Paralell Batteries on a Solar Ebike

[Little Voice]

Hi Folks my first post on this forum which I am new to, so sorry if this has been covered before (I have done some searching but not found an answer) and thanks for having me. Hopefully I am posting in the best section for this topic.

I am not an electrican, as will be evident from viewing my first sketch of a, for now, theoretical, Solar trailer powered, PAS E-bike with Bafang mid motor.
Hopefully it makes sense, it lacks some details but I thought best to keep it simple.

A small intro to my project:
As solar power tends to vary in temperate climates (bright and also cloudy days), then although down-stepping MPPT charge controllers are more efficient, boost ones are more likely to be working all day long (although only a little when completely overcast). The boost MPPTs obviously need the voltage to be lower than the target battery voltage. Whilst trying to maximize the solar power input, I therefore I have decided to try and run 2 smaller batteries in terms of Amp hours and run them separately until they meet at the motor (controller) connection obviously. I am not a programmer and want to keep the system as simple as possible (therefore not wanting to write a small circuit board program to manage things, plus it means less to go break and fix). So I am willing to monitor the batteries myself and switch manually between them. Bosch says of their dual battery system, that by alternating between batteries (say using 10 or 20% of one then switching to the other) it allows the battery to recover when not in use. In theory using this design will be getting a rest and a top up charge too, if it works.

Having not built an ebike before I have been studying up somewhat, in an effort to look before I leap.
I have read a posts on ebike forums etc that say with parallel batteries if one is in a lower state of charge the other may discharge into it.
This was a potential problem I was stuck on for a few days until I discovered the Luna Remote on-off Solid State Switch, so now I have hope again. Weblink to that here https://lunacycle.com/remote-on-off-solid-state-switch/
It is possible that I could make a mistake and put both batteries on at the same time, and I am hoping that wouldn't be an instant disaster if I do (and have the Luna Remote Solid State Switches installed).

I am not too sure if the remote on-off solid state switch's MOSFETs will block discharge from the other battery or not please?

Does it have an in-built diode or some other effect to stop or limit cross current between batteries please?

Also does the remote micro switch get powered by the main battery or do I need to add a small charge to it from elsewhere (and if so at what volt/amps) please?

Maybe this wiring is all wrong? As I said before I am new to e-bikes and keen to learn, although I have done a fair amount of cycling at least albeit it years ago.
I am very happy that ebikes have grown over recent years, and I have gone without a car for many years now and see E-bikes as a low impact solution that can help people manage our impact on our beautiful world and get some exercise too.
Please ask if I have left any important info out that's needed to respond to my questions, thanks.
Thank you very much for any help, advise, suggestions and tips that you may offer.

 

[wturber]

There is a wealth of knowledge in this thread. Probably too much. But you may find reading through it useful.

https://endless-sphere.com/forums/viewtopic.php?t=94721.

FWIW, I run 8 packs of 36volt 10s2p "hoverboard" battery packs in parallel and have for almost two years now. The always measure within about a 100th of a volt of each other. They are pretty much continuously left in parallel for charge and discharge through riding. The only exception is that I'll check the individual packs every three months or so. But my main check is to just monitor the voltage of the combined packs. I look at the voltage about a half hour after riding and if it isn't changing 12 hours later when I ride again, then none of the batteries are discharging into the others. Simple.

I think a major key to success is to use high quality cells and monitor voltage routinely so that you'll recognize any unusual behavior right away. I also suspect that you are better off not switching back and forth between two banks of batteries. Doing that puts the batteries under twice the strain. Instead, I'd parallel everything and figure out how to charge the whole system while riding or parked.

 

[billvon]

As solar power tends to vary in temperate climates (bright and also cloudy days), then although down-stepping MPPT charge controllers are more efficient, boost ones are more likely to be working all day long (although only a little when completely overcast).

There is very little difference between the two. Both will work "all day long." Both are efficient. About the only reason I'd go with boost over buck (i.e. standard MPPT) is if you are going to have a lot of shading issues AND you are going to use multiple panels.

Whilst trying to maximize the solar power input, I therefore I have decided to try and run 2 smaller batteries in terms of Amp hours and run them separately until they meet at the motor (controller) connection obviously. I am not a programmer and want to keep the system as simple as possible (therefore not wanting to write a small circuit board program to manage things, plus it means less to go break and fix). So I am willing to monitor the batteries myself and switch manually between them. Bosch says of their dual battery system, that by alternating between batteries (say using 10 or 20% of one then switching to the other) it allows the battery to recover when not in use.

