any response about the whole battery ID idea. Does that mean I'm the ONLY person who has any fear of plugging a battery into the charger with the wrong setup, and/or wants the convenience of not having to 'tell' the charger the desired parameters every time?
HealthYoung, I agree with what you say, but I was looking for suggestions specific to this project. I like to use a pot on some projects to adjust parameters 'on the go' for experimentation, but for this project up and down buttons would suffice for setup.
P. S. I know that's not the symbol for a zener, but didn't feel like drawing my own.
P. P. S. Man I'm tired of winter. -28F right now! Coldest winter I've ever seen!
Skippic wrote:I agree a BMS would be beneficial for this setup, just might be less messy to create a separate thread and link it.
As for the HVC, any way it would implemented should stay right in this thread. Since unless we want to burn stuff it's an integral part of the charger.
What would be the simplest way to implement HW HVC?
As for the 15 charging currents selectable by relays, I can say from experience, only three steps are needed:
- max amps
- BMS leveling, topping off
With Lipo you can charge 98% of the capacity at max amps and trickle charge the last 2%.My setup will be something like 400uF and 12uF.
oldswamm wrote:What I get for sleeping. (Lots of post to respond to.)
Let me reiterate, I'm NOT trying to push more sophistication you than you want. I'm trying to design a circuit with enough options that it will work for ANYONE.
Just because an option is there, doesn't mean YOU have to use it!
I'm surprised people think this is complicated. One processor, one op amp IC, however many shunts you need, and a handful of caps and resistors for the main board, plus a processor and a half dozen peripheral components for each serial ID unit (if used).
Rather than using quotes and responses, I just going to make random responses.
Thanks for the link to the diyelectriccar thread link, I'll pursue it farther, but the safety devices I saw so far have already been discussed here. (fuses, GFCI, HVC/BMS shutoff)
The caps should be modern UL listed RUN caps. They have a switch that opens if they build up pressure.
I have a lot of faith in my programing, especially if checked by members of this forum.
If you want true redundant safety override, it shouldn't share ANY components with the main controller.
A good cheap alternative would be a crowbar (a heavy SCR) across the output, with fuses in front and behind it. Could use a trip voltage higher than any of your packs ever see (could be switch selected, but there's the danger of it tripping when you forget to change it), and it could have a timer that energizes it if the controller doesn't send regular 'watchdog' resets (not to be confused with the processors internal watchdog).
You don't need to use all 6 relay control outputs. Would only be for those who have more than one 'size' battery. You surely aren't going to use the same current on your lite 4Ah battery as on the 40Ah+ motorcycle battery, are you? At an EXTREME it would provide almost infinite current control 'like a commercial charger'. I rerepeat, you don't have to use them if you don't want to! My immediate plans call for 2 significantly different battery packs, so probably 3 caps would do me fine, but other people might have a wider need, so I'm trying to allow for them. Skippic, are you sure your setup would be ideal for both a 4Ah 4s and a 40Ah 24s?
Good point on the internal references. I'll probably build a prototype with the VR reference, and 'allow' for it's variability when setting my limits. (I don't have a 16F1938)
I can't see why we could possibly need a separate thread to include an HVC input, or why it's 'messy'. One pin on the processor, one line of code.
Evibus is an alternative to CANbus. Involves wires and is much more complicated than adding a serial # IC to the battery. Wake on interrupt would require some form of bidirectional com, which would unnecessarily complicate things (I can live with 20 to 60uA).
My thought on the zener is that it wouldn't have to conduct continuously or even accurately (maybe I'll experiment this afternoon). As long as the voltage stayed between 2.5 and 5.5 volts, even the old 12c508 would be happy.
The more I think about it, the more I like the idea of a wireless serial # ID, because it's simple (I've pretty much abandoned the idea of an extra connector at the charge connect as being unnecessarily complicated). Programming at the charger would be simple since the one I build would have up/down buttons and an LCD (I would require that the battery being programed for, be plugged in, so you would KNOW you were programing for the right one). And before you jump on me, YOU don't have to spend the extra $5 for an LCD if you don't 'need' readouts or programability.
The reason the processor has a watchdog timer is in case the micro 'takes a dump'. Like Alan pointed out, we just have to make SURE the reset shuts the relay(s) off.
Anybody who wants to is welcome to build a non computer controlled version, (they already are after all), but it's NOT what I'm designing.
oldswamm wrote:As far as I can see, the only part the charger needs to concern itself with is HVC, unless we want to tell it to go to trickle for balancing.
oldswamm wrote:Parabellum, I don't understand what you mean. Where would you put the zener(s), and how would they trip the HVC? I figured on the output of a set of methods HVC for myself.
Has anybody tried to measure the efficiency of these chargers. A watt meter can be mislead by the power factor, but if NO heat is generated, I would have to assume it's 100% efficient. I would like to see some actual temperature/time/ambient measurements so we could calculate heat energy vs. battery energy. Any other energy leakage (RF, light, xray )?
ga2500ev wrote:Actually with reactance for the capacitors it's simpler than that. Any unused energy flows back to the source. The only problem with this is that there's a meter between the source and the caps.
oldswamm wrote:ga2500ev, my board would be isolated from the mains with a transformer, but it's 'ground' would connect directly to the Batt-. Any other setup would make current measurement problematic. In that case, I don't see why a voltage divider wouldn't be fine for B+ measurement.
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