Hi!
I'm looking to charge an ebike using a solar panel mounted on a bike trailer - off grid, while travelling with the entire setup. So the topic of this post is basically, how to get solar energy into your ebike battery using as little weight as possible, and of course without damaging your equipment?
BACKGROUND
European standard ebikes use 250W pedal assist motors that typically work off a 36V LiFePO4 battery with 10-15Ah (ca 400 Wh) worth of power. High end manufacturers have their own proprietary drive lines, which means you can't just charge a Bosch or Shimano battery with a generic battery charger, or easily attach a generic battery to such a motor - they take steps to make sure you use all their stuff all the way with special connectors etc. This can probably be circumvented in certain ways, but I expect warranties to be void and ugly or perhaps even unsafe solutions to result.
Most manufacturers only provide chargers for on the grid. There are a few 12V chargers for those who want to charge their ebikes in their cars/RVs. These have the advantage of not having to go via an inverter, and being capped at reasonable amperages (since 12V car plugs are often max 10A, meaning a very transportable solar panel of roughly 100W would be sufficient). Modern on grid chargers often use 3-4 amps at 230V AC, especially for higher end systems, where more rapid charging is a constant development goal. Unfortunately, the 12V chargers are either sold out, hard to come by or unavailable for the systems I'm interested in, so it looks like high capacity AC chargers is what I'm stuck with for this project.
TRADITIONAL SOLUTION
The traditional way to charge a 36V bike battery using solar panels off grid would be:
Solar panels -> regulator -> battery -> inverter -> ebike charger -> ebike battery
This is safe, tested and it works. The problem is transportability. A 36V 10A LiFePO4 battery can be drained to almost all it's capacity, while - say - a 12V AGM battery can only be drained to about 50%. So, to safely put 36x10=360 Wh power into the bike battery, you'd need a 12V battery bank of roughly 700 Wh or 60 Ah. The battery alone would weigh 16 kg, adding panels and regulator on top of that. While the trailer + ebike solution does give me some room to travel with more weight, I'd like to keep it as low as possible.
A costly but much more attractive option would be to use a LiFePO4 battery for the solar panel as well. This would allow me to use a sleek 30 Ah battery bank that would weigh half as much as a corresponding AGM battery, cutting the weight by a factor of 4. A 4kg battery is much more acceptable. The battery cost is probably 4 times higher too, and I'd need to use a special (more expensive) regulator for LiFePO4 batteries.
However, this got me thinking: If I can charge a LiFePO4 battery bank with a solar panel, might I not be able to charge the ebike battery directly with solar panels too? This would have the cost and weight advantages of cutting out the intermediate battery. I'd need 3x 12 volt panels (of say, 50W each) and a regulator/charger that accepts at least 30-50 VDC input and output. Or is the bike battery so different that it really needs its own specially designed charger? Or is one LiFePO4 battery pretty much like the next one?
MICROINVERTER SOLUTION
A microinverter includes an MPPT regulator, attaches directly to a solar panel and outputs AC. When I first heard of these, it seemed to good to be true - and it probably was. The MIs I've found all seem to require grid installation - they simply don't work unless you connect them to the grid. Enphase (a manufacturer) is developing an off the grid MI to be launched in 2019. This would allow the following set up instead:
Solar panels -> microinverter -> ebike charger -> ebike battery
As long as the sun is shining, you basically have a mobile AC socket with you for all your needs - ebike battery included. One downside with this setup is that you would probably need to use (and travel with) more solar panels than you'd otherwise like. Microinverters are designed to be paired with run-of-the-mill 60 cell 250W panels. It's possible that an off grid model will be more flexible in this regard (to cater to the RV market for example), but who knows? Putting 250W of panels on the trailer might be doable, but kind of overkill for my needs.
Another concern with such a set up is, how will the ebike charger handle a limited available amperage from the microinverter? If the charger wants 3 amps at 230 volts, but it only gets .8 to 1 amp at 230 volts, what will happen? Ideally, the charger will work fine, no equipment will be damaged, and the ebike battery will simply charge more slowly than if it had been connected to a normal on-grid socket. Worst case, it won't charge at all and/or equipment may be damaged. Ideally, the battery charger might provide options of different charging speeds to anticipate varying charging conditions, but such solutions are rare for small ebike batteries while they are common for electric cars.
If it does work, it begs the question - why haven't we done this before in off grid applications? After all, there are simple regulators designed to be used without batteries. Why not connect a low capacity inverter directly to such a regulator and avoid batteries if you don't really need a 24h power source? I'm guessing the varying amperage would be problematic for almost all AC equipment you might be interested in connecting to such a setup, hence the battery bank is needed to provide steady current. But then again, if this is the case, why is Enphase developing a battery-less off grid microinverter?
Bottom line, if I could get away with this, I'd take this solution in a heart beat - no extra battery weight or cost would be awesome. Not having to replace an intermediate battery would also be great in the long run.
I'm basically wondering if I've missed any good alternatives and what you think about a solution without an intermediate battery?
