Hi all,

I'm trying to put together a small solar system to power an electric bike, also partly to learn more about putting together solar systems with an eye to later building a system to power a minimalist off-grid cabin. The bike has a Li-ion battery, which is charged by an AC charger.

My initial thoughts were that the output of this charger is 42 volts at 2.0 amps, so 84 watts of power, so I've bought an inverter, a panel (a flexible panel, as I've read that these are better at generating current in lower light conditions, though I understand it may not last as long as a rigid panel), a charge controller and a battery (I understand this isn't ideal for a solar system, I bought it because it's the same one I have in my car, so I've put the new battery in the car, and the 5 year old battery is intended for the solar system), and would be good to go. I need to recharge the battery on the bike a couple of times a week, and live near the Mediterranean, so get a fair bit of sunshine (something like 2000 hours/year).

Having ordered all this kit, I was looking at how to wire the inverter to the system, as I'd assumed I'd connect it to the load pins on the charge controller; thankfully some time spent reading this forum has educated me to the point that I realise I need to connect the inverter to the battery instead (also that I might have been better off buying a smaller inverter). What I'm currently unsure about is the difference between the input rating on my bike charger, and its output rating; the input states AC100v-240V and 1.8A, which means that the input is 414 watts, given that I'm on 230 volt AC power.

What's confused me is that I'd previously only considered the output; 42V DC at 2.0A for 84 watts of current. Does that mean it's dumping 330 watts as (mostly) heat, given that the input is 414 watts? I'm trying to understand how much current will be drawn from the battery when the charger is plugged in, and had previously assumed that the load would be equal to the stated output of the charger, i.e. I'd be looking at 2 amp load, which I believe means that my 62 amp hour battery should be capable of powering the charger for perhaps 20 hours, given that I understand that the inverter itself might use an amp of power, and that there's some electrical resistance along the way. That would give me around 3 full charges of my bike battery, as it takes approx. 6 hours to recharge from empty. But I'm now starting to second guess myself having seen the input figures. Hence I'm looking for a sanity check, I think; have I hopelessly misunderstood how all this works, or, given my fairly minimal needs (recharging a 36 volt, 8 amp hour li-ion battery twice per week), is my 50 watt panel, 500 watt inverter, 62 amp hour car battery going to do the trick? As I understand it, my panel is producing 4.17 amps under ideal conditions, so on a sunny summer's day, with 8 hours of sunshine, I'm going to get perhaps 30 amp hours into the battery, so it's going to take approx 2 sunny days to charge my battery from empty.

I've got a couple of other questions (e.g. my charge controller manual talks about a voltage setting for undervoltage recovery, as well as undervoltage protection; the latter I understand, but the former is somewhat mysterious, even after a search on this forum and a search engine), and why it is that my charge controller will only allow me to set an undervoltage protection of 11 volts maximum (I'd rather that figure were 12.2 volts, to maintain the life of the battery), but any thoughts on the sanity (or otherwise!) of the basic system I'm putting together are much appreciated first off.

I'm trying to put together a small solar system to power an electric bike, also partly to learn more about putting together solar systems with an eye to later building a system to power a minimalist off-grid cabin. The bike has a Li-ion battery, which is charged by an AC charger.

My initial thoughts were that the output of this charger is 42 volts at 2.0 amps, so 84 watts of power, so I've bought an inverter, a panel (a flexible panel, as I've read that these are better at generating current in lower light conditions, though I understand it may not last as long as a rigid panel), a charge controller and a battery (I understand this isn't ideal for a solar system, I bought it because it's the same one I have in my car, so I've put the new battery in the car, and the 5 year old battery is intended for the solar system), and would be good to go. I need to recharge the battery on the bike a couple of times a week, and live near the Mediterranean, so get a fair bit of sunshine (something like 2000 hours/year).

Having ordered all this kit, I was looking at how to wire the inverter to the system, as I'd assumed I'd connect it to the load pins on the charge controller; thankfully some time spent reading this forum has educated me to the point that I realise I need to connect the inverter to the battery instead (also that I might have been better off buying a smaller inverter). What I'm currently unsure about is the difference between the input rating on my bike charger, and its output rating; the input states AC100v-240V and 1.8A, which means that the input is 414 watts, given that I'm on 230 volt AC power.

What's confused me is that I'd previously only considered the output; 42V DC at 2.0A for 84 watts of current. Does that mean it's dumping 330 watts as (mostly) heat, given that the input is 414 watts? I'm trying to understand how much current will be drawn from the battery when the charger is plugged in, and had previously assumed that the load would be equal to the stated output of the charger, i.e. I'd be looking at 2 amp load, which I believe means that my 62 amp hour battery should be capable of powering the charger for perhaps 20 hours, given that I understand that the inverter itself might use an amp of power, and that there's some electrical resistance along the way. That would give me around 3 full charges of my bike battery, as it takes approx. 6 hours to recharge from empty. But I'm now starting to second guess myself having seen the input figures. Hence I'm looking for a sanity check, I think; have I hopelessly misunderstood how all this works, or, given my fairly minimal needs (recharging a 36 volt, 8 amp hour li-ion battery twice per week), is my 50 watt panel, 500 watt inverter, 62 amp hour car battery going to do the trick? As I understand it, my panel is producing 4.17 amps under ideal conditions, so on a sunny summer's day, with 8 hours of sunshine, I'm going to get perhaps 30 amp hours into the battery, so it's going to take approx 2 sunny days to charge my battery from empty.

I've got a couple of other questions (e.g. my charge controller manual talks about a voltage setting for undervoltage recovery, as well as undervoltage protection; the latter I understand, but the former is somewhat mysterious, even after a search on this forum and a search engine), and why it is that my charge controller will only allow me to set an undervoltage protection of 11 volts maximum (I'd rather that figure were 12.2 volts, to maintain the life of the battery), but any thoughts on the sanity (or otherwise!) of the basic system I'm putting together are much appreciated first off.

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