The overall scheme of charging on Zeros bikes is BMS monitored Bulk Charging. Identical to what electric bicycles do.
Ok, so the BMS is connected to each cell in the battery. It is always on, always watching, but very simple.
The BMS basically has 2 things it can control. 1 bleed off resistor on for each cell, and the contactor.
The flow of charging is identical regardless if you're using the onboard charger or an external one.
The BMS gets the "charger connected" command (It's just two pins connecting) and if everything is ok, it turns on the contactor.
The BMS does not regulate the power flow into the battery. It simply monitors it.
The power flow is regulated by the charger. The charger is nothing more than very simple CC/CV power supply.
The power supply is set for the full voltage of the battery when each cell is at 4.15v. With the 28 cell battery, the zeros full charge is 116.2v
First let’s look just from the charger.
When the battery is very low, the charger will hit is current limiter, and stay there. As it stays there at that set amperage, the voltage slowly rises as the battery charges. Eventually the battery voltage reaches 116.2.
Once this happens, the voltage remains constant, but the current starts dropping... it drops further and further. Once the current drops into the milliamp range, the battery is fully charged.
Any CC/CV power supply that doesn’t hiccup or crowbar itself when it hits its current limit will work. Earlier zero motorcycles used meanwell power supplies, similar to the ones I use to charge my homemade EVs.
(Btw, CC/CV stands for constant current and constant voltage)
Most power supplies for this use a buck power supply design. This means it can step the voltage down (and current up) but if the voltage is too low to start with, they cannot "boost" it up.
The first step in all of these supplies is to rectify the AC voltage to DC. You said you were an electrical engineer, but in case someone else is reading this, I'll mention AC to DC for a second.
DC power is measured in volts. It is really simple as it is just one set number. X volts.
AC power is commonly referred to in RMS voltage, but it’s actually never 1 set number. The voltage "alternates" between a positive number, drops down to zero, then keeps going to a negative number... and then it repeats.
To get that 1 number for an AC voltage we, are actually getting an average. This is the Root Mean Square. There are other numbers like Peak-to-Peak, but most of the time an AC voltage is written as the RMS.
If you want more info on any of this, Google the terms I mentioned above... they're cool to know.
Now, when you convert AC to DC with full-wave rectification, you are taking that AC voltage and making it a DC signal that pulses up and down. If you put a few caps on there, it evens out and makes a stable voltage.
If you take the AC voltage, and multiply it by 1.414 you will get the DC voltage. If you doubt me on this, Google "RMS AC to DC voltage". There are all kinds of pages that teach this much better than I could. It’s worth knowing if you have any interest in how this works.
So your household 120v is really feeding that power supply 170vDC (120*1.414= 169.68) in North America... and 340vDC elsewhere (240*1.414)
If you look at most of these power supplies, you will see that they take both AC and DC with the AC range of 90 or 100 to 250 or 260... But their DC range is much higher. This is because of what I said above. It’s all the same power.
The only catch is that running them on DC only uses half the rectifier... however this isn’t an issue on most modern supplies, since they have such a large input range. Just remember that higher input voltage is always better, because it lowers the amperage.
Ok. All that said, what does it mean?
It means almost any CC/CV power supply, set to the proper voltage, and kept under the max amperage (100a in your case) will work to charge the battery.
It means that same supply needs an input voltage over 116.2v to operate or you need a supply with a boost stage. You would have to buy a full buck/boost power supply.
We are trying to charge off of cheap 6/12/18/24v batteries. Why batteries and not directly off the solar cells? The short answer is consistency.
Solar panels have to deal with clouds, dust, and weather in general. Even when there is enough light to maintain the same voltage, the amperage available is constantly changing. This causes issues with most power supplies. They like to see a nice steady DC input, and they work very hard to clean up the AC signal coming in to make it a reasonable DC starting point.
So, by having the panels charge the batteries, we can use very cheap off the shelf parts to have them keep the batteries topped off... and then run the charger off a nice stable battery source.
Now we have two options.
1. Find a boosting CC/CV power supply. I have never found a commercial one, but I am sure one exists somewhere.
2. Generate the boost ourselves with cheap, common, off-the-shelf parts... then feed that boosted voltage into a regular charger.
Method 1 is a non-starter for me. I could design a supply to do it, but it would probably be a yearlong project and I wouldn’t want to do it.
Method 2 is pretty simple. Use a cheap inverter. Not a perfect sine wave high quality one, but a cheap "2000w" or more Chinese inverter. Anything that can handle the constant 1300w demand. I have one I used for years, I think it cost me 20 dollars US. It’s only rated for 800 continuous, but I pull 1250 from it continuous according to my kill-a-watt meter. I run 2 laptops, a phone charger and a camera charger off it at the same time for about 2 hours.
Now before you dismiss this "turning to AC and back to DC" again... think about this. In order to boost the voltage from your battery bank level to the charging level, you NEED to convert it to AC anyway. All boost converters convert DC to AC, then up the AC voltage. If you are making AC anyway, why not make it useful as AC for everything else?
Now I realize that most of the world works on 220-240vac and not 120vac that we use here in the US. This may make a large difference in how expensive inverters are. If so, don’t worry, your Zero will run off the lower US AC voltage, so you can order a "US spec" one from china.
Now there’s only one problem with that. My wife is from South Africa as well. I don’t know if things have changed, but from what she has been telling me, getting something mailed or posted to you can be difficult. On top of that, the rolling brownouts make inverters very expensive. I don’t know if that applies to you, but if it does, I understand your pain. It sucks when you cannot mail-order stuff like we can in the states.
Another nice thing about method 2 is you can use the outlet for anything else that plugs into the wall. Just be aware that if you pull more power than the solar panels provide, you will deplete the batteries over time.
That brings me to my next suggestion... panel size.
You said you have 1200watts of panels. This is less than the stock charger. Plus, most panels are overrated anyway.
That means that you need to either run a smaller external charger that provides less than 1200watts... or you need to run the internal charger on a timer so that the batteries get a chance to recharge.
I don’t like either of these options... so my choice would be to add another panel. One extra panel will not only allow you to charge at full internal charger speed, but it will also ensure the batteries stay topped up when the weather isn’t ideal.
All this was the quick thinking behind my recommendation to run batteries, an inverter, and one extra panel.
Now, on to how the BMS works.
If the overall voltage goes out of range, it opens the contactor and disconnects the battery. This is rare and shouldn’t ever happen.
If a single cell reaches its top charge, the BMS will apply a load with the bleed off resistor. This load will discharge the cell and keep the cell at the same fully charged voltage of 4.15v.
As each cell comes up to peak more and more watts are being converted to heat via the resistor. This is why it is important to not set your bulk charger too high.
If it overheats, it disconnects the contactor.
It all works off of zeros principle of simplicity. There really isn’t a whole lot to the theory of it...
Cliff notes:
1. Dont worry about how the BMS works. Just feed the bike the power at the proper voltage/amperage, either through the charger, or directly.
2. You need a stable power source that has enough wattage to sustain the charger. Batteries will be required for solar.
3. No matter what you do, you will have some form of inverter in the system to raise the voltage.
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