Battery sizing for electric bicycles

[wunder]

Hi everyone, I'm just getting my feet wet with solar power. I've got a 100w panel from Harbor Freight and I've been charging a 17ah 12v battery, but I need more capacity for what I'm trying to do. My wife and I have electric bicycles that I would like to charge exclusively with solar power. I've done some reading and searching and I think I have a solution, but I'd like to run it past more experience folks to see if I'm on the right track. We would like to ride several times a week, but we don't typically use more than about 1/2 the battery power at a time.

The bikes each have 11.6 ah @ 36 volts = 417 watt hours
My 100w solar panels apparently will generate about 400 watt hours per day from what I've read about solar hours and effective power generation.

So far so good- I think solar should be able to handle my needs.

I'm planning on buying 2 35ah batteries and hooking them up in parallel for 70ah total (840 watt hours)

The battery chargers for the bikes are 2 amps @ 42v = 84 watts

My questions: will 100w panel be able to keep 70ah worth of batteries charged? I have a 300w inverter which is working ok, but I'm also thinking it would be nice to be able to power my sump pump in the event of power outages as an emergency back up so I'm thinking about getting a 750w inverter for this purpose. Are there any downsides to a larger inverter?

Thanks in advance from this solar noob.

[SunEagle]

Hello wunder and welcome Solar Panel Talk

A real easy way to size the panel wattage and battery system is to have a charging system rated between C/8 and C/12 where C = the Ah rating of the battery. The middle ground would be C/10 or 1/10th the Ah rating of the battery as charging amps.

So a 70Ah battery system would require around 7 amps of charging. IMO most 100 watt panels probably have an Imp = 5.5 amps which would be in the C/12 area or just about the minimum amount of charging amps needed to keep it happy.

[wunder]

Thanks. I saw a blog post https://www.altestore.com/blog/2016/...w-solar-panel/ that mentioned using a 89AH battery- sounds like that wouldn't work terribly well. So it sounds like the ideal target would be 50ah for my panel. I can get these 2 batteries at a pretty good sale price this weekend ($50 each for agm's)- do you think I should hold off and look for a single 50 instead?

[SunEagle]

Thanks. I saw a blog post https://www.altestore.com/blog/2016/...w-solar-panel/ that mentioned using a 89AH battery- sounds like that wouldn't work terribly well. So it sounds like the ideal target would be 50ah for my panel. I can get these 2 batteries at a pretty good sale price this weekend ($50 each for agm's)- do you think I should hold off and look for a single 50 instead?

Actually Amy from Altestore is a member of this forum and I value her input. Maybe 100 watts will be ok for an 89Ah battery but IMO that battery will not last that long,

Also you have 4 cheap HF panels that suppose to equal 100 watts but that may be only under Lab conditions. So you should expect less output then they post which may or may not get up to 5.5 amps.

I also say that your first battery bank is a learning one. So if you can get them cheap or free the go for it. But don't expect them to last their posted life since we all "learn" how to keep our first bank happy.

[wunder]

Thanks much! I'll sleep on it before I pick up the batteries this weekend, but this is really a 'just for fun' project so all learning is good.

[SunEagle]

Thanks much! I'll sleep on it before I pick up the batteries this weekend, but this is really a 'just for fun' project so all learning is good.

Your welcome. I hope you enjoy solar technology like you do astronomy technology. I would kill for that scope you showed in the other post. Right now I have a cheap Celestron 102SLT which isn't anywhere close to being in the same ball park as what looks like your Cassegrain model.

[wunder]

I've been doing astrophotography for a lot of years If you want to talk about a money pit, that's the ticket! But it's very rewarding. Actually that SCT scope wasn't terribly expensive, but the mount was about $4k. If you want to just do visual astronomy, Dobsonians are the way to go- you can get a really nice 10 incher for just a few hundred bucks.

[SunEagle]

I've been doing astrophotography for a lot of years If you want to talk about a money pit, that's the ticket! But it's very rewarding. Actually that SCT scope wasn't terribly expensive, but the mount was about $4k. If you want to just do visual astronomy, Dobsonians are the way to go- you can get a really nice 10 incher for just a few hundred bucks.

Thanks for the tip. I now live out in the country so I do get some pretty good views of the stars when the moon isn't full.

[Sunking]

The bikes each have 11.6 ah @ 36 volts = 417 watt hours
My 100w solar panels apparently will generate about 400 watt hours per day from what I've read about solar hours and effective power generation.

Hate to tell you, but it is not going to work. You have a 12 volt battery panel trying to charge a 36 volt battery.

What is wrong with that picture?

It would require 3 of those panels wired in series using a Charge Controller that can charge a 36 volt battery. Good luck with that.

To do what you want would require you to have a 12 volt battery, a large one, and an Inverter to run a conventional charger. That can be done but very wasteful because of all of the conversions. To get 800 wh into a 36 volt battery is going to require the panels to generate 2000 watt hours per day. That will require 500 or more watts of panels, a 12 volt 400 AH battery (, and a expensive 40 amp MPPT charger. All that to generate less than 5 cents worth of electricity. You are looking at $2000 for a crappy charger.

It would be a lot less expensive and a lot better environmentally to use good ole commercial power and the bike charger. Off grid is extremely expensive and nasty nasty CO2 emission. Far more than any fossil fuel would generate. Not to mention a whole lot faster in 2 hours vs a day or two using solar.

I suggest you rethink this idea. Not well thought out.

[Mike90250]

What's the charger for each bike, consume from 120VAC ? You need a solid number, not a guess.
How long daily, do you need to charge ?