You get more power AND total energy out of a battery system by leaving them in parallel all the time. Batteries do not "recover when not in use."

I am not too sure if the remote on-off solid state switch's MOSFETs will block discharge from the other battery or not please?

Most DC SSR's use back to back MOSFETS and will block current in both directions.

Also does the remote micro switch get powered by the main battery or do I need to add a small charge to it from elsewhere (and if so at what volt/amps) please?

Most SSR's need from 3 to 32 volts to turn on. Almost no current; usually 5-10ma.

 

[Little Voice]

There is a wealth of knowledge in this thread. Probably too much. But you may find reading through it useful.

https://endless-sphere.com/forums/viewtopic.php?t=94721.

FWIW, I run 8 packs of 36volt 10s2p "hoverboard" battery packs in parallel and have for almost two years now. The always measure within about a 100th of a volt of each other. They are pretty much continuously left in parallel for charge and discharge through riding. The only exception is that I'll check the individual packs every three months or so. But my main check is to just monitor the voltage of the combined packs. I look at the voltage about a half hour after riding and if it isn't changing 12 hours later when I ride again, then none of the batteries are discharging into the others. Simple.

I think a major key to success is to use high quality cells and monitor voltage routinely so that you'll recognize any unusual behavior right away. I also suspect that you are better off not switching back and forth between two banks of batteries. Doing that puts the batteries under twice the strain. Instead, I'd parallel everything and figure out how to charge the whole system while riding or parked.

Thanks very much for your reply Wturber.
I have seen a few of the linked guys youtube vids I believe. As far as I am aware he is putting 4x12vdc SLA batteries in series as they are moduluar and can be taken out and transported on aircraft without breaking safety regulations, for his planned round the world trip. Maybe I am remembering the wrong person.
I will take the time to go through his thread when I have time, there are a fair few pages there

I don't understand how using 2 batteries on at the same time won't be problematic, as that could potentially double the available amps if I am not mistaken.

As there seems to be only one decent portable waterproof MPPT charge controller on the market and its max recommended volts/watts and amps are constricting, I came to the idea of splitting my batteries into 2, as otherwise when a solo battery is full the 2 MPPTs can start 'looking' at each other. Plus there's a potential issue of them not working together well.
So on the other end I am then left with the issue of trying to separate the batteries, as I am thinking of doing long hauls off grid, so balance charging would only be possible through the batteries' BMS (not the MPPT as far as I am aware).

I suppose I can run a small charge to an ON-OFF-ON master switch, on a waterproof DPDT in theory, but just how to do that is beyond my current level of knowledge.

 

[john61ct]

You have a lot of flawed assumptions behind decisions you appeared to have already made.

In other words you are making design mistakes due to your thinking you know a lot more than you actually do.

Suggestion: inspect those assumptions, turn them into questions and assume you you do not yet know the answers.

Do not worry about different types of batteries, or differences in use cases, focus on basic DC electrics at the 101 level.

 

[Little Voice]

You get more power AND total energy out of a battery system by leaving them in parallel all the time. Batteries do not "recover when not in use."

Thanks so much for your reply Billvon.
I got that idea from something relating to Bosch's dual battery system, a video I saw a few weeks ago i think. Maybe they are telling fibs, or whoever it was who said it is misinformed.

As I am thinking of long haul energy efficiency then I am not too sure giving more possible ampage to the (motor) controller is necessary. Maybe I do not understand paralell effects so well. I guess I may need to read more, this project is so big as it requires knowing about bikes, solar, ebike stuff, lightweight camping gear and so on

My e-bike project is an off grid touring bike with solar trailer where every scrap of energy counts. I therefore don't want to use the power up quickly, but say use pedal assist at about 50% of the actual work needed to turn the rear wheel, and get in on average what I am using. Maybe it would have been helpful if I had said before, apologies for that.

Maybe having heard what I have to say you still feel that I would benefit from running 2 batteries at the same time?

 

[Little Voice]

You have a lot of flawed assumptions behind decisions you appeared to have already made.

In other words you are making design mistakes due to your thinking you know a lot more than you actually do.

Suggestion: inspect those assumptions, turn them into questions and assume you you do not yet know the answers.

Do not worry about different types of batteries, or differences in use cases, focus on basic DC electrics at the 101 level.