I'm looking to charge an ebike using a solar panel mounted on a bike trailer - off grid, while travelling with the entire setup. So the topic of this post is basically, how to get solar energy into your ebike battery using as little weight as possible, and of course without damaging your equipment?
BACKGROUND
European standard ebikes use 250W pedal assist motors that typically work off a 36V LiFePO4 battery with 10-15Ah (ca 400 Wh) worth of power. High end manufacturers have their own proprietary drive lines, which means you can't just charge a Bosch or Shimano battery with a generic battery charger, or easily attach a generic battery to such a motor - they take steps to make sure you use all their stuff all the way with special connectors etc. This can probably be circumvented in certain ways, but I expect warranties to be void and ugly or perhaps even unsafe solutions to result.
Most manufacturers only provide chargers for on the grid. There are a few 12V chargers for those who want to charge their ebikes in their cars/RVs. These have the advantage of not having to go via an inverter, and being capped at reasonable amperages (since 12V car plugs are often max 10A, meaning a very transportable solar panel of roughly 100W would be sufficient). Modern on grid chargers often use 3-4 amps at 230V AC, especially for higher end systems, where more rapid charging is a constant development goal. Unfortunately, the 12V chargers are either sold out, hard to come by or unavailable for the systems I'm interested in, so it looks like high capacity AC chargers is what I'm stuck with for this project.
TRADITIONAL SOLUTION
The traditional way to charge a 36V bike battery using solar panels off grid would be:
Solar panels -> regulator -> battery -> inverter -> ebike charger -> ebike battery
This is safe, tested and it works. The problem is transportability. A 36V 10A LiFePO4 battery can be drained to almost all it's capacity, while - say - a 12V AGM battery can only be drained to about 50%. So, to safely put 36x10=360 Wh power into the bike battery, you'd need a 12V battery bank of roughly 700 Wh or 60 Ah. The battery alone would weigh 16 kg, adding panels and regulator on top of that. While the trailer + ebike solution does give me some room to travel with more weight, I'd like to keep it as low as possible.
A costly but much more attractive option would be to use a LiFePO4 battery for the solar panel as well. This would allow me to use a sleek 30 Ah battery bank that would weigh half as much as a corresponding AGM battery, cutting the weight by a factor of 4. A 4kg battery is much more acceptable. The battery cost is probably 4 times higher too, and I'd need to use a special (more expensive) regulator for LiFePO4 batteries.
However, this got me thinking: If I can charge a LiFePO4 battery bank with a solar panel, might I not be able to charge the ebike battery directly with solar panels too? This would have the cost and weight advantages of cutting out the intermediate battery. I'd need 3x 12 volt panels (of say, 50W each) and a regulator/charger that accepts at least 30-50 VDC input and output. Or is the bike battery so different that it really needs its own specially designed charger? Or is one LiFePO4 battery pretty much like the next one?
MICROINVERTER SOLUTION
A microinverter includes an MPPT regulator, attaches directly to a solar panel and outputs AC. When I first heard of these, it seemed to good to be true - and it probably was. The MIs I've found all seem to require grid installation - they simply don't work unless you connect them to the grid. Enphase (a manufacturer) is developing an off the grid MI to be launched in 2019. This would allow the following set up instead:
Solar panels -> microinverter -> ebike charger -> ebike battery
As long as the sun is shining, you basically have a mobile AC socket with you for all your needs - ebike battery included. One downside with this setup is that you would probably need to use (and travel with) more solar panels than you'd otherwise like. Microinverters are designed to be paired with run-of-the-mill 60 cell 250W panels. It's possible that an off grid model will be more flexible in this regard (to cater to the RV market for example), but who knows? Putting 250W of panels on the trailer might be doable, but kind of overkill for my needs.
Another concern with such a set up is, how will the ebike charger handle a limited available amperage from the microinverter? If the charger wants 3 amps at 230 volts, but it only gets .8 to 1 amp at 230 volts, what will happen? Ideally, the charger will work fine, no equipment will be damaged, and the ebike battery will simply charge more slowly than if it had been connected to a normal on-grid socket. Worst case, it won't charge at all and/or equipment may be damaged. Ideally, the battery charger might provide options of different charging speeds to anticipate varying charging conditions, but such solutions are rare for small ebike batteries while they are common for electric cars.
If it does work, it begs the question - why haven't we done this before in off grid applications? After all, there are simple regulators designed to be used without batteries. Why not connect a low capacity inverter directly to such a regulator and avoid batteries if you don't really need a 24h power source? I'm guessing the varying amperage would be problematic for almost all AC equipment you might be interested in connecting to such a setup, hence the battery bank is needed to provide steady current. But then again, if this is the case, why is Enphase developing a battery-less off grid microinverter?
Bottom line, if I could get away with this, I'd take this solution in a heart beat - no extra battery weight or cost would be awesome. Not having to replace an intermediate battery would also be great in the long run.
I'm basically wondering if I've missed any good alternatives and what you think about a solution without an intermediate battery?
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