Here's my suggestion
Look for a couple 200 -300w panels. Maybe a single 300w panel will work
Use the appropriate charge controller PWM or MPPT
Use a single 12 v marine deep cycle battery (less expensive than smaller "solar" batteries
use a suresine 300w inverter, and only charge batteries while sun is on the panels (or other efficient small inverter, nothing from autoparts store)

[wunder]

Hate to tell you, but it is not going to work. You have a 12 volt battery panel trying to charge a 36 volt battery..

I'll have to have a conversation with my existing equipment which has been working fine for weeks to occasionally charge 1 battery from 50% to let it know it's not supposed to do this.

It would require 3 of those panels wired in series using a Charge Controller that can charge a 36 volt battery. Good luck with that.

Only if I were not using an inverter which I already said I was using

To do what you want would require you to have a 12 volt battery, a large one, and an Inverter to run a conventional charger. That can be done but very wasteful because of all of the conversions. To get 800 wh into a 36 volt battery is going to require the panels to generate 2000 watt hours per day. That will require 500 or more watts of panels, a 12 volt 400 AH battery (, and a expensive 40 amp MPPT charger. All that to generate less than 5 cents worth of electricity. You are looking at $2000 for a crappy charger.

You're claiming this setup would only be 40% efficient at converting solar into the bike batteries? What is your basis for this? From what I've read, inverters are 85-90% efficient, and Lithium-Ion battery charging is close to 99% efficient. Being conservative, it seems like I should be in the ballpark of 80% efficient, not 40%, and as I already mentioned I'm targeting 50% recharge of both batteries a few times a week.

It would be a lot less expensive and a lot better environmentally to use good ole commercial power and the bike charger. Off grid is extremely expensive and nasty nasty CO2 emission. Far more than any fossil fuel would generate. Not to mention a whole lot faster in 2 hours vs a day or two using solar.

I already have most of this gear except I'm considering a bigger battery. I assume you're talking about CO2 created to produce the gear? AFIK, solar panels do not produce CO2 directly

I suggest you rethink this idea. Not well thought out.

I already own most of this equipment and riding a bike powered by sunbeams is fun. If you don't think so I think you should find a new hobby like yelling at kids to get off your lawn.

[wunder]

What's the charger for each bike, consume from 120VAC ? You need a solid number, not a guess.

from my original post:

2 amps @ 42v = 84 watts

That's not a guess, it's what is printed on the charger

[Sunking]

You're claiming this setup would only be 40% efficient at converting solar into the bike batteries? What is your basis for this? From what I've read, inverters are 85-90% efficient, and Lithium-Ion battery charging is close to 99% efficient. Being conservative, it seems like I should be in the ballpark of 80% efficient, not 40%, and as I already mentioned I'm targeting 50% recharge of both batteries a few times a week.

So you have an Inverter with a 12 volt battery to make it work. I told you that is the only way to make it work which is real silly and wasteful. Why you ask, because of everything you are ignoring. You have a PWM Charge controller, which changes your 100 watt panels into 66 watts if even that high because it is from Harbor Fright. Next you have a 12 volt battery which at best has a 80% charge efficiency. Next in line you have a cheap Inverter with any where from 70 to 90% efficiency. Then we have your battery charger with unknown efficiency but lets pretend it is top of the line and say 90% efficiency. Lastly we get to your bike's battery which has its own efficiency, and depending on battery type can be 60 to 95% efficient.

So you start with a 100 watt panel that never generates 100 watts but lets be ignorant and say it does generate 100 watts. 100 watts x .66 (PWM CC eff) x .8 (12 volt battery eff) x .8 (Inverter eff) x .9 (Bike charger eff) x .9 (bike battery) = 34ish watts from a 100 watt panel that never generates 100 watts.

I already have most of this gear except I'm considering a bigger battery. I assume you're talking about CO2 created to produce the gear? AFIK, solar panels do not produce CO2 directly

No I did not include all the CO2 used to make the panels, charge controller, Inverter, and bike charger. Hell if I did that it gets really nasty. I was just referring to the batteries alone. With just the batteries you are paying up some 10 times more for electricity because the ROI and EROI of a battery is negative. Throw in all th eother stuff and it makes you a very heavy polluter paying up to $2 per Kwh rather than buying it for 12-cents.

I already own most of this equipment and riding a bike powered by sunbeams is fun. If you don't think so I think you should find a new hobby like yelling at kids to get off your lawn.

Charging any battery is not fun regardless of the energy source. You must like watching paint dry. Me I want it charged as fast and cheap as possible so I do not have to hang around and watch paint dry and grass grow while loosing my kids money.

[Sunking]

from my original post:

2 amps @ 42v = 84 watts

That's not a guess, it's what is printed on the charger

That is the output, not the INPUT. The Input is HIGHER. No such thing as unity gain.

[Mike90250]

OK, sorry I missed the 84 watts. How many hours to recharge 3 hours each bike ? 84x3= 252 watt hours per bike ? x 2 bikes = 504 wh

When do you ride the bikes Noon ? Morning ? Evening ? It matters because you will either be charging directly from the sun, or storing power in an intermediate battery.

100w HF panel. Sadly, it's turned out that the cardboard the panel comes in, is more valuable than the panel. But while it's working, great

The most practical battery is a cheap battery, generally a 80ah 12v battery. 100w charging it, is barely sufficient, but it's all in the timing.
, using the inverter to charge the bikes in the morning so solar can recharge the battery in the day, is preferable to using the inverter to charge at night, and letting the battery sit drained all night.

The longer and deeper a battery is discharged, the shorter it's life, you want to maximize solar utilization.

Sump pump. Would a 12V marine bilge pump, if it meets the flow and lift requirements, be a option to avoid an inverter ? How often does Grid power pump fail and what expense are you willing to go to for a 80% solution or do you need a 99.5% solution ? If grid fails, that means storm which means no solar for recharge .