Thank you for your comments John61ct.
Starting from no or little knowledge, I have been reading up on solar power, MPPT charge controllers, bicycles, ebikes, bike trailers and camping for quite some time now.
The path I have taken is not based on any pre-assumptions on my part. Maybe I have been misinformed by some, but I generally get my info from more than one source (so maybe then it is common misunderstandings I am reading). There is not a massive amount of info on off grid solar ebikes, how to wire them and so on. I have studied a fair bit before posting, and am staring to post now to get informed advise on what I am not clear on.

Which assumptions that you see me making are the biggest mistakes please?
Thank you

 

[wturber]

I have seen a few of the linked guys youtube vids I believe. As far as I am aware he is putting 4x12vdc SLA batteries in series as they are moduluar and can be taken out and transported on aircraft without breaking safety regulations, for his planned round the world trip. Maybe I am remembering the wrong person.

SLA = Sealed Lead Acid. I'm sure he isn't using that. He may be using the Grin Ligos. They are a super-neat system (especially for traveling), but are a tad pricey.

I don't understand how using 2 batteries on at the same time won't be problematic, as that could potentially double the available amps if I am not mistaken.

Having a higher amount of amps available presents no problem. You motor controller will decide how much to draw and send to the motor. Just because a combined battery CAN deliver more amperage doesn't mean that it will.

The advantage to having everything in parallel is that each individual cell experiences a lower maximum discharge rate because the load is shared across more cells. That's good because the cells are most efficient at lower discharge rates. In fact, the rated amp/hours for a single cell (18650 type) is often given at a fairly low discharge rate.) They also tend to last longer since they are stressed less. Also, if you are using regen, they are better able to accept a large charge input because the input amps are spread between more cells.

As there seems to be only one decent portable waterproof MPPT charge controller on the market and its max recommended volts/watts and amps are constricting, ... <snip>

I can't speak to the solar charge side of things. But I'll point you to another thread for a solar powered "bike" went from France to Tehran. So you might want to look at what they are doing. Also, the Grin Cycle Analyst is able to help you track your solar input. You might want to look into that as well.

https://endless-sphere.com/forums/viewtopic.php?f=6&t=93482

https://endless-sphere.com/forums/viewtopic.php?f=6&t=93482&start=100#p1386620

I think billvon knows a bit about the solar side of things. So make sure to pay attention to what he has to say.

 

[billvon]

Maybe having heard what I have to say you still feel that I would benefit from running 2 batteries at the same time?

Yes. Two batteries running together can be thought of as one battery at a time, each running at half the needed power. So lower I2R losses, more capacity before you hit the low voltage limit and less heat in the battery if you run parallel instead of one at a time.

Of course there may be other reasons to run one at a time, like you want to completely discharge one so you can charge it partially later while leaving the second one full. But if your goal is efficiency and getting as much as possible out of the battery, go parallel.

 

[Little Voice]

Having a higher amount of amps available presents no problem. You motor controller will decide how much to draw and send to the motor. Just because a combined battery CAN deliver more amperage doesn't mean that it will.

The advantage to having everything in parallel is that each individual cell experiences a lower maximum discharge rate because the load is shared across more cells. That's good because the cells are most efficient at lower discharge rates. In fact, the rated amp/hours for a single cell (18650 type) is often given at a fairly low discharge rate.) They also tend to last longer since they are stressed less. Also, if you are using regen, they are better able to accept a large charge input because the input amps are spread between more cells.

I can't speak to the solar charge side of things. But I'll point you to another thread for a solar powered "bike" went from France to Tehran. So you might want to look at what they are doing. Also, the Grin Cycle Analyst is able to help you track your solar input. You might want to look into that as well.

https://endless-sphere.com/forums/viewtopic.php?f=6&t=93482

https://endless-sphere.com/forums/viewtopic.php?f=6&t=93482&start=100#p1386620

I think billvon knows a bit about the solar side of things. So make sure to pay attention to what he has to say.

Thanks for sharing your knowledge of how batteries in parralell work Wturber.
I will definitely think it over.
I am told that usually battery BMSs do not like to be charged in parralell.

I guess from the other end of the solar charge controller, without separation, it seems that the other battery looks in series (via the controller connection), which probably is not ideal either.
On the plus side good BMSs will limit the input charge to a battery at about 10A, at least the EM3EV ones do, so that is a nice safety feature on the Li-ion batteries.

I am erring towards looking at DPDT ON-OFF-ON marine switches and isolating the batteries though for now.
Its a work in progress so time will tell, I definitely appreciate constructive input.

Thanks also for the links to the thread by the guy from Grin who did the SunTrip 2018, haven't seen them before, so I will carefully go through that material at some point soon

 

[Little Voice]

Yes. Two batteries running together can be thought of as one battery at a time, each running at half the needed power. So lower I2R losses, more capacity before you hit the low voltage limit and less heat in the battery if you run parallel instead of one at a time.

Of course there may be other reasons to run one at a time, like you want to completely discharge one so you can charge it partially later while leaving the second one full. But if your goal is efficiency and getting as much as possible out of the battery, go parallel.

Cheers Bill Von for the helpful comments.
Much material online I have read is old and bike technology moves on.
But it is good to learn from the mistakes of the past, when still relevant.
So I have stayed clear of parralell battery set ups so far, you have made me think again, and I think that is good

Even though I plan to use same solar panels, same charger on same setting and same batteries, there may be disparities in the batteries so this has informed my thinking about trying to keep them separate.

Best wishes and thanks again

 

[billvon]

Even though I plan to use same solar panels, same charger on same setting and same batteries, there may be disparities in the batteries so this has informed my thinking about trying to keep them separate.

OK if you use separate batteries (different chemistry or series number) then that IS a good reason to keep them separate for charging. If you end up doing that, you may want to consider diode* ORing their outputs. That way they can charge separately but discharge together.

(* - diode can be simple diode or active FET based diode)

 

[amberwolf]

I am told that usually battery BMSs do not like to be charged in parralell.

Charging in parallel is perfectly fine as long as you disconnect the discharge connector (if they are separate. if they are the same connector it doesn't matter).

This presumes both batteries are the same chemistry and number of series cells (number of parallel cells doesn't matter).

On the plus side good BMSs will limit the input charge to a battery at about 10A, at least the EM3EV ones do, so that is a nice safety feature on the Li-ion batteries.

The BMS doesnt' really limit current. It may have a current limit but all that does is shut off input or output (whcihever it's designed to, or both), completely. It doesn't regulate the current.

So if you have a charger that is set for say, 20A output, and the battery can't handle more than 10A input, then *if* the BMS has an input current limit, it'll simply shut off the input completely, preventing the battery from charging at all.

I am erring towards looking at DPDT ON-OFF-ON marine switches and isolating the batteries though for now.

FWIW, I would recommend paralleling the batteries rather than switching between them; it will reduce the load on each battery, so they don't heat up as much, don't age as quickly, and don't have as much voltage sag (so you have more watts available, and potentially more watt-hours).


This presumes both batteries are the same chemistry and number of series cells (number of parallel cells doesn't matter). If they are differnet chemistries, or different numbers of series cells, then except for certain special cases, you can't connect them in parallel.

It will also eliminate some lossy resistive points in the system (extra connectors and switch contacts) that are also other potential failure points. If you're going to have a switch in there anyway, rather than just unplugging the battery, then that doesn't apply.

The plus side of independent (switched) batteries is that if one of them completely fails somehow and shorts out (very unlikely) it can't take the other one with it.

 

[Little Voice]

OK if you use separate batteries (different chemistry or series number) then that IS a good reason to keep them separate for charging. If you end up doing that, you may want to consider diode* ORing their outputs. That way they can charge separately but discharge together.

(* - diode can be simple diode or active FET based diode)

Thanks Bill Von I will keep this in mind

 

[Little Voice]

Charging in parallel is perfectly fine as long as you disconnect the discharge connector (if they are separate. if they are the same connector it doesn't matter).

This presumes both batteries are the same chemistry and number of series cells (number of parallel cells doesn't matter).

The BMS doesnt' really limit current. It may have a current limit but all that does is shut off input or output (whcihever it's designed to, or both), completely. It doesn't regulate the current.

So if you have a charger that is set for say, 20A output, and the battery can't handle more than 10A input, then *if* the BMS has an input current limit, it'll simply shut off the input completely, preventing the battery from charging at all.

FWIW, I would recommend paralleling the batteries rather than switching between them; it will reduce the load on each battery, so they don't heat up as much, don't age as quickly, and don't have as much voltage sag (so you have more watts available, and potentially more watt-hours).


This presumes both batteries are the same chemistry and number of series cells (number of parallel cells doesn't matter). If they are differnet chemistries, or different numbers of series cells, then except for certain special cases, you can't connect them in parallel.

It will also eliminate some lossy resistive points in the system (extra connectors and switch contacts) that are also other potential failure points. If you're going to have a switch in there anyway, rather than just unplugging the battery, then that doesn't apply.

The plus side of independent (switched) batteries is that if one of them completely fails somehow and shorts out (very unlikely) it can't take the other one with it.

Thank you Amber Wolf for sharing the knowledge you have gained through experience.